The History of Controlled Reluctance Microphones from the Shure Archives

There are thousands of treasures to choose from in the Shure Archives and each has its own story. In this installment, Shure resident historian MICHAEL PETTERSEN explores the microphone transducer technology known as controlled reluctance.

Some historical sources assign Shure credit for the development of the controlled reluctance microphone. While Ben Bauer, Shure’s Chief Development Engineer, invented the Uniphase Acoustical Network that led to iconic mics like the Unidyne I Model 55 and Unidyne III SM58, his controlled reluctance research instead advanced an existing technology to better serve the US military in World War II.

From 1942 through 1945, Shure’s factory in Chicago operated nearly around-the-clock manufacturing military mics and headphones. Wartime demand was so great that in 1942 alone, the number of employees at the company’s headquarters grew from 150 to well over 1,000. Keeping up with new contracts was not the only challenge; there were technical hurdles. Though the ubiquitous carbon microphones were able to withstand the blast pressures created by war weapons, controlled reluctance mics, preferred for their improved speech intelligibility, could not.  

Bauer was determined to create a controlled reluctance unit capable of withstanding battle condition sound levels. After placing a quantity of microphones near a Navy battleship gun firing sixteen-inch diameter shells, he examined every unit and found the sound pressures were bending the drive pin.

It took thought and experimentation before he found an elegant, yet simple solution: replacing the solid drive pin with a stiff spring. With the insignificant sound pressures of speech, the spring acted like a solid drive pin. During the extreme sound pressures from an explosive blast, the spring compressed for milliseconds, did not bend, then returned to acting like a solid pin. 

Ben Bauer had fabricated a controlled reluctance microphone that consistently withstood the deafening sounds of war. 

The Operating Principle 

The controlled reluctance cartridge is a variation of a dynamic mic. The mic diaphragm connects, via a drive pin, to a small lever made from ferrous material. The other end of this lever is positioned inside of a stationary coil of wire. Surrounding the coil of wire is a stationary magnet. As the ferrous lever is moved by the mic diaphragm, the lever disturbs the magnet field. This induces an AC signal (the audio signal) in the coil of wire.

Shure and Controlled Reluctance Mics

In spite of Bauer’s wartime experiments, there was little documentation that Shure supplied the armed forces with controlled reluctance microphones.

A deep dive into the Archives’ many thousands of documents provided the proof. A letter dated August 15, 1943 from the company’s Hugh Knowles to a Chicago patent law firm describes the technology resident in a security-classified “microphone we manufacture which has aerial shock wave or blast-proof resistant properties.” That makes it likely that some of the Battle Announce microphones Shure sold to the Allied Forces during World War II contained a controlled reluctance cartridge. 

The technology eventually made its way into consumer markets with the introduction of the Model 520 Green Bullet, Model 505 Ranger, Model 510 Hercules and Model 520 SL Dispatcher microphones in 1949.  

Designed for voice applications, advertising touted their ruggedness, “clear reproduction and the high output long needed for Public Address, Communications and Recording at an amazingly low price.” Unlike carbon and crystal microphones, they were immune to heat and humidity. Shure also offered, for $9, a controlled reluctance transducer to replace the crystal and carbon elements in existing Model 707A and Model 708 mics. 

A secondary market emerged for Shure as home tape recording and intercom systems gained in popularity. Manufacturers needed reliable and inexpensive microphones to pair with their products. Those OEM models, often recognizable by a CR prefix, are widely available on auction sites today.

Commando models, now identified as controlled magnetic microphones in Shure literature, followed in 1957. The handheld Models 415 and 430, gooseneck-mounted Model 425 and wearable Model 420 were offered until the 1980s. By the early 1990s, the controlled magnetic microphones (descendants of the handheld Ranger) still in production were sold as paging mics for communications systems. Controlled magnetic models made their final appearance in a 1995 distributor price list. 

Controlled Reluctance/Controlled Magnetic Mics in the Shure Archives

The omnidirectional controlled reluctance microphones that Shure manufactured spanned a period of 46 years. While considered low-cost general purpose or communications microphones, the 1949 catalog promised access to “vast new fields of applications: Announcing and Mobile Public Address Systems (including Carnivals, Circuses, Parking Lots, Paging Systems, etc.); Communications; Dictating Machines; Portable Recording Machines; Home Recording; high quality Inter-Communication.”

Here are three iconic models. 

GREEN BULLET

Model 520  - $16.50 in 1949  

Introduced in 1949, the Model 520 was designed for public address and two-way radio applications. By the 1970s, new technologies emerged and sales had eroded. The Green Bullet was headed for extinction.

What Shure discovered, thanks to customer letters, was that the Green Bullet had become a favorite of blues harp players the world over for its unique sound and mid-range punch.  Who was first to adopt the Model 520 as a harmonica mic is the stuff of legends. Little Walter? James Cotton? Theories abound.

Why is easier to explain. At $16.50, the Model 520 was affordable for most musicians. It could be comfortably cupped into a player’s hand, allowing the harmonica to be placed on top of the grille. The distinctive timbre of the Green Bullet became a signature sound of Chicago blues. 

Eventually, the controlled magnetic cartridge of the Model 520 model was replaced with a moving coil dynamic element tuned to replicate the frequency response of its predecessor. Today’s 520DX – the only surviving descendent of the original controlled reluctance models – still reigns, according to its legion of fans, as the definitive blues harp voice.

Listen to a Model 520 from 1949.

HERCULES

Model 510C - $12.95 in 1949 

Model 510S (with switch) - $14.95 in 1949 

Shure advertising touted its “ruggedness, clear reproduction, and high output long needed for Public Address, Communications, and Recording – at an amazingly low price”. It was, according to copywriters, “especially ideal for general-purpose use in tropical countries and all coastal areas.” Indoors or outdoors, controlled reluctance mics were not susceptible to moisture. (It is worth noting the Shure REX Model 710, often promoted next to the HERCULES, featured an identical shell with a crystal cartridge.) 

The small footprint (2 2/3” wide x 3 ¼” high x 1 ½” thick) and visual appeal of the Hercules made it popular with manufacturers of recording, paging, intercom, and dictating systems. It explains why there are vintage Hercules microphones, like the ones pictured here, in a variety of colors designed to match the equipment paired with the Hercules.

COMMANDO LINE

Model 415 (High impedance) - $27.50 in 1957  

Model 420 (Lavalier, Two impedance levels, H & L) - $30.00 in 1957    

Model 430 (Two impedance levels, H & L) - $38.50 in 1957

At the 1960 Electronic Parts Show in Chicago Sidney N. Shure touted the advancements and versality of the Model 425 as being “adapted for a flexible mounting for use in language study laboratories, commercial public address systems, paging systems and other installations.” Commandos were offered in handheld, stand-mounted and suspended (lavalier) versions and could be adapted for different applications, “without the use of tools!”

Google the iconic image of Dr. Martin Luther King as he delivered the ‘I Have a Dream’ speech at the March on Washington for Jobs and Freedom on April 23, 1963. There, next to a Shure Unidyne II Model 55S, is a Shure Model 415 microphone, providing the controlled magnetic version of the Commando a place in history.

One of the Survivors 

Over time, carbon, crystal, and ceramic cartridges disappeared as moving coil condenser, ribbon, and MEMS technologies evolved. Controlled reluctance/controlled magnetic/ balanced armature technology did not suffer the same fate. Its modern application may be a surprise.

For example, balanced armature drivers can be found in most Shure earphones, including the SE325, SE425, SE846 and the new Aonic3, Aonic4 and Aonic5 models. Favored for their small size, superb sound, and low power requirements, balanced armature drivers are used in many premium priced earphones.

Thanks to the controlled reluctance research by Ben Bauer during World War II, the balanced armature transducer optimizes personal listening in the 21st century.