From: Richard D Pierce <DPierce@world.std.com>
Newsgroups: rec.audio.high-end
Subject: re: Time Alignment
Date: Sun, 24 Jan 1993 18:53:15 -0500 (EST)
Organization: University of Wisconsin - Milwaukee
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Originator: tjk@csd4.csd.uwm.edu


John Lipp writes:

>?>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.
>
>You are right that this is non-sense.  Sound is the excitation of air
>particles, so sound originates where the speaker diaphram (or cone or
>whatever is moving) contacts the air.  However, the effective sound
>"center" which you try to align in time aligned speakers can be very
>difficult to determine.  Consider the cone of your standard speaker
>driver.  Sound is originating all over the cone which does not have a
>unique center.  Do you use the average of the distance the dustcap is from
>the front and where the outer edge of the surround?  Or is it the
>geometric mean of these two.  And what about cone resonances? 

No, John, you are wrong. It makes perfect sense.

There is indeed a unique single position (which is indeed frequency
dependent) which can be considered the acoustical center
of the driver. If you sum all of the wave paths from all points in the
cone to the listening position, the result will be a single averaged path
to a single virtual point. For the most part, it will NOT be somewhere
between the dustcap and the edge of the cone, it will not even be on the
cone anywhere at all. It will be somewhere down near the mechanical center
of the voice coil/magnet system, since that's where the impulse
originates. 

Imagine, if you will, the voice coil moving the apex of the cone. The
entire cone does not move instantly, but the movement essentially travels
outward from the apex at some speed. This speed is usually much faster
than the veolcity of sound in air. But the cone is angle (and that angle
is somewhat dependent, when the designer knows what he is doing) on the
actual propogation velocity in the cone material. So while the motion is
propogating along the cone towards the edge, it's also launching a wave
that is, for the most part, keeping up with the cone. S0 where's
the origin, the acoustic center? Where it was started, in the voice coil.

In an ideal situation, the base angle of the cone should be the arctangent
of the ratio of the propogation velocity of air to the propogation
velocity of the cone material. Thus, to form a flat diaphragm, where the
angle is zero, requires a cone material with an infinite propogation
velocity. A material with a propogation velocity equal to that of air (342
m/s at STP) requires a cone with a base angle of 45 degrees. 

Secondly, you completely ignore the fact that for most drivers, you're
operating at frequencies where the wavelength is substantially larger than
the depth of the cone anyway. Thus the effect I described above and your
claim of stuff being radiated all over the cone is really insiginificant,
since the cone, as far as a wavelength is concerned, is one piece and
effectively flat. It's only at high frequencies that the effect I describe
above is valid. 

The whole issue of measuring acoustic centers is something that was well
covered by Richard Heyser 20 years ago. The phenomenon is actually well
understood by those who care to take the time to research the relevant
literature or actually measure the drivers (I've done both). It most
assuredly is not the black-magic, empirically based guesswork that you
seem to imply.

>The point I am trying to make is that by physical inspection it is hard to
>find the acoustic center of a driver.  In general, it has to be measured
>and can be a function of frequencies due to resonances and nulls in cone
>response (some high quality drivers are intentionally designed to exhibit
>specific resonance and null patterns and acheive superior performance; do
>not conclude that these phenominon are always bad). 

Well, in one sense, you are partially right. In fact it's impossible to
find the acoustic center by inspection. Why on earth would you presume to
even do so? It's like looking at a driver and determining by simple
inspection what the Thiele-Small parameters and the 30 deg off axis
response curve is (actually, some of us with the experience can make a
reasonable guess).

As to some "high quality drivers... intentionally designed to exhibit
specific resonance and null patterns..." you're on pretty thin ice with
this one. Care to mention any specific examples? I don't know of any, and
I've seen a hell of a lot of drivers. 

>Also, time aligned systems suffer from the same problem that stereo
>imaging does--there is always a sweet spot based on speaker geometry,
>location and room acoustics.  And often some of the design aspects of
>time-aligned speakers make them very sensative to position (much more so
>than conventional speakers). 

Bullsh*t! This is only true in the sense that spending the effort
providing time alignment will result in ideal time alignment occuring over
only a fairly narrow range of positions, while not spending the effort
probably results in the alignment happening nowhere. So I guess you could
say that since it's the difference between beeing good in one spot and bad
everywhere else, as opposed to being bad everywhere, the latter case is
better since you can't ever loose the good spot.

Now none of this discussion addresses the issue of whether time alignment
is good or not. That's a whole different subject altogether.

|                Dick Pierce                |
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