🚀 go-pugleaf

I have been playing with a largish linear motor, basically a voice-coil
from an old washing-machine sized HP disk drive, and its behaviour started
me thinking.  This motor weighs about 22 Kg (50Lb) and the coil has a 2
Ohm impedance.

I connected 1.2 Volts, expecting to see the coil *accelerate* along its
15 cm (6") throw, but instead it moved at a steady rate determined by
the input voltage.  Ok, I understand that - the motion induces a reverse
EMF that exactly counters the input voltage (inertial effects
notwithstanding).

Consider this effect in a speaker - a rising voltage corresponds to
acceleration and hence sound pressure.  AC voltage peak means peak
acceleration and hence peak sound pressure (once again, inertial effects
notwithstanding).  This is sensible in retrospect, but I had always
considered that peak cone excursion (not peak acceleration) would occur
at the voltage peaks so it was a surprise.  Obviously speakers will
self-centre, so there must be some spring constant in play too.

Now consider the recording end.  A moving coil microphone with equivalent
mechanical characteristics will respond in a matching fashion (meaning
accurate reproduction is conceivable), but not an electret.  The electret
measures diaphragm *displacement* (capacitively), not rate of displacement,
doesn't it?  Won't this cause phase shifts and frequency response anomalies
etc in the reproduction?  Or is it that the diaphragm displacement is a
force measurement (sound pressure against an hopefully linear spring
constant) that the effect is the same?  And in this case, the moving-coil
microphone had better have a very small spring constant or else it measures
the sum of two functions:  pressure against the spring constant and actual
sonic displacement.  These two functions are 90 degrees out of phase as I
have described.

It seems to me that the balance of the displacement and acceleration
functions should match from the recording to the reproducing device.

Thoughts?

I had vague ideas of using the linear motor as a driver for a servo-
controlled subwoofer... after all it was made to deflect a head assembly
weighing two hundred grams up to 10 cm and stop to an accuracy of under
.05mm within about 10 ms, and this it has done more or less continuously
for the last ten years.  Its continuous power handling must be over 600
watts, and its linearity and long throw make it an attractive idea.

Clifford Heath,   cjh@osa.com.au,         +61 3 871-1694    _ _
Open Software Associates Limited,              fax -1711   / / \
Box 401, Ringwood, Victoria, Australia 3134.               \_/_/