My seven-year-old was delighted to figure out how to snap her fingers. I'm talking about the hypnotist-waking-you-up kind of snap, not cracking your knuckles. She wanted to know what caused the sound.
What causes the snap?
The most plausible answer I could find was that the sound comes from the pad of your finger tip smacking against part of your palm near the base of your thumb. You can see a high speed video of a finger snap and see the palm rippling in slow motion, although it doesn't actually have the sound to go with it.
Some people think the snap comes from the friction of your fingers rubbing quickly. But if you isolate that action by snapping with fingerless mittens or something to dampen the finger smacking the palm, you don't get much of an effect.
Some other people think your fingers even move faster than the speed of sound! The crack of a whip or a snapped wet towel does go fast enough, but the dampening test for snapping also applies here. Also, you can calculate from the finger snap video that it moving nowhere near fast enough.
What affects the loudness of the snap?
A post on how to snap your fingers suggested your non-dominant hand would produce a louder snap. I would have thought your dominant hand would be stronger and produce a louder snap. Some people I asked couldn't snap at all with their non-dominant hand. Right-handed people tend to have stronger right hands, but lefties tend to be more even handed. I wonder if some people have stiffer joints in their non-dominant hand that might make it louder.
I have found that a wet hand results in a louder snap. This may be helpful in places that prefer finger snapping to hand clapping so you can use one hand to hold a drink. It might allow your finger and thumb to grip better and build up more pressure before the snap, like certain earthquakes. And/or maybe a wet surface vibrates more on the smack.
The Guinness World Record for the loudest finger snap is 108 decibels by Bob Hatch in California, in 2000, which is considered comparable to a rock band loud.
What affects the pitch of the snap?
I was expecting some difference related to finger strength and muscle mass affecting the resonance. In a lame little Facebook survey, more of my friends indicated the right-handed snap sounded higher. I think my right might be slightly higher, but I am left handed. So I don't know what to make of this, although there is that even handed thing. I would expect that people with bigger palms and maybe longer fingers would have a lower sounding snap. A musician I know noted that curling your remaining fingers also affects the tone of the snap.
Seems this subject requires further study. What are you waiting for, snap to it!