BBC Focus

EVERYDAY SCIENCE

hand clap is an odd thing, when you

starttothinkaboutit.Weclapallthe

time, to accompany music, to

communicate approval, to set a

rhythmortoattractattention.Aclap

canbethesharpcrackmadebya

flamboyant flamenco dancer, or the

polite pattering of the unimpressed. But we rarely think

about how that sound is made. Recently, I had to clap to

synchronise cameras repeatedly over a few days.

Inoticedthatthesoundofmyclapvariedalot,even

thoughIthoughtIwasdoingthesamethingeverytime.

So I started to experiment.

I hope that you’re not reading this on a train or in a

library,becauseyou’reprobablygoingtowantto

experiment too and you don’t want to attract grouchy

Paddington Bear stares from those around you.

Startbyclappingwithyourhandsparallelandlevel

with each other, and then move one hand downwards

bit by bit. My claps are noticeably louder when the

fingers of one hand meet the palm of the other. Then go

back to the starting point, and this time rotate one hand

byabitoneachclap.You’llnoticethatthepitchofthe

clap goes down as you rotate. You might also see that

you’re starting to cup your hands, instead of letting

them stay flat. The more you cup your hands, the deeper

and softer the sound. So what’s going on?

It turns out that there are two major contributions to

thesoundofaclap.Whenyoubringtwoflatsurfaces

together, you squeeze the air in the gap between them

outwards. Right at the last minute, when your hands are

almost touching and the gap is very thin, the outward

airflow becomes extremely fast. In addition, your hands

aren’t perfectly flat and so after the first edges have

touched, air has to be squeezed out between the small

gaps that are left, and this adds extra speed. The first

sound happens when that outward flowing air gets very

closetothespeedofsound,possiblyexceedingitfora

tiny fraction of a millisecond. That generates a weak

shock,likethecrackofabullwhip.Thisiswhatthe

flamencodancersexcelat,andthatweakshockisthe

reason that a clap can be so startlingly loud. I’ve found

thatitworksbetterwhenI’vegotwethands,andIthink

that’sbecausetheedgesofmyhandssealbetter,making

oneverysmallgapfortheairtoescapefrom.Afterthat

initial crack, there are some extra oscillations as the air

reverberates around the space between your hands,

but the whole sound probably only lasts for five

thousandths of a second.

When you cup your hands, the sound comes from air

oscillatinginandoutofthecircularspaceinbetween

your palms. The bigger the space, the deeper the note. It

still depends on your hands sealing briefly around the

edges, but you need a small gap to be left for air to move

inandout.Thisdoesn’tmakeasharpcrack,becauseit

comes from air flowing in and out a few times, so the

soundofthisclapwilllastabitlongerthanthesharp

crack of the weak shock.

It’s astonishing that such a simple movement is

capable of generating airflow (however brief) that is

HELEN CZERSKI... WHY ARE HAND CLAPS SO LOUD?

faster than the speed of sound. But

this is what generates the loud

flamenco clap.

I’d better leave you to apologise to

the people around you, now that

you’ve almost certainly got their

attention. I recommend telling them

t hat it’s all in t he na me of science.

Feel free to blame me!

A

ILLUSTRATION: DARIA SKRYBCHENKO

Dr Helen Czerski is a physicist

and BBC science presenter. Her

book, The Storm In A Teacup, will

be out in November.

NEXT ISSUE: WHY DON’T ALL

CERAMIC BOWLS GET REALLY HOT

IN THE MICROWAVE?

“THIS SIMPLE MOVEMENT IS CAPABLE OF GENERATING

AIRFLOW THAT IS FASTER THAN THE SPEED OF SOUND”