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How does a wall make things louder? (One of the laws of busking.)

Imagine that a source of sound with power P is sitting on hard ground, in the open air, radiating sound uniformly in all directions. Let's assume that the ground absorbs no sound. Now draw a hemisphere centred on this source, with radius r. All of the power P passes through the surface of the hemisphere, whose area is 2πr2. So the intensity at distance r is I = P/2πr2. Let's now move the source against a perfectly reflective wall: it's now radiating through a quarter-sphere, with area πr2, so its intensity (P/πr2) has been increased by a factor of 2, so the sound level is increased by 3 dB. Now move into a corner, still on the ground. The power nnow passes through one eighth of a sphere, so intensity is now another factor of 2 higher, and the sound level increased by further 3 dB, which is 6 dB higher than the open air.

Okay, now what about if you're not on the ground? Then we run into inference and filtering, which are discussed above. For sufficiently long wavelengths (low frequencies) the '3 dB for a wall' still works, but it becomes more complicated for high frequencies. Further, some surfaces absorb high frequencies nore than low, so the high frequency gain may not be as high.

I've been asked about the other laws of busking. Well I've not busked for quite a while but some of the important things are (2) smile, energy and eye contact (look like you're having fun), (3) duos and (4) nucleation. Why duos? My experience is that a duo makes more than twice as much. Partly because of the interaction: there's something good and live going on betweenthe two of you, and passers-by find that interesting, even enviable. Further, it's easier to keep your energy and spirits up, and easier to take a breather. Nucleation: if you can get three people to stop, others are more likely to stop. ('Hey, this must be worth stopping for'.) If you can ask friends or family to drop by and stop for a while to nucleate a crowd, it can be really helpful. Conversely, if you like a busker, then stopping to help nucleate might be more valuable than any coins you can spare. Source: Busking http://newt.phys.unsw.edu.au/jw/musFAQ.html#wall