Tuesday, July 09, 2013
Good screens make good neighbors?
A local columnist raised a random question, triggered by his observation that lots of windows are open at night in this weather: Have you ever been awakened by snoring from a neighboring house? I thought about it for a moment and realized it was fairly interesting after all. I've never heard any sounds from inside a neighboring house! I hear plenty of sounds from neighboring yards, but never from inside. (Apartments are a different matter. Walls and floors act as diaphragms to transfer ALL sounds directly from one apartment to another!)
What's going on? I wonder if there's a sort of Helmholtz resonator effect that prevents sound from escaping easily through a small hole like a window? In order to get out, the waves have to bend and diffract through the hole, which uses up some of their energy. Sound inside House A loses energy on exiting through a window, then spreads out through the air, then loses more energy on entering House B through a window. And screens would cause extra diffraction of higher frequency components.
While I was thinking, I noticed this work in Materials Science that agrees with the idea:
An important factor in this is how efficiently the sound can get into the chamber and here Kima and Lee have another trick. To maximise this efficiency, they drill a 50 millimetre hole through each piece of acrylic. This acts as a diffraction element causing any sound that hits the chamber to diffract strongly into it. The result is a double-glazed window with a negative bulk modulus that strongly attenuates the sound hitting it.They were using diffraction the other way around, trying to guide sound into a chamber where it can dissipate, but the basic point is the same. ¶ 3:16 AM
