From: DPierce@world.std.com
Subject: What is an LS3/5A?
Originator: tjk@csd4.csd.uwm.edu
Organization: University of Wisconsin - Milwaukee
Date: Mon, 31 Aug 1992 13:14:34 GMT

Bill Vermillion asks:
>
>But I do have a question.  You say LS3/5A's.   I own a set of
>LS3s by Fostek.  Another studio in town has LS3As and another
>the LS4s (talk about big - these are about the size of a
>standard 20 cubic foot refrigerator), and another has the LS2s.
>(Those suckers aren't cheap either - the LS4s - about $4k each)
>
>Is the LS3/5A a Fostex or another brand.

The designation LS3/5A refers generically to a series of design licenses for
monitoring loudspeakers designed by the BBC in the mid and late 1960's for
use in BBC facilities. Several companies were contracted by the BBC to produce
the designs for use in BBC facilities, chief among them was KEF. Several
other companies, such as Rogers and Spendor, produced their versions for
the home market.

The designs were based on some pretty radical though at the time: Paper for
cones is no good because it's too variable, is environment sensitive, and 
doesn't really have the mechanical and acoustical properties suitable for
high-accuracy monitoring purposes. New cone materials and configurations
were used (leading to the use of "Bextrene", a specific brand name for what
is otherwise generic extruded polystyrene sheet that is then thermo-formed to
the correct shape, and "Plastiflex PV-2000", an acrylic dopant used to add
mechanical loss to the cone).

Further, the low frequency configurations were based on the works of people
like A. Nevill Thiele in Australia, who published in the Journal of the I.R.E
in the mid 60's what has now become the definitive foundation work of bass-
reflex design (it's interesting to note that this article was not available
in the United States until about 1972, when it was reprinted by the Audio
Engineering Society, some 7 or so years after it's international publication!)

There were about a dozen so-called BBC designs, designated by an "LS" number.

The LS3/5A refered to here is undoubtedly the Rodgers version, which is not
a BBC sanctioned version, but rather their interpretation of it. It's a small
(about 7 liters internal volume) two-way system using the KEF B-110 5" bass/
midrange driver (which is a BBC sannctioned unit) and the KEF T-27 3/4"
dome tweeter (I am not sure if this was ever part of the official galaxy of
BBC parts, but it is a very good tweeter nonetheless, whe used properly).

It is a relatively low effiency system, primarily because of the incrediably
comp[lex and lossy crossover network. It has a low end cutoff (real F3) of
about 85 Hz, with a bit of a hump around 110 Hz. The front grill and decorative
molding of the Rodgers version very much spoil the great high frequency 
dispersion and imaging characteristics the T-27 is capable of. Otherwise it's
a pretty good (but dated) little speaker which, in spite of it's shortcomings,
is probably worlds better than the Fostex systems you mentioned.

|                Dick Pierce                |
|    Loudspeaker and Software Consulting    |
| 17 Sartelle Street   Pepperell, MA  01463 |
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