Do room reflections come only from a localized source?

[BrentDomann]

[P]ressure change takes time to propagate, it does not happen uniformly across the entire room volume at the same time. Instead, near the wall the pressure will build up first, and it will then take the form of a pressure wave with will travel inward across the room. At such a point it's in basic principle no different than a similar pressure wave that came from somewhere inside the volume and simply reflected off the wall.

Extrapolate that to 6 surfaces, and the interactions get a bit more complicated, but the basic idea stays the same; you will have pressure waves moving through the volume and not instantaneous pressure change across the whole volume.

The atmosphere is like a liquid, changes in it can only propagate through it like waves, not occur simultaneously across the whole volume.

[A] pressure change in a medium will always have a localized source. This is because any change in pressure requires a transfer of energy, and even the fastest-moving energy we know of has a top speed.

This, I think, is the answer. There will always be waves that must propagate through a medium. Talking about pressure changes, even when an atomic bomb appears to instantaneously vaporize a building, the heat from the bomb will reach the near side first. Although we can't perceive it because of its speed, the wave of expansion of the building's material as it vaporizes nevertheless moves across the building as it vaporizes.

OK. So I think from that we can say that as long as there is an enclosed space, there will always be room reflections. In other words, don't record dry mono sources.

Here's another question so we don't kill the thread: If you have a volume of gas suspended in a vacuum (say, like a cloud of gas in space), it won't have any walls. Don't say this is impossible - think about the edge of our atmosphere. If a wave travels toward the edge of the cloud, can it still reflect? There wouldn't be any wall against which the wave can bounce, but just as compression will bunch up against a surface and reflect, would rarification get to the (normal pressure) boundary, expand out a bit beyond it, and then, because of gravity, re-compress as it stretches out and then "snaps back"?

 

[miroslav]

Okay, good night, but to answer your question.. YES, it's an effect on the water, not the waves!!

Well...that's what I was getting at...that we may be looking at this from different perspective.
Personally...I don't care what happens to sounds in a "box" (lab experiment)...I care what happnes to them as to what my ears hear.
So...while the waves may not be individually affected...they are causing an effect on the water, and with sound waves, on what I hear...as-in harmonic overtones from the summing/interaction of the individual soundwaves.

 

[miroslav]

So I think from that we can say that as long as there is an enclosed space, there will always be room reflections.

Not if it's an anechoic chamber (room)...and isn't THAT what your original question was asking...if it's possible to have NO reflections inside a room.

But...listening to or recording music in a totally non-reflective room would actually suck!

 

[miroslav]

Here's another question so we don't kill the thread: If you have a volume of gas suspended in a vacuum (say, like a cloud of gas in space), it won't have any walls. Don't say this is impossible - think about the edge of our atmosphere. If a wave travels toward the edge of the cloud, can it still reflect? There wouldn't be any wall against which the wave can bounce, but just as compression will bunch up against a surface and reflect, would rarification get to the (normal pressure) boundary, expand out a bit beyond it, and then, because of gravity, re-compress as it stretches out and then "snaps back"?

You have to consider the amplitude of all these waves and how far they can travel before dissipating and/or being absorbed by whatever is there.
Sound will not reflect endlessly even in a 100% reflective space.

 

[SouthSIDE Glen]

This, I think, is the answer. There will always be waves that must propagate through a medium.

Yep. And once you know that, the question about the source really becomes pretty irrelevant, because regardless of the source location or type, the waves will all act moror less the same way.

So I think from that we can say that as long as there is an enclosed space, there will always be room reflections. In other words, don't record dry mono sources.

Not necessarily, because the reflections are determined by the enclosing medium, not by the waves themselves. As miro pointed out earlier, there are anechoic chambers that via a combination of irregular surface angles and sound absorbing material reflect nothing.

While there's no substitute for a good-sounding room, there are plenty or recordings made in virtually dead spaces with the purpose of either applying artificial reverb later. There are also times where at least one track of the mix sounds best absolutely dry. Sometimes bone dry vocals work (*sometimes*, not usually ), and 99% of us record bass or leyboards direct (no reflections in the direct electric signal.)

And let's not forget nature's anechoic chamber: the great outdoors. Find yourslef some nie flat farm land or prarie or desert with no major structures anywhere near, and you'll have no reflections. Which brings us to:

If you have a volume of gas suspended in a vacuum (say, like a cloud of gas in space), it won't have any walls. Don't say this is impossible - think about the edge of our atmosphere. If a wave travels toward the edge of the cloud, can it still reflect? There wouldn't be any wall against which the wave can bounce, but just as compression will bunch up against a surface and reflect, would rarification get to the (normal pressure) boundary, expand out a bit beyond it, and then, because of gravity, re-compress as it stretches out and then "snaps back"?

Well, first the technicalities: a small volume of gas in a larger vacuum will not last very long because the gas molecules will diffuse into the vacuum to fill it evenly; very thinly, but evenly.

The atmosphere sticks around because of the macro effects of the earth's gravity, but make no mistake; there is some leakage into space. But even within earth's gravitational field, like in your room or office, if you pumped all the air out of a box to create a vacuum and introduced a balloon filled with gas, and popped that balloon, the gas would not stay as a cloud the size of the balloon, but would disperse and diffuse to eventually evenly fill the full volume of the vacuum.

But as to the question of whether the boundary between the atmosphere and the vacuum of space is a reflective bondary, the answer is no. First off, because there is no "boundary" per se. The atmosphere just keeps getting thinner on a curve. Where one wishes to draw any kind of "boundary" is purely arbitrary. Second, even if there were a definitive boundary, gravity would not cause any minuscule "expansion" (if any) caused by sound pressure waves to spring back with any kind of resiliancy. Eventually that boundary might re-normalize to the same level, but it would take a lot of time.

Note that the gas in a vacuum container won't be pulled to the bottom of the container by gravity either, because at the molecular level there are forces in control stronger than the pull of gravity.

G.

 

[BrentDomann]

And let's not forget nature's anechoic chamber: the great outdoors. Find yourslef some nie flat farm land or prarie or desert with no major structures anywhere near, and you'll have no reflections.

Won't you get a reflection off of the ground? Even if you can't hear it, won't some measure of sound waves bounce back?

Which brings us to:Well, first the technicalities: a small volume of gas in a larger vacuum will not last very long because the gas molecules will diffuse into the vacuum to fill it evenly; very thinly, but evenly.

The atmosphere sticks around because of the macro effects of the earth's gravity, but make no mistake; there is some leakage into space. But even within earth's gravitational field, like in your room or office, if you pumped all the air out of a box to create a vacuum and introduced a balloon filled with gas, and popped that balloon, the gas would not stay as a cloud the size of the balloon, but would disperse and diffuse to eventually evenly fill the full volume of the vacuum.
. . .

Note that the gas in a vacuum container won't be pulled to the bottom of the container by gravity either, because at the molecular level there are forces in control stronger than the pull of gravity.

Clearly the gas will diffuse to fill a vacuum, but I dare say "not evenly". If you had a box with a popped balloon as you describe, the gas inside would be for all practical purposes evenly distributed. But on a larger scale, the weight of gas compresses the gas between it and the source of gravitational pull. In other words, the air at the bottom of the box is going to be very slightly more dense, just like the atmosphere is densest at sea level.

Eventually that boundary might re-normalize to the same level, but it would take a lot of time.

That's a much better way to describe what I was talking about when I said "snap back". Certainly the energy can't just be whiffed off into the vacuum. It has to go somewhere. While it seems you wouldn't say the sound wave was reflected, would you say that the energy is reflected? Diffused?

Not to be a thorn or anything, just keeping things going waaay beyond the edge of reason.

 

[SouthSIDE Glen]

Won't you get a reflection off of the ground? Even if you can't hear it, won't some measure of sound waves bounce back?

Well, technically the ones going straight down will bounce back up towards you (assuming a truly reflective surface and not loose topsoil or sand). But anything else will bounce off the ground away from the source at an angle coincident to the the angle at which it hits the ground and not back towards the listener or microphone. With no surrounding structures to reflect that back, t will never come back to the source to be heard. And again, even that's assuming one is standing on ice or concrete or hard-packed mud that actually will reflect the sound at all.

Clearly the gas will diffuse to fill a vacuum, but I dare say "not evenly". If you had a box with a popped balloon as you describe, the gas inside would be for all practical purposes evenly distributed. But on a larger scale, the weight of gas compresses the gas between it and the source of gravitational pull. In other words, the air at the bottom of the box is going to be very slightly more dense, just like the atmosphere is densest at sea level.

OK, you got me on a technicality there. But I doubt the difference would be even measurable in a container the size of a shoebox, and probably negligible even in a container the size of a room. But yeah, get the scale large enough, and the effects of gravity do become apparent.

But in a way, that even reinforces the point I meant to make there, that gravity is important only at a large, macro world scale. But when you get down to the physics of sound and pressure waves and atmospheric boundaries, which require looking at things on the molecular scale, other forces such as Brownian motion, electrostatics and thermodynamics and fluid dynamics swamp the effects of gravity.

That's a much better way to describe what I was talking about when I said "snap back". Certainly the energy can't just be whiffed off into the vacuum. It has to go somewhere. While it seems you wouldn't say the sound wave was reflected, would you say that the energy is reflected? Diffused?

Diffused and converted, probably. Picture this hypothetical "border" (which is really miles and thick, but we'll ignore that just for the sake of discussion), the boundary between where gravity holds a molecule in and where it's hold is not so sure.

There is constant leakage of molecules that get bumped just enough to go past the boundary. There are also those molecules that change direction or lose just enough kinetic energy to be re-trapped by gravity.

When a pressure wave comes along, that wave is actually composed of a tighter packing of the air molecules. So when it reaches the "boundary", it doesn't reflect against anything, you simply have a higher air pressure at the boundary. This higher pressure will result in more molecules managing to "jump the boundary" which would result in a release of energy in the form of a dissipation of kinetic heat.

G.

 

[SouthSIDE Glen]

now, just to confuse matters, in the ocean there are layers of oecan density and temperature, not completely unlike the layers of the atmosphere (troposphere vs. stratosphere, etc.). The boundaries of some of these ocean layers do manage to reflect sonar; i.e. they are reflective to sound waves traveling through water.

As to whether a similar boundary layer effect for sound waves can or does exist in the atmosphere, I don't know. Nor am I sure that a sharp boundary between atmosphere and vacuum wouldn't have some such reflective properties. This makes your question all the more interesting.

But what I do believe is that in reality such a boundary between gas dense enough to carry sound an a vacuum does not exist; there is going to be extreme fuzziness at that "boundary". this fuzziness, which I imagine would be like putting thick open-cell foam on a solid wall, i would believe would tend to absorb and dissipate any reflective properties any such boundary might otherwise have.

BTW, This is mostly all thinking out loyud for the fun of it. An atmospheric scientist might just come along any time now and tell me I'm full of shit . But it's thinking to the best of my knowledge - an honest effort, at least.

G.

 

[noisewreck]

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.

Actually at the quantum level the light waves do interact with each other. Also, due to their nature, you cannot compare soundwaves to those of photons. Soundwaves will interfere with each other somewhat. Sure, they will hit the opposite wall, but there will be some difference between the wave that had crossed another wave, and between the one that didn't.

 

[noisewreck]

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

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

Well, not entirely true. It would depend on the material. If the air inside a body would prevent expansion/contraction, then how would you explain vacuum chambers?

Still though, I am not quite getting the point of this thought experiment of the OP. You are assuming that the gas expansion/contraction (which in turn would translate in drop/rise in internal pressure) would follow the walls instantaneously, which I don't think is the case. No matter what, there is time needed for information to travel from one side of the room to the other, in other words, the gas molecules will get the information about the room expanding from their neighboring molecules. This is not instantaneous. At the very best, it will still take the speed of light for information to travel from one side to the other, but my guess would be it is more like the speed of sound, which is much slower. So, yeah, based on this, I would say the gas molecules closer to the walls would be moving much faster than the ones closer to the center, which would then catch up to the rest. Then the molecules that were following the walls, will hit the walls once the walls stop moving, due to inertia, and bounce back, which will in effect result in a wave. In the absolute center of the room, you will likely have a standing wave.

Now, if you had slight variances in the room (let say it wasn't pure gas, but you had some dust particles here and there, the walls weren't perfectly smooth down to the single molecule), then you would have slight irregularities, which would in return make up for a more complex wave. In other words, the contraction and expansion will put the internal gas in motion, or disturb it's equilibrium, for at least a little while. The result will be "sound" waves of some sort, until that equilibrium was reached again in the new pressure environment.

So, what is the point of this discussion again

 

[SouthSIDE Glen]

Well, not entirely true. It would depend on the material. If the air inside a body would prevent expansion/contraction, then how would you explain vacuum chambers?

Or the opposite, compressed gas canisters like scuba tanks, helium tanks for balloons etc. Of course gasses can be compressed or expanded. We'd never have the internal combustion engine otherwise!

Still though, I am not quite getting the point of this thought experiment of the OP. You are assuming that the gas expansion/contraction (which in turn would translate in drop/rise in internal pressure) would follow the walls instantaneously.

That was kind of his question. Ignore the walls for a second; he was wondering if a global cyclical pressure change to the entire volume would be perceived as a sound, as opposed sound "emanating" from a localized source.

The answer was that pressure in a volume *cannot* change globally instantaneously, just like you said, but rather has to propagate through the volume, regardless of the size or location or cause of the source, rendering the question kind of without meaning.

As far as the purpose of the question; I don't think there was any real application intended, it was just a question that came to him that he just threw out there for consideration.

That, and he's concerned his mixes are not as loud as Lady Gaga's CD.

G.

 

[Chili]

Actually at the quantum level the light waves do interact with each other. Also, due to their nature, you cannot compare soundwaves to those of photons. Soundwaves will interfere with each other somewhat. Sure, they will hit the opposite wall, but there will be some difference between the wave that had crossed another wave, and between the one that didn't.

You're right in the fact that light does react with each other, but light is different in that it follows the properties of both wave and particle theory. I was using the wave portion as an example and it is still valid.

I don't see how sound waves can interact with each other. You'd have to elaborate on that to convince me.

 

[Chili]

now, just to confuse matters, in the ocean there are layers of oecan density and temperature, not completely unlike the layers of the atmosphere (troposphere vs. stratosphere, etc.). The boundaries of some of these ocean layers do manage to reflect sonar; i.e. they are reflective to sound waves traveling through water....

Just nitpicking a bit... but sound in water refracts not reflects, as you described it. It's a change in speed of the sound in water that causes it to bend. The change in speed is caused by temperature, salinity, and something else I can't remember right now.

Sound will reflect off the bottom or the surface... and, of course, Targets!!! (ex-submariner - everything in the water is a target!!)

Okay, back to the topic at hand. Which was what, again??

 

[SouthSIDE Glen]

Just nitpicking a bit... but sound in water refracts not reflects, as you described it. It's a change in speed of the sound in water that causes it to bend. The change in speed is caused by temperature, salinity, and something else I can't remember right now.

True, it was an incorrect choice of words on my part.

I'm not personally a submariner, ex- or otherwise, but I did first learn about this reading Tom Clancy's "Hunt For Red October" .

But while you're right, it is refraction and not reflection, the end result can be in the sonar waves being channeled within layers or zones of depth and never reaching the bottom or the submarine; the point being that differences in layers of liquids like the ocean can "bend" sound waves in less-than-intuitive ways.

For a pretty good treatise on this, check out http://www.fas.org/man/dod-101/navy/docs/es310/SNR_PROP/snr_prop.htm

the only reason I borught it up was that, while I don't know of this first-hand, I suppose it may be possible that similar things could happen between layers of the atmosphere. Though I still don't see it happening that the fuzzy boundary between a gas and a vacuum.

G.

 

[noisewreck]

I don't see how sound waves can interact with each other. You'd have to elaborate on that to convince me.

Since this reaches the limits of my theoretical knowledge, I'll defer to the following:

http://www.mediacollege.com/audio/01/wave-interaction.html Make sure to click on "wave interference patterns" link towards the bottom of that page as well.

http://www.maths.gla.ac.uk/~fhg/waves/waves1.html

BTW, here's a link showing a quantum experiment with light particles... to show that a quantum particle could appear to be in two places at once and interfere with itself!

http://en.wikipedia.org/wiki/Double-slit_experiment

 

[miroslav]

Of course gasses can be compressed or expanded.

I wasn't saying they couldn't...somehow the point has been inverted.

I was saying that if you have an airtight container...like a room (since this whole thread was about sound in a room)...
...the air already inside would fight against your attempts to "pull"/expand all sides of the room simultaneously, or to "push"/compress them if the room was *airtight*.
Yeah...you can by force expand/compress the air...but, only up to a point, and then the container will explode/implode.

But like noisewreck...I haven't been able to get a handle on how any of this would actually tie into a practical application of sound/reverberation...????

OK...it's fun to just come up with some non-real-world, almost fictional "theoreticals" ...but then we might as well be talking about "warp drive" and "dilithium crystals"

 

[noisewreck]

OK...it's fun to just come up with some non-real-world, almost fictional "theoreticals" ...but then we might as well be talking about "warp drive" and "dilithium crystals"

I would like to have a warp drive for my car!

 

[miroslav]

Yeah...but without the dilithium crystals....

 

[SouthSIDE Glen]

I've said it before, and I'll keep on saying it:

When I was growing up, they promised us flying cars by now. Where's my flying car??? I WANT MY FLYING CAR, DAMNIT!!!

G.

 

[noisewreck]

I've said it before, and I'll keep on saying it:

When I was growing up, they promised us flying cars by now. Where's my flying car??? I WANT MY FLYING CAR, DAMNIT!!!

G.

Only if they put the SMSing women in a separate lane!