What causes the sound (snap) when you snap a towel?

It is supersonic…

“Traditional simple means of generating shock waves are examined by high-speed imaging. A leather belt is folded upon itself at mid-length and the ends are grasped firmly in each hand. When pushed together a loop forms, and when quickly pulled apart the loop closes rapidly, producing a sharp ``crack’’ similar to the cracking of a whip (Shock Waves 8(1), 1998). The towel-snap mimics whip cracking by causing the towel end to rotate supersonically. We investigated these phenomena using a high-speed digital camera (10k and 30k frames/sec, 4 microsec exposure) and a sensitive schlieren optical system of 1m aperture. Results show that compression of the air between the two rapidly-approaching leather belt bands first causes a spherical shock wave to form near one hand. The compression then runs along the belt length toward the other hand at supersonic speed, producing an oblique shock wave that is responsible for the audible crack. In the towel-snap, shock waves are visible from tip motion in open air as well as from the compression due to snapping the towel against a surface. There are no known useful applications of these simple phenomena, but they do address how weak shock waves can be generated by muscle power alone. Several other related examples are also mentioned.”

courtesy of NASA

That explanation is exactly right. It also explains similar phenomena like the crack of a whip.

It’s a tiny Sonic Boom !

Is the explanation the same for the snap heard when you whip your thumb and middle finger across each other with some force? Is there a reason I can only generate the noise with my left (non-dominant) hand?

@gailcalled Nope that’s a percussive strike ! I doubt you could move your fingers above Mach 1

http://www.fluther.com/disc/5822/what-makes-the-snapping-sound-when-you-snap-your-fingers/

@gailcalled Is the explanation the same for the snap heard when you whip your thumb and middle finger across each other with some force?

No, it is more like a hand clap – the noise is the impact, it is not created by supersonic movement.

I do remember the earlier question, now that @Pretty_Lilly reminds me.

So why can’t I get the same result with either hand? (I can cock only one eyebrow, if anyone cares.)

@gailcalled So why can’t I get the same result with either hand?

Usually it is simply that the dominant hand has far more dexterity. It needs a degree of dexterity to do it correctly.

I can cock only one eyebrow, if anyone cares.

(Guys always raise an eyebrow when a girl talks about “cock” and skill in the same sentence.)

@DarkScribe : But I get results only with my non-dominant hand (and non-dominant eyebrow).

I can penis only one eyebrow? That has a nice ring to it. Can you put rings in penises?

I’m waiting for an unpredictable answer. What was the question?

@gailcalled But I get results only with my non-dominant hand

That’s ok, I couldn’t imagine you as a Dominatrix – the boots and whip yes, but where would you put the flower vase?

Well, I consider us a team of Puss and Boots