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Path: utzoo!linus!philabs!cmcl2!seismo!harvard!talcott!panda!teddy!rdp
From: rdp@teddy.UUCP
Newsgroups: net.audio
Subject: Re: ribbon vs. electro statics
Message-ID: <1038@teddy.UUCP>
Date: Mon, 29-Jul-85 09:23:10 EDT
Article-I.D.: teddy.1038
Posted: Mon Jul 29 09:23:10 1985
Date-Received: Wed, 31-Jul-85 23:23:06 EDT
References: <42400009@hpfclo.UUCP>
Reply-To: rdp@teddy.UUCP (Richard D. Pierce)
Organization: GenRad, Inc., Concord, Mass.
Lines: 67

In article <42400009@hpfclo.UUCP> rjs@hpfcla.UUCP (rjs) writes:
>A salesperson at a local audio store recently pointed out to me that
>there is a difference between electro-static and ribbon speakers. Could
>someone explain to me the difference, and how electro-static speakers
>work? Thanks in advance.
>
>Bob Schneider
>{ihnp4|hplabs}!hpfcla!hpfclo!rjs

Ribbon tweeters consists of a small ribbon of metal, or some metallized
plastic (mylar, etc) held in a magnetic field. The direction tf the magnetic
field is parallele with the plane of the ribbon thusly (a front view):


			+----+
		N  <---	|    |  <---  S
		O  <---	|    |  <---  O
		R  <---	|    |  <---  U
		T  <---	|    |  <---  T
		H  <---	|    |  <---  H
			+----+

WHen a current is passed through the ribbon, from the top to bottom or
vice-versa, a force perpendicular to the magnetic field and the current
is applied to the ribbon, so it moves in or out

Electrostatic speakers have two grids spaced evenly apart, with a tightly
stretched film of metallized plastic in between them. The seperation is on
the order of a fraction of an inch (say 1/4 inche between the film and
the front and rear grids)

The front and rear grid have a moderately high (few thousand volt) charge
applied. The signal is then applied to the diaphram. The difference in
potential between the grids and the diaphram cause the diaphram to be
attracted to either the front or rear grid (and repusled by it's opposite)
causing the diaphram to move in a manner (supposedly) proportional to the
applied signal.

Other differences that are important:

    o	Ribbon drivers are often VERY low impedance devices, requiring
	an input coupling transformer. For the most part, these devices are
	used for tweeters, as it is nearly impossible to generate a
	large and sufficiently linear magnitic field over the wide area
	needed for a low=frequency driver. In addition, they are frequently
	of very low efficiency, requring horn loading or sone other such
	kludge. They can have, because of their very low moving mass, a
	very wide bandwidth.

    o	Electrostatic loudspeakers are very HIGH impedance devices, also
	usually requiring an input matching transformer (with the exception
	of things like Acoustats, which driver the speaker directly from
	a high voltage, high-impedance, high power source). The can be made
	large sizes, so have been used successfully for full-range use.
	The necessity of the polarizing voltage makes for all sorts of
	problems in humid or dusty environments. Again, because of the low
	driven mass (or more specifically, a low mass-to-force ratio) thay
	can be capable of very wide bandwidth

In both case, the addition of the input transformer almost totally negates
any bandwidth advantages. In fact, many otherwise good designs are trashed
by simply awful input transformers.

Oh, well...


Dick Pierce