What is pressurization? - AVS Forum
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post #1 of 25 Old 02-16-2014, 06:51 AM - Thread Starter
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In all my reading about subwoofers, I've never seen a true definition of "pressurization" in any forum from a "technical" (yet accessible) standpoint. Every characterization has been of the "feeling" of pressurization. Surely there must be a ballpark formula to measure whether a measured subwoofer(s) will "pressurize" a given room volume (assuming it's sealed). I understand about the concept of wavelength wrapping back on itself depending upon room size, but since wavelengths are different at different hertz, I'm not sure I understand exactly which waves cause pressurization and what part SPL may play in this.
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post #2 of 25 Old 02-16-2014, 07:11 AM
 
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It's not barometric pressure but a building up of bass waves as bass waves, like water, like to leak out through openings when ever they can and then there's the whole degrade thing where a sound starts out loud and decays to nothing. If you look at REW, it has a feature that graphs this, RT-60 or how long it takes for a signal to decay 60dB.

I'm a layperson so I hope you'll forgive me for the non-scientific response.
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post #3 of 25 Old 02-16-2014, 08:47 AM
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to pressurize a room you need a sealed space (at least reasonably sealed). Then, AIUI, when the wavelength of a sound is greater than the longest room dimension, no "wave" atually appears in the space. Instead, you get the space pressurizing and depressurizing.

you can go here http://www.sengpielaudio.com/calculator-wavelength.htm to see wavelengths of particular frequencies, or the frequency of a particular wavelength. 40 Hz is a little over 28 feet.

that, at least, is the way I understand it.
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post #4 of 25 Old 02-16-2014, 09:38 AM
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Quote:
Originally Posted by JHAz View Post

to pressurize a room you need a sealed space (at least reasonably sealed). Then, AIUI, when the wavelength of a sound is greater than the longest room dimension, no "wave" atually appears in the space. Instead, you get the space pressurizing and depressurizing.
The wave appears at the speaker. SPL stands for Sound Pressure Level; when a meter measures it what it actually is measuring is changes in air pressure, which is then extrapolated to a level in decibels. The smaller and tighter the room the less work it takes for the air pump, that is the speaker, to realize a given change in air pressure and therefore SPL. It's just like blowing up a small balloon versus an air mattress.
What we refer to as the feeling of pressurization is a combination of frequency and SPL. On average it takes about 105dB in the low frequencies. Reaching 105db at the listening position can be realized at a rock concert with massive subs and power, or it can be realized in a car with modest gear because of cabin gain. Cabin gain gives a theoretical maximum boost of 12dB per octave as frequency goes down starting at where the longest room dimension is 1/2 wavelength. It also requires a reasonably tightly sealed room. Real HTs tend not to be sealed all that tight, so cabin gain of 8 to 10dB/octave is a realistic expectation. 'Pressurizing' a room, which means you can get that 105dB or more at the LP, means having a specific combination of sub output capability and room size. If the room is big you don't get the cabin gain, so the system must be large enough to compensate for that. This has a direct correlation with the choice of sealed versus ported. Sealed subs don't have the low end output that ported do, but if there's sufficient cabin gain it will compensate for that. But if you use sealed subs in a room with a longest dimension of more than 20 feet or so you can figure on needing at least twice as many of them for the same SPL below about 35Hz.
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post #5 of 25 Old 02-16-2014, 12:45 PM - Thread Starter
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Quote:
Originally Posted by Bill Fitzmaurice View Post

Cabin gain gives a theoretical maximum boost of 12dB per octave as frequency goes down starting at where the longest room dimension is 1/2 wavelength.


Thanks Bill. This is the kind of information I'm looking for. The statement quoted is still over my head though. I understand the general concept of cabin gain as a driver in a room as opposed to open space, however I'm not sure where this boost you're talking about takes effect. Given 30Hz with a 1/2 wavelength of ~18 feet, does that mean a theoretical boost of 12dB at 18 feet? What about dB at other distances. Does that depend upon where on the wave the distance lies. In other words, what would be the 30Hz dB gain at a source distance of ~9 feet?


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Originally Posted by Bill Fitzmaurice View Post

On average it takes about 105dB in the low frequencies.

Are you saying that the common advice of buying a sub that pressurizes your room is only applicable if you listen at transient levels that reach ~105dB? So at sub-pressurization levels you'll hear the frequency, but not feel the pressure?
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post #6 of 25 Old 02-16-2014, 12:57 PM
 
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My understanding an octave is a increase of frequency, from, as an example, 10Hz > 20Hz > 30Hz > 40Hz, et cetera.

So doing the math, 12dB > 24dB > 36dB > 48dB. Thasss a lotta energy but a nice even slope up/down.

Speed of sound/distance in feet = frequency.

Speed of sound = 1130'/sec

Length of room 24'

1130'/24' = 47.08Hz

47.08Hz/2 = 23.54Hz.

If I screwed up my understanding of the math, I'll look forward to all corrections.

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post #7 of 25 Old 02-16-2014, 01:33 PM
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An octave is 10 Hz then 20 Hz, then 40 and so on. There is a formula for room gain: C/2*L, C is the speed of sound and L is the length of the room.
Example: 1130/2* 20 ft. =28.5 Hz so you will start gain 12 db/octave starting at 28.5.. This mean at 14Hz you will be up 12 db. Room gain is not boundary gain which is usually 2-5 db.

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post #8 of 25 Old 02-16-2014, 01:34 PM
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I believe an octave is double or half of the frequency. 10, 20, 40, 80, etc.. 5 hz is an octave below 10 hz. I also could be wrong but I get about 21 dBs of gain at 8-11hz.
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post #9 of 25 Old 02-16-2014, 03:43 PM
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Quote:
Originally Posted by SmittyJS View Post

Thanks Bill. This is the kind of information I'm looking for. The statement quoted is still over my head though. I understand the general concept of cabin gain as a driver in a room as opposed to open space, however I'm not sure where this boost you're talking about takes effect. Given 30Hz with a 1/2 wavelength of ~18 feet, does that mean a theoretical boost of 12dB at 18 feet? What about dB at other distances. Does that depend upon where on the wave the distance lies. In other words, what would be the 30Hz dB gain at a source distance of ~9 feet?
The gain will be the same at any listening distance. Assume the longest room dimension is 20 feet. That's 1/2 wavelength at 28Hz. You could reasonably expect 9dB of gain one octave down, at 14Hz, and 18dB of gain two octaves down, at 7Hz. To see what your sub is capable of you'd add those gain figures to the measured half-space SPL chart, if you can find one that is.
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Are you saying that the common advice of buying a sub that pressurizes your room is only applicable if you listen at transient levels that reach ~105dB? So at sub-pressurization levels you'll hear the frequency, but not feel the pressure?
That depends on the frequency. 105dB is pretty much what you need at 50Hz, and that's what you're getting when your chest thumps at a rock concert. The level required to feel the thump goes down as frequency goes down, but so does the output capability of most subs. It might only take 90dB to give you a healthy thump at 30Hz, maybe 85dB at 20Hz. So getting thump while listening to music takes a lot more SPL than it does with HT, but it takes a lot more sub and/or a lot smaller room to do 85dB at 20Hz than it does 105dB at 50Hz.

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post #10 of 25 Old 02-16-2014, 03:56 PM
 
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Originally Posted by derrickdj1 View Post

An octave is 10 Hz then 20 Hz, then 40 and so on..

I must have been thinking of harmonics.
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post #11 of 25 Old 02-16-2014, 04:00 PM
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can the pressurization happening at open space?
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post #12 of 25 Old 02-16-2014, 04:04 PM
 
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Quote:
Originally Posted by WereWolf84 View Post

can the pressurization happening at open space?

No. You'll have output but not pressurization. Think of pressurization as the free lunch you always wanted as a kid. tongue.gif

An example, for viewing purposes, to help the process, I close adjacent doors when I can.
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post #13 of 25 Old 02-16-2014, 04:58 PM - Thread Starter
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Now I understand.

I've felt the pressurization at many rock concerts in small venues, but I'd never seen it explained anywhere. I just like to know this stuff from a technical standpoint and it might come in handy when I design my new house.

Although I haven't measured my dual subs' output, on the limited occasions when I crank up the volume, I definitely feel it in my ~2000 cu. ft. room.
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post #14 of 25 Old 02-16-2014, 05:16 PM
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Quote:
Originally Posted by SmittyJS View Post



Now I understand.

I've felt the pressurization at many rock concerts in small venues, but I'd never seen it explained anywhere. I just like to know this stuff from a technical standpoint and it might come in handy when I design my new house.

Although I haven't measured my dual subs' output, on the limited occasions when I crank up the volume, I definitely feel it in my ~2000 cu. ft. room.
your 2000 cu. ft. room is sealed?
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post #15 of 25 Old 02-16-2014, 05:21 PM
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Quote:
Originally Posted by WereWolf84 View Post

can the pressurization happening at open space?
With sufficient power, sure. Hand grenades and rockets do so, as do rock concerts.

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post #16 of 25 Old 02-17-2014, 08:37 AM - Thread Starter
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Quote:
Originally Posted by WereWolf84 View Post

your 2000 cu. ft. room is sealed?

No. It has a 4-foot doorway that is usually open. Sealing a room requires a format that generally isn't compatible with a "living" room. My new house will probably have a much more open concept, so my small dual subs may have to go in a basement entertainment room and I'm considering Bill's table tuba(s) for the living area.
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post #17 of 25 Old 02-17-2014, 11:08 AM
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Pressurization is a bad term in that to pressurize something requires either more molecules in a fixed volume, decreasing volume with a fixed number of molecules or a heating of a fixed number of molecules in a fixed volume. Volume in this case is cubic dimensions, not what we hear.

When talking about subs, you're creating sound waves which is a temporary increase in local pressure followed by a corresponding decrease in pressure. This is what our ears detect as sound. You're not really pressurizing the room, you're unevenly distributing the air molecules. The smaller the time interval between the peak and trough, the higher the frequency. The greater the difference between peak and trough pressure, the louder it sounds. Pressure in this sense is really just higher volume. If the volume is high enough, we not only hear the sound, but we can also feel it. I think this is what people mean by pressure. Feeling the sound in addition to hearing it.

I don't think a sealed room is required to feel sub pressurization, but it helps.
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Quote:
Originally Posted by Bill Fitzmaurice View Post

With sufficient power, sure. Hand grenades and rockets do so, as do rock concerts.

Gotta watch out for those pressure waves.

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post #19 of 25 Old 02-17-2014, 01:25 PM
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Quote:
Originally Posted by KidHorn View Post

Pressurization is a bad term in that to pressurize something requires either more molecules in a fixed volume, decreasing volume with a fixed number of molecules or a heating of a fixed number of molecules in a fixed volume. Volume in this case is cubic dimensions, not what we hear.

When talking about subs, you're creating sound waves which is a temporary increase in local pressure followed by a corresponding decrease in pressure. This is what our ears detect as sound. You're not really pressurizing the room, you're unevenly distributing the air molecules. The smaller the time interval between the peak and trough, the higher the frequency. The greater the difference between peak and trough pressure, the louder it sounds. Pressure in this sense is really just higher volume. If the volume is high enough, we not only hear the sound, but we can also feel it. I think this is what people mean by pressure. Feeling the sound in addition to hearing it.

I don't think a sealed room is required to feel sub pressurization, but it helps.

I think people have different sensitivity to sound pressure waves. For instance, I did my subwoofer crawl using a very low volume piece of music (SACD of Francis Poulenc Concerto in G minor … Tempo Introduction Largo) that has a 2 minute sustained organ pedal at 16 Hz. Db readings never reached 60 dB. Both my son and I felt the pressure and open our mouths in a yawn a couple of times to help pop our ears. This is in a room volume of over 3,000 cu. ft. and fairly open with cathedral ceilings. I was quite surprised because I simply wasn't expecting it.

Oh yeah, this piece of music was also great for helping find a half dozen paintings rattling throughout the house. Because of its low volume of sound the rattles became easily apparent.

BTW, I was auditioning a single SB13u, which I kept.
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post #20 of 25 Old 02-19-2014, 01:52 AM
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Quote:
Originally Posted by WereWolf84 View Post

can the pressurization happening at open space?
I was wondering that.

Say for example you'd dragged your speakers etc outside on the patio for a bbq etc
Would someone sitting across the garden not hear any of the bass from the sub?
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post #21 of 25 Old 02-19-2014, 02:06 AM
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Quote:
Originally Posted by SilverSteve View Post

I was wondering that.

Say for example you'd dragged your speakers etc outside on the patio for a bbq etc
Would someone sitting across the garden not hear any of the bass from the sub?
hear bass & feel pressurization are different
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post #22 of 25 Old 02-19-2014, 03:13 AM
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They may hear the bass for several hundred feet but, they most likely won't feel anything.

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Quote:
Originally Posted by derrickdj1 View Post

They may hear the bass for several hundred feet but, they most likely won't feel anything.

That's because they're cold hearted and don't appreciate good bass in music. tongue.gif
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post #24 of 25 Old 02-19-2014, 06:04 AM
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Quote:
Originally Posted by SilverSteve View Post

Say for example you'd dragged your speakers etc outside on the patio for a bbq etc
Would someone sitting across the garden not hear any of the bass from the sub?
Why wouldn't they?
Once again, but this time with feeling:"Pressurization' is when the SPL is sufficient for you to both hear and feel the sound. If the dB level is high enough you'll feel the sound, if it isn't you won't. You don't have to feel it to hear it. The exception to this is in the range below 15Hz or so, where your ears can't hear it, but you can feel it at sufficient levels.

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post #25 of 25 Old 02-19-2014, 06:28 AM
 
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