The BBC-STC 'Bomb' Condenser Microphone: a curatorial perspective

by Iain Logie Baird, 25 June 2021

Among the assorted microphones used by the BBC in the 1930s, the ‘teardrop’ or ‘bomb’ condenser type has a particularly powerful presence in photographs—conveying radio modernism and an Art Deco aviation aesthetic that harmonised with Broadcasting House's interior designs. Today, surprisingly little is known about these microphones, not only because in the 1930s other types were publicised more by the BBC, but also because today, condenser microphones are inexpensive and commonplace. This is the inverse of the situation in the 1930s where the technology had not been perfected and experiments with condensers were being run concurrently with their use in broadcasting. In this article, I look at the history of this microphone, describing its configurations, its built-in amplifier, and its application in broadcasting. I also suggest that a condenser microphone used by the American film industry may have influenced its design.

What are condenser microphones?

Condenser microphones consist of two plates of a condenser, whose distance apart is altered by the sound waves impinging upon one of them. The consequent variations in capacity affect an external circuit where they can be amplified to any desired extent. These variations, however, are so infinitesimal that it is preferable to amplify them immediately, thus most such microphones had small one-stage ‘head’ amplifiers built-in.

The idea of measuring sound with a condenser originated from an American physicist, E.C. Wente (1889–1972). He worked at Western Electric/Bell Labs from 1914 until 1954. His early attempts at developing a 'telephone transmitter', starting around 1916,1 were unsuccessful and during the early years the technology was used mostly for audio experiments.2 By 1922, the company was able to produce the first commercially available condenser microphone—the 394-W condenser capsule developed by Wente (which used an aluminium alloy diaphragm) combined with a single-valve Western Electric 47-A amplifier.

A Western Electric 394 condenser capsule, BBC Handbook 1929, London: British Broadcasting Corporation, 1928, p. 302.

Condenser microphones were used in various forms by the BBC between 1927 and 1939.3 The first type, tried at the Birmingham station, was a Western Electric model.4

The M.G.M. ‘bomb’ microphone

In 1931, Wesley Miller, a sound engineer for the Metro Goldwyn-Mayer company came up with the idea of building a spherical casing for the Western Electric 47-A amplifier and fitting the 394 condenser capsule into this. The 47-A factory casing had been cylindrical but Miller believed that a spherical enclosure would further reduce internal turbulence. The black-painted spherical shape meant that it was soon referred to as the ‘bomb’ microphone.5 It was used by M.G.M. in a small number of Hollywood films and still exists today, preserved in a private collection.6 A similar spherical condenser microphone unit of the era recently surfaced in Florida—slightly smaller, with a black wrinkle finish.7

It is possible that one of these American-made spheres inspired the BBC’s ‘bomb’ microphone. This would have likely happened at the same time one of RCA-Photophone’s ribbon microphones is well-known to have inspired the BBC Type A ribbon microphone—on the occasion of BBC representatives attending the annual convention of the Society of Motion Picture Engineers in 1931.8 M.G.M.’s microphone differed from the later BBC ‘bomb’ in that its shape was to reduce internal resonance and hence valve noise—thus it was for purely acoustical reasons.

Building the amplifier

In 1932, the ‘teardrop’ one-stage single-valve amplifier was developed by BBC engineers, consisting of a cylindrical chassis embodying the components, mounted in rubber sponge inside a streamlined aluminum housing. It was noted at the time that the streamlining itself ‘did not possess any acoustical advantage, other than enabling a neat and pleasing unit to be designed, in accordance with the acoustical requirements’.9

The microphone capsule inside was a Western Electric 394 or a similar unit manufactured under license in Britain by Standard Telephones and Cables (STC), a company that had been created in 1925 from British Western Electric.

Condenser microphones tended to be susceptible to humidity, reacting to it by making ‘frying noises’, therefore they were never used for outside broadcasts. It was found that even the slightest trace of moisture needed to be excluded from the wiring and components of the amplifier by sealing up as much as possible of the latter in paraffin wax.10 In addition, the greatest care needed to be taken in the selection of the type of valve used in the amplifier. The head amplifier introduced noise because low-noise valves were not then available.11 BBC engineers tested different types of valves in this application to try to fix the problem. By early 1933, certain types of separately-heated cathode valves were found to be the most satisfactory.12 Despite these precautions, valve noise remained a difficulty owing to the low sensitivity of the microphone itself.13


According to BBC historian Edward Pawley, ‘its quality appears to have been no better than that of the [Marconi] Reisz [carbon granule] microphone at its best’.14 The source for this statement appears to be an assertion made in the BBC Year-Book 1933.15 Contradicting this, it was argued in the BBC Year-Book 1930 that condensers outperformed the carbon microphones in terms of both signal-to-noise ratio and frequency response.16 It also appears that significant improvements were made during late 1933 and early 1934. In March 1934, wireless reporter Leonard P. Laurie wrote,

Probably most listeners must have noticed a marked improvement in the B.B.C. transmissions over the past few months. A great deal of experimental work has been carried out with the new condenser microphone, which is now, I believe, used almost exclusively in place of the older carbon, Reiss type. The condenser microphone has the ability to handle large volume without overloading and moreover, its absence of hiss is important. When the microphone is “open” and no actual matter is being radiated, the background is remarkably free from hiss in comparison with a year or so ago.17

The B.B.C. Year-Book 1934 describes two new types of moving-coil microphones capable of a high-frequency response up to 10,000 cycles thus some of the improvements observed by Laurie may have been due to these microphones. Condensers at this time were not able to pick up such frequencies:

The principle of construction is that the diaphragm is stretched until its natural frequency is near the upper limit of the useful range. In actual practice, however, it is not in general practicable to obtain a resonant frequency above about 4500 cycles per second. Above this frequency the response falls off rapidly, so that it becomes negligible above 6,000 cycles. This constitutes one of the limitations of the condenser microphone.18

The Voigt slack-diaphragm condenser

The BBC tried different condenser microphones with their teardrop amplifier casing. Efforts were made to utilise microphone designs originating in Britain.19 One of these was the slack-diaphragm condenser designed in 1931 by the British engineer P.G.A.H. Voigt (Paul Gustavus Adolphus Helmuth, 1901–1981).20,21 Voigt’s design differed from other condenser microphones because instead of the conventional stretched diaphragm, a foil diaphragm was wrapped around a central electrode with an elliptical cross-section. The thickness of the air gap was thus irregular. The air cavity communicated with a chamber that could be filled with calcium chloride as a dampening agent if desired.

The 1931 Edison Bell condenser microphone, cable connector, and spare calcium chloride, image courtesy History of Science Museum, Oxford.22

The most important characteristic of Voigt’s microphone was that it would respond equally to sounds from any direction. The disadvantage was that it was insensitive, consequently, a high degree of amplification was required. The screened leads between the microphone and the amplifier needed to be kept as short as possible, since their capacity was in parallel with the microphone capacity. Increasing the length of the leads from 3ft to 12ft. reduced the output further from 2.5 to only 0.7 millivolts with a full orchestra.23 Edison Bell had already supplied full directions for setting up the microphone and constructing a suitable amplifier to anyone who purchased a microphone, which was affordably priced at 6 guineas.24 Fitting it to the BBC’s new amplifier was an obvious experiment to try.

The 1932 BBC single-stage amplifier fitted with a 1931 Edison Bell slack-diaphragm condenser, BBC Year-Book 1933, p. 381.

The hole in the wall

Referring back to 1924, Voigt recalled:

In those early days, I had done a mental preliminary survey, not of what bits and pieces should be put together to get good audio, but more fundamentally of what good audio would be like if we ever got it.

My 1924 answer was my hole in the wall theory, which was controversial for a long time. Some people thought perfect reproduction should sound as though the sound originated in the room you were in. This overlooks the fact that your room has one set of reverberations and the studio or concert hall a totally different one. The latter set can easily be made negligible by having the announcer come right up to the hole on his side of the wall and, as it were, talk direct to you through that hole. That theory and the consequent understanding of what to aim for has been fundamental to my outlook.

I had a portion of the wall between the studio and the lab removed and a shelf put across the opening. On that shelf I put a specially designed mike … and I hid the opening with thin silk or something. The mike’s square frame did not fit the opening, so I closed the space around it with strips of carpet. The face of the mike was in the plane of the partition wall, simulating a closed window.

From what I learned from the excursion requirements, any velocity operated mike, moving iron or moving coil, already had to be free to move about with any appreciable restraint. The ordinary arrangement of a diaphragm clamped around its edge was out.25

The BBC, as one would expect, had developed a detailed policy on studio acoustics,26 which essentially agrees with Voigt’s hole-in-the-wall concept. Almost a decade later, in the B.B.C. Year-Book 1933, it should therefore be unsurprising that his new microphone was a topic of discussion.

It will be seen that the slack-diaphragm condenser microphone possesses one considerable advantage from the point of view of studio technique in certain types of broadcast, in that it is practically non-directional in a plane at right angles to the principal axis of the microphone. In a dramatic production, for example, it is possible for a number of actors to be grouped comfortably round a single microphone, instead of trying all to get into the field of a fairly directional microphone or being obliged to use two or more of such a type.27

Use in broadcasting

Condensers were used by the BBC in significant numbers from 1932. The BBC identification numbers for the ‘teardrop’ types seem to have been CM/2/1 up to at least CM/2/16. In addition to the Edison-Bell microphone described earlier, other less commonly-used condensers included RCA’s 4AA (or 4AP) introduced in 1928. These had a cubic wooden case that contained a three-valve amplifier. There was also a ‘BBC-Marconi’ condenser produced in relatively small numbers around 1935, manufactured in London by Technical & Research Processes Ltd. (Visatone).28,29

The carbon and condenser microphones were sometimes transferred between different studios. A standardised plug and socket system was developed to facilitate this.30

Owing to the difficulty of allocating a studio to particular types of programme and adhering to these decisions rigidly, it was considered advisable to design a Microphone Plug and Socket which could be used for both types of microphone. An eight-point plug and socket [see illustration below] is employed, and by using certain pins on the plug for certain microphones, either type of microphone can be plugged into any microphone socket. The eight pins used are as follows:—

  • Two for L.T. supply to Condenser Microphone Amplifier.
  • Two for two different values of H.T. supply to Condenser Microphone Amplifier. H.T.—is common to L.T.—
  • Three for Microphone output, one pin being common to the two different types of microphone.
  • One for earth.

A BBC microphone plug and socket, A Technical Description of Broadcasting House, 1932, p. 71.

Most studios were wired for more than one microphone. 'In some studios the microphone points are wired in parallel, and in others separately. In general terms this means that, in the former case, one microphone only can be used at a time, whereas, in the latter case, any number of microphones, up to the number of sockets available, can be used at once'.31

Use in television

The hole in the wall between studio and scanner was a feature of mechanical television studios, a feature which had started in 1926 in John Logie Baird’s studios at Motograph House and later, 133 Long Acre. It was emulated in the BBC's first television studio (BB), which was located in the basement and sub-basement of Broadcasting House. Studio BB had been given the ‘normal’ acoustic treatment as it had originally been designated as the dance music and jazz studio. Its wall surfaces had been treated with building board, felt, and wallpaper, and a certain amount of decorative fabric.32 However, rather than Voigt’s slack-diaphragm condenser, the condenser microphone used was the conventional Western Electric/STC stretched-diaphragm type, installed in a BBC ‘teardrop’ amplifier. There was also a Marconi-Reisz carbon granule microphone and a third microphone in the studio for announcements. Photographs indicate that the condenser microphone was usually placed alongside the performers. Once a television programme began, the studio was almost completely dark because the studio operated on the ‘flying spot’ system. In this system, a narrow beam of light would rapidly scan across the scene and the reflections would be picked up by photo-electric cells. The cells were on the studio wall or wheeled stands. It was awkward to move the microphone while a performance was in progress, so the positioning of it before the beginning was quite critical.

One reason that the condenser microphone was chosen for television was that it was omnidirectional—it could pick up sound from all directions. This allowed performers to move around in the studio freely without constantly worrying about drifting out of range. The B.B.C. Year-Book 1933 notes: ‘In a dramatic production, for example, it is possible for a number of actors to be grouped comfortably round a single microphone, instead of trying all to get into the field of a fairly directional microphone or being obliged to use two or more of such a type’.33

The non-directional microphone has also been used in the production of Vaudeville performances where the theatrical atmosphere and lay-out of artists and orchestra have been preserved. A single non-directional microphone of the type under discussion has been suspended over the “footlights,” or the position usually occupied by such accessories, at a height just above the heads of the performers. In such a position both artists and orchestra are correctly reproduced and suitably balanced, an impossibility with a more directional microphone.34

John Logie Baird and the Danish film star Carl Brisson preparing for a special long-distance telecast from Studio BB to a large screen at the Arena Theatre in Copenhagen on 8 November 1932. Gordon Ross, Television Jubilee, p. 17.

A performing sea-lion televised in Studio BB circa late 1932 a bank of photo-cells at far left. B.B.C. Year-Book 1933, p. 440.

Regular television broadcasts began from this studio on 22 August 1932 using Baird’s 30-line system. These were semi-experimental in that the service was broadcast to only a few thousand ‘lookers-in’, using existing sound transmitters during off-hours. It was expected that in a few years, bespoke television transmitters would enable television broadcasting to move to a more advanced stage.

Later use in radio

Although condensers were superseded by the extremely successful Type A ribbon microphone introduced in 1935, before they were phased out they were used by several prominent figures including the Prince of Wales (later Edward VIII) and Neville Chamberlain, particularly from Studio 3B which was outfitted with one of these microphones in an elegant chromed suspension mount from about 1933 until 1936.35 Out of twenty-two studios in Broadcasting House it was one of three small studios on the third floor called ‘Talks Studios’—3B, 3C, and 3D. They were all approximately 15 ft. by 11 ft. and 9 ft. high. Each was provided with a chair and reading desk, and the furnishing was ‘carried out so as to suggest the atmosphere of a library or study and thus create a favourable atmosphere for the speaker’.36 So far as talks were concerned, it was believed that ‘the voice must be modified only by the acoustics of the listener’s own room’, in other words ‘acoustically dead’.37 Despite this policy, 3B and 3D were treated with ordinary building board because ‘persons with little experience of broadcasting’ were ‘likely to be disturbed by the lack of reflected sound’—only studio 3C was given the full treatment for use by the professional broadcasters with mineral wool behind a decorative fabric.38

The Prince of Wales (later Edward VIII) in Studio 3B circa 1935.

Today it is often implied (due to misinterpretation of the photograph above) that one of these microphones was used when Edward the VIII gave his abdication speech on 11 December 1936, however, the abdication broadcast was made from Windsor Castle and a condenser microphone would not have been used. Similarly, the photo below is sometimes misinterpreted to relate to Chamberlain's announcement that Britain is at war. That announcement was broadcast on 3 September 1939 from the Cabinet Room at 10 Downing Street.

Chancellor (later Prime Minister) Neville Chamberlain in Studio 3B circa 1935.

Condensers were often considered experimental and used in small numbers compared to other microphone types such as moving-coil, carbon granule, and ribbon, however, by the 1950s the technology had improved significantly and condensers made a comeback—to be used by the BBC in regular service. Sadly, due to the miniaturisation of valves and other components, the neat and pleasing teardrop shape would never return.

A fairly accurate replica of the microphone appears in the feature film The King’s Speech (Tom Hooper, 2010), with actor Colin Firth portraying the Duke of York (the future King George VI) making a broadcast. In the opening scene of the film, there is an anachronism. The microphone is seen in use in a BBC studio in 1925, seven years before it was introduced.

1 Edward C. Wente, 'Telephone-transmitter', American patent application US1333744A, 20 December 1916.

2 E.C. Wente, ‘A condenser transmitter as a uniformly sensitive instrument for the absolute measurement of sound intensity’, Physics Review (1917) 10, pp. 39–63.

3 Asa Briggs, The Golden Age of Wireless, The History of Broadcasting in the United Kingdom, Vol. II, London: Oxford University Press, 1965, p. 97.

4 ‘The Performance of Microphones’, B.B.C. Handbook 1929, London: British Broadcasting Corporation, 1928, p. 302

5 ‘Bomb-Shaped Ash Tray Inspiration of New ‘Mike’’, The Nassau Daily Review (5 August 1931), p. 7.

6 For more information on the M.G.M. ‘bomb’ microphone, see the YouTube videos: and

7 ‘Vintage Western Electric 394 Studio’,,

8 Iain Logie Baird, ‘The BBC Type A Microphone: a curatorial perspective’,,

9 ‘Microphones’, B.B.C. Year-Book 1933, London: British Broadcasting Corporation, 1932, p. 380.

10 Ibid.

11 Edward Pawley, BBC Engineering 1922–1972, London: BBC Publications, 1972, p. 119.

12 ‘Microphones’, p. 380.

13 Ibid.

14 Pawley, p. 119.

15 ‘Microphones’, pp. 372–375.

16 ‘Advances in Microphone Construction’, B.B.C. Year-Book 1930, London: British Broadcasting Corporation, 1929, pp. 307–308.

17 Leonard P. Laurie, ‘Condenser Microphones’, Falkirk Herald (24 March 1934), p. 5.

18 ‘Recent Developments in Microphone Design’, B.B.C. Year-Book 1934, London: British Broadcasting Corporation, 1933, p. 411.

19 ‘Notes of the Year’, B.B.C. Year-Book 1933, London: British Broadcasting Corporation, 1932, p. 328.

20 The principles were described in British patents no. 263,300 and 324,152.

21 ‘Microphones’, pp. 375–376, 379–381.

22 History of Science Museum, 'Condenser Microphone, by Edison Bell',,

23 ‘Edison Bell Condenser Microphone’, Wireless World (26 August 1931) XXIX(9), p. 205.

24 Ibid.

25 Bruce C. Edgar, ‘P.G.A.H. Voigt 1901–1981, Loudspeaker Pioneer: An Interview with P.G.A.H. Voigt’, Speaker Builder (March 1981), p. 13.

26 A Technical Description of Broadcasting House, London: British Broadcasting Corporation, 1932, p. 40. 27 ‘Microphones’, p. 380.

28 Marconi’s Wireless Telegraph Co., Ltd. and H.J. Round, ‘Improvements in or relating to audio frequency transformers’, British patent application 7695/34, 10 March 1934.

29 The Staff of the Engineering Training Department, BBC Engineering Training Manual: Microphones, London: Illife & Sons Ltd., 1951, pp. 88–90.

30 A Technical Description, p. 71.

31 Ibid.

32 Ibid., p. 46.

33 ‘Microphones’, p. 380.

34 Ibid., p. 381.

35 Broadcasting House, London: The British Broadcasting Corporation, 1932, p. 62.

36 A Technical Description, p. 35

37 Ibid., p. 46.

38 Ibid.