Do room reflections come only from a localized source?

[BrentDomann]

I can't think of where else to post this, but as I was trying to get info out of noisewreck in the thread about dynamics, my example of a room with a speaker in one wall got me thinking:

Imagine a room that is airtight and where all six walls (including the ceiling and floor) are one piece. By some miracle, the whole room can expand and contract in volume at once. That is, the room can get bigger and smaller as a whole, and it's airtight, so its expansion and contraction will affect the air pressure in the room. If the room expands and contracts, will there be any room reflections?

My gut feeling is that theoretically, there won't be any. Sound waves travel from a localized source like ripples from a pond, and thus distortions in pressure level are more localized themselves. If a whole volume is changing in pressure simultaneously, then there are no "ripples".

If it's the corners that cause you concern, what about a spherical room? A localized source inside a sphere will create reflections, right? But what if the whole sphere is expanding and contracting?

 

[Chili]

My gut feeling is that theoretically, there won't be any. Sound waves travel from a localized source like ripples from a pond, and thus distortions in pressure level are more localized themselves. If a whole volume is changing in pressure simultaneously, then there are no "ripples".

If it's the corners that cause you concern, what about a spherical room? A localized source inside a sphere will create reflections, right? But what if the whole sphere is expanding and contracting?

What do you mean by "distortions in the pressure levels are more localized themselves"?? A distortion is an unwanted change. How would you be distorting the sound waves in your theoretical room??

If i'm understanding your theoritcal room correctly, you certainly would get room reflections. I think the only way you wouldn't was if the walls continually expanded at the speed of sound in air. Otherwise the sound would reflect off the walls at some point. Maybe I'm not understanding fully what you're asking.

 

[miroslav]

I like the first one better...but it's raining outside, so the pizza has no sauce on it.

 

[BrentDomann]

Distortion is just variance, not necessarily unwanted change. But that's beside the point.

The spherical room is a better example, I think (actually, any non-spherical room probably wouldn't work the way I thought). Think about a sound source in the middle of the sphere. If it makes a sound, the reflections will hit all points in the sphere at the same time, and will all reflect back to the center. What if the sound waves emanate from all points on the sphere at the same time? They're going to meet in the middle. Do they pass through one another, reflect off of one another, or what might they do?

 

[miroslav]

...a sound source in the middle of the sphere. If it makes a sound, the reflections will hit all points in the sphere at the same time, and will all reflect back to the center.

Only if the sound source itself is a sphere, emanating the sound equally in all directions

What if the sound waves emanate from all points on the sphere at the same time? They're going to meet in the middle. Do they pass through one another, reflect off of one another, or what might they do?

The would probably do a bit of everything...pass through, reflect off each other, and mostly hit the other side of the sphere perpendicular from their source point...and reflect back.

If it's a continuously generating sound...you would have almost instant feedback....IMO. If it was just a single pulse...it would stutter and die off...IMO...of course a lot depends on the source sound(s) and the size of the sphere.

 

[leddy]

You mention an airtight room that can expand and contract...

By how much? Do you want to actually be in it with the changing air pressure rupturing your eardrums?.

 

[Chili]

Distortion is just variance, not necessarily unwanted change. But that's beside the point.

The spherical room is a better example, I think (actually, any non-spherical room probably wouldn't work the way I thought). Think about a sound source in the middle of the sphere. If it makes a sound, the reflections will hit all points in the sphere at the same time, and will all reflect back to the center. What if the sound waves emanate from all points on the sphere at the same time? They're going to meet in the middle. Do they pass through one another, reflect off of one another, or what might they do?

They'll just pass through without interacting with each other. Think of two flashlights; one red lens, the other blue. You point them at perpendicular walls so the beams cross; red to one wall and blue to the other. What would you see on the walls? One red spot and one blue spot. What would see if you there was something reflecting the light at the intersecting points of beams, like a dust speck? Purple. The light beams aren't interacting with each other, but both are reflecting off the dust speck, so our eyes sees both colors and perceives it as purple.

So the sound waves will pass through without interacting with each other, but all waves would have an affect on a microphone or the ear.

 

[miroslav]

Actually...I don't think an airtight room COULD expand/contract without cracking/bursting.

The existing air inside would prevent the expansion/contraction...

 

[Chili]

Actually...I don't think an airtight room COULD expand/contract without cracking/brusting.

The existing air inside would prevent the expansion/contraction...

Well, he did say theoretically.

 

[miroslav]

They'll just pass through without interacting with each other.

Mmmmmmm...not sure about that.

If you play 5 notes simultaneously...do they not interact with each other (at least as the human ear perceives it) and create harmonics?

 

[miroslav]

Well, he did say theoretically.

Yes...but that's why I said it's raining outside and the pizza has no sauce on it.



You can't ignore laws of physics and attempt to obtain answers that are governed by those same laws...so even "theoretically"...it's not a valid question.

 

[Chili]

Yes...but that's why it's raining outside and the pizza has no sauce on it.



You can't ingore laws of physics and attempt to obtain answers that are governed by those same laws...so even "theorethically"...it's not a valid question.

Yup, I'll concede that!!

 

[miroslav]

What...that it's raining outside and the pizza has no sauce on it?

 

[Chili]

Mmmmmmm...not sure about that.

If you play 5 notes simultaneously...do they not interact with each other (at least as the human ear perceives it) and create harmonics?

Nope, the waves themselves don't interact with each other, but all 5 notes will interact with the ear. Your ear will hear them, but they won't alter each other.

Think of waves on the ocean (i've spent quite a bit of time there). They come from all different directions and they keep on going to where ever they're heading. A wave set in one direction doesn't influence a wave set in another direction. But a ball floating on the ocean will bob by whatever waves are under it any point in time.

 

[Chili]

What...that it's raining outside and the pizza has no sauce on it?

Man, you've gotta stop mentioning the pizza!!! I'm trying to lose weight.

 

[BrentDomann]

Actually...I don't think an airtight room COULD expand/contract without cracking/bursting.

The existing air inside would prevent the expansion/contraction...

That depends on what the room is made out of. All you need is an enclosed, airtight space. You're not putting a roof on it. The point of the question is to remove the localized sound source but still changes in air pressure. How would one do that? He or she would change the air pressure in every point in the room at the same time.

The only way I can think of to do that in an enclosed space is to alter the volume of the space. In that case, is it the walls (or sphere, or whatever) that are resonating, or is it the mass of air? If it's the mass of air, I don't see how there can be reflections because there aren't any emanating waves. If it's the walls, then reflections abound.

This isn't all that important, I'm just bored on a Thursday and wondered if there was a way that sound could exist in an enclosed space and not reflect.

 

[miroslav]

A wave set in one direction doesn't influence a wave set in another direction.

If two waves come at each other...your saying they will just pass through each other without any affect on each other...?

Not sure about that.
I guess we have to discern what type of effect we are talking about.
They might not completely reflect/repel each other...but there is a mass and pressure/force there...so they must have some effect?

What about low and high air preassure masses...they sure DO interact with each other...they don't just pass through each other as though neither is there.

Not sure if we are both looking at this from the same perspective........?

 

[SouthSIDE Glen]

It's hard to separate the theoretical thought experiment from the actual physics of sound. Any cyclic change in air pressure with a cycle of greater than 20-30Hz (and less than 20kHz) that can cause the eardrum to move and wiggle the hairs in the cochlea will be perceived as sound.

I'm not sure just what type of physical size requirements and changes would be required to cause the air pressure to modulate that rapidly, if at all. I'm not an expert on Boyle's Law (I think that's the one), but I'm not sure how quickly the pressure change in the room would propagate through the volume even if the walls moved instantaneously.

Then you have the complication of wave cancellations and moding. If one wall moved, that would be one thing, but with all 6 surfaces moving simultaneously, you'd have waves propagating in every direction, and that's even before they reflect. I'd expect there to be a lot of phase complications going on that would effect the results.

But man, I love the theoreticals coming up in this forum the past couple of days! It sure beats yet another series of "why is my mix so quiet?"

G.

 

[BrentDomann]

Nope, the waves themselves don't interact with each other, but all 5 notes will interact with the ear. Your ear will hear them, but they won't alter each other.

Think of waves on the ocean (i've spent quite a bit of time there). They come from all different directions and they keep on going to where ever they're heading. A wave set in one direction doesn't influence a wave set in another direction. But a ball floating on the ocean will bob by whatever waves are under it any point in time.

Following your wave analogy, what if the whole surface of the ocean is moving at once up and down. The ball floating on the ocean will still bob, but there won't be any waves going to wherever they're heading. No waves = no chance for reflection.

But now that I think about it, if you were in a room and the whole room's air pressure changed, it might not produce any sound. A ribbon mic wouldn't pick that up, would it? The pressure on both sides of the ribbon would always be the same. The same would be true of a mic diaphragm, or an eardrum (I think).

So we must perceive sound as a change in air pressure, or a distortion in air pressure, relative to a baseline. It's a matter of waves, not overall air pressure.

 

[miroslav]

That depends on what the room is made out of. All you need is an enclosed, airtight space.

Nope...makes no dif what it's made of...if it's airtight...THAT'S what will keep it from expanding/contracting...and so the volume will NOT be able to change.

This isn't all that important, I'm just bored on a Thursday and wondered if there was a way that sound could exist in an enclosed space and not reflect.

Yes...inside an anechoic chamber...where most speakers/mics are usually tested to plot their frequency response curves.