Physics question

[Zylka]

In an enclosed space, sound is continually produced from a central source. When the sound waves (partially) rebound off the nearby walls and intersect newer waves, do they interfere or distort each others form? How are frequencies and pressures potentially altered?

Is this possible interference and distortion a negligible effect in terms of human hearing?

[Jon Miller]

yes, sound waves can reflect and interfere...

if you have everything set up right you could get beats or other such things.. (frequencies..) or nodes and antinodes (pressure)

if you set it up right, you could do this with frequencies that hold for human hearing...

when my cars sunroof was off, at certain speeds (depending on the wind and the like) my head would feel like it was being oscillated, this was from pressure.. (and sound is just pressure waves)

Jon Miller

[Zylka]

So what physically described alterations do you get from sound waves colliding?

[Jon Miller]

you can get deconstructive and constructive interfrence (nodes and antinodes basically)

not beats.. unless motion is taking place

basically all sorts of wave phenomenum can be observed.. all if you have the proper cavities and frequencies

Jon Miller

[Az]

As with any other wave, there should be interference, both constructive and destructive. Frequencies shouldn't be altered during this interference IIRC.

I am not sure whether the dispersal of the waves of the walls is completely "elastic" though ( is there a change of frequency when the sound bounces off the wall )

[Jon Miller]

Zylka, I would imagine oyu have been places, where if you move a little bit, because of the shape of the place you can hear somebody very well (At a position), but if you move a bit, even if it is closer, the shape of the space you share is such that you can't hear them at all (even though they have not moved)

JM

[Zylka]

Originally posted by Az

I am not sure whether the dispersal of the waves of the walls is completely "elastic" though ( is there a change of frequency when the sound bounces off the wall )

Yes, is there?

[KrazyHorse]

Basically, if you walk toward a wall, the sound will get louder and softer depending on where you stand. It will go from loud to soft in a distance which is one quarter of the wavelength of the sound being produced. If the sound is multifrequency then some of the frequencies will be loud at certain points and others will be soft there, so you might notice a change in perceived pitch depending on where you stand.

[Jon Miller]

well, in a simple model, no...

in a more complicated, it would be like a wave traveling down a string, going from a thin string to a very thick string.. (Very very thick)

so some would be reflected, and some would be transfered

I don't think that the frequency changes (it does within the wall of course, but the reflected wouldn't I think)

now lower index medium to a high index medium causes a 180 degrees phase change, I beleive

Jon Miller

[Jon Miller]

Originally posted by KrazyHorse
Basically, if you walk toward a wall, the sound will get louder and softer depending on where you stand. It will go from loud to soft in a distance which is one quarter of the wavelength of the sound being produced. If the sound is multifrequency then some of the frequencies will be loud at certain points and others will be soft there, so you might notice a change in perceived pitch depending on where you stand.

that reminds me of a test question one of the profs I TAed for gave his students

JM

[Zylka]

Originally posted by KrazyHorse
Basically, if you walk toward a wall, the sound will get louder and softer depending on where you stand. It will go from loud to soft in a distance which is one quarter of the wavelength of the sound being produced. If the sound is multifrequency then some of the frequencies will be loud at certain points and others will be soft there, so you might notice a change in perceived pitch depending on where you stand.

That sounds like it pertains to the "ambulance driving down the street" effect in some inverse way. How delightfully confusing!

[KrazyHorse]

Originally posted by Zylka


Yes, is there?

Depends what type of wall it is. If it's metal or rock then you'll probably get very good echoes. If it's some other material then it might insulate some frequencies better than others. If your sound source is single frequency then there should be no problem (insulators will offset the reflection in period, but not change the frequency)

[Jon Miller]

the ambulance driving down the street effect is beats, I beleive

JM

[KrazyHorse]

Originally posted by Zylka


That sounds like it pertains to the "ambulance driving down the street" effect in some inverse way. How delightfully confusing!

Not the same physical effect. Ambulance down the street is doppler effect. It happens even with pure tones. This is simply adding waves together.

[Az]

Zylka, I would imagine oyu have been places, where if you move a little bit, because of the shape of the place you can hear somebody very well (At a position), but if you move a bit, even if it is closer, the shape of the space you share is such that you can't hear them at all (even though they have not moved)

That's not due to interference nodes, I think.

After all, sound waves of various frequencies are emitted, and they create a mishmash of maximums and minimums, interfering with each other along the way, just like visible light.

Yes, is there?

I don't think so - I do think that the if we're speaking of a sound source that emits multiple frequncies, the fraction of returned frequencies will be different from the ones sent, because the surface will interact differently with different frequencies - prefering the absorbtion of certain frequency, and the reflection of other. But soundwaves is really foreign territory for me.