Seperate drivers for LF and ULF or one driver? - Page 4

"If the pressure pot theory is correct, how exactly does the room gain increase as frequency decreases?"

below the modal region, what you call the "magic frequency", the wave is so long relative to the room that all the molecules are compressed at the same time, so pressure goes up and down everywhere in the room. since the pressure is going up and down at the same level everywhere in the room, the spl, is flat.

what you are seeing is not so much "room gain" as the absence of rolloff.

Still works the same, at least at LF, which is what this discussion is about. A monopole is for all intents omni in the main frequency range of the discussion of this thread.

That Geddes quote uses words such as "like" and "apparent" when describing the pressure phenomenon. I read that as there being waves in the room, just that they act as one.

Thank you. I was going to ignore the obviously errant statements to try to focus the discussion, but this needed said. Just place a sub 1/4 wavelength of a specific frequency from a boundary and measure the result. But that would require an actual exercise and not some blanket edict.



Part of the frustration in these discussions is that the explanations aren't available in a one-liner sound byte. It would help if responders actually read what I write.

Thankfully, some do. Tux has the answer and I posted it twice thus far:

Nothing in these forums is a given until you repeat the answer a few dozen times against all sorts of opinions to the contrary. Years later, folks will quote it back to you as though they're telling you something everyone always knew and as though it's news to you, even if they quote it out of context or with a completely errant understanding.

Fwiw;

Here is Geddes chapter on acoustics from his killer Transducers work. It is but a cursory look, for sure.

---

Additionally, aside from some of the issues being discussed in this thread, there's his take with regard to a room's early reflected energy.

Thanks

If you like, I can replace 'magic' with 'unknown', 'infinitely variable', 'room-dependent' or some term that's acceptable to you. Tell us exactly what frequency is 'below the modal region' in your room.

I submit that there is no such thing as 'below the modal region' where sound waves in a room are concerned.

Have another (or a first) look at the scaled syringe view of my room and then tell me how all the molecules of air could possibly undergo compression (and rarefaction, let's not forget the second half of the sound wave) at the same time when a) the sound wave is traveling at the speed of sound, not the speed of light, b) the point source displaces eighteen one hundred thousandths of the rooms volume and c) 5% of the rooms boundaries is wide open.

I mentioned Josh's measurement methodology earlier for a reason. In order to simulate an anechoic response at a distance of 1 meter, he has to place the DUT far enough away from any reflective surfaces according to the entire BW of the sine sweep, on the ground, and then he has to place the microphone on the same ground at 2M distance to compensate for the ground-induced boundary gain of 6dB.

That's irrefutable evidence of precisely predictable, not theoretical, boundary gain at all frequencies and the inverse square law.

Now, if he repeats that process in a perfectly sealed room of infinitely stiff walls and ceiling placed over the same ground, your contention is that below the 'modal region', the increase in SPL is also precisely predictable and an increase in SPL as frequency decreases is also predictable. [Never mind for now that there is only an infinitely stiff wall in Star Trek episodes and that this 'modal region' is theoretical and that the ground is not infinitely stiff.]

Now, remove one of the infinitely stiff walls. Your contention is that there will be no difference in the results, except that the SPL will increase at some small slice of the BW because the room will have been converted into the interior of a bandpass subwoofer.

And I should be buying this story?

"Still works the same, at least at LF, which is what this discussion is about. A monopole is for all intents omni in the main frequency range of the discussion of this thread."

a theoretical point source on a plane is not frequency dependant as you shift from 1/2pi space to 1/4pi space where the point would now be in the fold or as it changes again from 1/4 pi space to 1/8 pi space where the point would now be located in the corner.

in the first case, the sound at all frequencies increase like a half sphere, then a quarter of a sphere, then as an 1/8th of a sphere.

"That Geddes quote uses words such as "like" and "apparent" when describing the pressure phenomenon. I read that as there being waves in the room, just that they act as one."

that is exactly how it works. imagine a wave leaving the subwoofer in slow motion. the pressure begins to rise as the wave moves across the room. but when it hits the opposite wall it is still increasing pressure because the wave is so long relative to the room. so now the wave is reflecting back toward the source and the pressure keeps increasing everywhere in the room. then the second part of the wave begins to exit the subwoofer and pressure begins to fall with the same effect. because the wave is so long, the negative side of the wave is everywhere in the room at the same time.

here are three quick sketches to illustrate. the first picture shows the high pressure and low pressure zones of a wave that is small relative to a room. the wave will ultimately bounce around the room creates points of constructive and destructive interference aka modes.

when the wavelength is long relative to the room, the pressure will be building and then falling everywhere in the room.







"Just place a sub 1/4 wavelength of a specific frequency from a boundary and measure the result."

that is a completely different topic.

"I submit that there is no such thing as 'below the modal region' where sound waves in a room are concerned."

what is meant by that is that if you calculate all the modes for a room, the one with the lowest frequency will be the mode that spans the long dimension of the room (or long diagonal). below that point, there are no modes and you get 12db/oct gain.

"That's irrefutable evidence of precisely predictable, not theoretical, boundary gain at all frequencies and the inverse square law."

boundary gain is a related, but different topic. the topic that i was addressing was how a subwoofer pressurizes a room.

also, please stop telling me what i am saying. i can speak for myself thanks.
Edited by LTD02 - 8/5/12 at 11:48am

"Have another (or a first) look at the scaled syringe view of my room and then tell me how all the molecules of air could possibly undergo compression (and rarefaction, let's not forget the second half of the sound wave) at the same time when a) the sound wave is traveling at the speed of sound, not the speed of light, b) the point source displaces eighteen one hundred thousandths of the rooms volume and c) 5% of the rooms boundaries is wide open."

as i mentioned before, you have created a giant, low tuned, helmholtz resonator.

the more windows and doors that you open up, the higher the tuning of the room and the less pressure vessel gain that you will get.

edit: as for how little pressure can have such big effects, see post #33.
Edited by LTD02 - 8/5/12 at 1:32pm

interesting read foh. thanks for the link.

geddes and i are using slightly different terminology.

it is the region from what he calls the 0,0,0 or "static pressure mode" aka zero hz up to the 0,0,1 mode (the first non-zero mode as he calls it, which would be the obliqe mode of the magic frequency) that i am referring to as the "pressure vessel gain" region. that region behaves quite differently than the region above.

"The response actually rises at lower frequencies below the first nonzero
mode (and the first anti-resonance). This is due to the 0,0,0
mode, i.e. a static pressure mode. Its level depends on the static pressure
release resistance and only goes to infinity for a completely
sealed (air tight) enclosure. Real enclosures, most notably cars, can
have significant low frequency gain as a result of this static mode."

edit: just in case "static pressure" isn't clear. it means the pressure (positive and negative) is changing (mostly) in the entire room at the same time as in pics #2 and #3 in my post above.
Edited by LTD02 - 8/5/12 at 2:41pm

I think you're still misunderstanding. 0, 0, 0, is 0hz. We don't make 0hz. Until then it's still a wave. Even in your drawings.

As I said before, Geddes has said, after writing Transducers, that no room have pressure vessel gain because all rooms leak.

So I suppose every room has a hemholtz resonator, because all rooms leak. ?

You aren't saying anything, you're repeating what you've read and including zero data as evidence, save the nice drawings. You haven't posted a shred of evidence to support your claims, you have zero experience with the phenomenon and you've done as much experimentation, yet you issue edict after edict, like: "Boundary gain is related, but a different topic". If the wave reflects, it is no longer a different topic.

At least you've actually finally mentioned reflections, which are curiously excluded in your illustrations. Your own drawing confirms that, without reflections there is no gain... just a sound wave equal to the subs displacement capability and assuming zero non linearities. As with all of your posts, the drawing is so lacking important details as to not even represent a starting point for your argument.

Since I have been posting about room gain for many years, please stop explaining things like modal region as though it's my first day at LTD school.

I know what it's supposed to be. I've calculated it in my room from physical measurements. I've measured the in-room sub response accurate to 4 Hz and matched that against the subs anechoic response. Your 'explanation' of the pressure pot theory doesn't jive with my results. I've looked closely at notnyt's measurements, which I believe to be quite accurate. His results do not jive with your assertions. So, now what? Are both our measurements and results grossly in error?

Your posts haven't explained how gain increases as frequency decreases. Is the frequency below your magic number? Then as you allege, it pressurizes the room. So, how does the pressure increase by a factor of 20 as the frequency is halved when the displacement of the wave generator is not changed?

"So I suppose every room has a hemholtz resonator, because all rooms leak. ?"

yes. didn't i mention that already?

the sealed room model is a reasonable approximation though.

bosso was perplexed when he opened his window and his bass at 5hz increased instead of decreased. he posted the "mystery" data. i modeled up what he did to a first order approximation and determined that he tuned his room to 5hz or so. as a result, one should expect to see a little more spl around that frequency and a little more rolloff below it. that is what his data showed. there are many effects at work, but i thought it interesting that my theory fit his data quite well.

"Your 'explanation' of the pressure pot theory doesn't jive with my results."

so what? every person who has listened to audio in a car or who has used earbuds or headphones has experienced "pressure pot theory", as you call it. i kind of like that terminology. :-) in a home theater environment, the levels have to be much higher because the magic frequency is much lower...very much lower, so what is going on is not as intuitive to many readers.

"Your posts haven't explained how gain increases as frequency decreases. Is the frequency below your magic number? Then as you allege, it pressurizes the room. So, how does the pressure increase by a factor of 20 as the frequency is halved when the displacement of the wave generator is not changed?"

that may be thinking about it backwards...although that isn't a sin given how we generally discuss these things. the pressure does not increase by a factor of twenty or the spl would increase by that amount. remember spl is sound pressure level. if the pressure went up by 20x, so would the spl. once you are in the "pressure pot zone", you are not so much experiencing magic gain as a lack of rolloff. when the wavelength is long relative to the room, the pressure rises and falls "everywhere", so you do not experience the drop in pressure by a factor of 20 as you would with a sealed sub at the same frequency outdoors or to a lesser extent in the modal region..

my posts were to show that a subwoofer can and does pressurize a room. i have provided an approximation of the frequency where this occurs, an approximation of the effect on spl, and some examples to help with the intuition for why it is observed. several other folks who are familiar with the science have confirmed the effect. foh pointed you to a paper by dr.g. whose resume is a little better than either of ours in acoustics, but you keep blasting away with such nonsense as:

"A subwoofer, regardless of its displacement capability, cannot, does not, will not 'pressurize' a room. That's so absurd a claim as to border on humorous."

your words. meanwhile 'nots walls are flexing when he turns up the dial, most folks with infinite baffle subs worry if they are going to blow out their windows, and the spl car audio guys are standing outside of their cars pushing on the sides because the flex of the windows and body panels is out of control...no pressure? come on dave.

given how this thread is going, i think it is important to say that dave (bossobass) and i don't really disagree on the fundamental principles. in my estimation, his designs and products are good, particularly for home theater...dollar for dollar, among the best. we are just debating the "hows and whys" things work...

now back to our regularly scheduled programming. :-)
Edited by LTD02 - 8/6/12 at 1:22am

Oh well. See you in the speaker cross over threads.

For the record, "dr" Geddes wouldn't know a 5 Hz sound wave if it bit him in the rear end, constructively reflected or stewed in a pressure pot or otherwise. I couldn't care less what he thinks about the subject. His 'paper' is irrelevant to the conversation as it, like all of your posts, is devoid of any data whatsoever and laced with lofty proclamations.

In the cases of crazies blowing out their windows, post the video in slo-mo and you'll see very clearly that there sound pressure waves hitting those windows. Just because you confuse that to support your silly theory... come on John. You can't prove it's progressively constructive wave reflection or pressure pot fluctuations in ambient air pressure from those silly "OMG! My windows are going to burst!" statements (which are mostly BS) or those car videos (which are all BS). Just the fact that you're resorting to those sorts of references is telling.

Since there is not a single shred of data to support your theory regarding ambient air pressure, I'll focus on one facet in which there is hard evidence in abundance:

"Pressure Vessel Gain begins just below the modal region".

Ed Mullen. 2k cubes. So-called modal region maximum: approx 24 Hz. Pressure pot gain onset: approx 35 Hz:



My room. 3.5k cubes. So-called modal region maximum: approx. 19 Hz. Pressure pot gain onset: approx. 30 Hz



I also have notnyt's transfer function. I don't know his theoretical so-called modal region max, but I do know his room is nearly double the volume of mine, so it has to be lower than 19 Hz. and we all know the anechoic response of his subs. His room gain deviation begins around 30 Hz.

IOW. your 'approximation" is so far off as to not qualify as an approximation. This is obvious. Everyone reading can see it. Yet, you cling to this facet of your theory.

Since it's essential to know this information to properly configure a subwoofer system for a specific space, I have done it hundreds of times from others measurements and I've found the same discrepancy with your theory every single time. It simply can't be explained away with "dr Geddes has a paper" and "so what?"

That's because when you have no topical reply, you change the discussion to something else and pour on reams of verbiage and data.

In this case you had no response to your uninformed insistence that sound has no effect on the pressure in a room, when sound *is* pressure.

You love to play the contrarian but add nothing.

Sound pressure waves originate at the sub drivers cone and reflect off the boundaries of a confined space in a pattern consistent with the subs placement in relation to those boundaries. Those reflections may be destructive, unity, constructive and everything in between and can result in standing waves, which represent all of the above. As frequency decreases and the sound waves become longer, more and more of the reflections become constructive than any of the other possibilities. As frequency decreases, the room loses the ability to support a standing wave for reasons LTD has stated several times in this thread. The more constructive virtual point sources there are and the less unity and destructive, along with the absence of standing waves,. the more sound pressure levels in that confined space will increase as frequency decreases.

Who said pressure in the room is unchanged? There would be no sound if that were the case. The pressure is local. This we know is the case because we know the original wave consists of a local pressure moving at the speed of sound and reflecting off boundaries.

That is the effect of sound on the pressure of the room. The ambient pressure does not change. That is an impossibility, like trying to change the ambient air pressure in a tire inner tube with a pneumatic pump when the inner tube has a gaping hole in it. Since you're the one clinging to that fantasy, and since you have said this is a simple phenomenon to measure, could you please produce those measurements and the methodology?

Why do you think the pressure region is called that?

Wow... all this debate over what terms are used for the same result that converges from two perspectives.

Analysis of reflecting waves and images to lower and lower frequencies within a confined space simply converge with the notion of very slowly increasing and decreasing barometric air pressure. Maybe bosso' misunderstood the assertions of others to imply that the room's pressure was continuously raising, which would be incorrect.

By definition, sound is the modulation of air pressure. With no starting air pressure to modulate, there is no sound.

Looking only at the frequency below which sound turns to a pressure modulation of a confined, leaky space does not give any good indication of the transition and transfer curve of the room to which bosso was pointing, but it is a useful understanding and concept to equate to a perspective of boundary loading. This is very similar to the understanding that below some frequency there is no functional difference between a continuous line source and a sufficiently dense string of point sources the same length of the line. For a given line length and low frequency, adding more sources in between has no affect on the radiation pattern.

oh dear...

Someone coined the phrase and folks nicked it as a convenience, like every other phrase that's been bandied about here. Sorta like 'there's no replacement for displacement'.

Why I believe it's a misnomer:

If the pressure increases everywhere in the room, there would have to be an area or areas where the pressure simultaneously decreases because you aren't introducing more air molecules. If all of the air molecules compress, there will have to be simultaneous ares of rarefaction. Of course, that would mean that the ambient air pressure does not increase.

If the pressure increases everywhere in the room, it wouldn't matter where the subs are placed in the room:



One sub in left corner vs one sub in right corner vs both subs. An increase in SPL of +3-6dB is seen, but a decrease in SPL is seen in the so-called pressure region. Both point sources inject the same wave at the same time, yet there is cancellation below the modal region at the mic location.

The pressure pot theory does not explain how SPL increases as frequency decreases.

Why do you think the open/shut, separate placement, stack vs floor placement and onset of room gain discrepancies are evidence that ULF increases ambient air pressure?

I need more than you or anyone else saying 'because it does' to shift the weight of evidence.

Nope. I understand the concept perfectly.

I'm struggling with this:

A room has a finite and measurable number/density of air molecules (ambient barometric pressure). A sound pressure wave moving through that air cannot increase the number of air molecules, regardless of its frequency. Therefore, the barometric pressure cannot possibly increase everywhere in the room.

It would have to draw more air molecules from outside the room (in the compression 1/2, which is where we seem to have concentrated the discussion). Well, that's a perfectly fine starting point, but then why is it considered to increase the effect if the room is better sealed and why does the effect measurably increase when the walls and floor are masonry vs other, both conditions being less leaky?

If the woofers were open dipoles in the room, I agree, you could not have the pressure increase momentarily (actually periodically).

With an enclosed woofer you do have clear volume displacement which on the outward excursion leaves reduced pressure inside the box and increased pressure externally. This happens outdoors locally to the subwoofer, but once confined in a closed (but leaky) space, you are in fact reducing the volume of the room by the displacement of the woofers. It may seem small in displacement compared to a room, but moderate sound levels are not from huge pressure variations. On the negative cycle, the pressure inside the box increases and the pressure decreases in the room as the room volume gets larger by the incursion of the woofer.

"A room has a finite and measurable number/density of air molecules (ambient barometric pressure). A sound pressure wave moving through that air cannot increase the number of air molecules, regardless of its frequency. Therefore, the barometric pressure cannot possibly increase everywhere in the room."

the answer lies in the ideal gas law. p=nrt/v. n is the number of molecules. r is just a constant. so there are two ways left to increase pressure, by increasing the temperature or decreasing the volume. in the pressure pot zone, there is an effective change in volume, so the pressure goes up, everywhere. i suspect that the temperature also rises, but i'm not as sure about that one. on the opposite side of the wave, the room volume increases and pressure drops everywhere. same with temp.

the "modes" that you are observing below the magic frequency are boundary effects, such as your walls or floor absorbing energy.

for intuition, pressure cookers work on this principle. the number of air molecules in the cooker doesn't change, but as you apply heat, the pressure rises. (ultimately the number of molecules goes down because some of the air gets released so the cooker doesn't blow up).

alternatively, if you have an airtight syringe and try to press it in, you will only get so far because each time the volume in the device goes down by 1/2, the pressure doubles.
Edited by LTD02 - 8/7/12 at 7:20pm

Debating the cause of cabin gain, or for that matter it's existence, is a waste of time, as it's been proven time and time again. It occurs beginning at the frequency where the longest room dimension is 1/2 wavelength long, at a rate of 12dB/octave when the room is tightly sealed and strongly constructed. Most rooms won't get that much gain, being either not tight enough or with too soft walls, but you'll still get something. If you doubt its existence just look at the crazy high dB levels achieved in auto sound competitions, far higher than the physics otherwise allow. The winners in those competitions aren't those with the most speakers or largest amps. It's those who weld the doors closed and fill the body cavities with concrete.

You guys will love this: add 4 point sources and crank the frequency to the lowest setting. Check out the 3d mode too. Have fun.
http://www.falstad.com/ripple/
(You'll need to have java installed for it to work.)

I'm just not buying that cone movement causes an increase in ambient air pressure.

As Fitz notes, the winner is he who stiffens the boundaries (reflection/transmission losses) and seals the cabin against air loss, both of which support constructive reflection as the mechanism by which SPL increases as frequency decreases. The pressure cooker theory, as it's been presented here so far, would dictate that the frequency is irrelevant and does not explain how there is an increase in cabin gain as frequency decreases when excursion is a constant.

BTW, Fitz... no one has suggested room gain doesn't exist, just as no one has definitively shown how it works, thus the discussion.

The syringe illustration is a prime example of using an illustration that disproves a theory to prove that same theory.

But, you don't have an air tight syringe. The syringe has a hole in it. As the plunger moves into the chamber, it pushes the medium out the hole, which is the whole premise of its operation.

I've been using it for years. I use it as an aid in sub placement and, although it's a ripple tank and not a sound wave cabin, it is predictive enough. The trouble with the program is trying to accurately draw the boundaries with a mouse... tedious stuff.

I recently used it to predict the effectiveness of my latest project, the anti-horn center channel loudspeaker. Using dual drivers while eliminating comb filtering vs the typical flat baffle result.

I've run the ripple tank at its lowest frequency with a single point source, corner loaded. You can set the distance from the corner to a point where there is compression/rarefaction with almost no destructive interference, but the animation will clearly indicate that it takes a minimum number of cycles to get there and the ambient water level does not change, because it can't, not even in an animation simulator.

Please clarify what you are defining as "ambient air pressure". You can't have sound if the air pressure doesn't modulate vs. time.

Actually both perspectives of boundary images/reinforcement and a source in an acoustically small space predict the increasing gain with lowering frequency. Actually the condition provides gain vs. operation in an open space or radiation angle. A driver which has constant displacement vs. frequency has a 12dB/octave roll off to the bottom end. Once you aren't driving an infinite space, this dropping rate of efficiency no longer occurs.

Once again, the different perspectives or analysis converge to the same answer when wavelength is much smaller than the dimensions of the confined space.

Yes, air pushes out the leaky syringe, but assuming the hole is small or has any resistance, the air inside must compress some amount before or while the air is escaping.

Let's take an extreme example... Does the air pressure not increase and decrease uniformly at low frequencies inside a sealed subwoofer?