# [PD] overdriven speaker

Martin Schied crinimal at gmx.net
Thu Oct 21 03:14:50 CEST 2010

```  Hi!

Sorry for answering this late. And I was wrong in my last mail. The
signal doesn't have to be differentiated but integrated (like you

The signal in pd represents the current flowing through the speaker's
coil (if we assume it isn't capacitive or inductive load), producing an
acceleration equivalent to the current on the speaker cone. The peak
output voltage of the amplifier is equal for all frequencies and defines
the maximum acceleration the cone can experience. So we can say the
acceleration is

a(t)= a_max * -sin(w t)             // w stands for omega = 2 * pi * f,
a_max is the (peak) amplitude

If you want to know the speed you have to differentiate it by time:

v(t) = a_max * 1/w * cos(w t)

and for its travel:

x(t) = a_max 1/w² * sin(w t)

so the cone is moving faster for low frequencies (1/f) and also has more
travel (1/f²).

It shouldn't be too hard to do this integration with basic pole / zero
objects. A problem using integration only is the lack of mechanical
damping. A real speaker goes back to x=0 if no signal is present. A
simple integrator doesn't - so the 'simulated' cone would just fly away
slowly. So some damping should be included in the integrator to make it
stable. However I'm no expert on designing filters yet...

Looking at power and air pressure - we don't have to care about them as
long as we don't want to include thermal effects or nonlinearities of
the air I think. The pressure directly in front of the cone is related
to the acceleration I think, but I'm not sure about that. Can anybody
confirm that? I think that's not trivial to answer anyways, because
already 10cm farther from the speaker the pressure and air velocity are
different. The power from a 1 kHz sine and a 2 kHz sine are the same
anyways, so  why care...

cheers
Martin
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.puredata.info/pipermail/pd-list/attachments/20101021/58c6358b/attachment.htm>
```