Roman Black wrote:
> David VanHorn wrote:
> >
> > At 06:33 PM 7/29/01 -0400, Olin Lathrop wrote:
> > > > 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.
> >
> > Interesting.
> > I must have lucked out with my speakers.
>
>
> I thought about doing closed-loop speakers, ie
> using a CD player laser or similar to measure
> cone travel, then using a high speed digital
> PWM to move the speaker cone to the precise
> position. More like a high speed voice coil servo
> than an analog speaker.
>
> Has anyone else done this or know anyone working
> on it??


If I remember correctly Philips did a system with specially constructed
speakers with 2 voice coils, one doing the driving in the normal way and the
other used to provide motional feedback. This was (is?) also a technique
used in sub-woofers in High End Car Audio Systems (JVC, I believe). It
seemed to be fairly effective in moving the small finite air mass within a
vehicle.

Bouncing a laser beam off a small mirror glued to a speaker cone should work
for bass speakers. I have used that technique with a rotating mirrors to
project an oscilloscope type display on a wall.

Regards

Chris Carr

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