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Discussion Starter · #1 ·
I enjoy visiting the dedicated home theaters of fellow enthusiasts. Lately, I've been wondering about some trends I have been noticing. This subjective observation is limited to 7 rooms which had used Audyssey MultEQ and whose trims were therefore calibrated to reference levels. In summary:


- The larger the room, the higher the volume you can listen comfortably. For rooms that are 1000 to 2000 cubic feet, the comfortable level limit is -15 to -18 db below reference. For rooms that are 5000 to 6000 cubic feet, the comfortable level is -5 to -3 db.


- The smaller the room, the stronger the bass shake, literally making you feel that there is a bass shaker strapped to your seat. In all cases, the subs were multiple SVS PB-12 NSD or PB-13 Ultras. The PB-12's were in the smaller rooms and they shook the rooms much more than the Ultras in the bigger rooms, even if the subs were not being driven hot.


I'm sure there is some logical and technical reason for this phenomenon but I really don't know what that is.


Can anyone explain the scientific reason for this phenomenon?


Mark
 

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Quote:
Originally Posted by streetsmart88 /forum/post/20806021


I enjoy visiting the dedicated home theaters of fellow enthusiasts. Lately, I've been wondering about some trends I have been noticing. This subjective observation is limited to 7 rooms which had used Audyssey MultEQ and whose trims were therefore calibrated to reference levels. In summary:


- The larger the room, the higher the volume you can listen comfortably. For rooms that are 1000 to 2000 cubic feet, the comfortable level limit is -15 to -18 db below reference. For rooms that are 5000 to 6000 cubic feet, the comfortable level is -5 to -3 db.


- The smaller the room, the stronger the bass shake, literally making you feel that there is a bass shaker strapped to your seat. In all cases, the subs were multiple SVS PB-12 NSD or PB-13 Ultras. The PB-12's were in the smaller rooms and they shook the rooms much more than the Ultras in the bigger rooms, even if the subs were not being driven hot.


I'm sure there is some logical and technical reason for this phenomenon but I really don't know what that is.


Can anyone explain the scientific reason for this phenomenon?


Mark

Simple answer; large room has less sound from wall reflections at listening position, small room has more pressurization buildup from bass frequencies.
 

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Discussion Starter · #3 ·

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Originally Posted by jackbuzz /forum/post/20806225


Simple answer; large room has less sound from wall reflections at listening position, small room has more pressurization buildup from bass frequencies.

I can intuitively appreciate that a large room would have less sound from wall reflections at the listening position but how would that affect the comfortable listening level?


If a small room has the same SPL at the listening position and the same bass extension, will more air be moved? Why?


Mark
 

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Interesting observation. When you say the SPLs were the same, surely that was not the case across all low frequencies, was it? I would think it would be impossible to match them. So maybe it is due to much larger peaks in some rooms.
 

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Discussion Starter · #5 ·
Quote:
Originally Posted by amirm
Interesting observation. When you say the SPLs were the same, surely that was not the case across all low frequencies, was it? I would think it would be impossible to match them. So maybe it is due to much larger peaks in some rooms.
I think I may have confused you. My observations for 4 particular rooms:


1) All rooms calibrated with Audyssey MultEQ, with 0.0 db as reference level.


2) In a room that was 6000 cubic feet with 2 SVS PB13 Ultras and 2 Paradigm Servo-15 v2's, master volume at -3 db, bass shake was evident - the sofa shook.


3) In 2 rooms that were 2000 cubic feet with 2 SVS PB12-NSD's, master volume at -12 db, bass shake was unbelievably strong.


4) In a room that was 1000 cubic feet with 2 SVS PB12-NSD's, master volume at -16 db, bass shake was way stronger.


Are you saying that in a smaller room, there will be greater variation in bass response from one position to another? In all cases, however, I was seated at the sweet spot which should therefore have been calibrated by Audyssey so that the bass SPL should not have been excessive.


Mark
 

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Very good questions.

Quote:
I can intuitively appreciate that a large room would have less sound from wall reflections at the listening position but how would that affect the comfortable listening level?

There are certainly far too many variables to determine the phenomena you reported, however the relative amount of direct and reflected sound you experienced may have been at play. When one sits relatively near-field, the direct sound energy is merely that,..direct sound. As one either sits further back from the mains, and the ratio of direct to reflected energy lessens, one hears more of the "room", and it's effects on the experience. Reflected energy adds spaciousness. Very early reflected energy of about 400hz and above, blurs the image specificity of the recording. So sitting in the front half of the space, with the speakers aimed at you and somewhat away from the adjacent boundaries help significantly.


Perhaps, in the larger space, you were more immersed in the direct sound-field, and less exposed to the colorations of the reverberant sound-field. Perhaps this would have imparted a more coherent experience, hence more comfortable listening level. All this said, if you didn't emphasize the room size, I'd be more inclined to examine at distortion aspects of the loudspeakers being used, in addition to the overall level of optimization wrt acoustics.


Quote:
If a small room has the same SPL at the listening position and the same bass extension, will more air be moved? Why?


There exists room gain, and boundary gain.


First, Boundary Gain comes from the driver operating in a constrained space. One begins with a theoretical full-space. This has no surfaces which interact with the acoustic output of the driver. Only a full tilt, entirely anechoic chamber can approximate a theoretical full space. That said, cutting a full space sphere in half with a ground plane, one creates half space. This is an outdoor environment, whereby zero acoustic interaction occurs between the driver, and the hemisphere above the ground. The ground merely halves the previous full space. This halving, theoretically mirrors another like powered driver, thereby resulting in a theoretical 6db gain. Each subsequent halving of space, with the addition of another boundary, also results in a theoretical 6db gain.


So Boundary Gain is essentially added acoustic support, by halving the existing acoustic quantity. After one accepts the benefits of the ground boundary, wall and corner loading is considered. The maximum additional SPL for each boundary is 6db each. So with a sub in the middle of the room, the floor adds 6db. Moving it to a mid-wall position would add an additional 6 db, totaling 12db. Full corner loading would yield 18db. However, one must subtract whatever movement is incurred by the boundary surfaces not being entirely inert. Any movement results in losses. A full reinforced concrete wall basement structure would typically yield the full 18db for corner loading. One must be mindful that boundary gain is limited to fully supporting 1/8 the wavelength. A sub-woofer crossed over at 80hz, with the 80hz wavelength being about 14 feet, the driver's acoustic center needs to be within 20 inches of the boundary to retain the entire benefit of the effect.


So in your situation, the smaller the space, it's somewhat more likely the subs were closer to multiple boundaries, thus the LF has an additional measure of boundary gain.


Now Pressure Vessel Gain (PVG), or room gain, is the scenario whereby the longest dimension of the room can no longer support full propagation of the waveform. At this point, the acoustic propagation transitions to acoustic pressurization. A typical myth is a small cabin cannot support the lowest frequencies.... nothing could be further from the truth. The manner in which the sound is reproduced into the space changes from a normal cyclic propagation, to pressurization because the wavelengths are too big for the space. The frequency at which this occurs is approximately the point whereby half the wavelength of a given frequency is equal to the rooms longest dimension. So, a 20 hz frequency has a wavelength 56.5 feet. So half of that, 28.25 feet, is the point of transition. Any frequency below that point pressurizes the room, and any frequency above that point propagates freely. So in this room that's approximately 28 feet in the longest dimension, from 20 hz downward, the room gives back acoustically. This is room gain, cabin gain, or PVG...Pressure Vessel Gain.


At this frequency, the results are a gain in acoustic pressures in the room that grows as the frequency decreases. This acoustic support reciprocity, is theoretically 12db per octave. The percentage of the 12 db/octave gain one achieves, entirely depends on the integrity of the boundary walls and surfaces. If it was the theoretical concrete bunker, a full 12db/octave boost would occur. Typically, somewhere between 6-10 db octave could result. Also, in addition to the walls and surfaces flexing, other aspects may affect the point at which room gain begins. Furniture, cabinets etc, anything that consumes a certain measure of cubic feet, may slightly alter the transition frequency merely because the items take up space.


This acoustic pressurization, room gain, is the proverbial free lunch. It is essentially headroom that's thrown back into the system. And unlike horn subs, the distortions and non-linearities are not magnified. Sealed alignments roll off second order. Room gain also is second order. So one can see how integrating a sealed alignment may offer substantial benefit when attempting to integrate the system to the room via time and frequency equalization. Properly adjusted, this would result in substantial headroom added back in for significant capability for the big LFE effects. Regardless of sub-woofer type, PVG is truly a free lunch.


This is most likely why, in the smaller room, and even with a lower playback level, you experienced more bass.


Good questions and good luck.
 

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FOH, Thx very much for the Pressure Vessel Gain / room gain primer, one of the best write ups on it.

You helped connect the dots for me and "clik" the light came on, very helpful.

I subscribe this thread - my simple bookmarks method.

Quote:
Originally Posted by FOH /forum/post/20809514


Very good questions.


Now Pressure Vessel Gain (PVG), or room gain, is the scenario whereby the longest dimension of the room can no longer support full propagation of the waveform. At this point, the acoustic propagation transitions to acoustic pressurization. A typical myth is a small cabin cannot support the lowest frequencies.... nothing could be further from the truth. The manner in which the sound is reproduced into the space changes from a normal cyclic propagation, to pressurization because the wavelengths are too big for the space. The frequency at which this occurs is approximately the point whereby half the wavelength of a given frequency is equal to the rooms longest dimension. So, a 20 hz frequency has a wavelength 56.5 feet. So half of that, 28.25 feet, is the point of transition. Any frequency below that point pressurizes the room, and any frequency above that point propagates freely. So in this room that's approximately 28 feet in the longest dimension, from 20 hz downward, the room gives back acoustically. This is room gain, cabin gain, or PVG...Pressure Vessel Gain.


At this frequency, the results are a gain in acoustic pressures in the room that grows as the frequency decreases. This acoustic support reciprocity, is theoretically 12db per octave. The percentage of the 12 db/octave gain one achieves, entirely depends on the integrity of the boundary walls and surfaces. If it was the theoretical concrete bunker, a full 12db/octave boost would occur. Typically, somewhere between 6-10 db octave could result. Also, in addition to the walls and surfaces flexing, other aspects may affect the point at which room gain begins. Furniture, cabinets etc, anything that consumes a certain measure of cubic feet, may slightly alter the transition frequency merely because the items take up space.


This acoustic pressurization, room gain, is the proverbial free lunch. It is essentially headroom that's thrown back into the system. And unlike horn subs, the distortions and non-linearities are not magnified. Sealed alignments roll off second order. Room gain also is second order. So one can see how integrating a sealed alignment may offer substantial benefit when attempting to integrate the system to the room via time and frequency equalization. Properly adjusted, this would result in substantial headroom added back in for significant capability for the big LFE effects. Regardless of sub-woofer type, PVG is truly a free lunch.


This is most likely why, in the smaller room, and even with a lower playback level, you experienced more bass.


Good questions and good luck.
 

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Quote:
Originally Posted by mtbdudex /forum/post/20813371


FOH, Thx very much for the Pressure Vessel Gain / room gain primer, one of the best write ups on it.

You helped connect the dots for me and "clik" the light came on, very helpful.

I subscribe this thread - my simple bookmarks method.

My pleasure
,.. what's really cool is the clean, distortion free manner in which PVG can possibly extend the bottom octave extension. The typical, systemic distortions and non-linearities associated with increasing excursion, don't continue in the normal exponential model. The various types of nonlinearities which are the physical causes for signal distortion in sub systems, are held at bay while the acoustic support reciprocity of the space manifests itself, kicking in and providing assistance in the most physically demanding octaves of the spec.


This illustrates the importance of signal path extension, particularly roll-off characteristics of every aspect in the chain. If you're employing a LT circuit in a small sealed rig, you don't want to fight an early roll off elsewhere in the chain. Likewise, an IB system, that has good native extension, can easily reveal the limitations of any bandwidth limitation in the signal path.



Thanks and good luck
 

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Quote:
Originally Posted by FOH /forum/post/20809514


Now Pressure Vessel Gain (PVG), or room gain, is the scenario whereby the longest dimension of the room can no longer support full propagation of the waveform. At this point, the acoustic propagation transitions to acoustic pressurization.

On first reading this, I fail to see why the full propagation of the waveform is the essential element. After reading it a few times, and trying to visualize the pressure waves, I'd like to explain what I think is the reason for this transition. Please humor me and point out where I'm wrong, as I imagine I will at least misuse some terms.



In a room longer than half of a wavelength, the high pressure region propagates away from the driver toward a boundary, leaving a low pressure region following (effectively sucked along) behind it. After reflection at the boundary, the high pressure region interacts with the trailing low pressure region, as well as the presumed next high pressure region, creating the interactions and interference patterns that characterize bass reproduction in small rooms (uneven response).


In contrast, if the room is shorter than one half the wavelength, the reflected high pressure region has returned to the driver before the driver can finish with the propagation of the low pressure region - or where the room is significantly smaller than half of one wavelength, the driver may in fact still be pressurizing the space immediately in front of it - still producing high pressure. At this point, the entire room exists within the doubled-over high pressure region.


I feel like that all makes sense in my head. As the driver moves into the next phase of the waveform (low pressure), does the pressure of the whole room actually drop? This becomes troubling in a couple ways I am trying to work through as I type; I'm failing. The first thing that occurs to me is that the reflected high pressure region is still reverberating within the room. The interactions that defined the response of a larger room still take place in the smaller room, so at some point in time and space there must be interactions among peak and trough, as well as trough and trough. Second, is the idea that the absolute pressure of the room could change. The ideal gas law simply prohibits a confined quantity of gas from changing pressure without changing temperature or volume - so which is it? It can't be that the change in volume of the air space in the room caused by the driver excursion into the room is sufficient to generate the pressure, can it? I think it can: see sidebar calculation afterthought below... (wow! somebody check my math and assumptions)


I hope I'm not derailing the OP's thread too much, but if FOH is right and PVG is the answer to the OP's question, I hope we can better understand it. ...and hopefully FOH or someone similarly informed and articulate can clarify my misunderstandings.


Fred


Sidebar: If a room is very small, let's say 10 by 10 by 8 - 800 cubic feet, and the driver is a rather large 1.5 feet in diameter, and goes through an excursion (just in one direction - from rest - not xmax from fully one extreme to the other - rather half that) of say 3 inches (that would be large, yes?), the displaced volume would be... a cone of height .25 ft and a radius of .75 ft. (.33 pi r^2 h) rounded up to 0.15 cubic feet (somebody check my math...). Allowing that in the ideal gas equation (PV=nRT) n and R are in fact constants, we are left with the combined gas law, (adapted, etc.) PV/T = PV/T. If we further assume temperature to be constant, we work directly with Boyle's law, adapted as PV = PV. The initial pressure (assumed as 1 atm) multiplied by the volume (800 cubic feet) must produce a figure equal to the product of the new volume (where driver is fully extended into the room) and the new pressure. 800/799.85 = 1.000188... atm. For comparison to wikipedia , converted to Pa yields about 19 Pascals above normal, or a peak SPL just about loud enough to do permanent damage to your hearing... WOW! I never would have imagined that small a change in volume could lead to that large a sound pressure! Of course, that is at zero distance from a point source and doesn't predict what the overall room does, in the context of PVG, but it does validate some of my thoughts above... right? or am I making some erroneous assumptions?
 

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Good explanation in post 6.


If I may offer a suggestion: for boundary gain, this link expands on it and has pics which may help some others visualise what you're talking about.


Otherwise bookmarked and will I'm sure be linked to later.
 

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At the risk of blabbering on into the night... My "thought experiment" and calculations have lead me to conclude that because the sound wave can travel all around the room (At the speed of sound, obviously) before the time for the driver to begin diminishing the absolute pressure in the room (the pressure vessel, as the name would imply) the pressure in the room will in fact oscillate in exact time with the excursion of the driver. If an infinite array of barometers were positioned within the room I proposed above, they would all report the same pressure at the same time, and that pressure would fluctuate up and down a total range of (from my example) 38 Pa, at whatever frequency the driver was driven, as though the walls themselves were compressing the room and releasing it. Awesome.


Of course, that would be theoretical perfect pressure vessel performance that would never happen in a home theater, but the transition to that behavior must be awesome indeed.


Fred
 
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