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From: "Al Hephy"
Subject: Re: Speaker phase and current vs voltage drive
Date: Mon, 28 Oct 2002 10:05:24 -0500
Organization: Posted via Supernews, http://www.supernews.com
Reply-To: "Al Hephy"
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
When the speaker cone is moving forward, there is air pressure wave in =
front, but reduced pressure in back. =20
At low frequencies, these are out of phase and 'leak' around the cone to =
cancel each other out.
That's why baffles are needed. The air path from front to rear must be =
large compared to the wavelength in air. =20
Baffles were much studied in the early days of 'high fidelity' and was =
an art as well as a science.
I'd guess that your disk drive coils would be an overkill even at =
Perhaps some experiments at triggering earthquakes are in order. Are =
you in CA?
Clifford Heath wrote in message =
> 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
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