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75 Years Of The Shure Unidyne 55

One Direction
Published September 2015
By Hugh Robjohns

The iconic, original Shure Model 55.The iconic, original Shure Model 55.Photo: Shure, Inc

In 1939, Shure revolutionised the music industry with a microphone so successful that it is still in production today!

The American Shure Brothers Company, now Shure Incorporated, sits proudly amongst the world’s longest-surviving microphone builders. The company’s history began in April 1925, when Sidney N Shure started supplying components and kits for home-build radios. A few years later, with radio broadcasting, public-address and two-way radio communications becoming more common, he took on the distribution of microphones from a now long-gone manufacturer. Realising the vast potential for this nascent market, Shure Brothers started making their own microphones in 1932, their first in-house design being the Model 33N, a carbon-button microphone (the dominant microphone technology of the day). The Model 70 crystal (piezo-electric) mic and Model 40D capacitor mic followed in 1935, and the company’s first dynamic microphone appeared in 1939.

To put these dates in context, Georg Neumann started his microphone company in 1928, the Russian manufacturer that became Oktava started making mics in 1932, Electro-Voice in 1933 and Beyerdynamic in 1939, while both AKG and Sennheiser were formed post-war in 1946. Of course, there were several pioneering companies making microphones before this, such as Western Electric, Westinghouse, RCA, Marconi, EMI and others, but none are still involved in the microphone business today!An interesting exercise in “what might have been”: this 1937 prototype of the mic that would become the Unidyne features very different styling.An interesting exercise in “what might have been”: this 1937 prototype of the mic that would become the Unidyne features very different styling.

Double Trouble

The microphone that catapulted the Shure Brothers Company into worldwide recognition was also the world’s first single-element dynamic cardioid microphone: the Unidyne Model 55, introduced in 1939. We take single-element cardioid mics for granted today, but in the ’30s, this was an incredibly significant innovation. Before the Model 55, the only practical way to obtain a cardioid polar pattern was to combine the electrical outputs from separate figure-of-eight and omni capsules mounted within the same mechanical housing, as was done in the Western Electric 639A, which combined a figure-of-eight ribbon element and an omnidirectional moving-coil capsule. However, this solution was far from ideal as it was difficult and expensive to manufacture, heavy and bulky, and typically suffered from an inconsistent polar pattern and an uneven frequency response. Consequently, Shure’s introduction of a compact, single-element cardioid microphone revolutionised the industry, and the Unidyne Model 55 quickly became as iconic within the music business as the Neumann U47 or AKG C12 in later years.

Some 75 years after its introduction, the Model 55 is still in production (albeit with a few updates and improvements!), and Shure claim that it is “the most recognised microphone in the world”! They probably have a good case, too: as well as the iconic image of a Model 55 cupped in the hand of Elvis Presley, there are countless photos in circulation of artists such as Buddy Holly, Patsy Cline, Frank Sinatra, Ella Fitzgerald, Billie Holiday and even Metallica and Mariah Carey — amongst many others — singing into a Model 55. This classic microphone also featured in many of the critical moments of 20th century history, capturing the speeches of such luminaries as Franklin D Roosevelt, John F Kennedy, Martin Luther King, Indira Ghandi and General Douglas MacArthur on the deck of the USS Missouri during the ceremony ending the war with Japan in 1945.

Such is the revered status of the Unidyne Model 55 that it received an IEEE Milestone Award at the start of 2014, marking its place in the history of the audio industry alongside the work of Alan Dower Blumlein, Marconi’s early radio experiments, and the invention of the CD player.

The Model 55 was an instant hit with singers from Billie Holiday to Hank Williams, who appreciated its revolutionary sound quality and small size.The Model 55 was an instant hit with singers from Billie Holiday to Hank Williams, who appreciated its revolutionary sound quality and small size.Photo: William Gottlieb

To help celebrate the 75th anniversary of the Model 55, Shure created a fabulous book detailing the design story and evolution of the Unidyne Model 55 microphone, with lots of superb vintage photography and original engineering and administrative documents. Apparently this unique and fascinating tome was primarily put together for use internally at Shure, and it seems that few outside the company were ever destined to see it. However, a PDF version can be found online at and I’ve drawn out the pertinent points below...

Designing The Unidyne

In the 1930s the most common kind of professional microphone employed carbon button technology. Although cheap to make, these microphones were inherently noisy, had a very limited bandwidth and required an external DC power source. A common alternative was the crystal (piezo-electric) microphone, but these were delicate and reacted badly to heat, humidity and physical shock. Early capacitor microphones were available, too, but they were very expensive to build, still relatively fragile, and again, required external power. The final option was the ribbon microphone, but these were also fragile, tended to suffer from electromagnetic interference, and needed a large amount of amplification.Hank Williams.Hank Williams.

Consequently, Shure invested considerably in the development of a high-quality moving-coil dynamic microphone: in essence, a miniature loudspeaker working in reverse. Although precision manufacturing techniques were required to construct the capsule in a way that delivered consistent performance, this technology offered considerably greater bandwidth than carbon mics, needed no power, could produce a strong output, could be robust and reliable, and could be built relatively inexpensively.

Shure’s first production dynamic mic capsule was the Model 50 ‘Rocket’, a pressure-operated microphone with a more-or-less omnidirectional polar pattern, albeit with some forward directionality at high frequencies due to the case design. However, what was really needed was a genuine cardioid microphone that would reject unwanted background noise and provide greater immunity from feedback in public-address applications — and that’s exactly what a young development engineer named Benjamin Bauer (see box) came up with for Shure.

Bauer passionately pursued the idea that, if he could find a way of partially blocking and delaying sound waves reaching the rear of a microphone’s diaphragm, it would be possible to create a cardioid polar pattern within a single microphone element. Put simply, if sound from a source at the rear of a microphone reached the back of the diaphragm at the same time as it also reached the front, the net pressure on the diaphragm would be zero, and so the microphone would ‘reject’ sound sources from behind. Arranging for the rear sound waves to reach the diaphragm at the same time as those travelling around to the front required an acoustical delay network in the rear cavity of the capsule, and it took Bauer and his colleagues two years of research and development to optimise the design so that the polar pattern was reasonably consistent at all frequencies. Moreover, by varying the acoustic resistance of the rear capsule openings, Bauer was able to achieve almost any desired polar pattern, such as cardioid, supercardioid, or hypercardioid.

Visual inspiration: the Oldsmobile 6, with its distinctive radiator grille.Visual inspiration: the Oldsmobile 6, with its distinctive radiator grille.Photo: Jan Ake Schiller

Again, the mic was available in two forms: the $72 Model 55S was the general-purpose cardioid version, still fitted with an impedance selection switch and, initially, a blue windscreen cloth, while the $100 supercardioid Model 556S, now fitted with a Canon XL connector and a maroon windshield cloth, was still aimed at broadcasters. The ‘S’ in both cases indicated the use of the new ‘small unidyne’ capsule.

The resulting ‘Uniphase Acoustical System’ allowed a far more predictable and consistent polar pattern than composite (multi-capsule) directional microphones, significantly improving the control of unwanted ambient noise and feedback. Additionally, when compared to the alternative directional mics of the time, Shure’s single-element solution was much smaller, which helped to make it more popular with both performers and public speakers.

This revolutionary Uniphase Acoustical System first saw the light of day in the short-lived Model ‘Uniplex’ 730A microphone, which employed a crystal capsule, but it was in the form of the ‘Unidyne’ Model 55 dynamic microphone that the concept gained instant success, and set a new sound-quality standard at the same time.

General Douglas MacArthur receives the formal surrender of the Japanese forces on board the USS Missouri, 1945. Note the Shure Model 55 on a  stand behind the desk!General Douglas MacArthur receives the formal surrender of the Japanese forces on board the USS Missouri, 1945. Note the Shure Model 55 on a stand behind the desk!

Miniature Marvel

The Unidyne Model 55 was revolutionary: the first compact microphone to provide a consistent cardioid polar pattern from a single dynamic element. The company’s proclaimed benefits included reduced feedback in PA systems, and less reverberation and background noise for broadcasts and recordings. The marketing claimed “wide angle coverage with excellent high-quality response at the front, yet... dead at the rear”. Simple, but accurate!Shure's product data sheet for the original Model 55 microphone, from 1939.Shure's product data sheet for the original Model 55 microphone, from 1939.

It seems that the ‘Unidyne’ term was coined by Sidney Shure himself to reflect several important aspects of the design. The ‘uni’ part indicates the use of a single mic element, of course, as well as the unidirectional (cardioid) polar pattern it delivers, and the genuinely unique design concept. The ‘dyne’ part comes straight from the imperial unit of force used in acoustical measurements.

Distinctively styled, the Unidyne Model 55 body was said, at the time, to have a ‘futuristic look’, but it was actually influenced by the Art Deco movement and, specifically, the bonnet grille of the 1937 Oldsmobile Six convertible coupe! The Model 55’s omnidirectional sibling, the Model 50 ‘Rocket’, shared similar space-age-meets-Art-Deco styling.


The Unidyne Model 55 was initially made available in three variants: the ‘A’ version had a low (50Ω) output impedance, and was intended for broadcast applications with a cost of $42.50. The ‘B’ version had a medium impedance (250Ω) better suited to public address and recording, while the ‘C’ version was high impedance (15kΩ) and intended for two-way radio equipment. The B and C models both cost a little more at $45.

Between 1939 and 1946, the company’s wartime manufacturing commitments meant that the Unidyne Model 55 barely changed, although the Model 555 Broadcast variant was introduced in 1940 (again with A, B and C impedance versions, and a price rise to $60). The most significant change was an integral shockmount fitted in the stand adaptor, and an optional radio station call-sign flag. This period also saw the 1942 release of the Model 55AV/BC/CV, with an improved high-frequency response to give better vocal clarity, as denoted by the ‘V’ suffix. This version was really aimed at public-address paging and two-way radio systems, but was made available in all three impedance variants. At the same time the Broadcast version acquired a supercardioid polar pattern and was renamed the Model 556 (in A, B and C impedance forms), with a price tag of $75.The Model 55 remains popular to this day in the hands of artists such as LeAnn Rimes.The Model 55 remains popular to this day in the hands of artists such as LeAnn Rimes.Photo: Sarah Hersel

In 1947, Shure amalgamated the three separate variants with different output impedances into a single model (now costing $55) fitted with an impedance selector switch under the case at the rear of the microphone. The same simplification was made for the supercardioid Model 556 Broadcast version, too, now costing $85. While the general appearance of the Model 55 and Broadcast 555/556 microphones remained the same, the colour of the windshield cloth behind the metal grille varied considerably over the years, with different maroons, blues, brown, grey and black all making appearances. The actual fabric changed often, too, with silk, cotton, nylon and polyester all being used — basically whatever Shure’s engineers could find in a nearby store that had the correct acoustical impedance!

Unidyne II

A far more significant design change was introduced in April 1951 following the development of a new Unidyne capsule, creatively called the Unidyne II. At a technical level, this evolution offered an improved frequency response and lower noise, partly through the use of much stronger magnetic materials which became available after the war, but also through better diaphragm suspension and capsule-isolation mechanics. The new magnetic assembly also allowed the entire capsule to be usefully smaller and lighter than the original. Consequently, the microphone case was scaled down in size accordingly, ending up roughly two-thirds the size of the original, measuring just under four inches high instead of four and a half.

The microphone’s support base was altered in 1961, and an on-off slide switch was added, the new model being designated the Model 55SW at a cost of $85 (the suffix ‘W’ signified the provision of a switch). Somewhat gaudy gold-plated versions of the 55S and 55SW appeared in 1967, for $95 each, and the PE55 variant appeared in 1970. Coming with a plastic carry-case, it was permanently configured for high-impedance operation.

The final revision of the 55S line appeared in 1978, when the original Amphenol connector was replaced with an XLR on both the PE55SH and 55SH models, which now cost $112 and $100, respectively. The internal windscreen fabric material was also changed to open-cell foam, and the Model 55SH lost the internal impedance selector switch. All models were shipped with a low-impedance output, although it was still possible to convert to high-impedance if required by changing the wiring connected to the microphone’s XLR output socket.

Series II

Impressively, the Unidyne II capsule remained at the heart of all Model 55 variations for nearly 40 years, until it was replaced in 1989 by a new cardioid capsule design, available only in a low-impedance configuration. Microphones with the new capsule were marked as ‘Series II’, and the first 55SH Series II microphone cost $189. Impressively, there were no further new models for another 20 years, with the Super 55 — a supercardioid version with improved frequency response, chromed body and foam wind shielding — appearing in 2009. This was joined a year later by the limited-run Super 55-BCR model, which had a very distinctive black body and red foam wind shielding.

There’s an almost complete collection of Shure microphone catalogues, dating from 1933, and discontinued microphone user guides at

Shure Designer Benjamin Baumzweiger

Benjamin Baumzweiger.Benjamin Baumzweiger.

Benjamin Baumzweiger was born in Odessa, Russia, in 1913, but his family fled to Poland when he was eight in an effort to escape anti-Semitism. Unfortunately, they found Poland to be no better, and so the family emigrated to Cuba a few years later. At the age of 17 Benjamin moved to New York to attend the Pratt Institute, receiving an Industrial Engineering degree in 1932, after which he moved to the University of Cincinnati to study Electrical Engineering. This course included a work placement programme, and Baumzweiger served an internship at Shure Brothers. When he graduated in 1937 he was invited to join the company as a full-time Research Engineer working on transducer development.

Four years later, on becoming a US citizen, Benjamin changed his surname to Bauer in 1941, and continued to progress within the Shure Brothers Company, eventually rising to the position of Vice President of Engineering. During his time with the company Bauer also designed throat microphones which were used extensively in WWII aircraft, as well as disc cutters and phonograph pickups.

Bauer left Shure in 1957 to join CBS Laboratories as Head of Audio Technology Development, initially working on stereo discs and magnetic tape recording. In the 1970s his team developed the company’s SQ-quadraphonic matrix surround system. Bauer became the Vice President and General Manager of the CBS Technology Centre at Stamford in 1975, but died just four years later with over 100 patents to his name covering microphones and transducers, audio processing for recording and broadcasting, acoustic measurements and calibration, sound recording, transmission and reproduction, and quadraphonic disc technology.

Unidyne III & The SM57

The Unidyne III capsule, which was first introduced in the Shure Model 545 microphone in 1959, was developed primarily by Shure engineer Ernie Seeler. This new evolution of the single-element directional capsule represented a significant development in the Unidyne story, as it incorporated a very sophisticated pneumatic shockmount system which improved handling noise dramatically.

Key to its operation was arranging for the trapped air volume in the cavity at the rear of the capsule to exert a damping back-pressure on the diaphragm in the presence of mechanical vibration, thus preventing the diaphragm from moving and minimising the low-frequency electrical output caused by handling vibrations. This concept might seem simple enough, but the practical realisation is incredibly complicated, due to the fact that the rear cavity also serves a critical role in the acoustic circuit determining the microphone’s polar pattern. Shure claims that the Unidyne III design is so complicated that it has never been successfully duplicated by any other microphone manufacturer.

In 1965, the success of the Unidyne III element and the slender handheld design of the Model 545 microphone became the basis for Shure’s legendary SM series of stage mics, starting with the ubiquitous SM57 which celebrates its 50th anniversary this year! The SM57 is still one of the most popular and widely used microphones in the world, along with its sibling, the illustrious SM58, which followed a year later.

Model 55 Anniversary Edition

The Model 55 is still in production today, and retains its distinctive appearance despite some fairly major internal changes.The Model 55 is still in production today, and retains its distinctive appearance despite some fairly major internal changes.As an inevitable part of Shure’s 75th anniversary celebrations, a special limited-edition version of the mic has been released, called the 5575LE. With a run of an appropriate 5575 units, this anniversary set includes a bespoke 5575LE mic (employing the current Unidyne III cardioid element) with the instantly identifiable ‘birdcage’ microphone body and vintage badging.

It is supplied in an aluminium flightcase with a 75th anniversary logo, a certificate of authenticity, and some collectors’ photos of the vintage Unidyne Model 55. The mic also ships with a nice retro-styled desk stand (with a zinc die-cast finish) and an XLR cable with a right-angled socket to connect with the microphone itself.

Published September 2015

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