The physics of hand clapping | Hacker News Reader

The physics of hand clapping

physicsworld.com

78 pointsRafelMri30 comments

>As two flat hands collide, the air between them is forced out increasingly quickly, ultimately exceeding the speed of sound. This creates an abrupt pressure change, resulting in shock waves that make up a large part of the noise we hear

So the clap is a mini sonic boom between your hands?

That sounds surprising to me, too, because I can vary the loudness by moving one hand faster or slower towards the other. My intuition says that a sonic boom is a "catastrophic" event, so there should be a threshold. I'm wondering how they measured that.

The most interesting part of this statement is that you physically move both hands to clap… I let my dominant hand “hit” my other hand… is that not how everyone does it lol

I mostly move both, but vary a bit depending on intensity and frequency. It's faster to clap against, but I get more sound moving both. It also spreads the effort, so I prefer it for low intensity clapping.

I think you'll be ok, considering at least one person does this: https://twitter.com/MetroUK/status/748854767938473984

Look around sometime. I never see everybody clapping the same way. There are tons of variations, including what you do, and plenty of people use them.

The sonic boom comes from the object going faster than its own soundwave and crossing it, this just seems to be the product of a big pressure differential between the clap and ambiant air

I do not believe there is a literal "sonic boom" when clapping. It's possible to cap very slowly and still make a small noise and I don't believe my slow-clapping is pushing air around at 343 meters per second.

Why not? The size of the channel it has to move through goes to zero as your hands meet, but the volume reduction per second is roughly constant. That means the velocity goes to "infinity."

Finger snapping is fascinating too. I always assumed the sound happens when your finger rubs quickly against your thumb. But it’s actually after that, when your finger strikes the fleshy part of your thumb. You can verify this by blocking your finger from striking your hand.

So it’s actually more like finger clapping. The “snap” part just lets you build energy for the “clap” part. Maybe obvious to some but I amazingly only learned this later in life.

I’m curious how finger snapping started. Someone must have been the first and thought it was useful. I can’t remember when I learned it myself, i must have been very young when i was taught how to do it.

Edit: and then there is Beshkan, which TIL.

Just as side note, I clap as in figure A2.

However, it's one of those things in life where I don't remember when or why I started doing it like that. Surely I didn't clap like that before my earliest memories (3 or 4 yo).

What's with all the fake science content lately trying to claim all sound is caused by sonic booms?

Interesting! Presumably you have a source backing up the claim that the article and paper it’s based on are fake?

Try this: do a small clap. Then do a big clap. Now slap the table, chair, or wall you are near. All of them are obviously not sonic booms. QED.

You can make sonic booms with bull whips. They are quite a different sound than just slapping and clapping. Check them out on YouTube.

Just because the thing you hit continues to vibrate and emit sound after it is struck, doesn't mean there wasn't a small sonic boom caused by the air escaping the contact surface.

And a supersonic plane flying overhead doesn't sound like a bull whip. Therefore supersonic planes are a lie, QED.

For one, I can't move any part of my body at supersonic speeds.

It's a good thing that's completely not what is being claimed then.

I can't move any part of my body at the speed of sound, and yet - somehow - I still manage to produce vibrations that move at the speed of sound.

It's a good thing that pushing air at the speed of sound is not how you make sonic booms. You need to physically move an object at a supersonic speed to make a sonic boom.

So when you cast doubt on the claim that claps do not involve sonic booms you then have the burden of proof to demonstrate that a hand has supersonic motion. Please provide your argument and stop wasting time with pointless rhetoric.

> You need to physically move an object at a supersonic speed to make a sonic boom.

What do you think the object is pushing? “Speed of sound” is relative to a medium, it doesn’t happen _unless_ you are pushing air around.

It’s funny, this entire thread is literally “Please provide a source for this claim”, for which you are the one only providing rhetoric.

You think a gentle clap is pushing air around at 343 meters per second?

Well, not around. The sound of a clap is travelling exactly with the speed of... sound. But not one bit faster, of course.

I dunno, unlike - apparently - you, I'm not an expert on the last few microseconds of the microfluidics of what happens to small volumes of air suddenly evacuated from a hand-sized area under constant compression.

I take it that this means your source for refutation is "Lack of imagination"?

Why would the air move 1000X faster than the hands are moving?

You don’t need air moving at the speed of sound to create noise. I can gently scratch a table surface and create noise and it’s not because of a sonic boom happening.

If I gently tap my fingers together slowly in a quiet room I can hear the sound. It’s not a sonic boom clearly. Neither is a sonic boom happening when I scratch my elbow and can hear it.

Still doesn't explain the sound of one hand clapping.

Half-sonic boom.

I didn't get this was a koan until I realized that most people cant clap one handed.

Well, in some countries, instead of clapping two hands, people clap on tables or armchairs.

One of the few articles you’d write and AI stock photos wouldn’t be helpful

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