In the sixties, I bought my first wide range stereo speaker system, two "sand filled baffle" four way Wharfdale 70 E speakers.

They had the sweetest high end sound, although the low end was typically British, "conservative".  Nonetheless, they served me well until one of the them blew a tweeter when my brother and I used them as playback monitors during a 1962 recording session in a church in North Hollywood of the Jack Halloran Singers (I was a member).  They were very comfortable to listen to.
--- On Fri, 2/8/13, Don Cox <[log in to unmask]> wrote:

From: Don Cox <[log in to unmask]>
Subject: Re: [ARSCLIST] Passive radiator loudspeaker was Mono but Out-of-Phase
To: [log in to unmask]
Date: Friday, February 8, 2013, 4:04 AM

On 08/02/2013, L. Hunter Kevil wrote:

> Pace Don Cox, electrostatic speakers, by Quad or any other
> manufacturer, are definitely NOT infinite baffle designs. They are
> planar speakers, completely different from the conventional cone and
> voice-coil drivers used in box speakers. Magneplanar speakers, while
> not electrostatics, are well known American-made planars. Planars
> don't even look anything like infinite baffles.
I do know how electrostatic speakers work. I've used them for decades.

A baffle is a large board on which a speaker drive can be mounted, so
that out-of-phase sound from the back doesn't wander round to the front
and interfere with the direct sound. 

Ideally, the board is infinite in area. As said, it could be (as a
compromise) a wall between two rooms.

In practice, a board a few feet square gives useful results, as the
Wharfedale design showed. 

Some people think that a closed box full of wool or similar material can
give the same result as an infinite baffle, and they market such boxes
as "infinite baffle" designs. I think this is a misnomer.

Picture of the Wharfedale design here:

Don Cox
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