As I watched the holiday fireworks recently, I noted the time lag between what I saw — the bombs bursting in air — and what I heard — the pops of the explosions. One sees the fireworks burst a notable time before hearing them, and the reason is obvious: light travels lots faster than sound, and you're far enough away from the fireworks that the sound takes a non-negligible amount of time to reach you, whereas the light gets there essentially instantaneously (the distance is insignificant with respect to the speed of light).
But I also got to thinking of a related thing, which was highlighted when people clapped along with the Sousa marches that the US Army band played. The summary is that I wish people wouldn't do that, because it makes it sound awful for everyone. Here's why:
In the crudely drawn (sorry) diagram at right (click to enlarge), assume that the sound comes from the point source on the stage that's marked with a black dot. Suppose there's a downbeat generated at time 0. The people at the green dots, marked a and b, hear the downbeat at time 1, and they clap then. Those of us at the red dots hear it at time 2, so that's when we clap. The guy in the back at the yellow dot hears it at time 3, and that's when he claps.
Meanwhile, a's clap arrives chez moi at time, oh, say, 2.5; b's and c's get to me around time 3, though probably not exactly; e's reaches me at time 4, and d's shows up at time 5. Give or take. Further, the claps from c and e are the loudest, while d's clap will have faded the most and be the quietest.
The result is that by the time of the next downbeat at time 8, I'll have heard all these other claps that are out of synch with my own (they'll all lag my own clap, which will seem to me (but to me only) to be at just the right time). Just using the example claps, I'll hear them, at varying volumes, at times 2 (mine), 2.5, 3-ish (two of them), 4, and 5... and that's not counting all the other people you see there, who I haven't included in this tally. And my clap, in turn, will annoy others by arriving at their ears with an awkward delay.
In other words, everyone in the audience will hear a muddy cacophony of off-the-beat clap-like sounds that will ruin any sense of musical rhythm, and all the while, everyone's perception is, “It's not my fault. I am clapping on the beat!”
Resist the urge, please. For the good of the music, just don't do it!
[Next up: Why too many people who sing along with The Star-Spangled Banner can't seem to get the words right....]