# B Why do things produce sound even though no pressure is applied to them?

#### leoneul

I understand that sound is a propagating pressure through a medium. We produced pressure and it’ll propagate through the medium as “sound”. How can we use this model to understand everyday-life experiences like opening a chips packaging and the packaging makes a sound, and alike. In the chips example, where does the pressure even come from so that the sound we hear is produced?

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#### andrewkirk

Homework Helper
Gold Member
This is just an educated guess:

The crinkly plastic of a chip packet ('crisp packet' to our UK users) is a bit stiff, so that it does not deform smoothly like a piece of soft cloth does. Accordingly, when we bend it with our hands, as we do when opening it. it resists briefly and then gives way in a triggering sort of movement. A triggered movement is much faster than a smooth one, and hence will compress the air it is moving towards much more than a slow gradual movement will. That brief but fast compression creates a high-pitched pressure wave that we hear.

The crinkling sound we hear when somebody is handling a packet of chips is the combination of many pressure waves from lots of little triggerings, as the packet briefly resists and then triggers in many different places.

#### davenn

Gold Member
I understand that sound is a propagating pressure through a medium. We produced pressure and it’ll propagate through the medium as “sound”.

Not quite .... Sound is a propagating pressure wave in a medium
It is mechanical wave generated by any mechanical operation, from the slipping of rock on a fault line
causing seismic P waves, to a hammer striking metal, to your vocal cords vibrating to produce sound waves

Dave

#### sophiecentaur

Gold Member
where does the pressure even come from
If any surface moves against a body of air, then there will be a force against the resistance of the surrounding air (i.e. pressure as the air is slightly compressed), locally. In the long term, the air will flow and equalise the pressure. The step increase in pressure will propagate outwards and gradually disperse. The individual small parts of a crisp packet surface will move in small steps which will produce many small pressure peaks (whereas a piece of cloth will tend to flow silently). We hear those peaks as a crackling sound.

#### Periwinkle

We produced pressure and it’ll propagate through the medium as “sound”.
Feynman - perhaps not to say in which book - in the second paragraph of chapter 47-2 describe the propagation of sound in a manner described above.

How the strange sound is created can be thought of, but in its own way, every voice is weird.

#### davenn

Gold Member
Feynman - perhaps not to say in which book - in the second paragraph of chapter 47-2 describe the propagation of sound in a manner described above.
seriously too complex for a B level thread .... Half of it was even over my head

"Why do things produce sound even though no pressure is applied to them?"

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