[LTD02]

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

 

[notnyt]

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.

 

[HopefulFred]

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.


Fred

 

[LTD02]

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.

 

[LTD02]

"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.

 

[LTD02]

"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.

 

[notnyt]



Quote:
Originally Posted by LTD02  /t/1521106/spl-nearfield-subwoofers-and-brutality/0_100#post_24442422


"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.

 

[LTD02]

"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.

 

[Bill Fitzmaurice]



Quote:
Originally Posted by notnyt  /t/1521106/spl-nearfield-subwoofers-and-brutality#post_24442243


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.

 

[Scott Simonian]



Quote:
Originally Posted by notnyt  /t/1521106/spl-nearfield-subwoofers-and-brutality#post_24442243


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.

Quote:
Originally Posted by Bill Fitzmaurice  /t/1521106/spl-nearfield-subwoofers-and-brutality#post_24442586


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

 

[BassThatHz]

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

)

 

[LTD02]

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.

 

[Scott Simonian]



 

[LTD02]

 !!

 

[Bob7145]

Maybe conduct that test in a Smokey room? Strategicly placed sticks of incense?

 

[dominguez1]

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

 

[dominguez1]



Quote:
Originally Posted by notnyt  /t/1521106/spl-nearfield-subwoofers-and-brutality#post_24442500


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.

Quote:
Originally Posted by LTD02  /t/1521106/spl-nearfield-subwoofers-and-brutality#post_24442567


"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.

 

[dominguez1]



Quote:
Originally Posted by LTD02  /t/1521106/spl-nearfield-subwoofers-and-brutality#post_24445709


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.

 

[BCRSS]

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.

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.

 

[Martycool007]



Quote:
Originally Posted by BCRSS  /t/1521106/spl-nearfield-subwoofers-and-brutality#post_24448273


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.

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)