From: DPierce@world.std.com (Richard D Pierce)
Newsgroups: rec.audio.high-end
Subject: re: Time Alignment
Date: 20 Jan 93 16:52:51 GMT
Organization: University of Wisconsin - Milwaukee
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>I was reading a book on Hifi (Good Sound, by Dearborn) which gave an
>explanation of time aligning speakers.  It implied that this entailed
>always putting the speaker magnets in the same plane because this is where
>the sound originates.  The reasoning struck me as faulty for the following
>reason:  If the voice coil is reasonably stiff, the cone will move at the
>same time as the voice coil.  There would not be an appreciable lag before
>the cone moved.  As a thought experiment, imagine a speaker with a ten
>foot infinitely stiff voice coil former (or whatever the tube the coil is
>wound on is called, with the coil wound at opposite ends from the speaker
>cone.  According to the align the magnets theory, the coil of this speaker
>should be aligned with that of the other speakers.  But if the cone starts
>moving simultaniously with the coil (which it should if the former is
>stiff) this will give sound waves coming from the speaker a ten foot head
>start on those coming from other speaker drivers, thus not time aligning
>them. 
>
>My question:  Is my reasoning here flawed?  I'm not an engineer by 
>profession, so there may well be something I'm missing.

Your reasoning and your assumptions are both flawed. Take for example your
though experiment, which includes a "ten foot infinitely stiff voice
coil". An infinitely stiff voice coil simply does not exist. All materials
(air included) have a finite propogation speed. In addition, since the
devices in a driver (voice coil, adhesives, cone, dust cap) are far from
infinitely stiff, and because they have real mass, there is a delay
associate with pushing on something thats flexible, then having it push on
something that has some mass. Such is the case with loudspeaker drivers.

There are further sources of delays. For example, the crossover (be it
active or passive) has electronic delays as well.

The net result is that the cone not only does not start moving
simultaneously with the voice coil, it doesn't even move uniformly either.
The voice coil is often the least of the problems. There exist
sohpisticated measurement techniques that even allow us to visualize the
"impulse" radiating outwards from the voice coil to the edge of the cone.
If the driver designer has his act together, then the cone will be
designed so that the propogation velocity in the cone material corresponds
axially to the propogation velocity in air. This is a tough trick to pull
off, considering the mountain of variables that control the cone's
behaviour (for example, the propogation velocity in paper cones is
dependent upon relative humidity, in some plastices, it is temperature
dependent, and so on).

In any case, it is not the magnets that will be aligned, but the voice
coils. Then they need to be offset accordingly due to delays from the
crossover, delays in the materials themselves, and so on.

|                Dick Pierce                |
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