The second objection concerns typical acoustical effects, like the Doppler effect, which are perfectly accounted for by appealing to (medial) sound waves. The Doppler effect is a shift in frequency of the sound heard by an observer who moves relative to the sound source. As waves in the direction of movement are compressed, and waves in the opposite direction are expanded, the frequency drops dramatically when the hearer and the source go past each other. Such explanations of the Doppler effect