A car sounding a horn of frequency moves straight towards a stationary listener who…
PHYSICS · Single correct · AP · Physics 2: Algebra-based · Waves Sound and Physical Optics · Waves Sound and Physical Optics
Problem
A car sounding a horn of frequency 400Hz moves straight towards a stationary listener who is simultaneously sounding a tuning fork of the same frequency. The listener hears 6 beats per second. Which statement best explains why beats are produced even though both sources have the same frequency at rest?
(a) Air currents produced by the moving car slow down the sound waves, so the horn frequency appears slightly lower than that of the fork.
(b) Sound from the horn reflects off the road surface and combines with the direct sound, producing a second frequency that beats with the fork.
(c) The motion of the car increases the frequency of the horn as received by the listener, creating a small difference from the fork frequency.
(d) Beats arise because the horn is much louder than the tuning fork, and the ear interprets the alternating loud and soft sounds as beats.