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Many years ago I heard a radio broadcast featuring a beeping sound that always seemed to come from behind me. The announcer said that the sound would have this quality and it did, even when I turned around. It was a plain beep, and the radio only had one loudspeaker. It was a complete mystery to me then and it still is, so can anyone explain the effect?

(Previously posted at https://www.newscientist.com/topic/lastword/its-behind-you/.)

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The answer can be found at the sound stackexchange - if you take a pure tone and reverse the phase of one of the stereo channels, there is no "sensible" direction in front of the listener that the sound could come from. We then conclude that the sound comes from behind us (because we have poor ability to figure out the direction of sounds behind us because of the shape of our ears)

It's mostly a perception effect, not physics.

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  • $\begingroup$ But the radio definitely only had one loudspeaker. I can remember that very clearly. (Sorry for posting in an unsuitable forum - should I move it to sound stackexchange?) $\endgroup$
    – user105814
    Commented Feb 2, 2016 at 12:16
  • $\begingroup$ In that case it is probably a clever use of the HRTF filter - see the linked posts for more details. Yes maybe the sound exchange is better as this is sound perception more than pure physics. I will flag the question to be moved... $\endgroup$
    – Floris
    Commented Feb 2, 2016 at 12:18

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