A punk-rocker walks by a listener with a portable stereo playing music at full volume. The pitch of the recording will seem lower:
I. as the punk-rocker moves away from the listener.
II. as the listener moves away from the punk-rocker.
III. under no circumstances, as the pitch of the recording is fixed when the recording is made.
A. I only
B. II only
C. I and II only
D. III only
Click for Explanation
The doppler effect causes the apparent pitch of a sound to change. The change is governed by the equation:
Where f’ is the apparent frequency of the sound, f is the true frequency of the sound, v is the speed of sound in the surrounding medium, vd is the velocity of the sound detector, and vs is the velocity of the sound source. While specific numbers may be calculated, qualitatively a source and detector moving towards each other will hear a pitch higher than the actual pitch of the sound. If only the source or the detector is moving, the actual apparent pitch, f’, is slightly different but still higher than stationary sources and detectors. Sources and detectors moving away from each other experience a lower apparent pitch. Immobile sources and detectors experience the true frequency of the sound. I and II (or answer C) are correct.
- I only, incorrect, II is also correct.
- II only, incorrect, I is also correct.
- I and II, correct.
- III only, incorrect, This answer choice disregards the doppler effect.
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