A loudspeaker sits in a large room (of length L) against a
wall and faces towards the opposite wall. The speaker emits a
resonant frequency of the room, thus setting up a standing wave.
Assume that the wavelength is much smaller than the size of the
room, so that the mode number n is high. Use vsnd for the speed of
sound.
a.) Your friend starts at the speaker and runs toward the
opposite wall at constant speed v. As she does, she hears a
loud-soft-loud modulation of the sound intensity. From your
perspective, sitting at rest, she is running through nodes and
antinodes of the standing wave. Find an expression for the number
of sound maxima she hears per second, only in terms of the
variables given above.
b.) From your friend’s perspective, the two sound waves are
not the same frequency, so they don’t create a standing wave. Find
an expression for the beat frequency that she hears, again, only in
terms of the variables given above.
c.) Are your answers in (a) and (b) the same, or different?
Should they be?