[Physics] How does sound propagate up a tall building


I live in a flat in a 22-storey tall building. It is a common observation that loud people at level 1 can still be audibly heard even at this height.

Could someone suggest a reason for this counter-intuitive observation? Doesn't sound intensity decrease linearly with the square of the distance from the source? Is it possible that the building is acting as a surface for the sound to bounce and echo off?


Best Answer

I think that the echo idea (with a bit of finesse) is the best bet. As the original sound propagates as a sphere, as you suggested, every place it touches becomes a transmission point for another spherical wave front; this is basically Huygens Principle. The sound bounces back and forth among all the surfaces in the area creating echos ad infinitum - these are called multiples. Fresnel explained how interference of these waves could produce a coherent signal at another location. Because there are so many multiples, constructive interference compensates for some of the spherical energy loss - thus you can still hear it on your balcany.

Sound and light are sufficiently analogous at this level for the principles to work for both.

This article has some great illustrations and more information: https://en.wikipedia.org/wiki/Huygens-Fresnel_principle