BBC Research Department loudspeaker development

[tuga]

A small selection of old papers by the BBC Research Department.


A Survey of Performance Criteria and Design Considerations for High-Quality Monitoring Loudspeakers
D.E.L. Shorter - January 1959

Loudspeakers used for monitoring purposes in broadcasting and recording studios are designed to give the nearest practicable approach to realistic reproduction. The paper discusses the various criteria which can be applied to the performance of such loudspeakers, together with the relationship between the measured free-field characteristics and the response as subjectively assessed in the working environment. While the degree of realism achieved in sound reproduction can only be judged aurally, even subjective assessments can be misleading unless carried out under controlled conditions and with clearly defined terms of reference; the precautions necessary in such tests are discussed. Some of the less obvious design considerations are reviewed and illustrated by examples.

http://www.aes.org/e-lib/browse.cfm?elib=579 ($)


Design of a new free-field sound measurement room: Specification and performance
D.E.L. Shorter, H.D. Harwood (Harbeth), C.L.S. Gilford, J.R. Chew, R.L. Deane - 1965

The various requirements laid down in the design of the new free-field room at Kingswood Warren are discussed. Details are given of the building, the acoustic treatment and the associated technical equipment. Acoustic tests on the completed room are described; the frequency range over which free-field conditions can be obtained depends to some degree on the direction of sound propagation, but in favourable circumstances extends to below 50 c/s.

http://downloads.bbc.co.uk/rd/pubs/reports/1965-17.pdf


The design of a low-frequency unit for monitoring loudspeakers
H.D. Harwood - 1966

The present state in the design of low-frequency loudspeaker units is reviewed and the areas where improvement is desired are indicated. Experimental details are given leading to the design of 12 in. (305 mm) unit incorporating a vacuum-formed cone of toughened polystyrene with a p.v.c. (poly-vinyl chloride) surround and it is shown by objective and by listening tests that this design is superior to existing units. An analysis of the price indicates that the new unit should not cost any more than those at present in use.

http://downloads.bbc.co.uk/rd/pubs/reports/1966-28.pdf


The design of studio monitoring loudspeakers Types LS5/5 and LS5/6
H.D. Harwood, S.A. Hughes (Spendor) - 1967

Details are given of the various factors which have led to the design of two new monitoring loudspeakers suitable for use in studios and outside broadcasts. The loudspeakers are much smaller than those of the present type; a floor standing model is designated type LS5/5, and one intended for hanging is called type LS5/6. In the course of the design, the questions of bass pre-emphasis and of directivity have been examined in some detail.
The quality of reproduction and the directional properties are appreciably in advance of those obtained for the LS5/1A and the maximum sound level is also higher. The spread in frequency characteristics between development specimens is extremely small, and the level of non-linear distortion is low.

http://downloads.bbc.co.uk/rd/pubs/reports/1967-57.pdf


Aspects of High-quality Monitoring Loudspeakers
H.D. Harwood, C.L.S. Gilford, S.A. Hughes - September 1969

In Part I of this monograph a description is given of the design and performance of a compact two-unit loudspeaker originally intended for use in mobile control room.
Part II describes investigations to determine why the reproduction given by the latest hanging studio loudspeaker type LS5/6 became noticeably coloured when it was suspended above the picture monitors in a television sound-control room, as dictated by considerations of layout. It is concluded that the colouration was due to the build-up of multiple reflections from the walls and windows when the loudspeaker was suspended symmetrically in the corner of the control room and that the colouration was more noticeable in the case of the LS5/6 primarily because of the higher quality of this loudspeaker.

http://downloads.bbc.co.uk/rd/pubs/archive/pdffiles/monographs/bbc_monograph_78.pdf


The design of the LS3/4 loudspeaker
H.D. Harwood, S.A. Hughes - 1969

A description is given of the design and performance of a compact two- unit loudspeaker for use in mobile control rooms. Both the frequency range and the quality of reproduction are similar to that of the studio monitoring loudspeaker type LS5/5 from which this design was developed; the power handling capacity, whilst less than that of the LS5/5, is quite adequate for the purpose.

http://downloads.bbc.co.uk/rd/pubs/reports/1969-05.pdf


New BBC Monitoring Loudspeaker
H.D. Harwood - March 1968 (Wireless World)

1. Design of the low frequency unit
2. Bass equalisation: The cabinet: Frequency response characteristics of the units

http://www.keith-snook.info/wireless-world-magazine/Wireless-World-1968/New B.B.C. Monitoring Loudspeaker.pdf


Stereophonic Image Sharpness
H.D. Harwood - July 1968 (Wireless World)

Results of experiments to discover how the width of the sound image is related to the image position; also how image position is related
to inter-channel level difference.

http://www.keith-snook.info/wireless-world-magazine/Wireless-World-1968/Stereophonic Image Sharpness.pdf


Loudspeaker distortion associated with low-frequency signals
H.D. Harwood - January 1972

Three differing forms of distortion, which are associated with low-frequency signals in loudspeakers, are investigated.
It is shown that distortion due to the Doppler effect can be compared with that due to wow and flutter in recording machines and subjective data obtained for this purpose can be applied to loudspeakers. Generalised design limits for loudspeakers are calculated and it is seen that present BBC designs lie within these limits.
In loudspeakers designed to reproduce low frequencies, the voice coil is made longer than the magnetic field. At low frequencies, when the amplitude of vibration of the cone exceeds the difference in length, it is shown that instead of the peaks of the waveform being clipped, expansion of the input/output curve takes place. This effect, with its associated distortion, can be compensated by employing an appropriate non-linear suspension and thus a much greater useful output can be obtained than by using a linear suspension.
Finally, a vented cabinet is often used to reduce the magnitude of the undesirable effects previously mentioned as well as to extend the bass response. However, a vented cabinet is a resonant system and high sound pressures and particle velocities are produced in the vent. These are liable to give rise to distortion from the inherent non-linearity in the air and from turbulence at the orifice and in the pipe. Existing data are used to estimate the sound levels which may be generated in a typical listening room before distortions from any of these causes is audible. It is also shown that this form of distortion is not a troublesome factor in the design of the BBC Studio monitoring loudspeaker.

http://downloads.bbc.co.uk/rd/pubs/reports/1972-25.pdf


The design of the miniature monitoring loudspeaker Type LS3/5A
H.D. Harwood, M.E. Whatton, R.W. Mills - October 1976

This report describes the design of a miniature two-unit loudspeaker of adequate sound quality and loudness to serve as a monitor in conditions where larger existing designs would be unusable.
Details are given of the construction and performance of the loudspeaker which is shown to be equally suitable for monophonic, stereophonic and quadraphonic purposes.

http://downloads.bbc.co.uk/rd/pubs/reports/1976-29.pdf


Factors in the design of loudspeaker cabinets
H.D. Harwood, R. Matthews - January 1977


The mechanical properties of timber, wood products and other materials potentially suitable for the construction of loudspeaker cabinets have been measured and details of the results are given. Various commercially available damping materials have also been assessed and their relative efficiencies are listed. A new method of test for the cabinets of completed loudspeakers has been devised and a tentative performance specification has been produced.

http://downloads.bbc.co.uk/rd/pubs/reports/1977-03.pdf



Some factors in loudspeaker quality
H.D. Harwood, R. Matthews - May 1976 (Wireless World)

Some of the factors in loudspeaker design have been dealt with in the technical press many times but there are others which have received comparatively little attention, at any rate quantitatively. In this article it is proposed to deal with a few of the latter and to add some subjective data which is new.

https://pdfhost.io/v/Ff4nP~MG_Some_...arwood_R_Matthews_May_1976_Wireless_World.pdf


A new distortion measurement - Better subjective-objective correlation than given by THD
R.A. Belcher - May 1978 (Wireless World)


This article describes a new technique for measuring non-linearity distortion which gives much better correlation with subjective assessment of sound quality than does the conventional total harmonic distortion measurement.
 Known as the double comb-filter method, it uses pseudo-random binary test signals and largely digital processing, so the cost of instrumenting it can be expected to fall with the increasing availability of l.s.i. circuits. Ultimately, the hardware may cost less than that used at present for THD measurement. Ways are suggested for using the new technique to measure cross-over distortion, transient intermodulation distortion and other parameters such as "wow" and "flutter" and linear distortions. The BBC is now testing it on sound-signal transmission circuits and studio equipment.

http://www.keith-snook.info/wireless-world-magazine/Wireless-World-1978/A new distortion measurement.pdf


Design of the high-level studio monitoring loudspeaker type LS5/8
C.D. Mathers - November 1979

A new studio monitor is described which can provide high-quality sound at sufficiently high levels to fulfil the complete range of monitoring requirements from pop to serious music. The design principles and criteria are outlined, and the performance of the prototypes is assessed both subjectively and objectively.

http://downloads.bbc.co.uk/rd/pubs/reports/1979-22.pdf


The design of the prototype LS5/9 studio monitoring loudspeaker
K.E. Randall, C.D. Mathers - August 1983

The prototype of a new medium-power studio monitor is described which is physically small enough to be accommodated in areas where a large high-power monitor would be inappropriate.
The main design and performance features are as follows:- A vented cabinet having a volume of 28 litres (1 cubic foot). Two drive units; a proprietary 34mm soft-domed high-frequency unit and a BBC designed 200mm low-frequency unit having a polypropylene diaphragm and a high temperature voice-coil. A low-level cross-over circuit feeds a 50watt stereo amplifier which drives the units separately; the cross- over frequency is 2.4 kHz. The axial frequency response is +/-3dB from 56Hz to 16kHz and the maximum sound level is 100dB(A) at 1m on axis measured in a typical listening room using light music.

http://downloads.bbc.co.uk/rd/pubs/reports/1983-10.pdf



On the design of loudspeakers for broadcast monitoring
C.D. Mathers - December 1988


By designing its own monitoring loudspeakers for about the past forty years, the BBC has achieved a degree of continuity and consensus in a subject where there are almost as many opinions as there are designers. This report describes the approach that has evolved within the Corporation towards design and assessment, and indicates how calculation and objective measurement are supplemented by experience and subjective judgement to arrive at a design that is acceptable to users in the broadcasting service. Each component of the loudspeaker is considered, and the problems of achieving consistency and reliability are addressed.


http://downloads.bbc.co.uk/rd/pubs/reports/1988-14.pdf

 

[tuga]

 

[tuga]

This snippet was taken from "On the design of loudspeakers for broadcast monitoring" by C.D. Mathers 12/1988 (link above):

9 LOUDSPEAKER EVALUATION

9.1 introduction

The obvious and definitive means of evaluating a loudspeaker is of course by listening to it. An expert listener auditioning known programme material can learn a great deal from a listening test. If all of the sound balancers who use a particular loudspeaker declare it to be excellent, then by definition it is excellent. In the author's experience at least, such universal approbation is rare. Although a group of users in an organisation like the BBC usually show remarkable accord in their evaluations, they tend to use adjectives like 'woolly, ‘hard’, or ‘chesty’, and nouns like 'honk', 'quack', or ‘lisp’. One can often hear what they refer to, but such quirks can rarely be identified by objective measurement, and are very poor guides indeed to any design modifications that might effect significant tonal improvements. (Very rarely, complimentary expressions like 'clean' or 'uncoloured' are applied; perhaps one reason for the rarity of these is that a perfect loudspeaker should presumably have no perceptible characteristics of its own.)

What is required, of course, is a well-defined relationship between subjective peculiarities, measurable deviations from 'ideal' acoustic output, and oddities in physical behaviour. A 'dreadful quack at 800 Hz' should be confirmed by a disturbance in the otherwise serene acoustic time-frequency-acceptability plot, and by an agonised writhing at 800 Hz to disturb the otherwise exemplary piston-like movement of the diaphragm.

Reality is otherwise. 'Good' loudspeaker drive units appear to exhibit just as complex mechanical and acoustic behaviour as 'bad' ones.
The author is currently engaged in a project to try to find some relationship between the subjective, acoustic, and mechanical facets of loudspeaker behaviour. This has been undertaken in the knowledge that previous attempts during four decades have not yielded a final solution. Results (positive or negative) will be published in due course. Two reference works only are listed relating to this subject, each includes an extensive bibliography.

9.2 Subjective evaluation

Experience shows that comparative judgements of loudspeaker quality can be made more consistently than absolute ones. An absolute assessment of a new design is something which emerges gradually out of weeks or months of use in control rooms. Often, a pair of new loudspeakers sent out for 'field trial' will be received with cautious approval, yet returned after a month or two with a list of criticisms detailing points that have emerged only gradually from continuos use. For comparative tests, a reference loudspeaker is of course needed. This is provisionally selected during the early stages of commercial production as being a typical unit of acceptable quality; once production is well established, a new reference may be adopted as a clearer picture emerges of what is 'typical'. In fact, at least three such units are selected in normal BBC practice, to provide a working standard for acceptance testing: a spare (which is carefully stored): and a standard by which the manufacturers can assess the consistency of their output, whether by listening or by measurement. An established standard is also of course the only reasonable reference available in appraising a new design.

In listening tests, it is important that the listener should begin with as few preconceived ideas as possible. For example, a look at a response plot may cause him, consciously or otherwise, to listen for some expected peculiarities. Normally, an A/B switch is provided, and the loudspeaker to be used as reference is indicated. The loudspeakers are placed behind an acoustically transparent but optically opaque curtain, especially if any aspect of the units under test might be visually identifiable. To help eliminate room effects, the test may be repeated with the loudspeaker positions interchanged. If several units are to be tested, it is useful to include one twice — anonymously — to test the listener's consistency. (Experienced listeners expect this.)

Finally, it is essential that the listener delivers his judgement before any additional information is given to him; not (one would trust) that he might 'cheat', but rather that he might re-interpret what he thought he had heard in the light of further knowledge. Subsequent discussion may well prove valuable, but must be subsequent.

Formal tests involving a number of listeners may need further care, particularly if, as is likely, they permit less in the way of personal communication between subjects and test organiser. Past experience suggests that a particular hazard is the use of descriptive terms whose meaning seems obvious to everyone, but which can actually mean different things to different people.

 

[milw50717]

Interesting links and reading on the evaluation of speakers. I think I have certainly described speakers as sounding 'wooly'.

The photo could, quite easily, be mistaken for a photo from an early Doctor Who episode!

Are they really using chairs as speaker stands ?

 

[SMc]

Interesting links and reading on the evaluation of speakers. I think I have certainly described speakers as sounding 'wooly'.

The photo could, quite easily, be mistaken for a photo from an early Doctor Who episode!

Are they really using chairs as speaker stands ?

They didn’t have enough interns to hold them up by hand!

 

[milw50717]

I thought that was a clipboard!

When I was a mere slip of a lad the father of my best friend was an accountant at the Beeb. He got a few pairs of their speakers over the years and I recall listening to the first pair that he got with my friend. I have it in my mind that we said something along the lines of "The silences sound so black". These were on speaker stands you understand, not handheld or chair mounted

Edit - I found another version of that image and I think the intern is Rowan Atkinson

Image from https://kknews.cc/news/mgxl59p.html

 

[Frank Dernie]

Are they really using chairs as speaker stands ?

I am not sure speaker stands were a "thing" back then.

 

[tuga]

Are they really using chairs as speaker stands ?

I've read that they liked "lossy" stands, in keeping with the cabinet design philosophy.

 

[milw50717]

The New BBC Monitoring Loudspeaker link you posted indicates the LS 5/5 was designed to be mounted on a plinth - one is shown in the PDF - and the LS5/6 was designed for hanging. It also has some details on the tests and the music used. It's pretty cool to be able to read all of this.

 

[martin900]

I have an original pair of Rogers LS3/5A 15 Ohm, one of the last 'true' ones and they are very special.
Not the best speaker in the world, obviously but still astonishing in terms of timbre.

 

[whazzup]

Always had this question when I read this about the LS3/5a (linked from Wikipedia):
"This is why the frequency response is not flat; the speaker was critically voiced octave-by-octave, so that the whole radiated sound, driver and cabinet effects included, sounded most lifelike."
http://www.g4dcv.co.uk/ls35a/pics/HFC3_LS3.pdf

What is the rationale for doing that? If it sounded lifelike on the ls3/5a, did it mean it sounded bad on other speakers? Isn't that bad for monitoring purposes?
And when it's 'voiced octave by octave', they are doing that with measuring instruments? Some sound engineer sits and tunes the speaker response by ear? When tuned by microphones / machines, the result wasn't acceptable?

I'm not an engineer, so don't know enough about the process....

 

[sergeauckland]

Always had this question when I read this about the LS3/5a (linked from Wikipedia):
"This is why the frequency response is not flat; the speaker was critically voiced octave-by-octave, so that the whole radiated sound, driver and cabinet effects included, sounded most lifelike."
http://www.g4dcv.co.uk/ls35a/pics/HFC3_LS3.pdf

What is the rationale for doing that? If it sounded lifelike on the ls3/5a, did it mean it sounded bad on other speakers? Isn't that bad for monitoring purposes?
And when it's 'voiced octave by octave', they are doing that with measuring instruments? Some sound engineer sits and tunes the speaker response by ear? When tuned by microphones / machines, the result isn't acceptable?

I'm not an engineer, so don't know enough about the process....

Bear in mind that the LS3/5A was designed as a speech / drama effects monitor for use in restricted spaces like Outside Broadcast vehicles. It was never intended for music reproduction. As such, the correct rendering of speech at 'natural' levels was considered the most important thing.

S.

 

[tuga]

Bear in mind that the LS3/5A was designed as a speech / drama effects monitor for use in restricted spaces like Outside Broadcast vehicles. It was never intended for music reproduction. As such, the correct rendering of speech at 'natural' levels was considered the most important thing.

S.

These beasts:

 

[whazzup]

Bear in mind that the LS3/5A was designed as a speech / drama effects monitor for use in restricted spaces like Outside Broadcast vehicles. It was never intended for music reproduction. As such, the correct rendering of speech at 'natural' levels was considered the most important thing.

S.

Right. I get that. But from the article, if they deliberately make the speakers sound life-like, in a (albeit poor) monitoring setting, does it mean it'll be inaccurate in other speakers? That's the part that confuses me.

 

[sergeauckland]

Right. I get that. But from the article, if they deliberately make the speakers sound life-like, in a (albeit poor) monitoring setting, does it mean it'll be inaccurate in other speakers? That's the part that confuses me.

I not sure I understand the question. The LS3/5A was one of the first 'high quality' loudspeakers specifically designed for small spaces. All previous monitoring loudspeakers were much larger, so they wouldn't have fitted conveniently in the small spaces. If an LS3/5A was compared with a much larger loudspeaker,in a larger space, the LS3/5A would come off quite badly, especially with music. Equally if a larger loudspeaker was shoehorned into a small OB van, then whilst it would work, it would be inconveniently large and of no real extra benefit for speech and effects.

The LS3/5A was very much a 'horses for courses' loudspeaker, designed for a specific function. It's amazing to me that they became so popular, as they were never intrinsically a very good loudspeaker.

S.

 

[Soniclife]

Always had this question when I read this about the LS3/5a (linked from Wikipedia):
"This is why the frequency response is not flat; the speaker was critically voiced octave-by-octave, so that the whole radiated sound, driver and cabinet effects included, sounded most lifelike."
http://www.g4dcv.co.uk/ls35a/pics/HFC3_LS3.pdf

What is the rationale for doing that? If it sounded lifelike on the ls3/5a, did it mean it sounded bad on other speakers? Isn't that bad for monitoring purposes?
And when it's 'voiced octave by octave', they are doing that with measuring instruments? Some sound engineer sits and tunes the speaker response by ear? When tuned by microphones / machines, the result wasn't acceptable?

I'm not an engineer, so don't know enough about the process....

I read that as they deliberately prioritised in room response at the low end in the intended space, over flat on axis anechoic response. But it's interesting, they have over cooked the baffle step compensation, which sounds the opposite of what you would expect.

 

[AudioSceptic]

I am not sure speaker stands were a "thing" back then.

No. they weren't. As I recall, Spendor was one of the first to offer stands (actually trolleys with castors) custom made for a speaker, first for the BCI/II and later for the BCIII. I still have BC1s with the Spendor trolleys.

 

[tuga]

No. they weren't. As I recall, Spendor was one of the first to offer stands (actually trolleys with castors) custom made for a speaker, first for the BCI/II and later for the BCIII. I still have BC1s with the Spendor trolleys.

This is from the Spendor BC1 manual:

I've overlaid the "optimum position" response in red on the bottom plot.

 

[whazzup]

Ah.....think I get it now. It must have been a problem with my interpretation of the words in the article then.

My original interpretation of the words 'make voices life-like', led me to think that the speakers were tuned to mimic realistic voices as close as possible, in the van / small spaces.

But a more accurate interpretation should be that they made the speakers 'sound neutral' (and by extension, made voices 'sound life-like') in the van, compensating for the in-room response. Makes sense?

 

[sergeauckland]

Ah.....think I get it now. It must have been a problem with my interpretation of the words in the article then.

My original interpretation of the words 'make voices life-like', led me to think that the speakers were tuned to mimic realistic voices as close as possible, in the van / small spaces.

But a more accurate interpretation should be that they made the speakers 'sound neutral' (and by extension, made voices 'sound life-like') in the van, compensating for the in-room response. Makes sense?

That would be my interpretation too. Male speech is especially difficult to get right. It's so often 'chesty' with 'cupped hands' coloration. Both terms now somewhat passé.

S