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2 hz tuning - pros and cons - Page 9

post #241 of 302
Thread Starter 
Quote:
Originally Posted by noah katz View Post

Actually diy is correct for the current point of discussion; the port velocity is high because of resonance, but the discussion is about the implications of the velocity, not what causes it.

Not to mention the fact that the lungs and nasal passages are also a resonant system so it's not a bad analogy. You are correct, the cause of the velocity is not the issue here, the effect of the velocity itself is.
post #242 of 302
I haven't read much of this thread, and I think this is a crazy idea. But I'm kinda wondering about the idea. I just modelled my CSS Trio 12 which are in sealed 72L boxes. I could add an external 4" diameter, 12m long (yes, 12m long) port for a 5hz tune. This (according to a simulation) cuts excursion significantly below 10hz. I'm sure my system acts as a fairly good 5hz high pass. And the port velocity is down around 2.5m/s at 10hz (unibox doesn't show below 10hz).

Now, where do I put a 12m long pipe in my room biggrin.gif

I'd test it. But I honestly don't know where I'd put the pipe. Or where I'd cut a hole in my box...
post #243 of 302
Quote:
Originally Posted by bass addict View Post

Man. I forgot where I was for a moment. For a second I imagined I was in my living room listening to my wife and pre teen daughter arguing back and forth over something that would never come to fruition. biggrin.gif

Hey!
post #244 of 302
What will actually happen is higher bass notes will start bouncing around inside that tube and the resonances will build up and you'll hear the bass come out with that 'piped-in' sound because that's exactly what's going on.
Quote:
Originally Posted by tuxedocivic View Post

I haven't read much of this thread, and I think this is a crazy idea. But I'm kinda wondering about the idea. I just modelled my CSS Trio 12 which are in sealed 72L boxes. I could add an external 4" diameter, 12m long (yes, 12m long) port for a 5hz tune. This (according to a simulation) cuts excursion significantly below 10hz. I'm sure my system acts as a fairly good 5hz high pass. And the port velocity is down around 2.5m/s at 10hz (unibox doesn't show below 10hz).
Now, where do I put a 12m long pipe in my room biggrin.gif
I'd test it. But I honestly don't know where I'd put the pipe. Or where I'd cut a hole in my box...
post #245 of 302
Thread Starter 
Quote:
Originally Posted by tuxedocivic View Post

I'd test it. But I honestly don't know where I'd put the pipe. Or where I'd cut a hole in my box...

I'd sure be greatful if you did. But where to put the port is the least of your worries. How do you measure velocity?
post #246 of 302
Thread Starter 
Quote:
Originally Posted by imagic View Post

What will actually happen is higher notes will start bouncing around inside that tube and the resonances will build up and you'll hear the bass come out with that 'piped-in' sound because that's exactly what will be happening.

Exactly, just like every other ported box in the world. Not sure why you think this is so different. Group delay might be worse than most ported boxes but it's debatable how much that matters below 10 hz.
Edited by diy speaker guy - 12/11/12 at 3:49pm
post #247 of 302
Quote:
Originally Posted by diy speaker guy View Post

Just to let everyone know, this conversation has frustrated me to the point that looking at a pic of these drivers now makes me feel physically ill so I don't think I'll be buying them at all for any purpose. So thanks for that. You (collectively) have raised my blood pressure quite a bit but saved me a bunch of money at the same time. To be clear though, this is not an admission that there is anything wrong with the design as presented in post 1.

Welcome to the internet, and the AV SCIENCE forum. Go ahead and use the scientific method and test it. How else are you going to prove everyone wrong?
post #248 of 302
Thread Starter 
Quote:
Originally Posted by notnyt View Post

Welcome to the internet, and the AV SCIENCE forum. Go ahead and use the scientific method and test it.

I can't afford to buy supporting equipment to set it up and test it as I've said several times. I wouldn't know where to look for suitable equipment and I have no idea how to measure velocity. This is exactly why I brought up the real world analogy of a sneeze (high velocity coming out of a tiny hole attached to a resonant system). This has been measured and it supports the theory that the port will not compress like the unsupported speculation here suggests. If it's so easy why don't you build it and test it?

But thanks for the welcome. I just wonder why it doesn't make me feel all warm and fuzzy inside. Feels a lot more like a swift kick in the ass.
Edited by diy speaker guy - 12/11/12 at 3:49pm
post #249 of 302
build box, test at high level, plug port, test again, compare results. You should visibly be able to see the excursion difference or hear any port noise. Am I missing something here?
post #250 of 302
Who cares what the velocity is. That's not what you wanna test. I'd do 8 things.

1. Nearfield of woofer at roughly 1 watt.
2. Nearfield of port at roughly 1 watt.
3. Nearfield of woofer at roughly 100watts
4. Nearfield of port at roughly 300 watts.
5. Room measurement at 1 watt.
6. Room measurement at 100watts.
7. Tones at about 150 watts and watch cone movement.
8 Listen.

If people didn't think that was satisfying data, well, eff 'em. If any of those showed an issue, the idea doesn't work. If it measures the same at high and low output. And sounds fine. And the excursion really does drop. Well, it works. Biggest issue would be mic clipping with a nearfield at those levels. Might have to go outside with it and measure GP at 1m.

But alas, I won't be doing this. Cause even if it's true and does work, I wouldn't build this type of subwoofer because of port length.

EDIT - Ah, nyt posted as I posted. There you go. Just watch the thing and see if excursion falls.
post #251 of 302
Long tubes create problems with resonance and with friction that are less prevalent in shorter tubes. That's the crux of this discussion. There are very real limitations with long tubes when it comes to carrying sound and moving volumes of gas. Shorter/wider tubes don't have this issue, not as much anyhow. The sound of compressed air, that's what you'll be hearing if you build that sub, and if you don't hear that sound, it's because turbulence is blocking up the works. It's going to be one or the other, probably a mix of both (that's why it's turbulent!).
Quote:
Originally Posted by diy speaker guy View Post

Exactly, just like every other ported box in the world. Not sure why you think this is so different. Group delay might be worse than most ported boxes but it's debatable how much that matters.

Edited by imagic - 12/11/12 at 4:09pm
post #252 of 302
Thread Starter 
Quote:
build box, test at high level, plug port, test again, compare results. You should visibly be able to see the excursion difference or hear any port noise. Am I missing something here?
What does that prove if you don't map out the frequency response of the entire system chain and accurately measure the input power? For example, if I use a really terrible signal chain at a power level lower than simulated this test will show a pass whether the concept is sound or not.

I'm pretty sure that's not what you are hoping for, so why not do it properly if you are going to do it at all? Including a real velocity measurement.
Edited by diy speaker guy - 12/11/12 at 4:11pm
post #253 of 302
Quote:
Originally Posted by diy speaker guy View Post

but find it very hard to ignore posts directed at me.

However, you did SEEM to ignore mine regarding your nasel analogy, and you still continue to use it...
post #254 of 302
Thread Starter 
Quote:
Originally Posted by imagic View Post

Long tubes create problems with resonance and with friction that are less prevalent in shorter tubes. That's the crux of this discussion. There are very real limitations withlong tubes when it comes to carrying sound and moving volumes of gas. Shorter/wider tubes don't have this issue, not as much anyhow. The sound of compressed air, that's what you'll be hearing if you build that sub, and if you don't hear that sound, it's because turbulence is blocking up the works. It's going to be one or the other, probably a mix of both (that's why it's turbulent!) but not neither.

This is actually a very useful comment, and it's been alluded to. Flare It does not consider length.

I'm still waiting for someone to point to some evidence showing how this changes things.

And to be clear, if it does change things, it will change things with ALL long ports, not just mine, and nobody seems to distrust Flare It for more conventional but still very long port designs.

See? I'm totally willing to admit I'm wrong. Just post the evidence.
post #255 of 302
Thread Starter 
Quote:
Originally Posted by PsychoM3 View Post

However, you did SEEM to ignore mine regarding your nasel analogy, and you still continue to use it...

I didn't respond to you because you didn't say anything except that the analogy is invalid, which doesn't help. Why is it invalid?

It's a resonant system made up by a chamber of air, a duct, and a hole, with a given velocity of air passing through. The chamber (lungs) are very well damped (fleshy) and changes size as you sneeze but it's still a resonant system that passes air. How is this invalid in any way?
Edited by diy speaker guy - 12/11/12 at 4:08pm
post #256 of 302
The truth is I have a pair of old P.A. cabinets that I've taken all the parts out of. One of them could be sealed up, it's 20"x24"x49" external so that's in the ballpark. Could stick an 18" into one cab and add a the long 2" pipe and do the test. I don't have a driver for it but maybe one of these days something can be worked out. If it works, bravo and if it does not, no foul because I'd love to turn those old cabinets into subs (perfect for gigs, they have wheels and handles). Should only take a few extra minutes and a few extra bucks to try out this theory. I already have four subs handling 16hz on up so the only range I'm interested in is the very bottom. Maybe this February.
post #257 of 302
Quote:
Originally Posted by diy speaker guy View Post

I didn't respond to you because you didn't say anything except that the analogy is invalid, which doesn't help. Why is it invalid?
It's a resonant system made up by a chamber of air, a duct, and a hole, with a given velocity of air passing through. The chamber (lungs) are very well damped (fleshy) and changes size as you sneeze but it's still a resonant system that passes air. How is this invalid in any way?

It is invalid in that it does not support your theory, not that it doesn't represent a vaguely similar system. The point I made was that the nasel passages DO indeed compress, to the point of pain IF the nasel passages are the only "port". I went on to point out if you seal off the larger "port" of the mouth and just depend on the nasel passages to channel the air, pain will insue. The nasel passages alone can not transmit the pulse and compression results. If you try my "experiment" it won't be fun. My point is simply if you accept your analogy as valid you have not proved your point, you have proved exactly the opposite of what you want to prove. In the case of a sneeze the large volume of air moved at a "high" velocity causes significant compression in the relatively small nasel passages. Again I do not see it as being a very valid analogy since the physiological system you are comparing the sub vent to is FAR more complex than you are giving it credit for but if YOU think it is a valid amalogy/comparison then you have to deal with the fact that system shows very high levels of compression. Since we are talking about reflex ports the port in the KEF LS50 shares more in common with the "sneeze" port, but still far from a resonably similar situation.
post #258 of 302
^^^In a nutshell, a sneeze is the same thing as very heavy chuffing. That's why those tiny little holes (nostrils) make so much noise.
post #259 of 302
Thread Starter 
Quote:
Originally Posted by PsychoM3 View Post

It is invalid in that it does not support your theory, not that it doesn't represent a vaguely similar system. The point I made was that the nasel passages DO indeed compress, to the point of pain IF the nasel passages are the only "port". I went on to point out if you seal off the larger "port" of the mouth and just depend on the nasel passages to channel the air, pain will insue. The nasel passages alone can not transmit the pulse and compression results. If you try my "experiment" it won't be fun. My point is simply if you accept your analogy as valid you have not proved your point, you have proved exactly the opposite of what you want to prove. In the case of a sneeze the large volume of air moved at a "high" velocity causes significant compression in the relatively small nasel passages. Again I do not see it as being a very valid analogy since the physiological system you are comparing the sub vent to is FAR more complex than you are giving it credit for but if YOU think it is a valid amalogy/comparison then you have to deal with the fact that system shows very high levels of compression. Since we are talking about reflex ports the port in the KEF LS50 shares more in common with the "sneeze" port, but still far from a resonably similar situation.

The theory it was supposed to support is that even well past 10x the Flare It core velocity prediction, the tiny tiny hole NEVER seals right up.

I fully admitted there is much compression in this situation but you can't sneeze hard enough to effectively seal the hole.

The claims made here suggest my design will act completely sealed at 1x the Flare It core limit prediction

So the analogy supports the theory that the port will not become ineffective nearly as soon as many people think.


Please take my comments in context, otherwise I repeat myself over and over and over and over and over and ...
Edited by diy speaker guy - 12/11/12 at 5:06pm
post #260 of 302
Thread Starter 
Quote:
Originally Posted by imagic View Post

^^^In a nutshell, a sneeze is the same thing as very heavy chuffing. That's why those tiny little holes (nostrils) make so much noise.

Yes, but they don't seal up and effectively close the hole like in the video you posted and like people are claiming my design will.

Forget the chuffing. Yes, a sneeze is one big chuff. That's not the point.
post #261 of 302
Thread Starter 
Quote:
Originally Posted by imagic View Post

The truth is I have a pair of old P.A. cabinets that I've taken all the parts out of. One of them could be sealed up, it's 20"x24"x49" external so that's in the ballpark. Could stick an 18" into one cab and add a the long 2" pipe and do the test. I don't have a driver for it but maybe one of these days something can be worked out. If it works, bravo and if it does not, no foul because I'd love to turn those old cabinets into subs (perfect for gigs, they have wheels and handles). Should only take a few extra minutes and a few extra bucks to try out this theory. I already have four subs handling 16hz on up so the only range I'm interested in is the very bottom. Maybe this February.

Unless you just want to test the theory (which I hope you do) there isn't much point unless you have a couple high excursion 18's. It won't put out enough spl to be useful for anything except this test.

Even with a couple high excursion 18's this is not going to be loud enough to be useful in any setting other than a smallish home theatre (depending on room gain). You need multiple boxes. I wouldn't bother unless I had at 2 (with 4 drivers total) right behind my couch in my living room.
post #262 of 302
That's all it would be, a momentary chance to test the theory. It absolutely hinges on whether or not I wind up with a couple of high excursion 18's. I hope I do, right now those cabinets are stacked in a corner of my HT, world's biggest cat scratching post when they could be doing double duty as earthquake generators. biggrin.gif
Quote:
Originally Posted by diy speaker guy View Post

Unless you just want to test the theory (which I hope you do) there isn't much point unless you have a couple high excursion 18's. It won't put out enough spl to be useful for anything except this test.
Even with a couple high excursion 18's this is not going to be loud enough to be useful in any setting other than a smallish home theatre (depending on room gain). You need multiple boxes. I wouldn't bother unless I had at 2 (with 4 drivers total) right behind my couch in my living room.
post #263 of 302
Quote:
Originally Posted by diy speaker guy View Post

What does that prove if you don't map out the frequency response of the entire system chain and accurately measure the input power? For example, if I use a really terrible signal chain at a power level lower than simulated this test will show a pass whether the concept is sound or not.
I'm pretty sure that's not what you are hoping for, so why not do it properly if you are going to do it at all? Including a real velocity measurement.

This is why you a/b test vs sealed. Signal chain will not matter, you will be able to see the DIFFERENCE between them.
post #264 of 302
Thread Starter 
Quote:
Originally Posted by notnyt View Post

This is why you a/b test vs sealed. Signal chain will not matter, you will be able to see the DIFFERENCE between them.

I get that, but I have a feeling that no results would be accepted as valid unless measured accurately. So we can each do this for our own edification (assuming we have the necessary parts) but it isn't going to prove anything to anyone but the person doing the test. Unless I prove myself wrong, in which case I will openly admit it, and I don't think anyone would argue with that.

If I had the parts I would have done this long ago. Drivers are not even available yet so best case scenario it will be several months before I could put this all together since I don't build anything big in winter.


So... unless someone else does it, it's not going to happen anytime soon, if ever.


As I mentioned, even a scale model would be tricky because I'm not sure what I could use for a tiny tiny port that would be strong enough. I don't have anything at home so I'll check the hardware store next time I'm out. Maybe copper pipe or something, I'm not sure what type of pipe is available in what sizes when they get that small.
Edited by diy speaker guy - 12/11/12 at 5:52pm
post #265 of 302
To be 100% certain, the excursion tests can be captured with a long exposure on a DSLR - the resulting images can be overlayed in Photoshop to show the exact difference in excursion between configurations at any given power level - I have to create an example with my subs, it's definitely easy. If I do convert the cabs, sticking a 2" tube in there will only cost a few bucks and take a few minutes. I could also do it with twin 15" subs (which is what the current baffle has cutouts for)... still not sure exactly what I'm doing with those cabs but they need a serious makeover to become decent subs (mainly bracing) I don't see the harm given how popular this thread has become.
Quote:
Originally Posted by notnyt View Post

This is why you a/b test vs sealed. Signal chain will not matter, you will be able to see the DIFFERENCE between them.
post #266 of 302
Thread Starter 
Quote:
Originally Posted by imagic View Post

To be 100% certain, the excursion tests can be captured with a long exposure on a DSLR - the resulting images can be overlayed in Photoshop to show the exact difference in excursion between configurations at any given power level - I have to create an example with my subs, it's definitely easy. If I do convert the cabs, sticking a 2" tube in there will only cost a few bucks and take a few minutes. I could also do it with twin 15" subs (which is what the current baffle has cutouts for)... still not sure exactly what I'm doing with those cabs but they need a serious makeover to become decent subs (mainly bracing) I don't see the harm given how popular this thread has become.

I can't do any of that, so I hope you do. But if it passes, don't expect anyone to accept your results. At this point there are a lot of people that have way too much invested in the dismissal of this concept to admit they are wrong.

I have no problem being wrong, I'm just after correct information.
post #267 of 302
Quote:
Originally Posted by diy speaker guy View Post

Now the results are right there and I don't know wtf they mean. It's clear from this excercise that length increases back pressure, but how that relates to velocity or compression I have no idea.

It's similar to Ohm's Law except instead of electron flow it's airflow.

The higher the flowrate (and therefore velocity for a given duct size), the greater the pressure drop.

So as flowrate increases, so does the pressure drop and you get less output per unit of pressure input, which is to say compression.
post #268 of 302
Thread Starter 
Quote:
Originally Posted by noah katz View Post

It's similar to Ohm's Law except instead of electron flow it's airflow.
The higher the flowrate (and therefore velocity for a given duct size), the greater the pressure drop.
So as flowrate increases, so does the pressure drop and you get less output per unit of pressure input, which is to say compression.

Thanks, but I deleted my post due to using the wrong calculator. That was a fluid caluclator I guess, it wasn't clearly marked.

I'm trying to work my way through the gas calculator now.
Edited by diy speaker guy - 12/11/12 at 10:23pm
post #269 of 302
No matter, what I said still holds.
post #270 of 302
Thread Starter 
Ok, I think I have some results now. I studied up on this for what seems like about the last 8 hours (probably closer to 2 hours in reality though) so I hope I did this right and I hope the results are helpful to the discussion.



Ok, above the orange line is a random pic of a Moody diagram. Never seen this thing before so I'm not sure I'm qualified to read it correctly.

Below the orange line is the results of a random air flow simulator. Duct is simulated as PVC with 2 inch diameter, 90 inch length.

I wasn't sure what to enter for cfm so I used a couple different velocity to flow (m/s to cfm) calculators to make sure the results match each other (as I usually do with simulators and calculators when I don't know the underlying math.)
http://www.pumpcalcs.com/calculators/view/72/
http://www.1728.org/flowrate.htm
Both these calculators say 10 m/s through a 2 inch diameter hole = 42.894 or 42.946 cfm (slightly different results in each), so that's what I entered. This seems to make sense because the calculated velocity is 1966.2 fpm, which is roughly 10 m/s, as expected.

Then I used the calculated Reynolds number and the calculated friction factor to plot out where this design falls on the Moody chart (at the point where my design is pushing exactly 10 m/s) with a big red X.

I'm not sure if I'm reading the Moody chart right but it appears that when my design is at pushing exactly 10 m/s the red X is almost exactly midway between the completely laminar flow line and the completely turbulent line.

I think this proves my point, but like I said I'm not sure I'm reading the graph right. The Reynolds number is pretty high but I'm not sure what that means on it's own. A Reynolds number of 4000 indicates the onset of turbulence as far as I can tell from my quick research, but I have no idea how this calculated Reynolds number as shown in the pic translates to compression or chuffing.

So I would appreciate some input on this but I'm assuming that if anyone knew what this meant exactly they would have brought up the Moody diagram, Reynolds number and friction factor a long time ago.

As always, I will still be quite happy if this info proves me wrong as long as I end up with the correct answer in the end.
Edited by diy speaker guy - 12/11/12 at 11:29pm
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