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From: pmr@drufl.UUCP (Rastocny)
Newsgroups: net.audio
Subject: Re: More Speaker Questions - (nf)
Message-ID: <750@drufl.UUCP>
Date: Wed, 21-Dec-83 11:50:26 EST
Article-I.D.: drufl.750
Posted: Wed Dec 21 11:50:26 1983
Date-Received: Thu, 5-Jan-84 01:30:39 EST
References: <2993@hp-pcd.UUCP>
Organization: AT&T Information Systems, Denver
Lines: 54

Recently in one of the big three audio magazines there appeared a
good article on the several different types of ways to transduce
electrical energy into the air.  Good arguments were stated at the
time as to the attributes of each type.  The driver you are referring
to is still basically a dynamic radiator (linear motor connected to
a radiating diaphragm).  All dynamic radiators have a set of
problems associated with this principle independent of the radiator
type.  The problem lies in the ability of the diaphragm to act as a
piston at all frequencies without flexing or distorting in any way.
While it is true that some diaphragms may operate more
like a true piston (e.g., Celestion's tweeter), there is more to
this transducer type than meets the eye.  New materials
(polypropylene, laminated graphite, et.al.) and new concepts
(ferro-fluid filled air gaps, rolled-edge cone suspensions, et. al.)
all reduce the magnitude of these unwanted nodes, but the cone is still
the stiffest geometric shape available to mechanically couple the
large diaphragm excursions encountered in low frequency reproduction.

Planar radiators act as a true piston when the entire diaphragm is
driven uniformly (e.g., the EMIT tweeter of your own loudspeaker).
This class of drivers is known as "ribbon" and "leaf" drivers.
If the entire diaphragm is not driven, the diaphragm may flex much worse
than the cone it is was trying to replace.  Another way around the
flexing problem is to use more than one voice coil and magnet assembly
(linear motor) and attatch them to different points on the diaphragm.
This helps but each of the linear motors must operate identically.
(You never get anything for nothing.)  Think of how your car acts when
one of the cylinders starts to miss.  A similar problem occurs when the
linear motors are not well matched.  The slow one will drag down the
faster ones.

Historically, other manufacturers have attempted to keep the diaphragm
from developing nodes of vibration, all unsuccessfully.  (How many
people remember styrofoam planar drivers?)  In summary, it is not the
shape of the diaphragm that is important but rather the ability of
this diaphragm to act as a piston throughout the operating range.

The best way to reproduce sound then would be to eliminate this
diaphragm and the associated problems and move the air electrically.
This type of driver is known as an ionophone and it has another set of
associated problems.  Since large radio frequency signals are
needed to polarize the air to be moved, the driver interferes with
radio and television reception in the area.  (Some also create ozone
that may be harmful to your health.)  Another way to ionize
the air is to use a flame.  But then you have to buy "gas" for your
stereo (AND your car).

Welp, this is a lot longer than I thought it would be but I hope you
can see some of the real world problems facing speaker designers.
Hope it answered your question.

		Yours for higher fidelity,
		Phil Rastocny
		..!drufl!pmr