# Sound waves interference

1. Apr 12, 2013

### GT1

If two sound sources (A, B) are placed in a water tank and emit sound waves at the same frequency, phase and amplitude, if I measure the sound intensity at point A, will the detected signal will be ~double than the emitted signal of each sound source?
I think that the answer is yes because of the constructive interference between the sound sources, but what is not clear to me is why- all the sound waves that are emitted from source A have momentum in the direction of point B, if the net movement of the water molecules is in the direction of point B, how do they interact with a detector at point A (they only move away from it..)
See the attached drawing.

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2. Apr 12, 2013

### Claude Bile

The intensity at "A" will be infinite (as you have posed the problem) because you have a point source of finite power localised over an infinitesimal area/volume.

I think question you were trying to get at is "What is the intensity at the midpoint of A and B?". The answer, as you rightly allude to; is zero, because the interference between A and B is destructive if the waves are longitudinal.

Claude.

3. Apr 12, 2013

### GT1

I'm asking about a real world problem - the sound sources and the detector have finite dimensions.
What is the sound intensity very close to point A? Why?

4. Apr 12, 2013

### nasu

In a real problem, you will need to use parameters that you measure in the real setup.
The answer will depend on the phase difference between the two waves at the point of interest.
"A point near the source" is not a specific location.
And the phase difference will depend on the wavelength of the sound, so even for the same point, the answer depends on both specific location and wavelength.

The particles in the medium do not move "forward" but oscillate back and forth. If the oscillations are in phase, the amplitude is larger and you have a maximum of interference. The oscillations may not be along the same direction, true.