> -----Original Message-----
> From: Peter L. Peres [SMTP:plp@ACTCOM.CO.IL]
> Sent: Wednesday, June 04, 2003 11:43 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [EE]: Can a speaker be Input Devices
>
> > distortion in common base preamp
>
> The distortion generated with the small signals encountered will be
> negligible wrt the distortion generated by the speaker proper. There are
> few small speakers that get under 1% distortion and they are unlikely to
> be used as microphones in such a circuit.
>
> The amplitude on the collector will most likely be 100mVpkpk at most while
> speaking directly into the speaker at 10 cm or so. This is a small
> percentage of the output capability and distortion will be low. You can
> model it in SPICE if you like.
>
> 3% distortion is acceptable for speech (some say 10%). A cheap small
> speaker will give 5% distortion at least. Remember that the emf in the
> voice coil depends linearly on the magnetic field in the gap and the field
> depends on the (inverse) square of the apparent gap (wrt voice coil
> position). 1% distortion in this context means gap accuracy of 0.01^2 =
> 0.0001. A 1mm gap would require machining and geometrical tolerance to 100
> nanometers for that. Oops, not your average far eastern tiny speaker. Oh,
> by the way, this assumes everything else is perfect, like suspension,
> damping, centering, coil geometry, and acoustics.
>
> The 0.01% distortion implied by some HiFi amps implies speakers with a
> field that accurate and the gap parts should be machined to 1e-8 which is
> 20nm for a 2mm gap. Re-oops. Better don't look at your expensive speakers.
> 20nm surface quality and geometry is mirror grade (good enough for laser
> mirror I think). And there are no shortcuts I know of here.
>
> One of the less published reasons for which good speakers go for huge
> magnets is this. Big magnets afford a larger gap and that puts precision
> within machinability limits, besides increasing efficiency and
> electrical coupling/damping. However big magnets are expensive. If the
> manufacturers could get to these results otherwise, they would. The
> ferro-fluid gap fillers etc play a role here but I think that an air gap
> is cleaner.
>
> Peter
>
I guess this is where the NTX flat panel speakers, and electrostatics have
an advantage?

Mike


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