A very clever first-year physics student I know, who had just been learning about the Doppler effect, asked me a question. If a plane were flying a straight trajectory at Mach 2, playing a song on its speakers very loudly, and an observer with incredibly sensitive recording equipment were to position themselves in the wake of the plane just after it had passed, would they hear the song playing backwards? There are a number of interesting aspects to this question. One that I found myself puzzling over was whether one could hear any sound at all from the plane itself, other than the sonic boom as it passed. I am not interested in questions of relative loudness, such as that the roar of the engines would drown out the song. One imagines that, with the right equipment, one might be able to separate the two waveforms and detect that of the song - at least in theory, if not in practice. What I was wondering was whether the sonic boom phenomena around the plane actually prevents one from detecting any sound wave emitted by the plane itself. What made me think that was that behind the plane there will be a near vacuum, preventing the transmission of sound backwards. In front of the plane is the very high pressure front of the sonic boom, and any wave that would otherwise be transmitted forwards is immediately destroyed by the plane crashing into it. Maybe sound can only escape in directions that are close to perpendicular to the plane's direction of travel - away from the rear vacuum and out of the way of getting smashed by the plane. But in that case one would only detect the sound very briefly as the plane passed directly overhead (again ignoring issues of volume). My initial reaction was that of course that is wrong because a ground observer hears the sonic boom and then hears a lower, continuous roar. But I thought that what we might be hearing there is not the engines themselves, which are shielded from us by the vacuum/boom-front, but the noise of the jet stream that is fired backwards by the engines, as it hits the air behind the vacuum. Because that collision occurs outside of the sonically shielded region immediately around the plane, it is free to propagate and so we can hear it. Anyway, my question is this: Could sound produced by a plane - such as if it had a very loud police siren blaring - be heard away from the plane with suitably sensitive equipment if the plane were travelling supersonically?