Sound interference True/False

  • #1

Homework Statement


Consider two sources of sound, each of which emits a sound wave of the SAME frequency and the SAME intensity. The sources are located on a straight and level road, relatively close to each other. You, the listener, are also located on this road, about 1/4 mile away from the two sound sources. All 3 objects (source A, source B, and you) are in a straight line. Which, if any, of the statements below is/are true about this situation?

For beats to occur, the sound waves from the two sources must have different frequencies.

If the two sound sources are very close together and you hear an intensity level of 10 dB from one source alone, then the two sources combined will give an intensity level of about 13.01 dB.

If one source is moving away from you and the other source is moving toward you, it is possible that there would be a zero beat frequency heard at your location from the two sources.

If one source is moving toward you, the wavelength of the sound detected from that source will increase.

If the two sources are separated by exactly 4-1/2 wavelengths, there will be constructive interference at your position.

If the two sound sources are rotated horizontally through 90o about their common center-of-mass, you will not hear any sound at your location because of destructive interference.



The Attempt at a Solution



I think the answers are:

T (beats are formed by waves with different frequencies)
T (unclear)
F (unclear)
F (frequency will increase, but wavelength will remain the same)
F (there will be deconstructive interference between it is between wavelengths)
F (there will be no change in the interference)

My buddy and I disagree about 2 and 3, but are at a collective loss.
 

Answers and Replies

  • #2
Ibix
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I've interspersed your answers with the questions

For beats to occur, the sound waves from the two sources must have different frequencies.
T (beats are formed by waves with different frequencies)
Agreed.

If the two sound sources are very close together and you hear an intensity level of 10 dB from one source alone, then the two sources combined will give an intensity level of about 13.01 dB.
T (unclear)
[strike]Check your maths. I think you've slipped a decimal point somewhere.[/strike]
Edit: no, I have, and I ought to know what 3dB means [kicks self]. Carry on. I'm taking "close together" to mean separation [itex]<<\lambda[/itex], by the way.

If one source is moving away from you and the other source is moving toward you, it is possible that there would be a zero beat frequency heard at your location from the two sources.
F (unclear)
Agreed. You get opposite Doppler shifts, so the frequency difference can never be zero.

If one source is moving toward you, the wavelength of the sound detected from that source will increase.
F (frequency will increase, but wavelength will remain the same)
Agreed it's false, but the speed of sound is not affected by the speed of the source and [itex]c=f\lambda[/itex].

If the two sources are separated by exactly 4-1/2 wavelengths, there will be constructive interference at your position.
F (there will be deconstructive interference between it is between wavelengths)
Agreed.

If the two sound sources are rotated horizontally through 90o about their common center-of-mass, you will not hear any sound at your location because of destructive interference.
F (there will be no change in the interference)
I'd draw this out, if I were you. Where are the sources relative to you in the new configuration?
 
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