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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?

Martin Harvey, London, UK

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This article appeared in print under the headline “It’s behind you”