Xref: utzoo rec.audio:5509 sci.electronics:2585
Path: utzoo!mnetor!uunet!psivax!torkil
From: torkil@psivax.UUCP (Torkil Hammer)
Newsgroups: rec.audio,sci.electronics
Subject: Re: Position Feedback On Woofers?
Message-ID: <2143@psivax.UUCP>
Date: 23 Mar 88 18:07:30 GMT
References: <3388@briar.Philips.Com>
Reply-To: torkil@psivax.UUCP (Torkil Hammer)
Organization: Pacesetter Systems Inc., Sylmar, CA
Lines: 41

In article <3388@briar.Philips.Com> ams@philabs.Philips.Com (M. Ali Shaik) writes:
#
#Does any of you know anything about woofers/ sub-woofers incorporating
#position feedback? (I believe Velodyne & Infinity make them).
#What are the merits/demerits? Is there any merit at all in doing it?

A friend of mine did that kind of job, back in 1970.  He used a pickup coil
mounted on the same tube as the voice coil.  This measures a velocity, which
was integrated to give the position, so the whole thing had a lower cutoff
frequency, which he managed to get well below 10 Hz.
The most remarkable effect was that the membrane became stiff to touch
when he switched in the feedback.  Also, the characteristics measured in an
anechoic chamber became flat within +/- .1 dB, though he was stuck with
small resonances around 80 and 300 Hz, which could be traced to a mechanical
resonance between voice and pickup coil.
Once he moved the contraptions back to his 3 bedroom speaker enclosure :-)
there was no audible and barely a measurable difference between the
listening position frequency response with or without feedback.
Window opening and the general position of pets and kids in the room
had all a much bigger effect.
Yet he claimed two acoustical advantages: He could run the membrane all the
way out and in and not cause speaker damage, because the driving limit
would depend properly on the waweform; and nonlinear distortion was
theoretically less, but for most music not audible and barely measurable
with state of the (then) art equipment.

Back then "high" wattage woofers were expensive, fragile and acoustically
distorting. I think that feedback gave a marginal edge in low frequency
"electronic music" (as it was called back then), and I know the protection
saved him at least one set of speakers.  On the other hand,
today's speakers cost less, have higher rating, seem more rugged and
are likely to be less distorting just as they are so I doubt that there is
anything to gain acoustically.

The stiff membrane will still be impressive, though.
And technology as substitute for religion has a potential for pleasure of
ownership.

Good luck and don't mess with those mercury filled cables :-|

torkil