> It always mystified me why we drive a device that converts current into
> sound pressure, with a voltage waveform.

I thought that too a long time ago -- until I tried it.  It didn't sound as
good.  Then I did some measurements and found the frequency response was
worse than when driven with a fixed voltage.  Then the obvious finally
became obvious to me as to why this is.

True, the force pushing the center of the speaker cone is directly
proportional to current, not voltage.  However, you hear the result of the
displacment of the whole speaker cone, not the force applied to it.  The
mechanical system has relative resonances and dead spots a various
frequencies.  When a resonance occurs, the speaker's impedance goes up, and
the inverse happens at a dead spot.  With constant current drive, when the
impedance goes up, more power is delivered, thereby exaggerating these bumps
in the frequency response.  With a constant voltage drive, the power goes
down as the impedance goes up, which compensates a bit for the bumps.

Sometimes the road less traveled is less traveled for a reason.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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