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post #271 of 354 Old 08-17-2012, 09:58 AM
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Originally Posted by LTD02 View Post

"Why does the driver excursion not follow your theory?"
as i explained before, it does.

Roomisnotabandpasssub.jpg

Yeah, sure it does. rolleyes.gif
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post #272 of 354 Old 08-17-2012, 11:51 AM
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Originally Posted by bossobass View Post

RiccisRoomGain.jpg

Way cool. This is what I was missing, making looking at a graph kinda hopeless. Thanks for filling in the details. cool.gif
I'm interested in a few things, but at the top of the list is the way the measurements funnel to a common point around 6-7 Hz, which I see in my room as well.
variousmicpositions.jpg
My graph is with the mic placed at various places, none of which are related to the seats. Close to a wall, inside one of the dormers and there's even one in the middle of the room with the mic cap face down on the carpeted floor. If you can get to that, I'd like to see any similarity/difference in your new room vs the old room.
BTW, my traces are 1/6 octave smoothed because the fine detail above 20 Hz id not relevant here.

If I level match all of the measurements near 7Hz they line up very well up until about 18Hz except for 2 outliers which are the back subs operated individually and measured from the main. There is some variation going on but not a lot. At 20Hz there starts to be a larger amount of variation and by 25-30Hz it starts to be huge depending on which combination of subs is measured. There are a couple of things that I find of interest. The modal region that either of our rooms support is no lower than maybe 25Hz yet we see lots of variation there. Also despite what should be similar basic construction and materials we see vastly different behaviors occuring below 25Hz. Your measurements show no sign of the 10-14Hz dip seen in mine. Yet I have not been able to duplicate the dramatic reduction in ULF that you measured when A/B/A+B ing subs. (I find this measurement especially peculiar.)

If you have alternative explanations for the drop out in my room and the ULF cancellation in yours I would love to hear them. I'm open to new ideas.

Don't forget I also have measurements down to ULF inside of a Jeep WJ as well. The mic was placed in three dramatically different spots: Kick panel, dash to windshield joint and at the drivers head area. There were some differences in the upper bass but everything was basically identical below 30Hz no matter the mic placement. It's on DB. I figure the longest possible diagonal dimension from the front dash to the rear cargo hatch can't be any more than maybe 13ft max. Also we know that the volume is far less and the walls are much more rigid and stiff than those in a typical room. I'd expect the seal of a car cabin to be much tighter than most any room as well.

To me it makes the most sense for the modal region to dominate the effects seen on the radiated sound as expected above the wavelengths corresponding with the half wave of the longest dimension of the room. As far as what is happening below that point and the sometimes strange results seen I expect that they are related to any number of odd phenomenon. The average room is far from sealed, equipped with far from infinitely rigid walls or floors, are usually more complex in shape than a simple oblong box and have various objects in them further deviating their internal space. Each of the walls and floors will operate like a spring system with various resonance points unless you are on a cement slab or in a concrete bunker. In essence the speaker system is closer to operating inside of a drum than inside of a inert rigid container and the major surfaces of typical wood frame constructed rooms will be excited and interact with the bass energy in complex ways . As would be expected from the sound waves interacting with a boundary that is not fixed itself but is also moving and vibrating. The openings from the room will each have their own various effects, etc. Yes I also believe that these openings can have various bandpass effects. In autosound this is commonly used to enhance the loudness of the bass outside of the vehicle in competitions. In the much larger and less rigid interior of a room the bandpass effects would occur much much lower in frequency. The sum total of what occurs to the output from the subwoofer before it reaches the ears or microphone is much more complex than the simple pressure pot or modal models alone can explain. It is all of these contributors combined. That's my 2 cents on it IMHO.

BTW what measurement is the green trace on your graph? Middle of the floor mic on the floor? I note that it does have a dip in about the same region as the one in mine that is why I ask.


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post #273 of 354 Old 08-17-2012, 02:34 PM
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Originally Posted by bossobass View Post

Roomisnotabandpasssub.jpg
Yeah, sure it does. rolleyes.gif

I see ~8dB more output when the window is shut.

Makes perfect sense to me.

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post #274 of 354 Old 08-17-2012, 03:21 PM
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Not all rooms are created equally.... not even close. If that's what was suggested, then wow.... I guess a 3rd floor is going to have the same resonances as a basement bunker. Not.

You don't think that there is a difference between single 1/2" drywall, or 5/8, or double 5/8 glued with a flexible adhesive between the layers... and no screws shorting the top panel to the studs? With the same stud spacings, and wall size, the resonances are going to shift.

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post #275 of 354 Old 08-17-2012, 04:16 PM
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"If the in room pressure theory is valid and as you say if a window "can not vent the pressure" then the subwoofer generated ULF can not be measured outside of the open window."

jpc,

the in room pressure theory is not mine. it is put forward by people who have a much deeper understanding of the physics than i do. i just learned about it based on some extensive digging around inspired by this thread.

i didn't suggest that that there would be no sound outside of an open window. there can be all kinds of effects at work from the energy leaving the room by vibrating the panels to helmholtz resonator effects.

"Yeah, sure it does."

bosso, you asked about driver excursion and then posted a frequency response plot. what is your angle?

"To me it makes the most sense for the modal region to dominate the effects seen on the radiated sound as expected above the wavelengths corresponding with the half wave of the longest dimension of the room. As far as what is happening below that point and the sometimes strange results seen I expect that they are related to any number of odd phenomenon. The average room is far from sealed, equipped with far from infinitely rigid walls or floors, are usually more complex in shape than a simple oblong box and have various objects in them further deviating their internal space. Each of the walls and floors will operate like a spring system with various resonance points unless you are on a cement slab or in a concrete bunker. In essence the speaker system is closer to operating inside of a drum than inside of a inert rigid container and the major surfaces of typical wood frame constructed rooms will be excited and interact with the bass energy in complex ways . As would be expected from the sound waves interacting with a boundary that is not fixed itself but is also moving and vibrating. The openings from the room will each have their own various effects, etc. Yes I also believe that these openings can have various bandpass effects. In autosound this is commonly used to enhance the loudness of the bass outside of the vehicle in competitions. In the much larger and less rigid interior of a room the bandpass effects would occur much much lower in frequency. The sum total of what occurs to the output from the subwoofer before it reaches the ears or microphone is much more complex than the simple pressure pot or modal models alone can explain. It is all of these contributors combined. That's my 2 cents on it IMHO."

i'll kick in my 2 cents in order to raise the pot the four. nice summary.

except that below the critical frequency, there aren't really any sound waves, just global pressure changes.

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post #276 of 354 Old 08-17-2012, 05:33 PM
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Originally Posted by LTD02 View Post

"Yeah, sure it does."
bosso, you asked about driver excursion and then posted a frequency response plot. what is your angle?

Can you and Bentz even read a graph? He sees +8dB with the window shut vs open and it makes perfect sense to him. Yeah, it doesn't just make sense that he can't read a graph, it makes perfect sense.

Look fella, I'm standing right in front of the drivers. If you'd like I'll video them for you with a throw gauge on them. The FR outside the open window is the same as the FR inside the closed window, just attenuated by the inverse square law and lack of boundaries. The window is not the port of a bandpass sub, so stop posting this nonsense, will ya?

Your theories have all been discounted by actual data, you know, real world facts. Yet you keep on, undaunted, transforming your story as it suits you with air to water, warp speed of sound, ported sub... no, bandpass sub and the rest of it.

I post data and you and Bentz misinterpret it. As I recall from earlier tutorials from Bentz, he's an audio measurements expert. When will you guys post something that backs your claims?
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post #277 of 354 Old 08-17-2012, 05:45 PM
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Originally Posted by Ricci View Post

BTW what measurement is the green trace on your graph? Middle of the floor mic on the floor? I note that it does have a dip in about the same region as the one in mine that is why I ask.

That green trace is for the flat earthers. I have a laundry room adjacent to the HT that shares the floor system. I set the mic in that room and closed the door. Anyone who can read a graph and has been following the debate will understand perfectly what it shows. The measurement taken with the mic face down in the carpet is for a similar reason.

When you take the various mic placement measurements, just keep the levels the same and let the traces fall on the FR magnitude graph as they will.
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post #278 of 354 Old 08-17-2012, 05:52 PM
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Quote:
Originally Posted by LTD02 View Post

"If the in room pressure theory is valid and as you say if a window "can not vent the pressure" then the subwoofer generated ULF can not be measured outside of the open window."

jpc,

the in room pressure theory is not mine. it is put forward by people who have a much deeper understanding of the physics than i do. i just learned about it based on some extensive digging around inspired by this thread.

i didn't suggest that that there would be no sound outside of an open window. there can be all kinds of effects at work from the energy leaving the room by vibrating the panels to helmholtz resonator effects.


The Nousiane article I linked to earlier mentions both pressure pot theory as well as the border proximity theory. Nousaine made no claims on which theory he prefers to go by. Either theory could be correct as well as a weghted combination of both theories.


Perhaps the size and construction of the room in question could prove either theory as being correct or could change the weighting of the two theories.





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Originally Posted by LTD02 View Post

i'll kick in my 2 cents in order to raise the pot the four. nice summary.

except that below the critical frequency, there aren't really any sound waves, just global pressure changes.



If the "pressure change" is global, then it should measure the same everywhere in the room. So if you run pink noise as a steady state stimulus rather than a sweep, should frequencies below the critical frequency measure the same everywhere in the room?
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post #279 of 354 Old 08-17-2012, 06:37 PM
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"If the "pressure change" is global, then it should measure the same everywhere in the room. So if you run pink noise as a steady state stimulus rather than a sweep, should frequencies below the critical frequency measure the same everywhere in the room?"

in theory yes, but you have to consider all the other schlock mentioned in ricci's post, there is more too it.

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post #280 of 354 Old 08-17-2012, 06:45 PM
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"Your theories have all been discounted by actual data, you know, real world facts. Yet you keep on, undaunted, transforming your story as it suits you with air to water, warp speed of sound..."

well, they are not my theories...see the treatise on acoustics.

if you wish to argue that there is no pressurization of a room below the critical frequency, then just put forward your theory for pressure vessel gain.

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post #281 of 354 Old 08-17-2012, 06:57 PM
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"The FR outside the open window is the same as the FR inside the closed window, just attenuated by the inverse square law and lack of boundaries."

you have a change of space there. the room is "small" relative to ulf. outdoors, not so much, so the spl will be lower. what's your point?

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post #282 of 354 Old 08-17-2012, 08:12 PM
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I think his point is that if it was a 5hz tuned port, the mic would hear a bit blast right at 5hz, which it does not.

And if the air is "trapped" in the room, then it wouldn't hear anything.



Facepalm. Quit while you still have your sanity. The only person posting any measurements is Ricci. The rest is misinterpreted hear say. Either way, thanks for the valuable measurements and dialogue.
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post #283 of 354 Old 08-18-2012, 06:13 AM
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I want to share my experiences with one vs. two drivers...

After a couple weeks of messing with 10", 12" and 15" drivers from Polk and Kicker (Polk DXi and Momo, Kicker CompVR) in an effort to use budget mainstream components to build a performance sub I've learned a few things.

1. You can't have 'enough' ULF capability, because low distortion is the friend of infrasonic sound. The vibrations should feel like they come out of dead silence, and to do that you can't have drivers operating at 50%+ excursion. 15" and 18" solutions are equally great, I have no issues with sealed, ported or IB so long as the SPL is there.
2. There should only be one subwoofer for bass. The ideal size range is 10" or 12". This subwoofer should be placed under the center channel, or between the mains in a 2-channel system.
3. The character of the bass driver is very important, it needs to mesh with the ULF drive(s) as well as possible... but the lower the crossover point between the ULF and the LF drivers, the less important this becomes... so I cross over at 28hz
4. The crossover point for the LF driver should be unusually high. I ended up with 160hz. The reason is that stereo speaker will continue to exhibit cancellation issues through this frequency range, having that bass come from a point source eliminates this issue. The reason for a single, centered LF driver is you can localize the source of this mid-bass. Putting it in the center allows audio cues from the mains to steer the bass in the right direction. The center LF sub disappears.
5. The timing of the center sub needs to be precisely adjusted to match the mains. The timing of the ULF content only needs to be approximate, which is fortunate for those using multiple ULF subs.
6. If possible, the center sub should be a dipole. Because of the dipole pattern, it's easier to eliminate nulls since there are no side-wall reflections. Because ULF content is not doing to the dipole, over-excursion risks are greatly reduced. The upshot is a cleaner, more transparent sound... and the option of using a flat baffle to reduce the footprint of the center sub.

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post #284 of 354 Old 08-18-2012, 07:03 AM
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"There should only be one subwoofer for bass."

that kind of runs against all the established science, so...why should only one sub be used?

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post #285 of 354 Old 08-18-2012, 07:26 AM
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"Facepalm."

tux,

the point that the bosso and i were debating was whether or not a sub could pressurize a room. the answer is...yes and the physics have been referenced.

which part of the treatise on acoustics that i posted is causing confusion? i'll try my best to help clear it up.

i admit, it is a difficult subject and i had my own facepalm moments trying to get through it all.

if you have an alternative theory for pressure vessel gain, kick it into the mix.

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post #286 of 354 Old 08-18-2012, 07:41 AM
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Are you guys still going at this nonsense? Sheesh, get lives already.

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post #287 of 354 Old 08-18-2012, 08:10 AM
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"Are you guys still going at this nonsense? Sheesh, get lives already."

who are you to judge what is worthy of discussion or not?

the bosso put forward a claim with which i disagreed. the bosso, seaton, ricci, tux, jpc, bentz, hurd, noah, the guy from germany, and others have weighed in with their data, perspectives, and references. it has been a good, if rough at the edges, conversation.

personally, i have learned a lot about the physics below the critical frequency inspired by this thread.

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post #288 of 354 Old 08-18-2012, 10:00 AM
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Room response is affected by reflections, resonances and transmission losses presented by the boundaries of the room.

Although a complex mechanism, it's not as unique to every room as its proclaimed to be and not so mysterious as to make a flat in-room response to 3 Hz a wet dream of pure chance.

The effects of RR&T don't change at some magic frequency. They are the driver of the final FR in-room across the subwoofer bandwidth, assuming a minimum performance from the subwoofer in the first place.

Over the decade I've been a member here and at other subwoofer forums, there have been some crazy theories being dictated as facts that included punishment including being striped, tarred, feathered and banishment for any disbelievers.

The idea that a sound pressure wave is so long that it doesn't have a beginning or and end, that its length causes it to accelerate to 5X the SOS, that it causes the barometric pressure to rise and fall everywhere at once but that all of those bizarre effects can be completely negated if the sound pressure wave excites any of the boundaries resonant frequencies is the stuff of a pencil neck looking for grant money.

This goes in the files with the folks who insisted everyone should tilt his in-room response by 16 times at the low end to re-EQ all source so that it complies with the Fletcher-Munsen curves. Like this thread, lots of really spiffy studies, white papers and graphs were presented to endow the rulers of the wolf pack an air of credibility and justification for the harsh reaction to any contrary opinion (and facts).

It goes in the files with those who persistently insist that the slice of bandwidth being discussed in this thread is irrelevant, inaudible-so-who-cares, doesn't belong on any source and if it is there it was a mistake (though over the years, the chief proponents of this baloney have been primarily sub makers whose products roll off at 20-25 Hz, one of whom referred to me as "a frequency response Nazi").

The highlights for me include: The cone moving into and out of the room causes the barometric pressure to rise and fall everywhere at once, BUT... if you open a door or a window, the pressure will rise and fall with greater amplitude because you are now not in a room but in the belly of a bandpass subwoofer. At first I laughed, but then it was being presented to me as though I was the class dunce who just wasn't getting it. W-O-W.

Also equally astounding is the idea that when a boundary that has an Sd of 420,000 cm^2 is sent into sympathetic vibration at its resonant frequency, it will always cause a complete attenuation (the tech-term is suckout) of that frequency from the in-room response. W-O-W.

The result is always the same. You get folks running around the world in every language crying out "You need 'X' number of subs to 'pressurize' the room down low!". "It has to be 135dB or you won't be able to hear it!". "That rooms really big so you'll never be able to 'pressurize' it!". "Just get a 'X' brand ported sub, 'cause the low stuff costs too much and you may not have any room gain down there anyway!".

This is why it takes forever to get to the actual bottom line. Too much noise and it's always protected by gatekeepers who are just parroting some crap published by a corporation with a sales quota or just quoted completely out of context.
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post #289 of 354 Old 08-18-2012, 10:04 AM
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I agree there is nothing wrong with debating these things. It's fun actually. It's DIY speakers IMO. But there isn't anything I need clarification on. I've thought through the physics myself. I've used the ideal gas law in real life to solve real engineering problems. I study earth pressure and atmospheric pressure for a living, and sound pressure as a hobby. I 100% agree with Bosso's position. Go back to the first few posts when this went nuts and you'll see I actually proposed the progressive constructive theory before Bosso did, albeit with different wording. No doubt he studied the theory long before I did also.

The ideal gas law is not useful in this engineering problem. That is my position.

The theories I see being discussed here are nonsense. And the only good data I'm seeing is from Bosso and Ricci. I'd post my own but it's not nearly as good as there's and I don't have my PC hooked up right now. Or my entire system for that matter. Just moved smile.gif
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post #290 of 354 Old 08-18-2012, 10:28 AM
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@ the bosso and the tux, what do you guys suggest is causing the observed 12db/oct (in theory, less in practice) gain below the critical frequency?

have you read the paper that i posted? which parts of it do you disagree?

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post #291 of 354 Old 08-18-2012, 10:32 AM
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"The idea that a sound pressure wave is so long that it doesn't have a beginning or and end, that its length causes it to accelerate to 5X the SOS, that it causes the barometric pressure to rise and fall everywhere at once but that all of those bizarre effects can be completely negated if the sound pressure wave excites any of the boundaries resonant frequencies is the stuff of a pencil neck looking for grant money."

so you are saying the two professors of physics that i linked to are wrong and just looking for grant money?

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post #292 of 354 Old 08-18-2012, 10:40 AM
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"...it was being presented to me as though I was the class dunce..."

who ever said you were the class dunce bosso?

we just pound on all ideas as hard as we can until the truth emerges.

i had it wrong half way through this thread, but that doesn't make me a class dunce either.

no need to quote it again, but the question that we were debating is whether a sub can pressurize a room.

if the weight of the evidence, changes your mind. that's cool.

if you are not convinced, that's cool too.

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post #293 of 354 Old 08-18-2012, 04:43 PM
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I thought I explained that in the post?. ULF needs multiple subs, but regular old LF, one sub can generate more than enough SPL so in my opinion the best way to integrate it is to put it front and center. Even better if that 'center sub' sub is a dipole. Anyhow some folks (like me) enjoy their bass in-phase and arriving on-time along with the sound from mains, other like fattening the sound by distributing their subs, making the bass more 'consistent' throughout the room. Both approaches have their merits and can be enjoyed for the specific result achieved. I like the 30-100hz range to be very tight and free of any boxy sound and that's the solution I found that works for me. I've mounted a 12" driver in a 1" thick MDF baffle and integrated it into an open-back box made of 18" paving stones. It sounds.... awesome. 10-30Hz is coming from a 15" in a sealed box, I'll likely add another 15" or swap for a very competent 18". I think multiple subs is the way to go for 10-30 hz because those waves don't need to be as accurately time-aligned, they are too big plus that low there is no way you can hear the sub's location but as the frequencies go up of course one's ability to pinpoint the sub increases.... but if the 'bass' sub is front and center, that issue is negated. It's working for me right now as I type, sounds completely awesome! I'll get a picture tomorrow.


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"There should only be one subwoofer for bass."
that kind of runs against all the established science, so...why should only one sub be used?

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post #294 of 354 Old 08-18-2012, 08:17 PM
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A single 12" in dipole radiation can't remotely approach reference level output at 30hz, especially when asked to provide output for at least the left, right and center channels. Not at 60hz. Not at....

Hey, I love dipoles as much as the next guy (more, probably), and your desire to keep higher bass spatially symmetric, up front, and time aligned with the mains is a worthy pursuit, but there aren't many free lunches in audio. I've started design on dipole mains to reach full reference levels at the seating position covering only down to around 60hz, and the design requires four aespeakers dpl12's. Per channel.

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post #295 of 354 Old 08-18-2012, 09:44 PM
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What you say is true, I admit I should've added a caveat: my room is very small. I don't know what your reference level is in decibels, I do know that what I'm listening to is completely satisfying throughout out the frequency spectrum. I should also have been more specific, 30 hertz is where the crossover point occurs, but the 12 inch is not taking full power at 30 hz. it's just 1 of many options, and I suggest it is the cleanest easiest way to have a subwoofer render that crucial mid-bass. I do so enjoy it as a concept, I would consider a highly articulate 18 inch for that duty, and still stick with the 1 central bipole driver concept. Otherwise, I would push up that cross over point a bit. I need my on-time non-ULF bass coming from a central point source.

Edit: MoMo 12" ($150 street) is an affordable, efficient neo sub that can get to the desired volume and stacks up quite nicely to the LMS-5400 in the range I'm targeting, thanks to it being efficient*:



*Closed box volume set to 9999

Second Edit: Putting a lid on the back of my dipole today. The box is made of stone, so if that sharply reduces or eliminates the 'box' sound... which is really the sound of MDF, I'll go sealed for my 12" center sub and take the extra watts it affords my. Stone enclosures is what I'll be examining over the coming weeks.



Hey, I love dipoles as much as the next guy (more, probably), and your desire to keep higher bass spatially symmetric, up front, and time aligned with the mains is a worthy pursuit, but there aren't many free lunches in audio. I've started design on dipole mains to reach full reference levels at the seating position covering only down to around 60hz, and the design requires four aespeakers dpl12's. Per channel.
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post #296 of 354 Old 08-19-2012, 04:20 PM
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Originally Posted by bossobass View Post

Can you and Bentz even read a graph? He sees +8dB with the window shut vs open and it makes perfect sense to him. Yeah, it doesn't just make sense that he can't read a graph, it makes perfect sense.

What amplitude difference do you see at the bottom of your graph between the two? I think it's at around 2Hz?

Which frequencies do you think are closer to the "pressure region", or if you prefer, which are further away from the modal region?

The problem here is you keep posting charts and graphs and random lines that have nothing to do with the actual theories being presented. I'm claiming that a lower coefficient of friction on your tire reduces your lateral G's on a skid pad, and you're showing skid pad results with a different suspension - it really is that wildly different.

Btw, please don't lump anything I'm saying with LTD02 - I don't agree with much of what he's saying either.

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post #297 of 354 Old 08-19-2012, 05:26 PM
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The effects of RR&T don't change at some magic frequency. They are the driver of the final FR in-room across the subwoofer bandwidth, assuming a minimum performance from the subwoofer in the first place.
Totally agreed: wave physics applies to all frequencies equally.

However, the result of the wave equation is different between short wavelengths and long wavelengths for a given sized room. There is no shortage of acoustics texts talking about this behavior. One of the advantages to splitting it up is it becomes easier to solve the wave equation by making different assumptions. Sure, any assumption is a deviation from the exact calculated result from the actual measured behavior, but we can quantity the worst case magnitude of that error and know that we're sufficient for any real application.

At the very lowest frequencies, below the modal region, the reflections effectively stay in phase with the direct sound. This has two distinct effects:
1) The listener experiences multiple acoustic sources (the reflections) summing together at the listening position
2) The subwoofer sees a larger acoustic load: this is really the same effect as horn loading. For the forward motion of the driver, the reflections from the room cause a larger pressure to exist in front of the driver, which in turns allows the driver to push even more air. Same thing happens in the reverse motion where it's trying to create a lower pressure, the negative pressure wave removes even more air molecules, and thus the woofer pulls to an even lower pressure. Basically, the subwoofer becomes more efficient. Eventually, this behavior gets capped at the point where the acoustic impedance is equal to the output impedance of the transducer - at which point you have the ideal case 50% efficiency. At this frequency, you no longer see any additional gain as you go lower in frequency. Then couple this with the lossy behavior of the walls and this is why you typically see a plateau type effect to the room gain. And if the walls are more lossy, then you'll see even less gains (which is why sometimes you don't get the ideal 12dB/octave).

If you want to solve the wave equation without making any assumptions, then go right ahead....you will find the exact same result within a calculable margin of error (which will be much less than the deviation any real room will impart).
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Also equally astounding is the idea that when a boundary that has an Sd of 420,000 cm^2 is sent into sympathetic vibration at its resonant frequency, it will always cause a complete attenuation (the tech-term is suckout) of that frequency from the in-room response.

You have yet to show any measurement where a large boundary sympathetic vibration has increased the amount of energy in the room.

Here's what the electrical circuit for a band-stop filter looks like:


At really low frequencies, the impedance of the LC section approaches an open-circuit because of the capacitor. At really high frequencies, the inductor makes it an open-circuit. At some frequency inbetween, there is a minimum impedance associated with the resonant frequency. It is at this frequency that it creates the acoustic short circuit. For a voltage source (the pressure wave), it's impossible to get a higher voltage than an open-circuit.

Below the modal region of the room, the acoustic source output impedance is no longer reactive (which would be the sum of the driver + all the other reflections). Therefore it is impossible for a band-stop filter to create more amplitude than comes into the node. The pressurization region is the only caveat I ever gave to the sympathetic resonances - and that's because most building resonances fall below the modal region.

Also, nowhere did anyone say that this resonance creates a huge -infinity suckout in the frequency response at the listening position. If you see -6dB at the listening position from a wall resonance, then that means half of the energy arriving at the listening position was probably being reflected off that wall. In other words, it essentially becomes the absence of a boundary - so no pressurization is happening at the surface of that wall at that frequency.

Inside the modal region, or near the reactive output impedance of the acoustic source, it is possible for resonances to have +/- effects, but there is still a net energy loss in the room. This is the fundamental basis of a Helmholtz resonator. Surely you aren't suggesting that I'm unaware of their positive effects in the modal region? Any graph that you might claim showing an increase, it is required that the acoustic impedance on the vibrating wall be reactive.

Anyways, none of the measurements here contradict any of these theories. Any extent to which you think they do, I would kindly suggest that you are mis-applying the theory. But hey, you're not formally trained, nor an engineer, and you certainly don't make your living based on how well you apply these theories - so I honestly expect no less and it's no discredit to the hobbyist. However, if you want to provide an actual rebuttal that offers any meaning, then let's bust out the wave equations and crank through the calculus. This stuff is actually one of the easier applications to solve the wave equations with. Or if you're afraid of the math, then let's stick to measurements showing actual causation instead of correlation - that would start with an actual rectangular room.

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post #298 of 354 Old 08-19-2012, 06:12 PM
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"Totally agreed: wave physics applies to all frequencies equally."

what about where the frequency is low (below the critical frequency) and air behaves more like an incompresible fluid?

that is what the guy in germany was noting as well, if the wave equation applied to all frequencies equally, his and others double bass arrays wouldn't fail below the critical frequency. it would be simple "throw" and "catch" front to back and that is how it works...until below the critical frequency.

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post #299 of 354 Old 08-20-2012, 09:37 AM
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Originally Posted by MBentz View Post

2) The subwoofer sees a larger acoustic load: this is really the same effect as horn loading. For the forward motion of the driver, the reflections from the room cause a larger pressure to exist in front of the driver, which in turns allows the driver to push even more air. Same thing happens in the reverse motion where it's trying to create a lower pressure, the negative pressure wave removes even more air molecules, and thus the woofer pulls to an even lower pressure. Basically, the subwoofer becomes more efficient. Eventually, this behavior gets capped at the point where the acoustic impedance is equal to the output impedance of the transducer - at which point you have the ideal case 50% efficiency. At this frequency, you no longer see any additional gain as you go lower in frequency.

I especially like this comment because when I originally proposed the idea, you pi$$ed all over it as "yeah, a really bad horn" . cool.gif

I completely agree (in fact, I believe it's the only thing you and I have agreed upon from day one) with the effects of sealed sub loading in a room. Where we seem at odds, or where I seem at odds with the theories being thrown around in this thread, is that I don't believe the loading is variably proportional to wavelength.
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You have yet to show any measurement where a large boundary sympathetic vibration has increased the amount of energy in the room.

Both Josh and I have posted detailed measurements of our respective rooms here and in other threads over the years that clearly show the resonance frequencies being excited by the subwoofer. They occur below the so-called modal region, which is positively in the ballpark for the structural construction and size of our rooms.

The effect of reflections does not get capped. It's the signal chain of the system that gets capped (literally, because it's the DC blocking caps that roll the signal off, which roll off is additive throughout the chain) below which it rolls off rather steeply.

You're not seeing any capping in the traces. You're seeing a spike at the rooms lowest resonance that's still in band, below which the subwoofer simply winds down quickly because it is no longer being fed a signal or can no longer amplify or reproduce the signal.
Quote:
However, if you want to provide an actual rebuttal that offers any meaning, then let's bust out the wave equations and crank through the calculus. This stuff is actually one of the easier applications to solve the wave equations with. Or if you're afraid of the math, then let's stick to measurements showing actual causation instead of correlation - that would start with an actual rectangular room.

I find it odd that you propose I rebut your flawed theory with math when I've posted data for years and you've simply misquoted specs and blustered a lot with the option to do a 180 degree shift as though you were right all along, posting no evidence whatsoever. For example, it's painfully apparent that you don't understand the increase in amplitude resulting from excitation of a boundary at its resonance frequency.

Yes, I would love to see you (and LTD) delve into the math. I will not do that. It takes up too much of my time for no good reason other than to debate folks who either don't own a subwoofer or own subwoofers that roll off above the band being discussed. Correct, I am not a paid acoustician and neither is anyone else who posts here. The rule is 'post em if ya got em' and let everyone agree or disagree until there's a consensus... or not.
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post #300 of 354 Old 08-20-2012, 08:30 PM
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Originally Posted by bossobass View Post

I especially like this comment because when I originally proposed the idea, you pi$$ed all over it as "yeah, a really bad horn" . cool.gif

Well as far as horns go it is a bad horn tongue.gif

The pressure loading from a reflection traveling 20ft is going to be at a much lower amplitude and delayed a lot more than a reflection traveling a few inches inside the compression chamber. It also doesn't work over as wide a bandwidth and you don't get the DI / polar control advantages either (not to say that's a free lunch for super low frequencies either).
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Originally Posted by bossobass View Post

Where we seem at odds, or where I seem at odds with the theories being thrown around in this thread, is that I don't believe the loading is variably proportional to wavelength.

I'm pretty sure you're not at odds with the concept that you need both phase and amplitude information to know how two acoustic sources will sum. In other words, what is the gain you get when the reflection is delayed by 90 degrees (let's just assume equal amplitude)? As you slide lower in frequency, the phase angle difference is decreasing, and therefore you get better summation.
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Originally Posted by bossobass View Post

Both Josh and I have posted detailed measurements of our respective rooms here and in other threads over the years that clearly show the resonance frequencies being excited by the subwoofer. They occur below the so-called modal region, which is positively in the ballpark for the structural construction and size of our rooms.

I dunno, for every so called "boost", I usually see two dips giving the appearance of a peak.....or I see characteristics of reactive impedance (like especially with the window open, showing a peaking high pass effect).
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Originally Posted by bossobass View Post

I find it odd that you propose I rebut your flawed theory with math when I've posted data for years and you've simply misquoted specs and blustered a lot with the option to do a 180 degree shift as though you were right all along, posting no evidence whatsoever/

Why post data when all the data available online already lines up very well with the theory? I would suggest that you're not fully wrapping your head around the existing theory....if you did, you wouldn't be so hung up on this notion that you're on the cutting edge of "new research" or however it is you go on about stuff that's already well quantified.
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Originally Posted by bossobass View Post

The rule is 'post em if ya got em' and let everyone agree or disagree until there's a consensus... or not.
Would you have preferred to believe in a flat Earth until the consensus was otherwise?

-Mike Bentz
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