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From: Clifford Heath
Organization: Open Software Associates Limited
X-Mailer: Mozilla 4.61 [en] (X11; I; Linux 2.2.17 i686)
Subject: Speaker phase and current vs voltage drive
X-Cache: nntpcache 2.3.3 (see http://www.nntpcache.org/)
NNTP-Posting-Date: Mon, 28 Oct 2002 10:29:45 EST
Date: Mon, 28 Oct 2002 10:29:45 +1100
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
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