Question

a speaker sliding across a table plays a sound with constant frequency, which is represented by the wavefront diagram below. there are stationary observers at points a, b, c, and d.
which observer is the speaker traveling toward?
○ a a
○ b b
○ c c
○ d d

Explanation:

The Doppler effect for sound waves: when a source (speaker) moves towards an observer, the wavefronts are compressed in the direction of motion, meaning the distance between successive wavefronts (wavelength) is shorter in that direction. Looking at the wavefront diagram, the wavefronts are closer together near observer D, indicating the speaker is moving toward D (since compressed wavefronts mean the source is approaching that observer).

Answer:

d. D