From: DPierce@world.std.com (Richard D Pierce)
Newsgroups: rec.audio.high-end
Subject: re: ESL & Ribbon Speakers
Date: 18 Nov 92 14:56:30 GMT
Organization: University of Wisconsin - Milwaukee
NNTP-Posting-Host: 129.89.7.4
Mime-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Originator: tjk@csd4.csd.uwm.edu


Andreas Yankopolous writes:
>
> ... brief description of operating principles of electrostatic
>     loudspeakers, essentially correct, a few minor technical
>     errors, but, generally correct ...
>
>In a ribbon speaker, the "voice coil" appears in the form of thin metal
>wires located in an insulating sheet.  The sheet is suspended in a
>strong magnetic field created by large permanent magnets.  The signal
>from the amplifier flows through the wires embedded in the ribbon.  The
>moving charges in the wire and magnetic field interact to produce a
>force on the wires and consequently move the ribbon.  Since the wires in
>the ribbon provide the resistance to current flow, ribbon speakers
>usually have very low impedances and usually require a high current
>solid state design.

No, in fact most ribbon drivers use a thin metallic foil, not a ribbon
with wires embedded in it. The entire ribbon is a conductor. Also, many
ribbon drivers come with matching transformers as well.

>There are many reasons for the clarity of these designs.  The vibrating
>sheet in an electrostatic speaker is very lightweight; Martin-Logan's
>weigh less than a cubic inch of air!

For some reason (I guess it's "intuitively obvious" to the average
audiophile) the lightweight diaphragm is somehow taken to be very
advantageous. People seem to think that being very light, it can be
accelerated very quickly, thus it has very good transient response, etc.

This is a myth. First, in an electrostatic loudspeaker, the amount force
applied per unit mass is typically on the same order as normal dynamic
loudspeakers. Therefore the net acceleration is about the same. Further,
the transformer that we end up sticking in between the amp and the driver
acts very well as a bandpass filter and, as such, provides the upper limit
on transient response. Essentially, no driver, no matter what principle
it uses, can be made to move any faster than the signal driving it asks. 

Further, the "mass" argument completely ignores the effect of the air load,
which can be significantly greater and pretty much nulifies any (really,
non-existant) advantage of low mass.

One further nit that can stand some debunking: the statement that the
diaphragm weighs less than a cubic inch of air seems a bit hard to
swallow. The density of air at STP is 1.18 kg/m^3. Thus, a cubic inch of
air weighs in at 0.000000491 kg or a whopping 0.000491 grams. Most
materials that are used for such diaphragms have a density damn close to
that of water, or 1000 kg/m^3. This implies that the total material used
must have a volume no greater than 4.91x10^-10 m^3. Using a material
that's, say, 5x10^-6 m thick (0.2 mil), that means its total area
cannot exceed about 1x10^-4 m^2, or a square diaphragm about 1 cm or 0.4
inchs on a side. Now, if they are saying a square inch of the material
weighs about the same as a cubic inch of air, I might allow them the order
of magnitude leeway, but by saying the diaphragm weighs that much,
well, somebody's off by at least a couple orders of magnitude.

>Also, with both of these designs, the force is applied more or less
>uniformly over the entire diaphram.  A dynamic speaker concentrates the
>force in the center of the cone or around the perimiter for the dome.

Yeah? So what? As several people over the years have pointed out, uniform
drive does not imply uniform motion. A variety of measurements (including
a number I have performed over the years) demonstrate very clearly that
large diaphragm electrostatic drivers not only to not move uniformly, they
are particularly terrible in that they suffer as much, if not more, from
non-uniform motion as other technologies.

There is a good, demonstrable reason why electrostatics and (to a lesser
extent, ribbons) sound clean and open, and that is (when done right) the
lack of a box. But, at the same time, it's amazing what a couple of well
placed braces and the right kind of absorptive material can do to make a
box-based system sound much better.

One of the finest, most open sounding and neutral speakers I've ever heard
was a modified double pair of older Quad ESL's. Another example were B&W
801's. Two examples of speakers that used radically different approaches,
but sounded more alike then they sounded different in many ways. One was
electrostatic, the ither was "conventional" (but very well executed)
dynamic driver/box technology.

|                Dick Pierce                |
|    Loudspeaker and Software Consulting    |
| 17 Sartelle Street   Pepperell, MA  01463 |
|       (508) 433-9183 (Voice and FAX)      |