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From: John Larkin
Subject: Re: Speaker phase and current vs voltage drive
Date: Sun, 27 Oct 2002 18:25:11 -0800
Organization: Posted via Supernews, http://www.supernews.com
References: <3DBC76E9.F9DA95C2@managesoft.com> <3DBC7B38.C7EB8A43@webaccess.net>
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On Sun, 27 Oct 2002 23:47:38 GMT, Chuck Simmons
>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.
The voice coil positioner is just a linear permanent-magnet motor, so
no-load speed is when the back EMF just about cancels the applied
voltage. Neglecting parasitics, no-load speed is proportional to
applied voltage, and loaded force is proportional to current.
>> 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.
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