|MadSci Network: Physics|
I think the most likely explanation is that the radio report you heard came on a day early: it must have been intended as an April Fools prank! I have definitely myself heard a duck's quack echo. The radio report is quite implausible anyway. Firstly there are a few hundred different species of duck around the world, and at least that many different species of quack. Does the report mean that all of this variety of noises will not echo, while the croak of a frog, or a squeak on a clarinet (both of which can be more like ducks' quacks than quacks are like one another) will echo? Echoing is caused simply by the reflection of sound waves off a hard surface. How good an echo is depends on the hardness and shape of the reflecting wall, and on the frequency spectrum of the sound. Usually higher frequency sounds reflect better (which is why the echo of a voice often sounds more like a whisper -- a voice without the low frequency components). It could be that a duck's quack is a noise that does not have a lot of high frequency components, and so does not reflect particularly well. It could be that the frequency spectrum is changed when the duck's quack echoes so that the echo does not sound much like a quack. I would believe that the echo of a duck's quack is much fainter than, say, the echo of a whip crack or a rifle shot. But I know that duck's quacks, like any other sound, can and do echo.
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