Small Room Reinforcement- Pressure Mode-Cancelation

[John White]

Inspired by my complete confusion in the tangent of this thread:
Pensados Place #21 - Bob Hodas

I'd like to better understand the relationship of large waves in smaller spaces.

It's clear (And well illustrated in several examples) that although it takes a greater amount of space/time in order for a large wave to fully form, we hear it as it passes us by.
However, what is meant by not having reinforcement of waves in a small space. I've always thought peaks AND nulls were more volatile in smaller spaces, yet Boggy suggests (forgive me if I read it wrong) that it is only because the waves don't have a chance to lose energy over time/distance.
Also, do waves have to be in a free field in order to intersect with each other in order to have constructive and destructive influences on one another?

Relevant posts include those after 16 with a few exceptions.

There seems to be several topics raised and I apologize in advance if I'm a bit thick in getting multiple principles confused. Still too fuzzy in order to even present a clear question.

Dumbing it down,

[localhost127]

Originally Posted by John White

Also, do waves have to be in a free field in order to intersect with each other in order to have constructive and destructive influences on one another?

Ripple Tank Simulation

Superposition principle - Wikipedia, the free encyclopedia

[John White]

Working along those lines:
Let's say there is a 0 degree phase shift at a particular frequency. In other words, an identical wave is superimposed so that the areas of high and low pressure coincide.
In this case there is a doubling of the energy, correct. Right? +6dB?

Would this be the case for all frequencies under all conditions?

[Ethan Winer]

Originally Posted by John White

In this case there is a doubling of the energy, correct. Right? +6dB? Would this be the case for all frequencies under all conditions?

Yes and yes. The power is actually 4x which is what 6 dB means.

Small rooms have worse peaks and nulls because the boundaries are closer and so the reflections are stronger. But even a 10x10 room is above the "pressure mode" for frequencies higher than 56 Hz. What kills you in a small room is mostly peaks and nulls above that frequency:



--Ethan

The Acoustic Treatment Experts

[avare]

Originally Posted by Ethan Winer

Yes and yes. The power is actually 4x which is what 6 dB means.

Small rooms have worse peaks and nulls because the boundaries are closer and so the reflections are stronger. But even a 10x10 room is above the "pressure mode" for frequencies higher than 56 Hz. What kills you in a small room is mostly peaks and nulls above that frequency:

+1

Powerfully,
Andre

[boggy]

Originally Posted by Ethan Winer

....
Small rooms have worse peaks and nulls because the boundaries are closer and so the reflections are stronger.

I agree...

Originally Posted by Ethan Winer

But even a 10x10 room is above the "pressure mode" for frequencies higher than 56 Hz.

I do not agree completely. It is not fully and always "above" because not all loudspeaker constructions are nearly "dead" below their f(-3dB) like ported/passive radiator constructions. I already said here and here, that it is possible with sealed box woofer to sum nearly flat in the small room, like graph which you can see below. Loudspeaker which is used in the room where the measurement took place, have f(-3dB) of about 50Hz in the free (half) space, and have a slope of about 12dB/oct. Room is about 3.5mx3.5mx2.6m.



(Actually, response is fairly flat down to 4Hz but that is not shown in the graph.)

Originally Posted by Ethan Winer

What kills you in a small room is mostly peaks and nulls above that frequency:
......

I agree.

EDIT: I edit picture to show approximate free space loudspeaker response and its response in the room.

[Jens Eklund]

Originally Posted by boggy

Actually, response is fairly flat down to 4Hz but that is not shown in the graph.

4 Hz ...? What kind of woofer/box, measured where?

[boggy]

Originally Posted by Jens Eklund

4 Hz ...? What kind of woofer/box, measured where?

Sealed box design, 12" long throw (sub)woofer. It is measured in the room with dimensions of about 3.5x3.5x2.6m (before treatment).
If you need a picture, here it is:


EDIT: more pictures are here: Pressed Lizard Studio | Facebook

[localhost127]

what mic did you use for measurement at 4hz?

[Jens Eklund]

Originally Posted by boggy

Sealed box design, 12" long throw (sub)woofer. It is measured in the room with dimensions of about 3.5x3.5x2.6m (before treatment).
If you need a picture, here it is:EDIT: more pictures are here: Pressed Lizard Studio | Facebook

Impressive. May I ask what woofer? Linkwitz transform (or equivalent)? XBL^2 motor (I’m currently looking for a replacement for the Exodus, perhaps CCS)?

[boggy]

Originally Posted by localhost127

what mic did you use for measurement at 4hz?

Earthworks M30

[boggy]

Originally Posted by Jens Eklund

Impressive. May I ask what woofer? Linkwitz transform (or equivalent)? XBL^2 motor (I’m currently looking for a replacement for the Exodus, perhaps CCS)?

Thanks!
I do not use Linkwitz transform (it is not needed, in this case). You actually see only (acoustical) "room gain" influence ("pressure mode" frequency region in the small room as Ethan said)
Subwoofer driver is actually an old Peerless XLS 830500... (maximum excursion is about 20-25mm in one way, nothing spectacular!).
New products, based on the same frame/basket, are slightly different (even XXLS series exist today), and as far as I know, they are produced by both Tymphany and Scan Speak.

[John White]

Originally Posted by Ethan Winer

Small rooms have worse peaks and nulls because the boundaries are closer and so the reflections are stronger. But even a 10x10 room is above the "pressure mode" for frequencies higher than 56 Hz...

Great dialogue so far. I'll have to take it slow, however, so I'll disregard the resulting discussion.

I.a.
So using this example, the 56Hz is 2x the length of the 10' axial. Below that, there is no conversion between pressure and velocity, pressure and velocity, etc. Only pressure? The wave doesn't have space to propagate so no interference with itself can occur? (How about tangentially or obliquely?)

Bonus: Why?

I.b.
Why is it half the wave length or rather 2x the axial length? (Sorry, I'm a bit dyslexic, but if this question is the opposite of what it should be just interpret accordingly.)

II.
The reflections are stronger with the boundaries closer because there is less energy lost in the space/time difference compared with a larger room. Yes? (Seems like a no brainer, but I just want to check if I'm on the same page.)
Thus the greater peaks and nulls associated with the relative strength of interference. Si?


III.

The power is actually 4x which is what 6 dB means.

Ooofda. What? I was just going by a doubling of power due to the superposition of the wave length.
PS. Not a big deal right now, but I do want to get back to it.

[DanDan]

A small but significant side point John.
Two same in phase waves will add to a maximum of +6dB.
And these can sometimes be Eqed down.
On the other hand....
Two same out of phase waves will cancel totally.
In the real world we see nulls of -20 to -30dB.
DD

[John White]

Yes, 100% increase and 100% decrease in a perfect scenario.

Still, I can't figure out what the "4x power" that Ethan stated when I mentioned a doubling of power. I'm sure he'll chime in on Monday.

[boggy]

Originally Posted by John White

.........
Still, I can't figure out what the "4x power" that Ethan stated when I mentioned a doubling of power.....

Me too.

[DanDan]

Perhaps he is alluding to the 20log vs 10log thing?
DD

[boggy]

Originally Posted by boggy

Me too.

I realized what Ethan meant with "4x power".... (I need to sleep sometimes... )
John, you need four times higher power (from amplifier through loudspeakers) to have a SPL that is two times higher (or +6dB).
So if you have 85dB with 25W of electrical power applied to your loudspeakers... for 91dB you will need 100W, at same distance from loudspeakers, in same room (and for loudspeakers of same impedance)

[John White]

Nice work! Obviously I was just referring to the wave itself.
See there's a reason you get your commission checks from GS.

[Ethan Winer]

Originally Posted by John White

So using this example, the 56Hz is 2x the length of the 10' axial. Below that, there is no conversion between pressure and velocity, pressure and velocity, etc. Only pressure? The wave doesn't have space to propagate so no interference with itself can occur? (How about tangentially or obliquely?)

I'm not a physicist, and I don't even try to play one on the Internet. But I'm pretty sure the above is true, though I'm not certain about the lowest frequency that can have interference via non-axial propagation.

The reflections are stronger with the boundaries closer because there is less energy lost in the space/time difference compared with a larger room. Yes?

Yes. Not only losses due to absorption by the air, but also the Inverse Square Law that spreads sound outward with distance. Again, I'm not a physicist, and others here might be better equipped to map out how the Inverse Square Law works inside a room with many reflections etc.

Originally Posted by John White

Still, I can't figure out what the "4x power" that Ethan stated when I mentioned a doubling of power.

Acoustic wave pressure is the equivalent of electrical voltage, and wave velocity is the same as current. Doubling the voltage to a loudspeaker quadruples the amount of power because the voltage is twice as large, and the current drawn is then also twice as large. Power = Watts x Volts, so when both double, the power quadruples. Back to acoustics, waves combine the same as voltages. So when the pressure doubles from two waves butting heads, the power increases four-fold just as when the voltage is doubled.

Or something like that.

--Ethan

[DanDan]

how the Inverse Square Law works inside a room with many reflections etc.

A fully hard concrete bunker will show very little loss of level with distance.
Such Reverberant Chambers are used for measuring Sound Power and many Building Acoustics issues. Very Reverberant is almost as useful as Anechoic in such testing.

However real rooms often have floppy boundaries. Drywall on studs. Drywall as in inner lining. And in Listening Rooms, Reflection Killing or Redirecting panels. I would wildly guess 3dB per doubling of distance, if the floor is hard.

The combination of distance and poor reflectivity at LF results in the very low level of reflections seen on ETCs of small highly damped rooms, particularly within the ZWR/RFZ

One sometimes finds levels as low as -15 to -20dB.

DD

[John White]

Originally Posted by DanDan

A fully hard concrete bunker will show very little loss of level with distance...
I would wildly guess 3dB per doubling of distance, if the floor is hard.

That certainly much lower than a textbook prediction. Why do you think this is so? Certainly not much of an argument for a larger room.

[Rod Gervais]

Originally Posted by Ethan Winer

Acoustic wave pressure is the equivalent of electrical voltage, and wave velocity is the same as current. Doubling the voltage to a loudspeaker quadruples the amount of power because the voltage is twice as large, and the current drawn is then also twice as large. Power = Watts x Volts, so when both double, the power quadruples. Back to acoustics, waves combine the same as voltages. So when the pressure doubles from two waves butting heads, the power increases four-fold just as when the voltage is doubled.

Or something like that.

--Ethan

I believe we are confusing power with amplitude here.......

3dB is a doubling of power......... thus 6dB is power quad......

6dB is a doubling of measured amplitude......... (roughly) 10dB is a perceived doubling of loudness.

Rod

[DanDan]

Hi John, I am not sure we are using the same Less and More....:-)

I did a lot of calculation of Noise sources over distance, nearly all outdoors though. I would have to pull out the books to be fresh on this. I normally got those calculations wrong BTW.....
One needs to watch definitions of Sound Pressure Level, vs simple pressure.
Squares turn into 20 log as opposed to 20 log and so on.

So, I usually get some arithmetic detail wrong, but generally...

The Near Field, typically between speaker and listener in a small room, has it's own unique behaviour. Moving away from that we may see a region of Free Field (very ish). Eventually we get to a region where sound level drops very very little with distance. In a fully reverberant room, this would be the Diffuse Field. The decay graph would flatline, no drop in level.

Sound in a Free field is expanding over a full spherical wavefront. It will drop 6dB per doubling of distance. The Inverse Square Law.

Introduce Solid ground, say concrete, half the universe has vanished. The same amount of Sound energy is now expanding over only a half sphere. 3dB per doubling of distance.

So a highly damped room with a solid floor, not quite as dead as outdoors, we would be lucky to see 3dB.

This is why I regard the Reflectivity of the Boundaries to be more significant than loss of level with distance.

DD

[John White]

I think I've got what you're saying in a simplistic way- the amplitude (Thanks Rod) of the signal over twice the distance in a small room would unlikely decrease by half due to boundary reflections.

[DanDan]

Yes John. Consider the extreme case, a fully reverberant room, a swimming pool will do. Kids shouting at the far end of the pool are pretty much the same volume everywhere. Except of course within very close proximity.
This transition from level dropping rapidly very close to the source, and the later evening out of the decay slope pretty much points out the Near Field.
Check out Critical Distance.
DD

[John White]

Good stuff

[Ethan Winer]

Originally Posted by Rod Gervais

3dB is a doubling of power......... thus 6dB is power quad......
6dB is a doubling of measured amplitude......

Yes, exactly. In this case, doubling the wave pressure represents a four-fold increase in power.

--Ethan

[DanDan]

I wrote that a bit speedily John. Critical Distance and all else that I referred to is presented really well here. Lenard Audio - Education - Acoustics
DD

[John White]

Yes, Dan. My biggest take away from Dc in a small room vs larger room is that the critical distance is frequency dependent so although early reflections can theoretically be managed, as the frequency drops, the time/distance required to hear a pure sound source becomes increasingly more difficult.

It seems that this is really more a matter of room layout/acoustic model than it is the size of the room, however. Not to mention listening in a near field.

Edit:
In other words, let's say we have managed early reflection in some form or another, speakers are flush mounted, and there is at least enough room for a LEDE approach. The amount of "reverberant" low frequency reflections from the front and sides of say a 1500cf room and 3000cf room seem minimal enough to make much of a difference in the quality of the listening environment.

(These examples may indeed be truly weak, but I'm really more interested in the concept of how smaller spaces negatively affect the listening environment.)