SPL, Nearfield Subwoofers, and Brutality - AVS Forum
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post #1 of 145 Old 03-05-2014, 07:47 AM - Thread Starter
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Something that has been a long running mystery is why some subs tend to produce more physical effects than other subs even when the SPL is kept the same.

A recent and rather striking experiment was conducted here:

http://www.avsforum.com/t/1488059/your-home-theater-ulf-score/1300_50#post_24096453

In essence, a near-field subwoofer produced much greater physical effects than a far-field subwoofer as measured by an accelerometer, even though SPL was kept the same.

This is not the first time that this observation has been made.

So what is going on?

One hypothesis relates to the way sound wave propagate. In the near field, or even out doors, the sound wave moves in one direction first compressing the air molecules on one side of the listener, then moving across the listener's position, and finally increasing pressure on the opposite side of the body. The pressure change is in ONE DIRECTION ONLY. As the pressure wave moves across the body, following compression of the air is rarefaction of the air--a negative pressure zone. As this rarefaction of air creates a negative pressure zone, the listener is then "pulled" back toward the sound source.

So, positive pressure gives a linear "push" and it is then followed by a negative pressure which gives a linear "pull". Thus the listener is rocked back and forth as the sound wave passes by. This creates the physicality.

Now, take a subwoofer placed in a car or in a home room. When a tone is played, the wave will begin to propagate in one direction. However, it will then be met with reflections off the boundaries. So what actually ends up hitting the listener are bass waves from ALL DIRECTIONS. The air molecules are compressed and rarefied, which creates the SPL, but there is no directional change. That is the key. Integrated across the entire body of the listener, there is no net directional change in pressure. It pushes in all at the same time, then it sucks out (rarefied) all at the same time. As a result, the pressure goes up and down. SPL is measured, but there is no movement back and forth of the listener. No physicality, at least in some sense.

---

A further ramification of this is that various subwoofer designs could have more or less physicality depending on the extent to which they produce linear bass waves. Horns, for example, are frequently described as having more punch than direct radiators all other things equal. Within this model of physicality, this could be explained by the fact that a sound wave must travel some distance before exiting the horn, thus putting the listener effectively into the "far field" where bass waves are nearly planar even while standing directly in front of the horn itself.

The same might also be true of large radiators and/or large numbers of them relative to small diameter radiators, as the former creates something much closer to the planar wave of the far-field.

---

Thoughts?

John
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post #2 of 145 Old 03-05-2014, 08:16 AM
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I think there's more to it than that. The couch experiment is flawed. Just because the SPL is the same at the accelerometer (phone), does not mean the SPL is the same where the speaker is near the couch and shaking it.

Also, I switched from sealed to ported in the same location and they're much more tactile.
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post #3 of 145 Old 03-05-2014, 08:35 AM
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John, this is interesting. I read some of that thread as it developed, but didn't weigh in at the time. There may be merit in the hypothesis you've proposed, but I'm not convinced (yet). There are a few things that should be considered as part of this theory.

First, is scale. Obviously, frequency will influence the response a great deal. Let's assume a 20Hz wave in free space (or I suppose half-space is fine assuming we're observing from within a couple feet of the boundary). With a wavelength around 56 feet, we should divide that into a 28' long zone of compression and a 28' long zone of rarefaction. At any position along that wave, an object experiences a net force defined only by the difference on pressure at each end of the body (that's simplified - I believe there is an integral expression for this, but my calculus skills are poor. Think of buoyant force exerted on a submerged body - the difference in pressure above and below the body defines the net force.) My point here is that the maximum force experienced by a person or piece of furniture is a small fraction of the total difference in pressure of the wave. HOWEVER, that force is also continuous in one direction (though varying in magnitude) during the entire compression portion, and then reversing for the entire rarefaction portion. Thus, the total acceleration experienced by the body is not simply defined by the magnitude of the force, but also by the time period over which it acts (More calculus I can't prove).

Related: when clothing or fabric is seen to flap in the wind of subwoofers, the motion of that fabric exceeds the motion of any single air particle, I believe (someone needs to check my math). The maximum displacement of a particle is calculated as the maximum pressure difference divided by the product of wave velocity, medium density, 2pi, and frequency. http://www2.cose.isu.edu/~hackmart/soundwaves100.PDF I calculate maximum particle displacement for a 20Hz wave at 123dB (30Pascals deltaP) as just over half a millimeter.

I'm not prepared to pose a complete competing theory, but I think notnyt's observation that ported subs don't behave the same as sealed may be relevant. I think there may be fluid dynamics related to what noah katz posted a couple weeks ago going on. http://www.avsforum.com/t/1518950/ported-boxes-shop-vacs-and-voice-coil-cooling Somehow the piston action of the driver cone or port air (as the case may be) is driving this phenomenon, I suspect.

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post #4 of 145 Old 03-05-2014, 08:55 AM - Thread Starter
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good points fred. thanks for weighing in.

with respect to point 1, it would seem that the ability to exert physicality is related to the total area under the curve for a cycle. this could explain why it is easier to "rock" something with low frequencies...an extended time period of "push" then followed by and extended time period of "pull" whereas with the high frequencies, the pull is so quickly following the push that the push doesn't really get a chance to get the object moving very much.

of course, within that is whether or not you are hitting the target on a resonance. each push/pull that hits on a resonance will build and increase, potentially dramatically, the amplitude of linear physicality (how much you are moving back and forth).

it does seem almost obvious after re-reading this now that what we call physicality is simply related to the linear component of spl, but i suppose that is way most things go. :-)

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post #5 of 145 Old 03-05-2014, 08:58 AM - Thread Starter
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"The couch experiment is flawed. Just because the SPL is the same at the accelerometer (phone), does not mean the SPL is the same where the speaker is near the couch and shaking it."

perhaps i misread what he did.

my understanding was that he played a tone at a given spl and took a reading on his accelerometer from the far field sub.

played the same tone at the same spl and took a reading on his accelerometer from the near field sub.

even though the two spl readings at the seating position were the same, the accelerometer readings were very different, with the far field sub producing pretty much no effect.

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post #6 of 145 Old 03-05-2014, 09:04 AM - Thread Starter
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"Also, I switched from sealed to ported in the same location and they're much more tactile."

in order to eliminate a whole bunch of other variables, plugging the ports and eq'ing the response, then re-running your test might be revealing.

some variables, such as the size of the baffles or changes in the internal air spring of the cab could be having an influence.
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post #7 of 145 Old 03-05-2014, 09:14 AM
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Quote:
Originally Posted by LTD02 View Post

"The couch experiment is flawed. Just because the SPL is the same at the accelerometer (phone), does not mean the SPL is the same where the speaker is near the couch and shaking it."

perhaps i misread what he did.

my understanding was that he played a tone at a given spl and took a reading on his accelerometer from the far field sub.

played the same tone at the same spl and took a reading on his accelerometer from the near field sub.

even though the two spl readings at the seating position were the same, the accelerometer readings were very different, with the far field sub producing pretty much no effect.

yes, but doubling distance drops spl by a given amount. If the subs are right up on the couch, it will be in much higher spl over more of an area than if the subs are across the room, even if the point on the couch where the phone is measures the same.


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post #8 of 145 Old 03-05-2014, 09:29 AM - Thread Starter
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"yes, but doubling distance drops spl by a given amount. If the subs are right up on the couch, it will be in much higher spl over more of an area than if the subs are across the room, even if the point on the couch where the phone is measures the same."

i see what you are saying. iirc, a few pages later he ran another test with no couch and similar results.

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post #9 of 145 Old 03-05-2014, 09:31 AM
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Originally Posted by notnyt View Post

I think there's more to it than that. The couch experiment is flawed. Just because the SPL is the same at the accelerometer (phone), does not mean the SPL is the same where the speaker is near the couch and shaking it.
Also, I switched from sealed to ported in the same location and they're much more tactile.
Here's a simple experiment. Play a low frequency tone at a given SPL , put your hand by the port, feel the air pulsating. Move away from the sub ten feet or so, turn it up to the same SPL at that spot, hold your hand in the direction of the port. Feel the air pulsating? Nope.
When very close to the sub there are two effects happening, the creation of an acoustic wave and the pumping of the air near the speaker by the driver and port. The effect of the latter dissipates much more quickly than the former as you move away from the source. With a ported enclosure you feel the pumping action even more due to Bernoulli's Principle, which causes a higher velocity of the air mass vibration in front of the port than in front of a cone. You can model what's happening with the acoustic wave with the principles of acoustics, but the other falls into the realm of fluid dynamics.

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post #10 of 145 Old 03-05-2014, 09:38 AM
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Quote:
Originally Posted by notnyt View Post

I think there's more to it than that. The couch experiment is flawed. Just because the SPL is the same at the accelerometer (phone), does not mean the SPL is the same where the speaker is near the couch and shaking it.

Also, I switched from sealed to ported in the same location and they're much more tactile.

Well.... it IS louder now. wink.gif
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Here's a simple experiment. Play a low frequency tone at a given SPL , put your hand by the port, feel the air pulsating. Move away from the sub ten feet or so, turn it up to the same SPL at that spot, hold your hand in the direction of the port. Feel the air pulsating? Nope.
When very close to the sub there are two effects happening, the creation of an acoustic wave and the pumping of the air near the speaker by the driver and port. The effect of the latter dissipates much more quickly than the former as you move away from the source. With a ported enclosure you feel the pumping action even more due to Bernoulli's Principle, which causes a higher velocity of the air mass vibration in front of the port than in front of a cone. You can model what's happening with the acoustic wave with the principles of acoustics, but the other falls into the realm of fluid dynamics.

Yup. +1


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post #11 of 145 Old 03-05-2014, 05:25 PM
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There are many variable that aren't being isolated and/or potentially taken into account during that "test".

1) Reduction of SPL over distance.
2) Planar wave in free-space vs an omnidirectional modal-oscillation, found in that of a room.
3) Differing resonant frequencies of differing materials.
4) Differing sound propagation velocities of differing materials.
5) The duration under which the particle's kinetic-energy is transfer (i.e a frequency dependance).

Put simply, if this were conducted in free-space and with only one frequency and one material (say a HW floor platform) and the SPL was kept constant for the distance under test and generated by only one source (and a lab-grade accelerometer was used)... then I would hazard a guess that the results would be far more consistent, if not found identical. No Difference. (Prove me wrong tongue.gif)

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post #12 of 145 Old 03-05-2014, 10:18 PM - Thread Starter
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compare this intuitive artistic representation:



to this particle velocity (sound intensity) model:



kind of looks like the intuition was right, but didn't know how to describe what they were observing.

[scott, insert ancient aliens graphic here.]

from this fairly decent explanation of sound intensity.

http://www.toyo.co.jp/file/pdf/mft/ebook/ebook_5_intensity.pdf

i'm kind of surprised that this topic isn't really discussed much.

iirc, it was neutro who introduced this idea in the other thread. somehow it didn't click the first time that i read what he said. props @neutro.

this is the key point:



intensity is different from pressure in that it is the net vector sum. pressure and direction. omnidirectional pressure can have a vector sum of zero, so it doesn't provide any lateral movement in the object affected.

sadly, this suggests using single microphone frequency sweeps to determine the nature of a sound field is critically flawed, as the single mic will record the same pressure (spl) regardless of the amount of intensity.
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post #13 of 145 Old 03-05-2014, 10:20 PM
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post #15 of 145 Old 03-05-2014, 11:41 PM
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Maybe conduct that test in a Smokey room? Strategicly placed sticks of incense?
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post #16 of 145 Old 03-06-2014, 12:35 PM
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Love this topic guys. Ltd, thx for bringing it up.

I've tried to summarize my findings in post 4 and 5.

http://www.avsforum.com/t/1488059/your-home-theater-ulf-score#post_23676269


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post #17 of 145 Old 03-06-2014, 12:37 PM
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Quote:
Originally Posted by notnyt View Post

yes, but doubling distance drops spl by a given amount. If the subs are right up on the couch, it will be in much higher spl over more of an area than if the subs are across the room, even if the point on the couch where the phone is measures the same.

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"yes, but doubling distance drops spl by a given amount. If the subs are right up on the couch, it will be in much higher spl over more of an area than if the subs are across the room, even if the point on the couch where the phone is measures the same."

i see what you are saying. iirc, a few pages later he ran another test with no couch and similar results.
This is correct. I removed the couch and it measured the same spl. See post 4 in that ULF thread.


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post #18 of 145 Old 03-06-2014, 12:41 PM
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Quote:
Originally Posted by LTD02 View Post

compare this intuitive artistic representation:



to this particle velocity (sound intensity) model:



kind of looks like the intuition was right, but didn't know how to describe what they were observing.

[scott, insert ancient aliens graphic here.]

from this fairly decent explanation of sound intensity.

http://www.toyo.co.jp/file/pdf/mft/ebook/ebook_5_intensity.pdf

i'm kind of surprised that this topic isn't really discussed much.

iirc, it was neutro who introduced this idea in the other thread. somehow it didn't click the first time that i read what he said. props @neutro.

this is the key point:



intensity is different from pressure in that it is the net vector sum. pressure and direction. omnidirectional pressure can have a vector sum of zero, so it doesn't provide any lateral movement in the object affected.

sadly, this suggests using single microphone frequency sweeps to determine the nature of a sound field is critically flawed, as the single mic will record the same pressure (spl) regardless of the amount of intensity.
The theory is that the fv15hps produces more particle velocity and thus sound intensity then the ftw21s in my room. Sound intensity is the driver for vibrating the couch more.


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post #19 of 145 Old 03-06-2014, 01:25 PM
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I find this all too interesting.
One thought I had was if a person was not interested in SPL, what would happen if you placed 2 subs across from each other but each sub ran on different polarity. My thinking would be that 1 sub would be pushing out while the other would be sucking in, creating a push- pull configuration. At least in my mind. rolleyes.gif

I probably am way off on this stuff and totally out of my league but my head keeps thinking about all the crazy things people have tried on here, so I thought I would ask.

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I find this all too interesting.
One thought I had was if a person was not interested in SPL, what would happen if you placed 2 subs across from each other but each sub ran on different polarity. My thinking would be that 1 sub would be pushing out while the other would be sucking in, creating a push- pull configuration. At least in my mind. rolleyes.gif

I probably am way off on this stuff and totally out of my league but my head keeps thinking about all the crazy things people have tried on here, so I thought I would ask.

I could be mistaken about this, so correct me if I am wrong, but, if you run two subs, across from each other, as you described, with one pushing out and the other pulling in, I believe that would make them 180 degrees out of phase, and you would get some cancellation in some form or another. (Like I said, I could be wrong on that, will wait on some of the more knowledgable folks to chime in)
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post #21 of 145 Old 03-07-2014, 05:26 AM
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I could be mistaken about this, so correct me if I am wrong, but, if you run two subs, across from each other, as you described, with one pushing out and the other pulling in, I believe that would make them 180 degrees out of phase, and you would get some cancellation
Correct. Push-pull does not wire the drivers reverse polarity unless they're also reverse mounted, one facing into the room, the other facing into the cabinet.

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post #22 of 145 Old 03-07-2014, 07:01 AM
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I wonder if it has to do with the SPL differential. Because we are not talking about an ideal pressure system, the SPL from a nearfield subwoofer has a different SPL distribution over the entire body than from a farfield source which is more homogenious.
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post #23 of 145 Old 03-07-2014, 07:10 AM
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SPL distribution over the entire body than from a farfield source which is more homogenious.

Good descriptive..

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post #24 of 145 Old 03-07-2014, 12:27 PM
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Hey John,

I read the paper and it seems to rule out any difference between sealed and ported. It also seems to conclude that intensity = zero with multiple gain matched subs in a diffuse environment.

I agree with that conclusion.

FR is king and necessary for accurate playback and ultimate listening pleasure.

As far as tactile feel goes, a simple platform is all that's needed for those who wish to feel the pressure waves underneath them. I think Nils proved that a proper platform does away with the negative effects the paper discusses on intensity by multiple source, phase, etc. If he reads this, maybe he can chime in.

Nearfield placement changes FR, weighting it to the top end, so that was never a viable option for me. Gross FR non-linearity is too high a price for a port blowing your hair dry from 1M.
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post #25 of 145 Old 03-07-2014, 12:37 PM
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Originally Posted by bossobass View Post


Nearfield placement changes FR, weighting it to the top end, so that was never a viable option for me. Gross FR non-linearity is too high a price for a port blowing your hair dry from 1M.

Bosso, can you explain this a bit more? I have been investigating placing two more subs nearfield, like directly under my couch, built in the crawlspace below and vented through the floor. My intent is to fill in my FR canyon that exists between 50 and 80Hz due to my couch being about right in the middle of a length dimension in a room ~24 feet long. Anything you can add to your statement would be appreciated.


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post #26 of 145 Old 03-07-2014, 12:48 PM - Thread Starter
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@dominguez1

thanks for summarizing your findings in posts 4 and 5 in your thread. that is a much easier read than digging through the thread!

one minor bit on the acoustic near field. not only is it within 1/4 of a wavelength of the source, but the source itself has to be no smaller than 1/4 wavelength. when that is the case, then you can get the weird effects, such as particle movement tangential to the wave, the air "slug effect", etc. since most sub drivers are relatively small relative to the wavelengths that they are producing, there really isn't much of that particular effect at work.

i'm not quite sure how that works with a line array or a wall array of subs all spaced less than 1/4 wavelength from each other. on the surface, it would seem a wall array would be close enough to produce an extended acoustic near field effect.

in any case, great work and great thread.

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post #27 of 145 Old 03-07-2014, 01:04 PM - Thread Starter
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"Hey John"

hey dave. :-)

"I read the paper and it seems to rule out any difference between sealed and ported."

i suspect that there may be at least two effects at work here. while the ported question is interesting, i was trying to keep focus on the difference between sound intensity and sound pressure. this is an important distinction that isn't really ever discussed, yet its effects seems to be observed in many different contexts (why horn subs may be more tactile even at the same spl for example).

"It also seems to conclude that intensity = zero with multiple gain matched subs in a diffuse environment."

that's a little bit tricky because while true in the middle of the room, where the vector sums may net out to zero, there is some net particle velocity at other points in the room simply because the air is being compressed and rarefied.

"FR is king and necessary for accurate playback and ultimate listening pleasure."

that is definitely the most widely held belief. while true, i would suggest that it is incomplete. time domain effects are the first case, because it seems that room resonances are not linear and so don't always present in frequency response data. these experiments and the discussion of intensity/particle velocity just add yet another dimension to it all, not replacing frequency response, but adding to it.

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post #28 of 145 Old 03-07-2014, 01:10 PM - Thread Starter
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"Nearfield placement changes FR, weighting it to the top end, so that was never a viable option for me. Gross FR non-linearity is too high a price for a port blowing your hair dry from 1M."

is that true for sustained tones?

i understand the idea that it takes a moment or two to build up the pressure from the room reflections, but that energy isn't missing in a near field placement.

if you run a frequency sweep near field, you show that the sub performs much like it does anechoically.

what happens when a test tone that is sustained is played? does the bottom end eventually "build up" just like anywhere else in the room?

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post #29 of 145 Old 03-07-2014, 07:20 PM
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Nearfield or not, the subject of how sound is experienced in the bass range is interesting.

And reading about others experiences and trying to relate that to physical facts and my own experiences is certainly interesting.

Sound intensity, phase relationship between air pressure and velocity, frequency response and impulse response is what we are looking into.
And it is true that mics only measure pressure, information about the velocity component is unknown until one rigs some device to actually measure it.

Experiences with different subwoofer set-ups indicates that they can sound different, even if they measure somewhat similar in frequency response.

Freedom of resonances and a flat frequency response may not be the most important, but high spl certainly makes a difference.
I remember when setting up two of the new subwoofers recently, before any eq, and perhaps not in the best locations.
On at least one occasion the punch was almost scaring, you know when you get this feeling that perhaps this is actually enough.
While when properly set up, and with double total spl capacity, the impact is nowhere close to that, but it is of course more balanced and still has some physical feel left.

I have a system with 4 subwoofers in the corners, this has a sweet-spot for bass exactly at the listening position, where response is flat and impact and punch is great, but the very lowest frequency punch is not as great as when the system fired alongside the room from two sources located along one sidewall.
Or maybe not?
Without any objective measurement, how can I really be sure.

I have done some initial experiments with phase reversal and time delay of the back subwoofers, and that was interesting.
When you do this there will be a wave launched along the room, rather that a pressure zone in the middle.
(This has been described by others, also on avsforum, as you probably right now just remember..).

I would not think the differences between ported and sealed is caused by different phase relationship between velocity and pressure, as they would be similar as soon as one moves only a short distance away from the speaker, if they were different at all to begin with.
That leaves impulse response and dynamics as likely candidates.

To judge audio only by listening is too vague and gives inconsistent and imprecise results, one need to find a way to measure things to make it possible to repeat experiments reliably and to have a common reference.
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post #30 of 145 Old 03-08-2014, 11:59 AM - Thread Starter
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any thoughts on whether or not that effect is somehow related to sound intensity?

it seems that the greater the change in the frequency response as one moves closer to the sub might be evidence for less sound intensity capability of the room. outdoors, for example, there would be very little change in response, and sound intensity outdoors is maximized.

i'm not sure if it was danley or beaver who mentioned that the sound emanating from a horn is a little different from a direct radiator in that it has had time to form more of the plane wave characteristic of the far field whereas direct radiators, particularly up close are radiating like a bubble. i'm not sure that effect would be captured in frequency response at all.

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