Reflection of sound. Changes to its properties.

[buggsndirk]

Hi guys and girls,

Just had a question I have been thinking about for a while.

Suppose you have a sound maker than makes a pure tone at a particular frequency. You play this tone for an instant directed directly at a flat wall (not absorbent at all). If you could measure the reflected tone would there be any change in frequency, intensity, wavelength or velocity?

My gut feeling is that the intensity will lower somewhat, but nothing else will change, but I have a feeling I'm missing something super obvious like a doppler effect or something.

Any and all thoughts are appreciated.

Thanks

 

[maajdl]

Is this wall moving?
If it is moving away, then each wavelength will be reflected a little bit later, which is the Doppler effect (called red shift).
If it is moving toward you, the each wavelength will be reflected a little bit earlier, also the Doppler effect (blue shift).

 

[buggsndirk]

Everything is stationary.

so nothing will change?

 

[Bobbywhy]

At longer distances from the reflecting surface, the only difference you will measure is some decrease in intensity, as you said in your Opening Post (OP). But, measure the sound intensity very close to the reflecting surface...expect a surprise!

“Sound intensity near a hard surface is enhanced because the reflected wave adds to the incident wave, giving a pressure amplitude that is twice as great in a thin "pressure zone" near the surface. This is used in pressure zone microphones to increase sensitivity. The doubling of pressure gives a 6 decibel increase in the signal picked up by the microphone.”
http://hyperphysics.phy-astr.gsu.edu/hbase/sound/reflec.html