# Hearing a sound backwards because of Doppler effect

Consider a supersonic plane (mach 2) aproaching a stationary sound source (e.g a fog horn on a boat).

If I understand it correctly, the passengers in the plane can hear the sound twice. First at a 3 times higher frequency, and then (after they passed the source) a second time at normal frequency but backwards. None of the textbooks or web sites mention this backwards sound. Yet I am quite sure it must be there.

Am I correct? And if so, is it actually observed (e.g. By fighter pilots) and why do textbooks never mention this?

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I doubt that fighter pilots can hear anything happening outside the plane (just because it's so loud), and I can't think of any other supersonic motion, so I wouldn't be surprised if this effect has never been observed. But it's an interesting question. – David Z Sep 19 '11 at 22:38
FYI Gareth Loy's Musimatics book mentions it at pg. 230; one gets it from Doppler shift eq. $f_d=f\frac{v_s}{v_s-u}$ (in 1d), where $v_s$ is sound speed, $u$ is the emitter's relative speed and $f$ the frequency being emitted if $u>v_s$. – eudoxos Sep 20 '11 at 9:12
i don't think this reversal of sound would take place. – Vineet Menon Sep 21 '11 at 4:59
@Vineet Menon: fully irrelevant what you think unless you give an argument. – eudoxos Sep 21 '11 at 18:15
@David Zaslavsky: how about moving surface wave source on water? (to be sure, I really mean surface waves, not acoustic waves; Doppler effect is the same) – eudoxos Sep 21 '11 at 18:17