Loudspeakers - The BBC collection

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LOUDSPEAKERS

The BBC collection

Graham Whitehead describes the range of BBC-designed loudspeakers, both past and present, used in programme balance and monitoring. The detail is brief but goes some way to explaining the reasons that the design came into being, and the applications for which they may have been designed. The current range is also described.

BBC loudspeakers are designed to provide a neutral sound which is consistent and repeatable: units can be supplied and maintained over a number of years without a change in performance.

Classification

BBC loudspeakers exist under four classifications:

Before these classifications came into use, loudspeakers were coded LSU/--. In future, only the LS1/--, LS2/--, and LS5/-- classifications are going to be used, ie OB and studio loudspeakers will be lumped together under the LS5/-- code. There are too many individual types to cover so, in this article, I shall confine myself to the more common types of monitor loudspeaker.

Cone Materials

The type of material used to form the cone of a loudspeaker has a marked effect on the overall sound and consistency of the loudspeaker. Over the years, a variety of materials has been used.

The original material was paper, which was pulped, compressed, and formed. This offered only the minimum of control over the resultant sound quality, especially in the critical midrange area, and effort was made to find an alternative.

In the mid-sixties, partially as a result of development work at Kingswood Warren, Bextrene - a kind of polystyrene - started to be used. Its major advantage was the ease by which more consistent results could be obtained by vacuum forming. The resultant sound quality could be controlled, was more repeatable from one sample to the next, and the hf response was superior. However, it was necessary to coat Bextrene with a damping compound to remove colourations in the region of 1.5 to 2 kHz. This process has never successfully been automated. The dope was applied by hand and hence the manufacturer was at the mercy of the skill and dexterity of whoever was wielding the paintbrush!

The use of plastics - notably polypropylene - developed considerably during the 1970s, again with pioneering work at Research Department. Polypropylene offered similar improvements over paper as did Bextrene. It did not need to be coated however, as the material was inherently self-damping. Because of its lower density, polypropylene yields greater efficiency than Bextrene and hence greater sound levels are possible for the same power input. This is the material currently being used for our Grade 1 monitors.

Yesterday's monitors

LSU/10

This large assembly, in an oak cabinet, was designed just after the war for general studio use. Some may still be found in service in talkback or PA applications.

The following units were used:

- Parmeko 18-inch dual concentric woofer/ horn tweeter
- Lorentz LPH65 tweeter (for extended hf)
- Modified Leak TL/ 12 valve amplifier: 12 watts

LS3/1

A grey-painted totally-enclosed assembly for OB use. HF units were mounted on a perforated sub-baffle in front of the lf unit, to enable accurate monitoring at close range.

The following units were used:

- Plessey 15-inch paper cone woofer
- 2 x Rola-Celestion HF1300 tweeters

LS3/1A

Similar to the L53/1 but with the bass unit replaced by a Goodmans C129/15pr.

LS3/4, and B

An angled-front assembly for mounting close to the ceiling where space was at a premium, eg. OB vehicles or above vt machines. Beware! Some of these are still in service but the surround of the bass unit fails with age and the unit is obsolete. The old ones have a gold coloured grille cloth. There are later, current types, with a black grille and the old may be modified into the new (see LS3/4C and LS5/11 on the next page, under "Today's monitors").

The following units were used:

- Spendor BC2/8, 203mm Bextrene cone woofer
- Rola-Celestion HF1300 tweeter

LS3/5

The predecessor to the L53/5A. Superseded when the particular bass unit used became unreliable.

The following units were used:

- KEF B 11O, 1 l Omm Bextrene cone woofer
- KEF T27 27mm Mylar dome tweeter

LS3/7

Similar in size to the LS3/ 1 and designed to replace it. Some are still in service for talkback, but none should now be used for monitoring. Max SPL at 1.5m: 104dB(A). Will become obsolete when repair stocks of drive units are exhausted.

The following units were used:

- Spendor 8-inch Bextrene cone woofer [Actually, it's a 12" unit!]
- Son Audax HD 12x9D25, 25mm fabric dome tweeter

The amplifier (AM8/15) is a modified Quad 303 where one channel drives the if and one the hf, by the use of an electronic frequency-splitting and equalising network.

LS5/1, and A

The studio versions of the LS3/1 and A, designed to replace the LSU/10. The cabinets were larger than the LS3/1 and the tweeters were mounted above the woofer, rather than in front of it.

The following units were used:

- Plessey (LS5/1) or Goodmans C129/15pr (LS5/1A), 15-inch paper cone woofer
- 2 x Rola-Celestion HF1300 tweeters

LS5/2, and A

Television versions of the LS5/1 and A. A different enclosure shape was used to enable the unit to be suspended.

LS5/4

A version of the LS3/1A for Bush House.

LS5/5

A large freestanding assembly for studio monitoring, built into a grey-painted cabinet. Max SPL at 1.5m: 103dB(A).

The following units were used:

- 12-inch Bextrene cone woofer (LS2/1)
- 8-inch Bextrene cone midrange (LS2/2)
- Celestion HF 1400 tweeter
- modified Quad 50D power amplifier (AM8/11)
- pre-amp and equaliser (AM1/20)

LS5/5B

A version of the LS5/5 with a Spendor BC2/8 mkII midrange unit and finished in teak veneer.

LS5/6

A television version of the LS5/5. Mounted in a different enclosure shape for suspending.

Today's monitors

LS3/4C

A medium sized triangular-shaped assembly, for mounting on a ceiling or wall where space is at a premium. Used in television in CMCRs/ MCRs or vt cubicles. Replaces the L53/4 and LS3/4B, or may have been converted from one of the above. Max SPL at 1.5m: 101dB(A). About forty are in BBC service.

The following units are used:

- 8-inch Bextrene cone woofer (LS2/13)
- Son Audax HD 13D34, 34mm fabric dome tweeter (LS2/12)

Requires a 50 watt amplifier.

LS3/5A

A small `bookshelf' assembly for use in restricted areas, such as OB vehicles. Extensively used in Local Radio. Max SPL from a pair at 1.5m: 95dB(A). Over 3500 are in BBC service and over 100,000 have been sold commercially, worldwide. Made under licence by Swisstone (Rogers), Spendor and Harbeth.

The following units are used:

- KEF B 110 SP1228, 1 l 0mm Bextrene cone woofer.
- Modified KEF T27 SP1032, 27mm Mylar dome tweeter.
- KEF SP2128 crossover (FL6/38)

Requires a 25 watt amplifier.

LS5/8

A large floor-standing assembly for highlevel, high quality studio monitoring. Replaces the LS5/1 and LS5/5. Max SPL at 1.5m: 116dB(A). Just under a thousand are in BBC service. Made under licence by Swisstone (Rogers).

The following units are used:

- Rogers 12-inch Polypropylene cone woofer.
- Son Audax HD13D34H, 34mm fabric dome tweeter (LS2/15)
- modified Quad 405 power amplifier (AM8/ 16)

The AM8/ 16 contains an equalising and frequency-splitting pre-amplifier. The woofer and tweeter are each driven from a separate amplifier channel.

LS5/8AL, and R

Similar to the LS5/8, but with the baffle angled in at 30 degrees for corner mounting in vehicles. Left and right handed versions make up a pair. Special acoustic treatment is required due to the close proximity of the loudspeaker to the walls. Eight pairs are in BBC service.

LS5/9

A medium-size studio monitor for high quality monitoring in areas where the LS5/8 would be too large. Designed to produce similar subjective sound balance to the LS5/8. Max SPL at l.5m: lO5dB(A). Over a thousand are in BBC service. Made under licence by Swisstone (Rogers) and Spendor.

The following units are used:

- 8-inch polypropylene cone woofer (LS2/14)
- Son Audax HD13D34H, 34mm fabric dome tweeter (LS2/ 15)
- crossover (FL6/ 36)

Requires a 50 watt amplifier.

LS5/11

This is a modified LS3/4, intended to produce a similar subjective sound balance to the LS5/9. It uses the same units as the LS5/9, fitted to a modified LS3/4 baffle, although the crossover is modified. Max SPL at 1.5m: 106dB(A).

Requires a 50 watt amplifier.

Commercial Loudspeakers

For some applications, there is no suitable BBC-designed loudspeaker so we have to go to the commercial marketplace. An example is high level monitoring of Rock music, where we have used a variety of types over the years. In general, though, it is preferable to stick to the LS-coded units as these undergo rather tighter quality control checks, at Avenue House, than is common commercially.

Supply And Repair

The L53/5A, LS5/8 and LS5/9 are available from Design and Equipment Department, although new LS5/8s will be made to order only. No new LS3/4Cs will be made: requirements will be met by the LS5/11.

If repairs are required to any BBC loudspeaker, it should be returned to D&ED who holds the Reference units of each type.

If you require any further information on BBC loudspeakers, and their associated amplifiers, please contact me on Avenue House ext 311.

Graham Whitehead
Support Group, D&ED

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