The BBC-Marconi Type A Microphone: a curatorial perspective

by Iain Logie Baird, 27 April 2021

The BBC-Marconi Type A is one of the most important microphones in the history of radio broadcasting in Britain. The story of its development is a microcosm of how the 1930s BBC Engineering Research Department innovated with the goal of providing the best available sound quality in both radio and early television. Its story also exemplifies how the BBC was influenced by other industries, in its case, the American film industry.

Part of our story concerns some of the magic of radio broadcasting experienced by the public in broadcast radio's earliest years, when microphones still presented an unfamiliar visual image. Broadcast microphones, prominently displayed in photographs and on magazine covers of the mid-late 1930s, often alongside celebrities, symbolised modernity and a collective hope against the backdrop of the Great Depression and fears of war. For Britain, it was usually the Type A microphone that filled this role, and its excellent sonic performance meant that it remained in regular use for many years, strengthening its cultural image.

Early development

In 1924, German scientists Walter H. Schottky and Erwin Gerlach co-invented the first ribbon microphone. They described it in German Patent 434855C, applied for on 21 December 1924 and assigned to the German electrical engineering company Siemens & Halske (S&H) in Berlin.1 Its basic principle was that an extremely fine metallic ribbon could generate electric signals corresponding to sound waves if suspended in a magnetic field.

Harry Ferdinand Olson, an engineer working for the Radio Corporation of America (RCA), designed their first ribbon microphones. Olson's relevant patent was US1885001A, filed on 31 March 1931.2 Manufacturing of a commercial model was already in progress through one of RCA's associated companies, 'RCA Photophone'. RCA Photophone was in the business of providing one of the systems for synchronising electrically-recorded audio to motion picture images. Four competing systems to do so had emerged in the American film industry in the late 1920s.

In May 1931, at the Spring Conference of the Society of Motion Picture Engineers (SMPE) held at the Roosevelt Hotel in Hollywood, representatives of the BBC (who I am still trying to identify) took notice of the RCA Photophone model PB-31 ribbon microphone (with a limited run of about 50 units) that was being used by the American film industry at the time.3 It had immediately become popular with filmmakers both because its sensitivity gave freedom of movement on the set and because it had an insensitive side that could be turned toward the camera—minimising camera noise in soundtracks.4 Unbeknownst to the BBC it seems, at least one example of the microphone was already in England, being shown to about thirty RCA recorder licensees also in May of that year.5 Six months later, two of the new microphones were supplied to the Associated Radio Pictures film studios at Ealing.6

The PB-31 was costly and even a new cheaper RCA model (44-A) introduced in 1933 would have cost the BBC £130 each (including its accompanying amplifier). This was deemed too expensive, because as a national radio broadcaster the BBC needed microphones in large numbers, unlike a film studio which could make do with two or three. It was at this point that it was decided that the BBC would need to design and build its own version.

The engineer selected to design the new microphone was Dr. Francis William Alexander. He had just joined the BBC Engineering Research Department, having moved from Dundee where he had worked as a church organist and choirmaster at South Church, Blairgowrie before successfully completing his Ph.D.7 Alexander would have had in mind the requirements of a singer and musician as well as those of an electrical engineer. The development work was done in 1934 and the new microphone was introduced into the BBC's London studios toward the end of that year.8 9 Having proven itself in London through 1935, it arrived in the regions in 1936.10 The Type A was the first ribbon microphone to be mass-produced in Britain, at a cost of only £9 each.11

Patents, and British Marconi's role

In Alexander's design, as with any ribbon microphone, minute differences in air pressure in front of and behind the ribbon cause it to vibrate, creating a corresponding electrical signal in conjunction with a powerful permanent magnet and its pole pieces. As a velocity microphone, it does not depend for its operation on the actual pressure of the sound wave (like most microphone designs), but, instead upon the differences in pressure between the front and back sides of the ribbon. The principle is called velocity because the ribbon's movement is proportional to the velocity of the air molecules which, in vibrating, transmit sound waves. As the ribbon vibrates between the pole pieces due to the effect of a passing sound wave, there is generated within the ribbon a voltage which varies in both amplitude and frequency in direct accordance to the changes of the sound wave.12

There was a risk of infringement of RCA patents but, after many meetings with patent agents, it was decided that the BBC had a valid patent, and this was taken out in Alexander's name in Spring 1934.13 As RCA was closely associated with the British Marconi Company, it was diplomatically decided to have the Marconi manufacture the microphones.14 Edward Pawley later summarised the distinction between the BBC and RCA designs as follows:

The BBC microphone (type A) differed from the RCA mainly in the design of the pole-pieces, which were made much thicker to increase the sensitivity. This lowered the critical frequency to about 4500 Hz and the resulting drop in the response above this frequency was counteracted by shaping the pole-pieces to provide a resonant cavity on each side of the ribbon [described in Alexander's original patent GB429307A]. Thus, the RCA version behaved as a velocity, or pressure-gradient, microphone up to 9000 Hz, whereas the BBC-Marconi microphone behaved as a pressure microphone at frequencies above 4500 Hz.15

H.F. Olsen had worked out the theory underlying the operation of ribbon microphones in general around 1930, which he had published in papers in the Journal of the Acoustical Society of America. Alexander later wrote that all of the efficient modern designs were based, to some extent, on Olsen's work.16

BBC-Marconi AXBT microphone innards, source: BBC Engineering training manual, 1951.

Subsequent improvements were made to the Type A microphones by Alexander and other engineers. The first of these was an extremely thin (1/10,000th of an inch) beaten aluminium X-foil corrugated ribbon introduced almost immediately. These second generation microphones were designated Type AX. The original duralumin foil ribbon material, although already very thin, had been found to create undesirable resonances starting from 35 c/s.

Use in broadcasting

The Type A was sensitive for about 120 degrees both at the front and the back, with two dead sectors of about 60 degrees on either side, (emulating the desirable 'figure 8' bidirectional characteristic of the RCA microphones). Asa Briggs later observed: 'Its value in variety—as in drama and radio discussion work—was obvious, and it became the standard studio microphone in 1936'.17 Since the microphone was equally sensitive to sounds coming from the front or from behind, performers could be placed on both sides of the microphone. This was a very convenient arrangement for interviews and cases where a soloist was performing with an orchestra. The 'dead' zones of the microphone could be put to good use in giving the effect of distance to a performer situated in the zone, and if the performer moved only a few feet out of the zone the semblance of rapid approach could be obtained.18

A 1951 BBC training manual advises that in order to avoid ‘an excessive bass response ... the microphone should never be used at a distance less than approximately two feet’.19 This was because a phenomenon known as 'bass tip-up' resulted from the pressure gradient design having to cope with the minute difference in phase on either side of the ribbon at low frequencies.20 The two foot rule was a carry over from issues experienced soon after the microphone was introduced, when it was only a one foot rule, the shorter distance probably due to the early Type A's lower sensitivity compared to the post-war version. Exemplifying the problem, Briggs quotes from a letter from F.W. Alexander to M.M. Dewar (a BBC Variety Department executive) dated 14 September 1936:

After considerable experience with ribbon microphones, it was decided that crooners would not be allowed to come nearer than one foot to a ribbon microphone. This obviously has meant that Producers and Balance Assistants have had more or less to train vocalists, in this new technique, sometimes under great difficulties. On the whole this has been carried out satisfactorily. However, one difficulty has arisen. In cases of American Variety artists, who are accustomed to working very much closer to the microphone, it has been found impossible to get these artists to change their technique for perhaps just one broadcast with the BBC, such artists as the street singer (Arthur Tracey) and Pop-Eye the Sailor (Mr Costello). The only thing to be done in these cases is to alter the electronic circuit, so that the singer may approach the microphone in the usual American way.21

The circuit alteration described above was probably an equaliser added between the microphone and its associated amplifier, the equaliser consisting only of a series condenser of suitable value that could easily be calculated for a 300 ohms termination, 300 ohms being the output impedance of the microphone.22

The Type A series microphones continued to be standard BBC studio equipment throughout World War II. One appears in a well-known photograph of Neville Chamberlain delivering one of his pre-war speeches, which is often said to depict the actual declaration of war against Germany at 11.15am on 3 September 1939. The background of that particular photo, however, is a Broadcasting House studio, while the declaration broadcast was made from the cabinet room at 10 Downing Street.

Bush House in London was used for radio broadcasts to Europe during the war, and several important broadcasts of European figures in exile—such as Charles de Gaulle of France, and Queen Wilhelmina of the Netherlands—were done using these microphones, providing information and some hope to those in the occupied countries.

Example of a balanced circuit, source: H. Burrell Haddon, High Quality Sound Production and Reproduction, London: Illife Books Ltd., 1962, p. 105.

In 1943, a balanced circuit was added. Microphones with this improvement were designated Type AXB. This circuit caused induced noise currents to cancel each other out if the microphone was used within an electromagnetic field.

The final improvement was made in 1944—a 'Ticonal' alloy (titanium-cobalt-nickel-aluminium) permanent magnet in place of the original cobalt steel, increasing sensitivity by 6 decibels. Microphones fitted with the new magnet were designated Type AXBT. A similar 'Alnico' alloy (aluminium-nickel-cobalt) had already been used for the smaller, circular permanent magnet used in the Type B ribbon microphone. The Type B was only about 4 in. in diameter, and had been brought into service a full seven years earlier in 1937, intended for use in outside broadcasts.23,24

(above) The Test Pilot sketch from Hancock's Half-Hour, broadcast and recorded on 30 December 1956.

Type A as archetype

Throughout the 1950s the Type A continued to be the BBC's standard studio microphone, (with the last unit manufactured in 1959), thus part of the Type A's significance results from its longevity. With its characteristic bass-proximity boost, no other microphone contributed more to the overall 'sound of the BBC' during those years. The ribbon mics in general also had a distinct sound amongst other microphone technologies used by the BBC like carbon, condenser, moving coil, crystal and so on.

In certain applications however, the Type A microphone was found to be visually obtrusive, too heavy, or subject to background noise. Starting with the Type B in 1937, a range of special-purpose ribbon microphones were introduced by the Research Department using the latest Type A's acoustic performance as their benchmark. The final AXBT version was 'never bettered for performance by any other ribbon microphone', Pawley boasts in a 1972 BBC Engineering monograph celebrating the 50th anniversary of the BBC.25

Although it was a difficult design to improve upon, there are three BBC-designed ribbon microphones that could be considered successors to the AXBT. (1) One of these was the Pressure Gradient Single (PGS, PGS/1) introduced into service at the end of 1952, manufactured for the BBC by Standard Telephones and Cables, Ltd.26,27 It was a downsized version of the AXBT largely to meet the needs of the Television Service.28,29 The 'S' described the single-ended horse-shoe magnet employed. (2) A significantly larger 'deluxe' version of the PGS was made in 1953 by S.T. & C. as part of a special agreement with the BBC. This microphone was designated model 4038-A (with manufacturing of it transferring in 1972 from S.T. & C. to Coles Electroacoustics Ltd.).30 (3) The third 'successor' was the Pressure Gradient Double (PGD, PGD/1), even though it was only made in very small numbers compared to the PGS. The PGD utilised two permanent magnets with one ribbon to increase the magnetic efficiency, smoothing the frequency response.

Microphones Type PGD (left), AXBT and PGS (right), S.T.&C. 4038 not shown, source: Shorter and Harwood, BBC Monograph 4, 1955

All of three of the new ribbon mics were smaller than the AXBT, yet with similar performance, while providing much better high-frequency response.31 In 1938, Alexander noted in an article in World-Radio how such a result could be achieved with a small baffle area and smaller pole pieces compared to the BBC-Marconi Type A/AX. He argued that the alteration resulted in a microphone of lower sensitivity and summarised this engineering compromise as follows: 'Various expedients have to be adopted in practical designs in order to combine good sensitivity and adequate frequency range'.32

Despite the parallel advances in design of other types of microphone, (notably the electrostatic/condenser variety), the Type A series even by the mid-50s were still used for studio broadcasting in Britain in greater numbers than any other kind, by reason of their simple construction, relatively low cost, and ease of maintenance. The Type A, with its characteristic lozenge shape and 25-year-history, was unquestionably the state-of-the-art microphone of its time and even today, it symbolises the BBC's broadcast tradition and its global reach.

1 Walter Schottky, 'Band membrane for band loudspeakers or band microphones', German patent DE434855C, granted 1 October 1926,

2 Harry F Olson, 'Apparatus for converting sound vibrations into electrical variations', US patent US1885001A, published 25 October 1932,

3 H.F. Olson, 'The Ribbon Microphone', Journal of the SMPE (June 1931) 16, pp. 695–708.

4 Edward Pawley, BBC Engineering 1922–1972, London: BBC Publications, 1972, p. 120.

5 'RCA Advances', Kinematograph Weekly (7 May 1931), p. 14.

6 'Latest RCA Recorders for A.R.P.', Kinematograph Weekly (12 November 1931), p. 72.

7 'B.B.C. Chooses Dundee Man as Head of New Department', Dundee Courier (14 April 1939), p. 3.

8 E.L.E. Pawley, 'BBC Engineering 1922–72', BBC Engineering (October 1972) 92, p. 31.

9 Pawley, BBC Engineering, p. 120.

10 'B.B.C. To Have Own Cinema Organ', The Nottingham Evening Post (12 February 1936), p. 6.

11 Asa Briggs, The History of Broadcasting in the United Kingdom: Volume II: The Golden Age of Wireless, Oxford: Oxford University Press, 1995 (reprinted 2000), p. 92.

12 G.L. Taylor et al., Lesson Two: Decibels and Microphones, Kansas City, Missouri: Midland Television Inc., 1938, p. 29.

13 Francis William Alexander, 'Improvements in and relating to electrodynamic vibratory devices such as microphones', British patent GB429307A, applied for 6 March 1934,

14 Pawley, BBC Engineering, p. 120.

15 Ibid.

16 F.W. Alexander, 'Ribbon Microphones: The Underlying Principles of their Design', World-Radio (13 May 1938) XXVI(668), pp. 10–11.

17 Briggs, History of Broadcasting, p. 92.

18 E.T. Wrathall, 'The B.B.C.-Marconi Ribbon Microphone Type AXBT', Marconi Review (July and September 1949) No. 94, p. 96.

19 The Staff of the Engineering Training Department, BBC Engineering Training Manuals: Microphones, London: Illife & Sons, Ltd. (for Wireless World), 1951, p. 74.

20 H. Burrell Hadden, High Quality Sound Production and Reproduction, London: Illife Books, Ltd., 1962, pp. 73–74.

21 Briggs, History of Broadcasting, pp. 109–110.

22 Wrathall, 'The B.B.C.-Marconi', p. 96.

23 Pawley, BBC Engineering, p. 120.

24 Alexander, 'Ribbon Microphones', p. 11.

25 Pawley, 'BBC Engineering 1922–72', p. 31.

26 D.E.L. Shorter, 'The Design of the PGD and PGS Ribbon Microphones', BBC Report 1953-16 (Jan 1953), p. 1.

27 D.E.L. Shorter and H.D. Harwood, 'The Design of a Ribbon Type Pressure Gradient Microphone for Broadcast Transmission', BBC Monograph (December 1955) 4, pp. 5–6.

28 Shorter, 'Design of the PGD', p. 2.

29 Shorter and Harwood, 'Design of', pp. 5–6.

30 Model 4038 and other ribbon microphones are still in demand today. Coles Electroacoustics Ltd., 'Our Heritage',

31 Shorter and Harwood, 'Design of', p. 15.

32 Alexander, 'Ribbon Microphones', p. 11.