Shure 55 Unidyne, the birth of a legend
When the Unidyne 55 was introduced 75 year ago, it was the first single element unidirectional dynamic microphone in the world. From entertainers to politicians, past and present, the Unidyne remains the world’s most recognizable microphone.
As we commemorate the Year of the Unidyne, we pay special attention to this day, April 29, the date in 1939 that the first Model 55 Unidyne microphones where shipped to customers.
The First Unidirectional Microphone – Goodbye Feedback
Shure engineer Ben Bauer is credited with being the driving force behind the creation of the first unidyne. Bauer began developing the microphone in early 1937. In undertaking the project, his primary objective was to create a unidirectional microphone which used a single dynamic element.
Prior to Unidyne, the common way of creating a microphone with unidirectional response was to use an omnidirectional element combined with a bi-directional (figure eight) pick up pattern element in a single housing. If the outputs from both cartridges were mixed together electrically in equal proportions, the result would yield a cardioid pattern.
These early dual-element microphones had many drawbacks. First and foremost, their size tended to be large and bulky. Performance was lacking as well.
Achieving the First Cardioid Polar Pattern
Since the omnidirectional and bidirectional elements didn’t possess the same frequency response, and they were in different locations in the housing, their resulting combined frequency response and polar pattern was irregular and difficult to control. Overall, the concept was far from perfect, but better than nothing if you were facing serious feedback or noise problems.
Ben Bauer realised that the best way to deal with these difficulties was to use just one element. He began by examining the physics. He knew that if a single element was only exposed to sound on its front side, you’d obtain an omnidirectional response. Conversely, a bidirectional microphone has both sides – the front and back – exposed to sound. With that in mind, Bauer knew that if he could partially block the backside of a microphone element, in theory he would achieve a response somewhere between omnidirectional and bidirectional which would be heart-shaped, or cardioid. He set about to create this hypothetical cartridge, and wound up with what Shure would later name the Unidyne.
When completed, Bauer’s Unidyne design was configured so that the microphone had a series of front and rear openings which allow sound waves to reach both sides of the element’s diaphragm. The sound waves reaching the diaphragm from the rear had a longer path and passed through openings which produced a delay between the sound entering from the rear and sound striking the front of the diaphragm. By varying the amounts of acoustical resistance encountered at the rear openings, Bauer was able to achieve cardioid, supercardioid, or hypercardioid patterns using a single element, and the first true unidirectional dynamic microphone became reality.
The Unidyne was the best sounding mic available, and set a new standard of high quality audio pickup combined with discrimination against unwanted sounds. True to Bauer’s design theories, the directional response was more predictable and better behaved than its predecessors, so it offered a tremendous new ability to control feedback and reduce ambient noise pickup. In addition, its size was small compared to competitive offerings.
During the years 1939 and 1946, the Unidyne remained largely the same. First unveiled in 1951, the new “Small Unidyne” microphones (featuring the Unidyne II cartridge) improved upon all the features which made the original Unidyne such a success. As their name implies, the Small Unidynes were lighter in weight and more compact than the originals.
Unidyne III – The Pneumatic Shock Mount
The Unidyne III microphone cartridge first appeared in the model 545 in 1960. Shure engineer Ernie Seeler is credited with being the driving force behind the creation of the Unidyne III. This design brought a substantial improvement to the Unidyne platform. A highly-sophisticated pneumatic shock mount was developed which provides best-in-class handling-noise reduction. It works by causing the air volume in the rear cavity to exert pressure on the back of the diaphragm when the system is stimulated with mechanical vibration. This “back pressure” prevents the diaphragm and coil from moving in the magnetic gap and the result is no output under vibration conditions.
This is due, in part, to the fact that the design works by adding another rear cavity volume which is integral to the acoustic circuit of the cartridge. Hundreds of pages of advanced mathematical equations balance every aspect of the cartridge; from diaphragm resonance, to cartridge mass, resistance values, inertances, cavity volumes, and the length of a sound wave’s path from back to front of the diaphragm. All of these variables are balanced and work together to influence not only the frequency response and polar pattern of the microphone, but the shock mount performance as well. In 1965, the Unidyne III element and the slender handheld design of the 545 became the basis for the SM57 microphone, the microphone that launched the famous SM series.In principle, the pneumatic shock mount is simple, but the science required to tune it is so extensive that it has never really been duplicated by another microphone company.