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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: Speaker phase and current vs voltage drive
Date: Sun, 27 Oct 2002 23:47:38 GMT
NNTP-Posting-Date: Sun, 27 Oct 2002 15:47:38 PST
Clifford Heath wrote:
> I have two large voice coils (3" coil by 4" throw with 50lb magnet)
> from old HP disk drives. Fun - hook up a 1.2 NiCd D cell and the
> coil rises about 1"/second fixed rate. The speed is determined by
> the current, not the voltage, because that's what determines the
> magnetic field strength. They'd make nice subwoofer drivers.
> So it got me wondering about loudspeaker phasing. Presumably the
> instantaneous air pressure at the speaker cone is caused by cone
> acceleration, what with the speed being so far sub-sonic and all.
> We try to build speakers with cones as light and stiff as possible,
> with supports that are as compliant as possible, so in a "good"
> speaker the acceleration comes "mainly" from change of current.
> Apart from the obvious mechanical deficiencies in our speakers,
> I can see many sources of 90 degrees phase shift here, presumably
> also in the recording devices. Do these phase shifts cancel out?
> Does it make sense that our audio amps are designed as voltage
> sources when the voice coils are clearly current devices? Can anyone
> share an end-to-end phase analysis that shows whether our recording
> technologies have a hope of reproducing the acoustic pressure waves
> Clifford Heath
Voice coils are usually spring suspended. Below the spring mass
resonance, the voice coil displacement is nearly proportional to current
(voltage also because the principle electronic resonance is much
higher). Above the spring mass resonance, acceleration is proportional
to current. This is really the nearly linear regime because sound
pressure is proportional to acceleration rather than amplitude or
velocity. Cross over is needed because the speaker may have to vibrate
in a higher mode than the fundamental.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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