Again, everything in that post is wrong (except perhaps the first line).
Again, SPL pressurization is a redundancy, spl is pressure.
Even if room gain were just room modes, it would still be pressure (all spl is pressure). But it's not. Room gain happens below the lowest room mode. That's why we don't call it room modes. We call it by it's proper name because that's how to be meaningful and coherent. Misunderstanding the phenomenon and calling it by different names is not meaningful and coherent.
Room modes are something else entirely and they are NOT trivial, in fact they are the leading cause of bad, sloppy, overbearing bass in one spot and nulls in another, and that's why increasing amounts of people use multiple subs spread around the room. Room modes show up in measurements as fairly sharp spikes (when frequency is low enough for them to be sufficiently spread out), not as a fairly steady increase as frequency decreases.
Room gain is sometimes trivial in ordinary homes, but not usually, and never non existent. All rooms have greater radiation resistance than an open field, there will always be some amount of room gain. Most people won't realize this though, because their signal chain doesn't go low enough, they can't measure, and simply don't care. But this is an audio forum where people know better.
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Perhaps I can point out the obvious. It is clear that Ben has a technical background and that he is also a bit playful in his posts. Please keep that in mind.
As it turns out, "gain" has a technical definition. When used by engineers, it actually has a more constrained meaning then simply "you get more" when the speaker is in the corner. Yes, you will measure "more", but that does not mean that that you now have "gain" (in its technical meaning).
No one is arguing that the frequency response of a speaker has "more" low end when measured in a corner and less when measured in an anechoic chamber. Unfortunately the term "gain" (remember it is a technical term) implies a mechanism. That mechanism may not be at play. Room gain is a commonly used term. That is convenient, but it suggests a mechanism that is not accurate.
As it turns out, "gain" has a technical definition. When used by engineers, it actually has a more constrained meaning then simply "you get more" when the speaker is in the corner. Yes, you will measure "more", but that does not mean that that you now have "gain" (in its technical meaning).
No one is arguing that the frequency response of a speaker has "more" low end when measured in a corner and less when measured in an anechoic chamber. Unfortunately the term "gain" (remember it is a technical term) implies a mechanism. That mechanism may not be at play. Room gain is a commonly used term. That is convenient, but it suggests a mechanism that is not accurate.
Why don't we call it the 0th room mode? 🙄
I remember many years ago struggling with the differential equations for torsional resonances in gearboxes. The shafts and gearwheels all have their own resonances, but there was one other solution to the equations that we called the "rolling mode". It had a natural frequency of 0 and represented the shafts spinning steadily at their respective speeds without vibration.
If we set down the equations for acoustic resonances in a sealed room, we would most probably find a similar solution with a frequency of 0 whose magnitude would represent the "room gain". If the room had an opening giving it a Helmholtz resonance, the frequency would not be 0 any more, but the 0th mode would not go away. If the room was just leaky, the natural frequency of the 0th mode might still be 0 but its magnitude would be less.
Intuitively it is easy to see that a subwoofer will give more output at low frequencies if it's placed in a confined space. Take a sealed box sub and test it in free space, it rolls off at 12dB/octave below its resonant frequency. Then test it again inside a tiny sealed room (imagine a chest freezer) If the room and the sub enclosure were both perfectly sealed, the response would be flat down to DC.
This is really just a very extreme case of the boost you get by placing a woofer in a corner. We speak of free space, half space, quarter space and so on, referring to the solid angle available for the sound waves to radiate away. A sealed room that's small compared to the wavelength is "zero space", the sound waves can't escape in any direction.
For most purposes this is irrelevant. As a concrete example, the 1st mode of my living room is at 34Hz, and my stereo speakers struggle to get down below 40. They are just plain old vented boxes, so they roll off at 24dB/octave below 40Hz and have no significant output that would interact with the 0th mode.
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OK, you hooked me.
How does one work out when room mode starts?
How does one define the frequency range over which it rolls in?
How does one work out when room mode starts?
How does one define the frequency range over which it rolls in?
I have no idea how to actually do the calculations, I am leaving that as an exercise for the reader.
I would guess the "0th mode" starts to show gain when the wavelength gets large compared to the room, that is at some frequency below the 1st mode, that being the frequency where 1 wavelength fits in the longest dimension of the room.
I would guess the gain would roll off 12dB/octave below any Helmholtz resonance the room might have (about 5Hz in the example given earlier in this thread, where an open door and window were acting as ports) or slope downward at 6dB/octave over the whole region if it has significant resistive leaks.
Some would say that the room gain rises 12dB/octave normally, and that is what gives a flat response when used with a sealed sub box that falls off at 12dB/octave. It really depends whether you think in terms of acceleration, velocity or displacement. If that is your mental model then just add 12dB/octave to what I said above. (The gain becomes flat below the Helmholtz resonance, or rises less than 12dB/octave in the presence of leaks)
I would guess the "0th mode" starts to show gain when the wavelength gets large compared to the room, that is at some frequency below the 1st mode, that being the frequency where 1 wavelength fits in the longest dimension of the room.
I would guess the gain would roll off 12dB/octave below any Helmholtz resonance the room might have (about 5Hz in the example given earlier in this thread, where an open door and window were acting as ports) or slope downward at 6dB/octave over the whole region if it has significant resistive leaks.
Some would say that the room gain rises 12dB/octave normally, and that is what gives a flat response when used with a sealed sub box that falls off at 12dB/octave. It really depends whether you think in terms of acceleration, velocity or displacement. If that is your mental model then just add 12dB/octave to what I said above. (The gain becomes flat below the Helmholtz resonance, or rises less than 12dB/octave in the presence of leaks)
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I entered the dimensions of my living room into an online calculator. 🙂 I also verified it with pink noise measurements while playing with room EQ.
Here is an example, there are several available. Note that the table indices suggest the possibility of an (0, 0, 0) mode but it doesn't calculate one.
Ben,
1A. I agree that you may "sound insane", but sometimes (not in this case) you do demonstrate a basic grasp of elementary logic ;^).
1. Agreed.
2. Unless otherwise stated, the technical term "gain" generally refers to gain for frequencies in the pass band, the intended operating frequency range of the equipment. In the case of the single 2x 12" sub I operate in my fairly normal home living room, the placement in the room, when it's two doors are open to the rest of the house results in significant gain at 20 Hz (relative to the near field response) at the listening position. "Room gain" would be a technically accurate term for the measured difference in response between the two, although point #1 is still true.
3. Sound Pressure Level (SPL) does not exist without pressure, pressure is necessary for any SPL measurement. A bass boost can be caused by any number of things, room gain being among them. In my particular living room, the low frequency Sound Pressure(ization) Level at the listening position is less with the doors shut than when open.
Art
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The SPL caused by pressurisation effect is frequency independant, but it makes most sense to only consider the effect below the lowest room resonant frequency (mode).
"Room gain" is flat (constant). It doesn't rise as the frequency drops. In a closed room with a sub, the amount of bass increases as the frequency drops because the speaker cone's movement (displacement) increases as the frequency drops. A sealed sub box where the (free air) response drops off at 12 dB/octave actually has a constant excursion (displacement) as the frequency drops. This results in a constant SPL in a closed room.
For an example:
A sub has a "measured" flat response from 40 to 20 Hz, and drops off 12 dB by 10 Hz.
The speaker cone might excurse 5 mm at 40 Hz, 20 mm at 20 Hz, and 20 mm at 10 Hz.
Assume a sealed room where the lowest room mode is higher than 40 Hz, so we're dealing purely with pressurisation effects.
At 40 Hz, the SPL might be, say, 100 dB. The SPL at 20 Hz will therefore be 112 dB. And the SPL at 10 Hz will also be 112 dB.
So, strictly speaking, Ben is right - there is no "room gain". That is, the room doesn't amplify the bass. It just captures the pressure generated by the sub that would normally dissipate into free air. But "room gain" is the term by which the effect is known, especially if you consider it to be the short version of "the increase (gain) in bass level that occurs when you put a sub in a closed room".
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Perhaps that was Ben's point all along?
As much as my lack of credentials allow me to opine on the subject is that the characteristics that make a room "less than perfect" - something with which almost all of us are blessed/cursed - for "gain", or whatever - is what makes them habitable
After plaything with several larger single subs over the years, I found multiples of the small (7-8") range a better compromise
As much as my lack of credentials allow me to opine on the subject is that the characteristics that make a room "less than perfect" - something with which almost all of us are blessed/cursed - for "gain", or whatever - is what makes them habitable
After plaything with several larger single subs over the years, I found multiples of the small (7-8") range a better compromise
Here's an explanation for those not caught up in adversarial bickering. Not sure I understand this all completely, but with the good help of Don Hills' post (which I rephrase a bit) here goes.
There are a variety of physical aspects that influence bass in a room. ONE of these (albeit of no practical value) is SPL pressurization (that some people in the past have called "room gain").
Rooms have resonances based on their dimensions. A living room might have its lowest standing wave at perhaps 30 Hz. Above 30 Hz, motions of the cone come and go and they average out to zero-increase in room pressure at your ear or at a mic because they are averaging out. Below 30, ain't nowhere to go and so the room gets pressurized..... if (1) the room is made of concrete on all 6 sides, (2) is tightly sealed, (3) you own a miraculous speaker than can move the cone as needed, and (4) the back wave from the cone is not also emptying into the room (typically because it is in a sealed enclosure).
Now, cones move more as the frequency goes down in order to produce the same loudness (this has everything to do with cone diameter below about 200 Hz, if I recall correctly* **). Since they move more, the pressurization goes up (at 12 dB/8ave). And that is the wonderful boost some used to call "room gain" assuming the cone can move the astonishing, inconceivable, and unobtainable amount needed at say 7.5 Hz for the same loudness.
The first reader who can name a dozen reasons why "room gain" it is of no practical value whatsoever in ordinary homes will gain everybody's admiration. (Hint: most of those reasons already hinted at above.)
Ben
*In theory, cones can't work and bees can't fly. But due to a mix of art and science, cones are known that handle the lower range OK.
** I'm not sure if an array of small drivers can sneak around this limitation a bit???
There are a variety of physical aspects that influence bass in a room. ONE of these (albeit of no practical value) is SPL pressurization (that some people in the past have called "room gain").
Rooms have resonances based on their dimensions. A living room might have its lowest standing wave at perhaps 30 Hz. Above 30 Hz, motions of the cone come and go and they average out to zero-increase in room pressure at your ear or at a mic because they are averaging out. Below 30, ain't nowhere to go and so the room gets pressurized..... if (1) the room is made of concrete on all 6 sides, (2) is tightly sealed, (3) you own a miraculous speaker than can move the cone as needed, and (4) the back wave from the cone is not also emptying into the room (typically because it is in a sealed enclosure).
Now, cones move more as the frequency goes down in order to produce the same loudness (this has everything to do with cone diameter below about 200 Hz, if I recall correctly* **). Since they move more, the pressurization goes up (at 12 dB/8ave). And that is the wonderful boost some used to call "room gain" assuming the cone can move the astonishing, inconceivable, and unobtainable amount needed at say 7.5 Hz for the same loudness.
The first reader who can name a dozen reasons why "room gain" it is of no practical value whatsoever in ordinary homes will gain everybody's admiration. (Hint: most of those reasons already hinted at above.)
Ben
*In theory, cones can't work and bees can't fly. But due to a mix of art and science, cones are known that handle the lower range OK.
** I'm not sure if an array of small drivers can sneak around this limitation a bit???
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As Don Hills showed in his last post, you can form consistent mental models of this based on either acceleration or displacement. The thing to understand is that when a cone speaker radiates into the free field, the SPL is proportional to cone acceleration. In the free field there is nothing to push against except the inertia of the air. But when it is trapped inside a small sealed room the SPL is proportional to cone displacement, as the air can't get away, the speaker can compress it. So the speaker's free field frequency response gets integrated twice. The sealed room is doing acoustically what the Bag End processor does electronically.
4 conditions for room gain were given above. In this thread we saw links to a system on some home theatre forum that fulfilled all 4 by simply cramming 4 18" long excursion woofers into a small room. I also question the practical use of single digit frequency response, but you can't deny that it can be done.
4 conditions for room gain were given above. In this thread we saw links to a system on some home theatre forum that fulfilled all 4 by simply cramming 4 18" long excursion woofers into a small room. I also question the practical use of single digit frequency response, but you can't deny that it can be done.
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Don,
1) In my living room, the measured LF response increases relative to the near field response of the 2x12" sealed sub and is by no means "flat (constant)", an obvious exception to your statement.
2) In my living room, opening the two doors increases LF SPL at the listening position compared to having the room doors closed, an obvious exception to your statement.
3)Sealed subs displacement typically increases quite a bit as the frequency drops, which may or may not result in constant SPL in an open or closed room, depending on dimensions of the room and the alignment and placement of the sub(s).
4) Agreed, "room gain" is the term by which the effect is known, the gain in bass level that occurs when a sub is located in a room, compared to in free space.
Art
If you take "amplify" to mean add power to the signal from an external power source, the room certainly doesn't do that. It is just trapping and concentrating energy from the speaker that would be lost in a free field situation. You could argue that "gain" is the wrong word for this, but that would be splitting hairs.
You know, I hadn't looked at this thread in a few days and got all excited because of the amount of replies.
Sadly, 9/10th of this thread is about "room gain". In the car audio world, it's called "cabin gain" and it is a very real phenomenon and it is used and leveraged in the design of SQ systems and SPL systems. Its starts around 80hz and adds ~12db or per octave below that frequency. It's how we get flat in car response out of small subs in tiny boxes. Tell an SPL guy that cabin gain does not exist and you will get launched into orbit by the brute force of all the laughter.
I appreciate the passion, but can we get back to the thread topic please? Or is this a situation were nobody has a good answer (which is fine) and so the only replies will be tangential at best?
Sadly, 9/10th of this thread is about "room gain". In the car audio world, it's called "cabin gain" and it is a very real phenomenon and it is used and leveraged in the design of SQ systems and SPL systems. Its starts around 80hz and adds ~12db or per octave below that frequency. It's how we get flat in car response out of small subs in tiny boxes. Tell an SPL guy that cabin gain does not exist and you will get launched into orbit by the brute force of all the laughter.
I appreciate the passion, but can we get back to the thread topic please? Or is this a situation were nobody has a good answer (which is fine) and so the only replies will be tangential at best?
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Nice of you to drop back in. Too bad you didn't take the opportunity to read more posts.
As a number of us have repeated, there is SPL pressurization at 12 dB/8ave below the room resonance in tightly sealed spaces. I specifically mention small cars a few times.
But the conditions that make it beneficial in small well sealed cars with high resonance modes (because they are small) are absent in home settings.
True you could make a room out of steel and have gaskets around the door and windows just like a car. But wouldn't it be easier just to crank up the bass tone control just one notch?
BTW, my ears have been assaulted by very loud bass coming out of the open windows of cars playing pop music. Funny thing is, a lot of people mistake the upper bass raucous thumping of dance music* for true low bass. Sure enough, with the windows open, that's all I hear. And I can't imagine exposing my ears to the enhanced hearing-destroying bass some people say is present in some cars with the windows rolled** up.... unless I was on the outside... far outside!
Ben
*easy to see that many people must find this esthetically pleasing: that's fine
** say, do cars still have "roll up" windows?
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Answer to OP's question:
Small subs distributed about a room will fill in some of the missing LF modes and the overall bass response will be smoother. The smoothing effect can be improved with EQ.
Whether a person likes the smoother response
is a subjective matter.😀
https://www.youtube.com/watch?v=gBzJGckMYO4
Yes, back to OP and FrankWW is right in most respects. But some re-thinking now about the limits to EQ to make substantial and hearable differences down low. For sure, some peaks can be lowered a bit but valleys are not as responsive to tweaking. Better to get the room and acoustic issues as right as you can first and then tweak. Judging from the hard-surface echo room pictures posted, not usual to care as much about acoustics as decor.
FrankWW also right about distributed subs and using smaller speakers facilitates that kind of distribution, as he says. But better to use the most heterogeneous choices of drivers, housings, and locations to get the best results. I know it makes engineers cry when i urge heterogeneity but you need to think statistically not elementarily.
Ben
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I once built a fairly large room audio playroom, paying particular attention to room modes and an even spectral decay (RT60). I simulated a bunch of different "golden ratios" and finally ended up with a room 39 ft by 23 by 11ft.
I forget the exact inches, but it was built to the inch, just for modal dispersion. Then the room was treated acoustically to tame or disperse reflections, and to level out decay with absorption). I did a lot of measuring...
The thing I learned with modes is....they are ALL bad, period.
If you don't think so, just take your gear outside and hear how much better the bass sounds, (and really the whole dang spectrum when devoid of reflections).
So multiple sub locations are really just trying to spread the badness out evenly 🙂
EQ can tame peaks, can't touch nulls. Placement can tame both peaks and nulls.
One thing about room gain, or cabin gain, or squished boundary gain, that may have been said, but may be worth repeating.... is that it's not 12 db/octave 'all or nothing' due to having a sealed enclosure or not.
There is clearly partial gain even when not sealed, and with whatever level of boundary absorption.
Here's a pretty interesting paper on subs and placement, if you haven't seen it...
http://www.harman.com/EN-US/OurCompany/Innovation/Documents/White Papers/multsubs.pdf
I forget the exact inches, but it was built to the inch, just for modal dispersion. Then the room was treated acoustically to tame or disperse reflections, and to level out decay with absorption). I did a lot of measuring...
The thing I learned with modes is....they are ALL bad, period.
If you don't think so, just take your gear outside and hear how much better the bass sounds, (and really the whole dang spectrum when devoid of reflections).
So multiple sub locations are really just trying to spread the badness out evenly 🙂
EQ can tame peaks, can't touch nulls. Placement can tame both peaks and nulls.
One thing about room gain, or cabin gain, or squished boundary gain, that may have been said, but may be worth repeating.... is that it's not 12 db/octave 'all or nothing' due to having a sealed enclosure or not.
There is clearly partial gain even when not sealed, and with whatever level of boundary absorption.
Here's a pretty interesting paper on subs and placement, if you haven't seen it...
http://www.harman.com/EN-US/OurCompany/Innovation/Documents/White Papers/multsubs.pdf