2 hz tuning - pros and cons - Page 10 - AVS Forum
Forum Jump: 
Reply
 
Thread Tools
post #271 of 302 Old 12-11-2012, 11:21 PM
AVS Special Member
 
robertcharles's Avatar
 
Join Date: Jul 2007
Location: New Orleans
Posts: 1,658
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 14
http://howthingsfly.si.edu/aerodynamics/air-motion

I hope this helps. GIve it a read and see if it makes sense. Just trying to help.

It has a very nice animation of what has been described.
robertcharles is offline  
Sponsored Links
Advertisement
 
post #272 of 302 Old 12-11-2012, 11:38 PM
AVS Special Member
 
Mark Seaton's Avatar
 
Join Date: Jul 2000
Location: Chicago, IL, USA
Posts: 5,978
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 12 Post(s)
Liked: 188
Wow. This has gotten ridiculous on many fronts. DIY, I get the feeling this thread is more about "winning" or being right over some argued point than actually getting feedback or gaining further understanding.

First and foremost models need to correlate to the real world. Professionals in any field know the point at which their models are subject to the assumptions being made and entered into the model. If a more accurate model is needed, correlation needs to be done with an applicable real device.

I'll make a few comments and direct to a few issues, but I'll only be at the computer briefly over the next day or two. Don't expect a volley of responses.
Quote:
Originally Posted by diy speaker guy View Post

Quote:
Originally Posted by Mark Seaton View Post

Varying Q related to exact geometries are separate from compression effects. Simple fact: A large diameter port that is a fraction of its diameter in length behaves rather differently at tuning than a port that is 5-20x its diameter in length. In general you will also see that real world losses mean that boxes which appear to have very high Q peaks at tuning never have peaks as high as the model.
Ok, but in your first post of this thread you said compression was the problem. I already mentioned q and losses way back on page 1 or 2 when I explained the WinISD velocity spike at 75 hz made me nervous but was likely not real. So what is the exact problem here? Q, compression, velocity? Where exactly does the model break down?

Compression and audible chuffing are related but not identical matters. You can most certainly have compression without chuffing when sufficient flaring is used, and you can have chuffing around sharp edges when there is very little compression. These are observations on real devices, not a hunch based on models.

Find the UniBox spreadsheet where you can play with Q values and see how much Q and loss/leakage factors can affect the response. In some designs it makes minimal difference, in others it will be much more. It would be incorrect to say the model breaks down, but rather the assumptions break down or need to be confirmed/correlated. The real device can quickly diverge from the model with the realities of where and how the port is terminated/flared, how many bends are used and how tight those bends are, along with stuffing of the box and any internal length interactions with the port and the box (which can be an issue in a 700L box).
Quote:
Quote:
The fact that we see a bunch of different programs showing different Q to the driver excursion minimum and port velocity points to the Q and loss assumptions I'm referring to. As with any model, the best use always comes from understanding the assumptions it was built on and variables involved. In this case we have at minimum Ql, Qa & % fill of the box to contend with, and then the effects of the end terminations which will all affect the sharpness or Q of the behavior, as well as losses.
It's not that the box won't work, it's that it is unlikely to be demonstrably/significantly better than the same driver in a sealed box of the same or bit smaller size. If you aren't leaving sine waves on for minutes at a time well below 16Hz, more power than the rating is not a problem thermally. For real listening without running hard into clip limits, it's unlikely to ever be a problem. The bigger concerns come down to real world excursion limits in a given box after suspension stiffening, BL drop and mechanical clearances are considered.

Thanks. In your first post, you mentioned that if I use a program that could simulate compression I would see this design can't handle more than 250 watts or so. Which program do you recommend?

There are a few programs which make attempts at modeling some of these factors. One of a few different programs I use is LspCAD which makes an attempt to estimate port losses which I have found to be much more accurate than those programs which make no attempt. Having built and measured many different designs, including boxes tuned in the 9-12Hz range, reality typically falls somewhere between the ideal and the predicted compression and losses. The particular details of a design's execution will determine how close the results are to the ideal vs. the maximum predicted loss/compression. The best way to test this is with a near field measurement of the driver to monitor the shape of the output/excursion minimum at tuning.

This brings us to your model:
Quote:
Originally Posted by diy speaker guy View Post


It would appear you have significant losses and leaks included in your model of the sealed box. The rise in excursion below 3-5Hz does not match your predicted response which drops 13-14dB/octave between 1-4Hz. If the box is reasonably sealed you will not observe this added rise in excursion and the benefits of your comparison change significantly.

Let's ignore any potential port compression and just look at the prediction of your ported design. Unless you have a perfectly straight port, flared heavily with those flares mounted to flat baffles, you won't see the shape of response and excursion profile modeled. Losses, port termination, and other realities will soften the magnitude peak somewhat. The important understanding is that as that peak softens, both the total level comes down *AND* the excursion minimum becomes less deep.

Taking a next step to consider possible port compression, there will be a small loss in observed SPL from the system, but that loss/compression comes from reduced port contribution and increased driver excursion vs. the ideal at tuning. If you did reach 10-15m/s, you would see the notch in the driver's near field response reduce to barely a saddle. This can be easily observed by driving a high output ported box at tuning with a sine wave. The model may suggest you would never see more than a few mm of excursion, but reality shows the driver moving a multiple of this.

If you correct your sealed model where the excursion has much less rise below 5Hz, and you then examine what happens with real world Q of the port and the onset of even minor port compression, you will see the differences quickly converge and the benefits minimize. If you do some careful searching on the net there are many measured and observe examples of these effects. I leave that for you to investigate.

Mark Seaton
Seaton Sound, Inc.
"Make no little plans; they have no magic to stir men's blood..." Daniel H. Burnham
Mark Seaton is online now  
post #273 of 302 Old 12-12-2012, 01:14 AM - Thread Starter
Senior Member
 
diy speaker guy's Avatar
 
Join Date: Jan 2011
Posts: 303
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 12 Post(s)
Liked: 25
Quote:
Originally Posted by Mark Seaton View Post

Wow. This has gotten ridiculous on many fronts. DIY, I get the feeling this thread is more about "winning" or being right over some argued point than actually getting feedback or gaining further understanding.

If you could only understand how untrue that is. I've been caught in an OCD nightmare for 3 days now. I literally cannot stop until I find the right answer. I almost threw up this morning when I woke up, that's how badly this has upset me. I hate confrontation but the need to have the correct answer won't let me stop until I get it.

Besides, I don't believe you would have taken the time to respond at all (much less in such detail) if you thought I was just a narcissistic bully at heart. What would be the point?

I admit I get extremely frustrated and angry when people don't pay attention and I have to explain the same details over and over. I've had to repeat some things over 20x and still people can't catch on to what was clearly said in post 1. I don't think anyone here is stupid, just not paying attention and dismissing the stranger with low post count and relatively new join date. It was a huge mistake to bring an unconventional idea to this forum with no street cred in this forum on my behalf, I think that alone caused most of the problems with people not paying attention to details even when repeated ad nauseum, and my increasingly anger fueled responses to such. I'd like to think I'm a pretty reasonable guy if people have the respect to pay attention to what I"ve actually said and offer constructive criticism (with supporting evidence). There's been not much of either from most people involved here.
Quote:
I'll make a few comments and direct to a few issues, but I'll only be at the computer briefly over the next day or two. Don't expect a volley of responses.

I don't think I'm going to need any more responses, actually. If you had posted this detailed information the first time you posted here, it would have ended the discussion right there in it's tracks.
Quote:
Find the UniBox spreadsheet where you can play with Q values and see how much Q and loss/leakage factors can affect the response.
Quote:
This brings us to your model:
It would appear you have significant losses and leaks included in your model of the sealed box.
The rise in excursion below 3-5Hz does not match your predicted response which drops 13-14dB/octave between 1-4Hz. If the box is reasonably sealed you will not observe this added rise in excursion and the benefits of your comparison change significantly.

It's actually the new WinISD, not Unibox. This is a new laptop and I forgot to even check Ql and Qa after downloading the program just last week. (I had the old WinISD on the old laptop.) Surprise, surprise, you are correct, Ql is way too low. I can't believe I didn't notice this before but I'm not going to beat myself up about it because no one else did either until you pointed it out. Here's the new model with Ql changed to 50.



So there you go, no one is going to argue that this ported box sim is worth building. Certainly not me.
Quote:
Let's ignore any potential port compression and just look at the prediction of your ported design. Unless you have a perfectly straight port, flared heavily with those flares mounted to flat baffles, you won't see the shape of response and excursion profile modeled. Losses, port termination, and other realities will soften the magnitude peak somewhat. The important understanding is that as that peak softens, both the total level comes down *AND* the excursion minimum becomes less deep.
Taking a next step to consider possible port compression, there will be a small loss in observed SPL from the system
, but that loss/compression comes from reduced port contribution and increased driver excursion vs. the ideal at tuning. If you did reach 10-15m/s, you would see the notch in the driver's near field response reduce to barely a saddle. This can be easily observed by driving a high output ported box at tuning with a sine wave. The model may suggest you would never see more than a few mm of excursion, but reality shows the driver moving a multiple of this.

All of this is true but the bolded part is the important part, for me at least. The main point I've been trying to explain this whole time has centered around compression losses. The losses will certainly be there but they will be small within the operational passband (compared to what's been suggested by others) and the model will operate with reasonable agreement with the simulation (with the stipulations as stated in the above quote) at least down to the 10 m/s point as long as it's built exactly as modeled, with all the correct oversized flares and flanges that something like this needs, no elbows, etc. I think the Moody chart shows this pretty well, but like I said, it's the first time I've seen a Moody chart so I could be wrong about that. (BTW, I was a bit surprised to see that elbows had so much effect, just learned that tonight as I was putting the Moody chart together.)
Quote:
If you correct your sealed model where the excursion has much less rise below 5Hz, and you then examine what happens with real world Q of the port and the onset of even minor port compression, you will see the differences quickly converge and the benefits minimize. If you do some careful searching on the net there are many measured and observe examples of these effects. I leave that for you to investigate.

It's already done and I see it. The design will measure reasonably close to the simulation down to at least the 10 m/s point with some small but not unexpected losses due to compression and q changes at high volumes (now that I've fixed Ql) but there's really nothing much to gain.

Thanks Mark, I just wish you had posted this 2 or 3 days ago.

So to anyone who would like to poke a bit of fun in my direction, you can go right ahead, I was at least half wrong, there's no benefit to this design. But before you poke too hard, just remember that Mark was the only one able to provide any useful criticism whatsoever.
diy speaker guy is online now  
post #274 of 302 Old 12-12-2012, 02:41 AM
AVS Club Gold
 
JapanDave's Avatar
 
Join Date: Jul 2009
Posts: 2,327
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 13 Post(s)
Liked: 98
Quote:
Originally Posted by diy speaker guy View Post


So to anyone who would like to poke a bit of fun in my direction, you can go right ahead, I was at least half wrong. But before you poke too hard, just remember that Mark was the only one able to provide any useful criticism whatsoever.
You are kidding right?

You need to lighten up. Having a thread on a forum do that to you is unhealthy.

My build thread

My 8 x RE XXX 18" Subwoofers, IB build
Couldn't pour water out of a boot with instructions on the heel.

Do you know what Nemesis means?

JapanDave is online now  
post #275 of 302 Old 12-12-2012, 03:32 AM
AVS Addicted Member
 
LTD02's Avatar
 
Join Date: Oct 2003
Posts: 15,977
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 136 Post(s)
Liked: 855
Quote:
Originally Posted by diy speaker guy View Post


...there's no benefit to this design.

;-)

Listen. It's All Good.
LTD02 is online now  
post #276 of 302 Old 12-12-2012, 03:52 AM - Thread Starter
Senior Member
 
diy speaker guy's Avatar
 
Join Date: Jan 2011
Posts: 303
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 12 Post(s)
Liked: 25
Kidding about which part? Lots of people presented the same POV as Mark but without the supporting evidence, which makes all the difference in the world. I don't care much for unsupported speculation. Some of the speculation was right (the part about the design not having much if any benefit over sealed) and some was wrong (the part about massive turbulence and reverting to sealed as soon as the red line in Flare It is crossed). The biggest mistake I made (I think the only real mistake I made) was a simple stupid modeling error and the compounded assumptions based on that error and Mark was the only one able to point that out.

Back on page 2 or 3 I gave that other dude a hard time about not being able to use WinISD properly, and it turns out that was my fatal flaw. Ironic, huh?

Anyway, I got what I needed here, a 10000000 lb weight has been removed from my troubled psyche, I admitted my mistake and I expect a bit of well deserved joking at my expense. It doesn't bother me at all.

And while I agree that the occasional OCD attack is not healthy, the cure isn't quite as simple as deciding to lighten up. I've tried, it doesn't work.
diy speaker guy is online now  
post #277 of 302 Old 12-12-2012, 04:12 AM
Senior Writer @ AVS
 
imagic's Avatar
 
Join Date: Dec 2005
Location: Philadelphia, PA
Posts: 5,276
Mentioned: 6 Post(s)
Tagged: 0 Thread(s)
Quoted: 725 Post(s)
Liked: 1746
Remember this chart? Glad this debate was finally settled. The lesson is to always check your data entry into a sim, and always treat sims as a less than complete representation of a real system.

http://www.avsforum.com/forums/threads/add/forumId/155

Find out more about Mark Henninger at www.imagicdigital.com
imagic is online now  
post #278 of 302 Old 12-12-2012, 04:32 AM - Thread Starter
Senior Member
 
diy speaker guy's Avatar
 
Join Date: Jan 2011
Posts: 303
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 12 Post(s)
Liked: 25
Quote:
Originally Posted by imagic View Post

The lesson is to always check your data entry into a sim,

Ql and Qa are not supposed to be normal data entry items, they should be set to a reasonable value in the program by default. I've never seen Ql set to 10 by default and certainly wasn't expecting it so I didn't bother even checking, although obviously I should have. That's probably the reason why it completely escaped my attention even though the graph clearly doesn't look right, as Mark pointed out.
Quote:
and always assume that sims are a less than complete representation of a real system.

This is true. Despite not knowing the math (assumptions) that each program uses I've got enough experience with the 4 or 5 simulators that I use regularly to know where they gloss things over or completely ignore things. A lot of this can be inferred by what inputs the programs ask for. For example, if it doesn't ask for baffle size and shape you won't get a diffraction profile.
diy speaker guy is online now  
post #279 of 302 Old 12-12-2012, 06:11 AM
AVS Special Member
 
mastermaybe's Avatar
 
Join Date: Sep 2007
Posts: 7,139
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 36 Post(s)
Liked: 149
"So to anyone who would like to poke a bit of fun in my direction, you can go right ahead, I was at least half wrong, there's no benefit to this design. But before you poke too hard, just remember that Mark was the only one able to provide any useful criticism whatsoever."

I wouldn't flatter yourself, I doubt anyone really gives a gnat on an elephants a$$, to be honest.


James

Actual phone call (see pic to left):

 

Tech (responding to laughter): "I'm sorry sir, did I miss something?"

Me: "Yeah, a case of Diet Mountain Dew walking across my living room."

mastermaybe is offline  
post #280 of 302 Old 12-12-2012, 07:30 AM
AVS Special Member
 
robertcharles's Avatar
 
Join Date: Jul 2007
Location: New Orleans
Posts: 1,658
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 14
Quote:
Originally Posted by diy speaker guy View Post

Kidding about which part? Lots of people presented the same POV as Mark but without the supporting evidence, which makes all the difference in the world. I don't care much for unsupported speculation. Some of the speculation was right (the part about the design not having much if any benefit over sealed) and some was wrong (the part about massive turbulence and reverting to sealed as soon as the red line in Flare It is crossed). The biggest mistake I made (I think the only real mistake I made) was a simple stupid modeling error and the compounded assumptions based on that error and Mark was the only one able to point that out.
Back on page 2 or 3 I gave that other dude a hard time about not being able to use WinISD properly, and it turns out that was my fatal flaw. Ironic, huh?
Anyway, I got what I needed here, a 10000000 lb weight has been removed from my troubled psyche, I admitted my mistake and I expect a bit of well deserved joking at my expense. It doesn't bother me at all.
And while I agree that the occasional OCD attack is not healthy, the cure isn't quite as simple as deciding to lighten up. I've tried, it doesn't work.
.

Now you know why I wanted to go over each and every step. Hope you can just let these guys offer help. I really am just trying to help. We all can offer each other some ideas. Seems you like the tapped horns. Those seem to be fairly cheap to build depending on the driver and you have lots of knowledge already. Maybe build one of those. I happen to like what the Gjallarhorn, and that would be a cool build to me.

Anyway,

Hopefully no hard feelings,


Rush made the Rock and Roll Hall of Fame!


Robert
robertcharles is offline  
post #281 of 302 Old 12-12-2012, 12:06 PM
AVS Special Member
 
mastermaybe's Avatar
 
Join Date: Sep 2007
Posts: 7,139
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 36 Post(s)
Liked: 149
^ Yeah I think the voters finally figured out the best way to sneak them in was to sandwich them between Donna Summers and Public Enemy. smile.gif

James

Actual phone call (see pic to left):

 

Tech (responding to laughter): "I'm sorry sir, did I miss something?"

Me: "Yeah, a case of Diet Mountain Dew walking across my living room."

mastermaybe is offline  
post #282 of 302 Old 12-12-2012, 09:33 PM
Member
 
mdocod's Avatar
 
Join Date: Apr 2012
Posts: 46
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Hi DIY Speaker Guy,

I will say based on what I have skimmed that there is absolutely no reason for this to have been drawn out as long as it has been.

Simulations are based on small signal levels. As the frequency goes lower or the port gets smaller (or both), the signal level also has to go lower for the simulation to retain accuracy. Your simulations are "accurate," under the right circumstances but none of those circumstances are useful to the real world application. These simulation programs allow us to hypothesize output and Xmax and velocity for various input levels but do not typically take into account the physics that begin to take over as we increase signal level, pressure, velocity, compression etc. The evidence is right in the program. Understanding where the simulation starts to drop out of reality is something that comes with experience but you can gain most of this experience if you just apply common sense while tinkering with the program. The relationship of port gain, and the resulting loading effect, is dependent on the real world air velocity in the port achieving something close to simulation (this is common sense). If your common sense tells you that the air velocity at real world signal levels is likely to come up short due to resistance, then you should follow that common sense to a logical conclusion. Lets try to work our way to that conclusion...


In WinISD Pro, Set the simulation output to "rear port gain." Set the port to an insanely small diameter (like a fraction of an inch, like a leak), tinker with signal level, then set the port to a realistic diameter, tinker with signal level, set the port to something insanely large, tinker with signal level. Note the complete and utter LACK of anything changing in the simulated port gain. The rear port gain is simulated exactly the same whether you simulate it over a 1" diameter port or a 100" diameter port, at 0.1W or 10,000W. Throw enough zeros on the input signal or shrink the port size enough and the program will tell you that the particle velocity has exceeded the speed of light. The simulation always assumes small signal characteristics and calculates for gain and velocity in a linear fashion with those small signal simulation results as the starting position. We would use common sense to know that when we simulate beyond linear Xmax, the simulation looses accuracy. We know that while the simulation can show us that enough input power can result in the speaker cone making cross country flights 50 times a second, we know that in the real world we were smelling the byproducts of combustion and the cone hasn't even left the garage let alone made it to the airport.


winISD will show you the same port gain for a 0.25" hole drilled in the wall of the sub as it does for a port the size of a office building. What you are having a hard time wrapping your head around is that something 2" in diameter could "behave" like a pinhole. Use winISD to simulate that 2" by 90" port at a "real world" signal level of say, 100W, and you'll see a peak particle velocity far exceeding what your common sense tells you is likely to occur after factoring in some real world fluid dynamics (ask your common sense whether it is likely for air speeds to accelerate to 200MPH, stop, and accelerate to 200MPH again, several times per second over something that has a cross section to length ratio that is worse than a drinking straw, with a "100W air pump").

We know that compression effect comes into play at a fraction of these simulated port velocities, it is well documented and has been discussed to death a million times over in books and among forums the world over. The resistance to flow prevents the port from producing the gain and effective loading that is simulated at small signal levels (the resulting losses we call compression). Common sense tells us that the resistance of the drinking straw in question is in play regardless of the frequency in question. As you bring the frequency above that "peak" around tuning, all that changes is the EFFORT to move the air through the port, the resistance that prevents us from hitting 200mph peaks also drags down all other efforts at all other frequencies to pump air back and forth. The EFFECT of the port is heavily compressed at ALL frequencies. So while the simulation might show us a reasonable sub 15m/s at 4hz at 100W, that does not mean that we will actually ACHIEVE that velocity. For the same reason we won't hit 100m/s at 2hz, we won't hit 15m/s at 4hz. The resistance of the drinking straw significantly effects the outcome at either frequency. The fluid (air) is the same at both frequencies, as is the temperature, the resistance of the pipe effects the unchanged fluid viscosity equally at either 2hz or 4hz. If you don't hit the simulated velocity, then you also won't hit the simulated effect on driver Xmax. They are correlated events.


I seem to recall reading a blerp about an oil pipeline. Ask yourself. "If I cut the pressure at the head in half, but the fluid viscosity remains the same, have I changed the effective resistance?" The answer is no.

Regards,
Eric
mdocod is offline  
post #283 of 302 Old 12-12-2012, 11:54 PM
AVS Special Member
 
robertcharles's Avatar
 
Join Date: Jul 2007
Location: New Orleans
Posts: 1,658
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 14
http://www.bodziosoftware.com.au/PORTS.pdf
http://www.hometheaterhifi.com/volume_5_2/cmilleressayporting.html

http://www.aes.org/tmpFiles/elib/20121213/11094.pdf

THE MOST COMMONLY ASKED QUESTIONS ABOUT BUILDING ENCLOSURES

Many JBL users build their own loudspeaker enclosures. Their audio
skills range widely from novice to expert. From the thousands of
letters and calls we have received addressing the subject of
loudspeaker enclosure construction, we have determined the most common
questions and present the following Questions and Answers. The
particular questions listed attempt to answer as many questions as we
feel are necessary to provide enough information to build an enclosure
which will allow your JBL loudspeaker to operate to its potential. The
questions selected here concentrate on vented "bass reflex" enclosures,
since low frequency horns are fairly complex, and many good tested
designs exist. Also, it is often more econonomical to buy a bass horn
enclosure than to build one. Vented box enclosures are by far the most
popular enclosure type. Vented boxes are finding increasing use by
touring sound companies, displacing existing horn enclosure designs
because of the greater low frequency power output and extended low
frequency capability they offer when used in arrays. In addition to
their simple design requirements, vented loudspeaker enclosures offer
flexibility of design in shape, weight and component complement, and
usually produce the best results obtainable from modern loudspeaker
drivers at the lowest cost.

[1]
Q: What makes a good vented enclosure?

A: Basically, an enclosure serves to partition the front and rear of
the driver's cone, preventing the opposing air pressure changes
produced by cone motion from cancelling, and allowing the radiation of
sound from the front of the driver only. In addition, vented
enclosures allow the compressibility of the air inside the enclosure to
work as a more active part of the "system" consisting of driver and
enclosure. Beyond these two basic functions, a low frequency
loudspeaker enclosure should do absolutely nothing, that is, it should
add no effects of its own--no vibration, no tonality, no motion--
nothing to interfere with or absorb acoustic energy produced by the
driver.

[2]
Q: Is it possible to get low, punchy bass from a small enclosure?

A: Yes, if the driver in the enclosure is designed for low bass
operation in a small enclosure. Unfortunately, it's usually a small
driver that can work properly in a small enclosure, and that dictates
that lower sound levels will result from the small amount of air such a
small driver can move. Larger boxes (with larger bass drivers) produce
more bass, smaller boxes produce less bass. It's a fact of life, like
the fact that it takes a bass viol, a tuba, longer piano strings, or
very large organ pipes to produce bass energy in the air. Low bass
requires that more air move, and bigger boxes contain more air that can
be put to work making low bass.

[3]
Q: Can I get more bass from my enclosure by installing a bigger driver?

A: A given enclosure will not automatically produce more bass when a
larger driver is installed, in fact the opposite is often the result.

[4]
Q: What about putting two drivers in the enclosure to increase bass?

A: Placing two bass drivers in an enclosure designed for one will
usually produce less bass and more midrange output, and will upset the
operation of the driver-enclosure system because each driver will
behave as though it is installed in an enclosure which has only half
the internal volume of the original enclosure (with one driver).

[5]
Q: What should I do to use two drivers (for more bass)?

A: There are two alternative possibilities. When using two identical
drivers, you can build an enclosure with twice the internal volume of
the original enclosure that contained one driver, or you can duplicate
the original enclosure and stack the two. As the latter alternative
suggests, when building the double enclosure, it's necessary to treat
the enclosure as if it were two enclosures--you must double the porting
used on the single smaller enclosure--although it is not necessary to
divide the volume of the double enclosure unless two different driver
models (e.g. E130 and E155) are used and their interaction would be
undesirable. A usable example of this might be a 227 liter (8 cubic
foot) enclosure divided into two chambers so that the E130 occupies 57
liters (2 cubic feet) and the E155 occupies 170 liters (6 cubic feet).
In this case, the ports tuning either chamber to the same desired
frequency will be quite different.

[6]
Q: What does port or enclosure "tuning" mean?

A: In exactly the same way the resonant note from a bottle can be
raised and lowered by adding or pouring out liquid to change the
bottle's air volume, enclosure tuning is affected by the ratio of air
volumes in the port (the bottleneck) with its attendant flow
resistance, and the enclosure interior volume. Tuning of loudspeaker
enclosures is a result of manipulating the differences in effective air
mass between the enclosure interior and the air in the port. The
bottle-like nature of a vented enclosure is known as a "Helmholtz
resonator." The ports or ducts in a vented enclosure work only over a
narrow band of frequencies near the chosen tuned frequency, producing
the same effect noted when blowing across a bottleneck--a single
distinct pitch.

[7]
Q: Is it always necessary to use a port for good bass?

A: JBL uses vented enclosure designs because they are superior to
sealed enclosure designs in several important ways--as long as it is
possible to tightly control the loudspeaker driver parameters in
manufacturing as JBL does. Vented designs produce lower distortion at
the lowest operating frequencies, afford the driver protection against
mechanically destructive large cone excursion, and better enable the
driver to absorb and utilize its full power rating from an amplifier
when operating at low frequencies. It is important to keep in mind
that porting and tuning an enclosure provides air loading for the bass
driver down to frequencies just below the Helmholtz frequency, but does
not provide any loading for the driver at frequencies below that, such
as subsonic turntable rumble, record warp or microphone wind pickup.
If you intend to operate a sound system at high power levels, we highly
recommend an electronic high-pass filter to eliminate subsonic input to
the power amplifier(s). This will substantially increase the available
useful power from the amplifier which will then only operate in the
audible frequency range. Such a filter is the UREI model 501 Sub Sonic
Processor, or the built-in sub-sonic switches of the JBL Electronic
Frequency Dividing Network model 5234A.

[8]
Q: Where should I locate the port(s) with respect to the woofer?

A: Bass reflex enclosures are usually designed to tune from about 100
hertz and down. The length of sound waves at these low frequencies is
over 11 feet, so port placement is not critical. Ports may be located
anywhere on the baffle with no change in bass performance; some designs
even locate ports on the back of the enclosure which works well as long
as the enclosure is not close to a wall (a couple of port diameters
away) and there is an unobstructed air path between the woofer and the
port. Overall, it's safest to locate the port somewhere on the baffle
with the woofer(s) far enough away from side walls to avoid interaction
between port and enclosure wall or the fiberglass insulation on the
wall.

[9]
Q: What should the ducts be made of? Is round better than rectangular?

A: Port ducts may be made of anything rigid, such as paper cardboard
with about a 1.5 mm (1/16") or larger wall thickness. They can be any
shape, square or rectangular (such that port area remains constant) and
made of wood or other suitable material. It is not necessary to use
PVC pipe for port tubing, particularly when most carpet stores throw
away large amounts of heavy carboard tubing of between 3 and 4-1/2
inches inside diameter.

[10]
Q: What is the relationship of duct length to port area?

A: When port area is increased, independently of other factors,
enclosure tuning is raised. If duct length is increased, independently
of other factors, enclosure tuning is lowered. To keep the same tuning
(Helmholtz frequency) you will need to increase duct length as you
increase port area.

[11]
Q: How big should the port be?

A: The bigger, the better. Any port causes some resistance to air
movement, and so introduces unavoidable losses in output to the system
as a whole. The ratios of port area and length and enclosure volume
determine the Helmholtz frequency tuning. Mechanical reactance
elements, stiffness and air mass, control the effective air mass
ratios. At very low operating levels, where air in the port does not
move very fast, a small short port will behave the same as a large
longer port as far as enclosure tuning is concerned. At high power
levels however, the restricted air flow of the smaller port will
produce output level losses, some de-tuning and at high enough levels a
small port will cause the enclosure to behave like a sealed enclosure
with little or no contribution from the port. To minimize resistive
losses, the largest practical port should be used. Computer listings
of port choices calculated to limit air velocity inside the port duct
will list duct sizes which are normally impractical. A 380 mm (15 in)
diameter port is not an unreasonable choice for a 380 mm bass driver,
however the necessary length would dictate that such a port might
itself have a volume of many cubic feet, sometimes equal to or larger
than the original enclosure. A good rule of thumb would be to avoid
ports whose circular area is smaller than at least 1/3 the diameter of
the driver such as a 127 mm (5 in) diameter port for a 380 mm (15 in)
driver. This will usually provide sufficient port area so that the
port will not "whistle" when the system is operated at high power
levels near the helmholtz frequency--a sure indication of severe system
losses and potential power compression and low-frequency output
limiting.

[12]
Q: Can I use several smaller ports instead of one big one?

A: Yes, however there is a phenomenon associated with air resistance
resulting from air drag on the internal surfaces of port ducts and
turbulence at the ends of the ports that requires a duct length
correction when several ports are used. For example, when using four
100 mm (4 in) tubes instead of one 200 mm (8 in) tube (which has the
same port area but one-quarter the internal surface area), the length
needed will be slightly less than that needed for the single 200 mm
tube, perhaps 5% to 10% less, depending on overall enclosure volume.
These effects exhibited by port ducts is exaggerated by proximity of
the duct to enclosure interior surfaces or any other type of boundary
that may cause air turbulence near the end of the duct, therefore it's
important to keep duct ends away from the rear of the cabinet or other
obstructions by an amount at least equivalent to or larger than the
dimension across the port. If you are using a rectangular port that
has as one of its sides, an enclosure wall, you might have to use some
correction.

[13]
Q: Is there a simple mathematical way of designing proper enclosures?

A: Yes, a JBL scientist, D.B. Keele Jr., simplified the work of A.
Neville Thiele and Dr. Richard Small so that anyone with a pocket
calculator and a ruler or straight edge can design the right enclosure
volume and choose the right port or duct for a given loudspeaker
driver. JBL offers, at no cost, a four-page "kit" containing detailed
step by step instructions, written specifically for non-mathematicians,
showing how to use published Thiele-Small driver parameters in
enclosure design. Examples are shown with their results graphically
represented. An enclosure design flow chart and enclosure venting
nomograph are included.

[14]
Q: Should the enclosure's baffle be removable?

A: This is a question of mechanical strength and rigidity. All
enclosures, particularly those intended for rough portable use, should
be constructed with all sides permanently fixed by glue and screws, and
sealed air-tight by virtue of well cut and glued joints. It is
preferable to mount loudspeakers from the front of the baffle board to
eliminate the possiblity of reflections from the inside of the
loudspeaker mounting hole, thus it becomes unnecessary to provide for
removing the baffle. Woofer openings are usually large enough to reach
through in order to work inside the box, for example, to install other
components.

[15]
Q: Is there a preferred shape for loudspeaker enclosures?

A: There are a number of shapes that improve performance and some that
cause distinct degradation in performance. For single, full-range
drivers (e.g. JBL's LE8T) a sphere is the ideal shape for an enclosure
because the curved surfaces avoid the diffraction effects of cabinet
edges, which bend sound waves in a manner dependent on frequency. For
multi-way loudspeaker systems, spheres are usually impractical because
of the large size needed and because of the precise orientation
required for optimal listening. Conventional enclosures work best
mounted flush into a wall where diffraction is controlled by virtue of
the wall surface, and for free-standing enclosures, tilting, angled and
curving surfaces may be employed to help reduce or control edge
diffraction. The overall shape of the enclosure is relatively
unimportant except where the shape makes it difficult to build a rigid
enclosure. It is best to avoid enclosure dimensions that are multiples
of each other, such as 1 X 2 X 4 ratios, and strive to use dimensions
that have somewhat unrelated ratios such as 1 X 1.23 X 1.41.

[16]
Q: What is the best material to use for building enclosures?

A: For home and permanent installation use, high density particle wood
is the most cost-effective material for general enclosure construction.
The best wood to use for portable enclosure construction is 14 to 20
ply per inch Finland birch type. Birch plywood is very expensive
however, and a carefully braced enclosure made of high grade void-free
fir plywood can do the job just as well in most cases. The thicker you
can make the cabinet walls, the better the results will be because of
reduced wall vibration and resonance, but the tradeoff is cost and
weight. Enclosure walls should be cut so that edges form an air-tight
seal when glued together. Cleats and caulking can also be used if
needed to insure a good fit and tight air seal.

[17]
Q: Is bracing necessary? How much should be used?

A: Bracing should be added to the enclosure interior to minimize
enclosure wall vibration. Enclosure walls simply cannot be stiff
enough since wall vibration indicates that energy is being wasted to
move enclosure panels rather than moving air. 25 X 76 mm (1 X 3 in)
pine bracing fixed on edge with glue and screws to the enclosure walls
will help provide the minimum necessary stiffening without affecting
the internal volume significantly. If you are building large subwoofer
enclosures, bracing with two-by-fours works better, though you should
take the bracing volume into account since a 3 m (10-foot) length takes
up 12.9 liters (0.36 cubic foot) of enclosure volume.

[18]
Q: How should I mount drivers on the baffle?

A: Mount drivers on the front of the baffle whenever possible to avoid
the reflections from inside the mounting hole. Heavy drivers should
normally be front-mounted using Tee-nuts and machine screws or JBL's
MA15 clamps. If Tee-nuts are used, apply a bit of Bostic or Pliobond
type rubber glue to the inside of the nut flange to help avoid losing
the Tee-nut inside the enclosure when installing the driver. Baffle
board construction is much easier if all baffle parts are assembled
prior to final box assembly.

[19]
Q: Do I need fiberglass inside the enclosure?

A: JBL uses a 25 mm (1 in) padding of 1/2-pound density fiberglass
stapled to the enclosure interior on all surfaces except the baffle.
You should use 100 mm (4 in) thick dacron or 25 mm (1 in) fiberglass on
at least three of the surfaces of parallel interior walls. Keep sound
absorbing materials away from the port(s) as the air velocity inside
the port can be sufficient to tear off bits of the material and squirt
them out of the enclosure. It is not necessary to cover the inside of
the baffle, but doing so will rarely degrade system performance. The
enclosure exterior may be covered with your choice of any suitable
finish or decoration; this will not affect bass performance and in some
cases (as with Formica) may help stiffen the enclosure walls.

[20]
Q: Does Fiberglass significantly affect enclosure tuning?

A: No, not unless the enclosure is stuffed full of fiberglass, in which
case the apparent volume of the enclosure increases by 12% to 20% as
seen from the point of view of the bass driver. Stuffing the enclosure
full with fiberglass is not recommended because it introduces system
losses, is expensive and interferes with port operation. The exception
to this would be a sealed "air suspension" type system enclosure where
more virtual volume is needed and actual volume is not available,
and/or where box dimensions which are multiples of each other can't be
avoided and the fiberglass stuffing will help absorb the internal sound
reflections.

[21]
Q: What is needed to mount a midrange on the baffle with the woofer?

A: For cone-type midrange drivers, a sealed sub-chamber should be used
to prevent interaction with the enclosure's bass driver. JBL drivers
suitable for sealed-chamber midrange use require only 10 to 40 liters
(.3 to 1.0 cubic foot) of chamber volume to operate at typical midrange
frequencies, above 200 hertz. Subchambers should be constructed
solidly and liberally lined with fiberglass. As in the case of
enclosure shapes, avoiding multiples of dimensions, subchambers should
be built so as to avoid square and cube shapes in favor of non-related
numerical ratios.

[22]
Q: Is there any special procedure for mounting a horn in an enclosure?

A: Use of a horn/compression driver does not require any subchamber
since these devices form their own air-tight seal. JBL horns such as
the 2344, 2370, MI-291 and 2380 horn family also seal their own cutout
opening in the enclosure when properly mounted on the baffle. Better
compression drivers are quite heavy, so a brace should be provided to
cradle the driver to prevent driver movement during shipping. In
combination with the length of a horn as a lever, driver mass can cause
the assembly to tear off the baffle or break the horn if the enclosure
is handled roughly or dropped. Driver mass can also tear off the horn
throat if cabinets are dropped on their backs.


CONVERSION CONSTANTS and USEFUL DATA
____________________________________


LITERS FEET^3 INCHES^3 METERS^3 MILLIMETERS INCHES METERS
___________________________________ _____________________________
1.00 = .03531 = 61.0 = .001 1.00 = .039 = .001
28.32 = 1.00 = 1,728 = .02832 25.40 = 1.000 = .0254
1000.00 = 35.31 = 61,016 = 1.00 1000.00 = 39.370 = 1.000

TO FIND SOUND WAVE LENGTH: divide velocity of sound by frequency (Hz)
(SOUND VELOCITY = 344 m/s, 1130 ft/s or 13,560 in/s)

AREA OF CIRCLE = 3.14 x (radius squared) Note: radius = 1/2 diameter

TO FIND THE DIAMETER OF A CIRCLE WITH EQUIVALENT AREA:
2 x square-root of (area divided by 3.14)
example: area of 9" tube = area of 8" square duct calculated:
(area) 64/3.14=20.37, square root = 4.51 x 2 = 9.03 (diameter)

VOLUME OF CYLINDRICAL DUCT = circular area x length

VOLUME DISPLACED BY JBL LOUDSPEAKERS: 8" = .05 cu ft, 10" = .1 cu ft,
12" = .15 cu ft, 15" = .2 cu ft, 18" = .3 cu ft.

JBL LOUDSPEAKER MOUNTING HOLE AND BOLT CIRCLE DIMENSIONS:
mounting holes:
8" = 7-1/16" 10" = 9" 12" = 11-1/16" 15" = 13-31/32"
18" = 16-13/16"

bolt circles:
8" = 7-5/8" 10" = 9-3/4" 12" = 11-9/16" 15" = 14-9/16"
18" = 17-3/8"


BIBLIOGRAPHY of RECOMMENDED AUDIO REFERENCES
____________________________________________

FOR AUDIO NOVICES:

BOOKS:

David B. Weems, "Building Speaker Enclosures," Radio Shack
publication, stock# 62-2309

"The CAMEO Dictionary of Creative Audio Terms," Creative Audio & Music
Electronics Organization, 10 Delmar Avenue, Framingham, MA 01701

F. Alton Everest, "The Complete Handbook of Public Address Sound
Systems," Tab Books #966, Tab Books, Blue Ridge Summit, PA 17214

David B. Weems, "Designing, Building & Testing Your Own Speaker
System," Tab Books #1364 (this is the same as the Weems book above)

Abraham B. Cohen, "Hi-Fi Loudspeakers and Enclosures," Hayden Book Co.,
0721

Alex Badmaieff and Don Davis, "How to Build Speaker Enclosures," Howard
W. Sams & Co., Inc., 4300 West 62nd Street, Indianapolis, IN 46268

Bob Heil, "Practical Guide for Concert Sound," Sound Publishing Co.,
156 East 37th Street, New York, NY 10016

PAPERS:

Drew Daniels, "The Most Commonly Asked Questions About Building
Enclosures," JBL Professional, 8500 Balboa Blvd., Northridge CA, 91329

Drew Daniels, "Using the enclosure design flow chart," JBL
Professional, 8500 Balboa Blvd., Northridge, CA 91329

FOR EXPERIENCED AUDIO PRACTITIONERS AND HOBBYISTS:

BOOKS:

Jens Trampe Broch, "Acoustic Noise Measurement," Bruel & Kjaer
Instruments, Inc., 185 Forest Street, Marlborough, MA 01752 (617) 481-
7000

Howard M. Tremaine, "The Audio Cyclopedia," 2nd Edition 1969, Howard W.
Sams & Co., Inc., 4300 West 62nd Street, Indianapolis, IN 46268

Arnold P. Peterson and Ervin E. Gross, Jr., "Handbook of Noise
Measurement," General Radio, 300 Baker Avenue, Concord, MA 01742

Martin Colloms, "High Performance Loudspeakers," A Halstead Press Book,
1978 John Wiley and Sons, New York and Toronto.

Harry F. Olson, "Modern Sound Reproduction," 1972, Van Nostrand
Reinhold Co., New York.

Harry F. Olson, "Music Physics and Engineering," Dover Publications,
180 Varick Street, New York, NY 10014

Don and Carolyn Davis, "Sound System Engineering," Howard W. Sams &
Co., Inc., 4300 West 62nd Street, Indianapolis, IN 46268

F. Alton Everest, "Successful Sound System Operation," Tab Books #2606,
Tab Books, Blue Ridge Summit, PA 17214

PAPERS:

Drew Daniels, "Notes on 70-volt and distributed system presentation,"
for the National Sound Contractors Association Convention, September
10, 1985, JBL Professional, 8500 Balboa Blvd., Northridge, CA 91329

Drew Daniels, "Thiele-Small Nuts and Bolts with Painless Math,"
presented at the 70th Convention of the Audio Engineering Society,
November 1981 AES preprint number 1802(C8).

FOR ENGINEERS:

BOOKS:

Harry F. Olson, "Acoustical Engineering," D. Van Nostrand Co., Inc.,
250 4th Street, New York 3, NY 1957 (out of print)

Leo L. Beranek, "Acoustics," Mc Graw-Hill Book Co., New York 1954.

Harry F. Olson, "Elements of Acoustical Engineering," D. Van Nostrand
Co., Inc., 250 4th Street, New York 3, NY (1st ed., 1940, 2nd ed., 1947
both out of print)

Lawrence E. Kinsler and Austin R. Frey, "Fundamentals of Acoustics,"
John Wiley and Sons, New York and Toronto.

N.W. McLachlan, "Loudspeakers: Theory Performance, Testing and Design,
Oxford Engineering Science Series, Oxford at The Clarendon Press 1934,
Corrected Edition, Dover Publications 1960.

PAPERS:

Don B. Keele, Jr., "AWASP: An Acoustic Wave Analysis and Simulation
Program," presented at the 60th AES Convention in Los Angeles, May
1978.

Fancher M. Murray, "An Application of Bob Smith's Phasing Plug,"
presented at the 61st AES Convention in New York, November 1978.

Don B. Keele Jr., "Automated Loudspeaker Polar Response Measurements
Under Microcomputer Control," presented at the 65th AES Convention in
London, February 1980.

R.H. Small, "Direct-Radiator Loudspeaker System Analysis," Journal of
the Audio Engineering Society (JAES), Vol. 20, p. 383, June 1972.

Mark R. Gander, "Ground Plane Acoustic Measurement of Loudspeaker
Systems," presented at the 66th AES Convention in Los Angeles, May
1980.

"Loudspeakers," An anthology of articles on loudspeakers from the pages
of the Journal of the Audio Engineering Society, Vol. 1 through Vol. 25
(1953-1977). Available from the Audio Engineering Society, 60 East
42nd Street, New York, NY 10165 Telephone (212) 661-8528

A.N. Thiele, "Loudspeakers in Vented Boxes," Proceedings of the IREE
Australia, Vol. 22, p. 487 August 1961; republished in the JAES, vol.
19, p. 382 May 1971 and p. 471 June 1971.

Fancher M. Murray, "The Motional Impedance of an Electro-Dynamic
Loudspeaker," presented at the 98th Meeting of the Acoustical Society
of America, November 19, 1979.

Mark R. Gander, "Moving-Coil Loudspeaker Topology As An Indicator of
Linear Excursion Capability," presented at the 64th AES Convention in
New York, November 1979.

Garry Margolis and John C. Young, "A Personal Calculator Program for
Low Frequency Horn Design Using Thiele-Small Driver Parameters,"
presented at the 62nd AES Convention in Brussels, March 1979.

Garry Margolis and Richard H. Small, "Personal Calculator Programs for
Approximate Vented-Box and Closed-Box Loudspeaker System Design,"
presented at the 66th AES Convention in Los Angeles, May 1980.

Fancher M. Murray and Howard M. Durbin, "Three Dimensional Diaphragm
Suspensions for Compression Drivers," presented at the 63rd AES
Convention in Los Angeles, March 1979.

R.H. Small, "Vented-Box Loudspeaker Systems," Journal of the Audio
Engineering Society, Vol. 21, p. 363 June 1973, p. 438 July/August
1973, p. 549 September 1973, and p. 635 October 1973.

JBL TECHNICAL NOTES:

The following are available at no cost from JBL Professional:

Vol. 1, No. 1 - "Performance Parameters of JBL Low-Frequency Systems"

Vol. 1, No. 2 - "70-Volt Distribution Systems Using JBL Industrial
Series Loudspeakers"

Vol. 1, No. 3 - "Choosing JBL Low-Frequency Transducers"

Vol. 1, No. 4 - "Constant Directivity Horns"

Vol. 1, No. 5 - "Field Network Modifications for Flat Power Response
Applications"

Vol. 1, No. 6 - "JBL High-frequency Directional Data in Isobar Form"

Vol. 1, No. 7 - "In-Line Stacked Arrays of Flat-front Bi-Radial Horns"

Vol. 1, No. 8 - "Characteristics of High-Frequency Compression Drivers"

Vol. 1, No. 9 - "Distortion and Power Compression in Low-frequency
Transducers"

Vol. 1, No. 10- "Use Of The 4612OK, 4671OK, And 4660 Systems In Fixed
Installation Sound Reinforcement"

Vol. 2, No. 2 - "JBL/UREI Power Amplifier Design Philosophy"

Instruction Manual - "Motion Picture Loudspeaker Systems: A Guide to
Proper Selection And Installation"

"JBL Sound System Design Reference Manual" ($15)



*********Pay special attention to question 11***********

I spent a little time going through my old computer files to find the papers I used when deciding to build my old PA cabinets and my more recent LMS cabinets. These are now most of the best sources that I found. Hope they help in understanding the importance of correct porting under real conditions.





These papers have all the information regarding port design and use. They will answer all the questions about air flow, fluid dynamics, and compression and port size. All the statements presented since the beginning of the thread are addressed. The answers are all here for everyone. Enjoy.

Keep cranking,

Robert
robertcharles is offline  
post #284 of 302 Old 12-13-2012, 07:12 AM
AVS Special Member
 
mastermaybe's Avatar
 
Join Date: Sep 2007
Posts: 7,139
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 36 Post(s)
Liked: 149
^ Wow. B2BBP's.

That's: Back to Back Brilliant Posts.

Thanks


James

Actual phone call (see pic to left):

 

Tech (responding to laughter): "I'm sorry sir, did I miss something?"

Me: "Yeah, a case of Diet Mountain Dew walking across my living room."

mastermaybe is offline  
post #285 of 302 Old 12-13-2012, 07:55 AM - Thread Starter
Senior Member
 
diy speaker guy's Avatar
 
Join Date: Jan 2011
Posts: 303
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 12 Post(s)
Liked: 25
Quote:
Originally Posted by mdocod View Post

Hi DIY Speaker Guy,
I will say based on what I have skimmed that there is absolutely no reason for this to have been drawn out as long as it has been.
Simulations are based on small signal levels. As the frequency goes lower or the port gets smaller (or both), the signal level also has to go lower for the simulation to retain accuracy. Your simulations are "accurate," under the right circumstances but none of those circumstances are useful to the real world application. These simulation programs allow us to hypothesize output and Xmax and velocity for various input levels but do not typically take into account the physics that begin to take over as we increase signal level, pressure, velocity, compression etc. The evidence is right in the program. Understanding where the simulation starts to drop out of reality is something that comes with experience but you can gain most of this experience if you just apply common sense while tinkering with the program. The relationship of port gain, and the resulting loading effect, is dependent on the real world air velocity in the port achieving something close to simulation (this is common sense). If your common sense tells you that the air velocity at real world signal levels is likely to come up short due to resistance, then you should follow that common sense to a logical conclusion. Lets try to work our way to that conclusion...
In WinISD Pro, Set the simulation output to "rear port gain." Set the port to an insanely small diameter (like a fraction of an inch, like a leak), tinker with signal level, then set the port to a realistic diameter, tinker with signal level, set the port to something insanely large, tinker with signal level. Note the complete and utter LACK of anything changing in the simulated port gain. The rear port gain is simulated exactly the same whether you simulate it over a 1" diameter port or a 100" diameter port, at 0.1W or 10,000W. Throw enough zeros on the input signal or shrink the port size enough and the program will tell you that the particle velocity has exceeded the speed of light. The simulation always assumes small signal characteristics and calculates for gain and velocity in a linear fashion with those small signal simulation results as the starting position. We would use common sense to know that when we simulate beyond linear Xmax, the simulation looses accuracy. We know that while the simulation can show us that enough input power can result in the speaker cone making cross country flights 50 times a second, we know that in the real world we were smelling the byproducts of combustion and the cone hasn't even left the garage let alone made it to the airport.
winISD will show you the same port gain for a 0.25" hole drilled in the wall of the sub as it does for a port the size of a office building. What you are having a hard time wrapping your head around is that something 2" in diameter could "behave" like a pinhole. Use winISD to simulate that 2" by 90" port at a "real world" signal level of say, 100W, and you'll see a peak particle velocity far exceeding what your common sense tells you is likely to occur after factoring in some real world fluid dynamics (ask your common sense whether it is likely for air speeds to accelerate to 200MPH, stop, and accelerate to 200MPH again, several times per second over something that has a cross section to length ratio that is worse than a drinking straw, with a "100W air pump").
We know that compression effect comes into play at a fraction of these simulated port velocities, it is well documented and has been discussed to death a million times over in books and among forums the world over. The resistance to flow prevents the port from producing the gain and effective loading that is simulated at small signal levels (the resulting losses we call compression). Common sense tells us that the resistance of the drinking straw in question is in play regardless of the frequency in question. As you bring the frequency above that "peak" around tuning, all that changes is the EFFORT to move the air through the port, the resistance that prevents us from hitting 200mph peaks also drags down all other efforts at all other frequencies to pump air back and forth. The EFFECT of the port is heavily compressed at ALL frequencies. So while the simulation might show us a reasonable sub 15m/s at 4hz at 100W, that does not mean that we will actually ACHIEVE that velocity. For the same reason we won't hit 100m/s at 2hz, we won't hit 15m/s at 4hz. The resistance of the drinking straw significantly effects the outcome at either frequency. The fluid (air) is the same at both frequencies, as is the temperature, the resistance of the pipe effects the unchanged fluid viscosity equally at either 2hz or 4hz. If you don't hit the simulated velocity, then you also won't hit the simulated effect on driver Xmax. They are correlated events.
I seem to recall reading a blerp about an oil pipeline. Ask yourself. "If I cut the pressure at the head in half, but the fluid viscosity remains the same, have I changed the effective resistance?" The answer is no.
Regards,
Eric

Everything in this post is true but the velocity (not talking about compression here, just velocity) in my design was predicted quite accurately by the simulation programs, it wasn't the speed of light or 200 mph, it was 10 m/s at the useful lower end of the bandwidth.

There are professionals in different fields that do these kinds of calculations every single day. I was hoping some of these type of pros were here that could put some real numbers on this, but no one did so I tried to. I used tools that HVAC designers use, a flow calculator and a Moody chart. These tools combined take length and full power flow into account so they are more advanced than Flare It. These tools say the flow is not completely laminar in my design (within it's operational bandwidth), but nowhere near complete turbulence. If you want to argue that, use real numbers, not the speed of light or 200 mph.

I never said compression won't happen. I said the issue has been overblown here. If you read Mark's post, nothing in there should really be news to anyone. Everything he said is true and well known and widely accepted. And it all agrees with everything you said here. But you need to use real numbers and the proper tools (they do exist) to figure out the answer, not glance at the situation and say it won't work.

I should have studied the design in more detail, done detailed comparisons against a sealed box of the same size before I ever posted anything, but I didn't, and then once I did, my simulations had the wrong Ql value. That caused a huge mess here and I don't try to hide that fact. But if we want to talk about compression (and to be honest I don't really - if the design concept doesn't yield real world benefits there's not much point), let's use real numbers and real available tools, not speculation.
diy speaker guy is online now  
post #286 of 302 Old 12-13-2012, 09:49 AM
AVS Addicted Member
 
LTD02's Avatar
 
Join Date: Oct 2003
Posts: 15,977
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 136 Post(s)
Liked: 855
while your attempt to employ the moody chart is admirable, you have misinterpreted its application to port functionality.

according to empirical investigation at harman and published in the jaes, ports hit a brick wall around reynolds between 50,000 and 100,000 even though this is still well within the transition zone and nowhere near full turbulence.

in your model, reynolds of 34,000 is where the ported and the sealed have equal cone motion, but the advantage of lower cone motion of the ported doesn't come on until multiples of that, in which case, you are into the brick wall.

so it seems that neither of us were fully correct.

http://koti.mbnet.fi/jahonen/Audio/Papers/AES_PortPaper.pdf

page 30 (as numbered inside the pdf) summarizes reynolds and port effectiveness.

Listen. It's All Good.
LTD02 is online now  
post #287 of 302 Old 12-13-2012, 10:06 AM - Thread Starter
Senior Member
 
diy speaker guy's Avatar
 
Join Date: Jan 2011
Posts: 303
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 12 Post(s)
Liked: 25
Quote:
Originally Posted by LTD02 View Post

while your attempt to employ the moody chart is admirable, you have misinterpreted its application to port functionality.
according to empirical investigation at harman and published in the jaes, ports hit a brick wall around reynolds between 50,000 and 100,000 even though this is still well within the transition zone and nowhere near full turbulence.
in your model, reynolds of 34,000 is where the ported and the sealed have equal cone motion, but the advantage of lower cone motion of the ported doesn't come on until multiples of that, in which case, you are into the brick wall.
so it seems that neither of us were fully correct.
http://koti.mbnet.fi/jahonen/Audio/Papers/AES_PortPaper.pdf
page 30 (as numbered inside the pdf) summarizes reynolds and port effectiveness.
Quote:
The conclusion is that compression is clearly related to turbulence and
that a Reynold’s number of about 50,000 is good indicator of when the system begins to
degrade.

Emphasis provided by me of course. My reynolds number is 16000 lower than the point that the system begins to degrade. The wall they mention is not a brick wall, it's the beginning of non linearity.

Of course this does not mean the design is any good.

Thanks for the link (and for providing real info instead of speculation), I did not previously know of it, or know how to relate reynolds number independently directly to this discussion.
diy speaker guy is online now  
post #288 of 302 Old 12-13-2012, 11:46 AM
AVS Addicted Member
 
LTD02's Avatar
 
Join Date: Oct 2003
Posts: 15,977
Mentioned: 1 Post(s)
Tagged: 0 Thread(s)
Quoted: 136 Post(s)
Liked: 855
i can selectively quote too...

"All designs seem to hit a wall near a Reynold’s number of about 50,000-100,000."

emphasis added by me. :-)

i think this is what they were talking about with those comments though.

you can see that except for the 50hz system, all these systems *begin* compressing before reynolds 10,000 and all seem to "hit a wall" somewhere in between 50,000 and 100,000.

notice also the trend for the lower tunings to compress earlier. extrapolate that down to 2hz and who knows what will happen.



at reynolds 34000, you are already way out into the transition zone and compressing. the question is how far out in the transition zone does compression kill the port completely.

by examing some of the other experimental data in that paper, it appears that the lower the frequency, the lower the reynolds number for a given amount of compression.

"Thanks for the link (and for providing real info instead of speculation), I did not previously know of it, or know how to relate reynolds number independently directly to this discussion."

no problem. it was actually one of the cites in the link robert posted.

i didn't know anything about reynolds prior to this thread either.

Listen. It's All Good.
LTD02 is online now  
post #289 of 302 Old 12-13-2012, 12:01 PM
Senior Writer @ AVS
 
imagic's Avatar
 
Join Date: Dec 2005
Location: Philadelphia, PA
Posts: 5,276
Mentioned: 6 Post(s)
Tagged: 0 Thread(s)
Quoted: 725 Post(s)
Liked: 1746
Try screaming (or better yet, beat boxing) through a drinking straw to see why this idea should have died a long time ago.

Find out more about Mark Henninger at www.imagicdigital.com
imagic is online now  
post #290 of 302 Old 12-13-2012, 12:22 PM - Thread Starter
Senior Member
 
diy speaker guy's Avatar
 
Join Date: Jan 2011
Posts: 303
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 12 Post(s)
Liked: 25
Quote:
Originally Posted by LTD02 View Post

at reynolds 34000, you are already way out into the transition zone and compressing.

On the Moody graph I posted I am midway between laminar and complete turbulence.

The graph you posted shows ~ 1 db down at 34000 Reynolds. So like I always said, yes there is compression there, but it is still acting like a port.

I didn't (and still don't) have time to read the whole paper, I only read the few paragraphs you told me to and posted the conclusion. So I don't have anything to say about this situation getting worse as freqeuncy lowers.

So between the graphs we've both posted, I hope we can agree that the design at 10 m/s will be AT LEAST 1 db down due to compression, possible more since it's tuned so low. But it's still a port with relatively low velocity so it shouldn't be noisy. Even so the design has no merit over a sealed box of the same size,

Can we agree on those points or no?
diy speaker guy is online now  
post #291 of 302 Old 12-13-2012, 12:29 PM - Thread Starter
Senior Member
 
diy speaker guy's Avatar
 
Join Date: Jan 2011
Posts: 303
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 12 Post(s)
Liked: 25
Quote:
Originally Posted by imagic View Post

Try screaming (or better yet, beat boxing) through a drinking straw to see why this idea should have died a long time ago.

If you actually could scream through a straw, there will be compression but not so much that it effectively seals the straw. Still going to hear you loud and clear.
diy speaker guy is online now  
post #292 of 302 Old 12-13-2012, 12:36 PM - Thread Starter
Senior Member
 
diy speaker guy's Avatar
 
Join Date: Jan 2011
Posts: 303
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 12 Post(s)
Liked: 25
Quote:
Originally Posted by LTD02 View Post

i didn't know anything about reynolds prior to this thread either.

Ironically, 34000 appears to be right on the ledge before the whole situation takes a dive over the cliff. If my Reynolds number was a bit higher, I'd agree the situation would be hopeless.
diy speaker guy is online now  
post #293 of 302 Old 12-13-2012, 01:01 PM
Senior Writer @ AVS
 
imagic's Avatar
 
Join Date: Dec 2005
Location: Philadelphia, PA
Posts: 5,276
Mentioned: 6 Post(s)
Tagged: 0 Thread(s)
Quoted: 725 Post(s)
Liked: 1746
Well then go try it. Then try the same thing with a 4 inch wide mailing tube. the straw is so restrictive, you'll finish yelling whatever you were going to say before you are out of breath. that is blockage from turbulence and friction... and it is also preventing any significant volume from emanating. Loud and clear is not how I would describe what comes out of a straw in comparison to a larger tube. It's one of those things where you really don't have to fully understand the science behind it to understand that it does not work. all you have to do is accept the practical example that disproves its effectiveness.
Quote:
Originally Posted by diy speaker guy View Post

If you actually could scream through a straw, there will be compression but not so much that it effectively seals the straw. Still going to hear you loud and clear.

Find out more about Mark Henninger at www.imagicdigital.com
imagic is online now  
post #294 of 302 Old 12-13-2012, 01:34 PM - Thread Starter
Senior Member
 
diy speaker guy's Avatar
 
Join Date: Jan 2011
Posts: 303
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 12 Post(s)
Liked: 25
Quote:
Originally Posted by imagic View Post

Well then go try it. Then try the same thing with a 4 inch wide mailing tube. the straw is so restrictive, you'll finish yelling whatever you were going to say before you are out of breath. that is blockage from turbulence and friction... and it is also preventing any significant volume from emanating. Loud and clear is not how I would describe what comes out of a straw in comparison to a larger tube. It's one of those things where you really don't have to fully understand the science behind it to understand that it does not work. all you have to do is accept the practical example that disproves its effectiveness.

Again, yes, yes, yes, there will be compression. But I think we are past analogies by now, and using real numbers and graphs. (If you want to continue the analogy though, try sticking a straw in your wife's ear and yelling into it and see what she thinks about your theory.)

Those numbers and graphs show 1 db down at 30 hz at 34000 Reynolds, but trend is downward with frequency. I'm not sure how bad it gets with decreasing frequency, it may level off at a certain point like the lines in Flare It do.

LTD has proven very successfully with actual proof (kudos to him) that there will be compression. This however was never in any doubt. Unfortunately the graph doesn't go low enough to predict how the situation will be at 2 hz.

We have almost all the data we need now, just need to see the graph LTD posted shown down to lower frequencies.

I think it should be obvious by now that I don't care in the least if I end up being right or wrong as long as the correct answer comes to light. I've even posted big pics showing exactly how, where and why I was wrong. Show me the numbers. LTD is almost there but the chart doesn't go low enough.

In lieu of this data, as always we can just agree there's not enough into presented here yet to say anything with 100 percent certainty below 30 hz and just drop it, Since I don't think anyone is going to attempt a Reynolds number this high anyway this info is not incrediby important.
diy speaker guy is online now  
post #295 of 302 Old 12-13-2012, 03:46 PM - Thread Starter
Senior Member
 
diy speaker guy's Avatar
 
Join Date: Jan 2011
Posts: 303
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 12 Post(s)
Liked: 25
I had a chance to look over the link in a bit more detail. I still didn't get through it all yet but I did notice that the chart you posted is by far the worst looking of the 3. Not trying to imply anything, I assume you posted that since it was the first one of the group, or because it best showed the brick wall effect, but to be fair there's this much more benign chart for comparison.



So after doing the math, it seems like the worse chart (as posted by LTD which is already 1 db down by 30 hz) loses about 1 db as frequency is lowered 1 octave. Extrapolate in linear fashion and you get 5 db down (already 1 db down at 30 hz minus 4 db more due to linear extrapolation) at 2 hz. Not impressive by any means but not nearly as bad as speculated. And the more benign chart doesn't paint nearly such an ugly picture.

Not sure if it's ok to use the charts in this fashion but even linear extrapolation doesn't look that bad if you use the more benign chart.

And like I said, it might not scale in a linear fashion, it might level off as frequency decreases like the core velocity does in Flare It. In fact if core velocity levels off at some point there's no reason to think compression wouldn't as well, since they are clearly related.
diy speaker guy is online now  
post #296 of 302 Old 12-13-2012, 05:47 PM
AVS Special Member
 
robertcharles's Avatar
 
Join Date: Jul 2007
Location: New Orleans
Posts: 1,658
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 14
I found a formula on the following link. If you plug in the numbers you get what the simulation says for the port, but as power goes up the percentage of Mach shoots through the roof.

It is assumed that the air within the vent moves sinusoidally as a rigid mass


http://www.carstereo.com/help/Articles.cfm?id=4


Give the caluculator a try.

http://portsgalore.stefanhinote.com/?do=ventmach

Also, this one has all the calculators also, and it shows all the ports that can be used from 2-10 inches, but when you put the vent Mach number in the speeds of the smaller ports are up to 75% of the speed of sound. Easy to manipulate the different sized ports and see if they will work or not. This is a good calculator to show which ports are too small.

Spend at least a couple of minutes manipulating different sizes and it can be easily seen which ones are too small for a given power input.

Thanks,

Robert
robertcharles is offline  
post #297 of 302 Old 12-13-2012, 06:12 PM - Thread Starter
Senior Member
 
diy speaker guy's Avatar
 
Join Date: Jan 2011
Posts: 303
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 12 Post(s)
Liked: 25
Quote:
Originally Posted by robertcharles View Post

I found a formula on the following link. If you plug in the numbers you get what the simulation says for the port, but as power goes up the percentage of Mach shoots through the roof.
It is assumed that the air within the vent moves sinusoidally as a rigid mass
http://www.carstereo.com/help/Articles.cfm?id=4
Give the caluculator a try.
http://portsgalore.stefanhinote.com/?do=ventmach
Also, this one has all the calculators also, and it shows all the ports that can be used from 2-10 inches, but when you put the vent Mach number in the speeds of the smaller ports are up to 75% of the speed of sound. Easy to manipulate the different sized ports and see if they will work or not. This is a good calculator to show which ports are too small.

Spend at least a couple of minutes manipulating different sizes and it can be easily seen which ones are too small for a given power input.
Thanks,
Robert

That's just a simple velocity calculator and as such it isn't as accurate as even WinISD's predicted velocity (MJK's would be more accurate than both). Regardless, the results are a bit higher than WinISD predicts but it's pretty close for such a simple formula.

WinISD predicts almost 450 m/s from the 2 inch pipe at 1.8 hz but of course nowhere near that up higher, at the beginning of the usable bandwidth where it's 10 m/s.

In other words, all the calculators agree (within reason) including this one. But this one only shows velocity right at resonance so it's by far the simplest and least useful. And we're not really talking about simple velocity anymore anyway, we're talking about compression.
diy speaker guy is online now  
post #298 of 302 Old 12-13-2012, 06:39 PM
AVS Special Member
 
robertcharles's Avatar
 
Join Date: Jul 2007
Location: New Orleans
Posts: 1,658
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 14
that calculator for vent mach is very useful. If you vary only the input wattage you see the velocity skyrocket at every value over 1 watt. try it please. just vary the wattage and the speeds are enormous.
robertcharles is offline  
post #299 of 302 Old 12-13-2012, 06:43 PM - Thread Starter
Senior Member
 
diy speaker guy's Avatar
 
Join Date: Jan 2011
Posts: 303
Mentioned: 2 Post(s)
Tagged: 0 Thread(s)
Quoted: 12 Post(s)
Liked: 25
Quote:
Originally Posted by robertcharles View Post

that calculator for vent mach is very useful. If you vary only the input wattage you see the velocity skyrocket at every value over 1 watt. try it please. just vary the wattage and the speeds are enormous.

I did try it. I told you, it agrees (within reason) with WinISD. There's no reason it wouldn't, one is a lot simpler than the other but they are doing the same job.

600 watts, 1.8 hz Fb, 1 inch radius, 186 percent mach, or 639 m/s. WinISD says just under 450 for the same conditions. I'm actually surprised they agree so closely since the formula you linked to is so simple and takes only a couple of details into account.

Nothing unexpected here. It's a high number, but remember, it's only 10 m/s at the useful lower end of the bandwidth.
diy speaker guy is online now  
post #300 of 302 Old 12-13-2012, 06:43 PM
AVS Special Member
 
robertcharles's Avatar
 
Join Date: Jul 2007
Location: New Orleans
Posts: 1,658
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 14
robertcharles is offline  
Reply DIY Speakers and Subs

User Tag List

Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page


Forum Jump: 

Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off