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Wed Mar 16 00:04:01 EDT 2011

## I don't know sound: 1. Pressure

```I think I don't understand pressure.  It's a statistical measure that
represents force per unit area in a collection of moving point-masses.

Note that it is really the force that is averaged:

Take a very small piece of area such that it gets hit infrequently.
The force necessary to keep it in place has a pulse-like nature due to
collisions: force is high when we're "pushing back" on an incoming
molecule, and zero when there is no contact.

Therefore it is simpler to go up and down the integral stairs once:
don't look at force but at impulse transfer per collision (integrate)
and then look at the number of collisions per time unit
(differentiate).

From this perspective, why is it that a speaker driver (i.e. a flat
surface moving in a direction perpendicular to its surface plane) has
high impedance, meaning it produces mostly pressure and no
displacement?

```
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