From: jch@teak.Cray.COM (J.C. Hamlin)
Subject: Isobarics
Organization: Cray Research, Inc.
Date: 7 Apr 92 16:37:00 CDT

		          Isobarik Enclosures
		       for Non-Technical Types
			    by J.C. Hamlin

Special thanks to: 
   Don Tillman (till%acid-rain@lucid.com) and
   Craig A. Blome (aardvark@ccwf.cc.utexas.edu) 
for detailed information about bandpass and isobarik enclosures.

			 ISOBARIK ENCLOSURES

"Isobarik" means "constant pressure".  An isobarik speaker configuration is
a design where two drivers have a small sealed volume of air at a constant 
pressure between them--the smaller the volume, the better.  This can be
achieved in a standard "helper" design, where both drivers face the same 
direction, a "push-pull" design (as above), or a "back-to-back" design, where
the drivers face away from each other.  The drivers are always moving in the
same direction.  (Shown below)

   "helper"     "front-to-front push-pull"  "back-to-back push-pull"
 +-----------+            +-------+         +------------+
 |____       |            |       |         |________    |
 |\ _ |\ _   |         _ /|\ _    |         |\ _  _ /|   |
 | | || | |  |        | | | | |   |         | | || | |   |
 | |_|| |_|  |        |_| | |_|   |         | |_||_| |   |
 |/___|/     |           \|/      |         |/______\|   |
 |           |            |       |         |            |
 +-----------+            +-------+         +------------+

To quote Don Tillman,
   "Well, isobariks are just a silly hack to reduce the size of a given design
    by trading off efficiency and weight.  You can isobarikize any design by
    doubling the speakers and halving the box size.  Note that it's a heck of 
    a lot easier to just buy a driver with a smaller Vas and otherwise 
    identical specs.  This driver will, incidentally, be less efficient.  So 
    isobariks are pretty much a hoax."

Sometimes in car audio though, you have to make due with what you have, what
you can afford, or what you can find.  Sometimes the ideal driver doesn't
exist, or you can't afford it, so you have to make due.  That's where isobariks
usually come in.  Also, sometimes it is just less expensive to purchase two of
one kind of driver and isobarikize them, than to purchase the one driver to
suit your needs.  For whatever reason, isobarik design is an option for you
to consider.

In any isobarik design, the "front" driver acts as if it were in "FreeAir" or
infinite baffle mounting, and the "rear" driver acts normally (with some
modifications).  The actual modifications you need to know about isobarik 
setups with identical drivers are:
1) The Vas for the driver gets cut in half, thus effectively cutting the 
   enclosure size in half.  Simplified reason:  you now have twice the
   "pushing" power, so the volume of compliance is cut in half.
2) The efficiency gets cut in half, (thus dropping 3 db).  Simplified reason:
   efficiency is usually measured at 1w/1m.  You are now feeding two drivers,
   at 0.5 watt each, and you can't hear the sound waves from the rear driver,
   so you hear half the energy.
3) The power handling doubles.  Simplified reason: two drivers can handle twice
   as much power as one.
4) All other specifications remain the same.

With that information, you can now consider isobarik speaker configurations in
your decision for an enclosure.

Please, questions and comments welcome (E-mail).  Any feedback is welcome.

Until then, happy listening! :)

J.C. Hamlin
jch@Cray.COM
Newsgroups: rec.audio.car