The History of Carbon Microphones and Artifacts from the Shure Archives
There are thousands of treasures to choose from in the Shure archives and each has its own story. In this series, Shure resident historian MICHAEL PETTERSEN discusses carbon microphones and some of his favorites from the collection.
The history of carbon microphones goes back to the 19th century. An Englishman named David Edward Hughes fabricated one of the first carbon mics in the 1870s. While Thomas Edison and Emile Berliner fought over patent rights for perfecting the design around the same time, Hughes tends to get the nod as the inventor of carbon microphone technology. Perhaps more altruistic than Edison and Berliner, Hughes never sought a patent, preferring to share the technology with the world. Hughes’ other contribution? Coining the word “microphone”.
In 1877, Alexander Graham Bell was looking for a way to improve the sound of his recently patented telephone. According to legend, Graham bought the rights to Berliner’s carbon mic for $50,000. Carbon mics replaced Bell’s liquid-based mic and were a mainstay in telephony for the next 100 years.
The Operating Principle
All microphone cartridges – regardless of type - are transducers that convert sound waves into electrical energy. To understand how carbon microphones work, here is an excerpt from “Microphones Explained for Beginners”, an article that appeared in the August 1938 issue of “Radio-Craft” magazine:
The carbon microphone depends for its operation on the varying resistance of a carbon element when subjected to varying pressure. The usual arrangement of this type unit, for best fidelity, consists of two carbon buttons one on either side of the diaphragm. This metal diaphragm - in a properly built carbon microphone - is stretched and air damped so that the effects of self-resonance vibrations are negligible, giving a reasonably uniform output at all ordinary audio frequencies.
Simply stated, anthracite coal (carbon) granules are contained within a small half-sphere covered with a thin metal diaphragm. External power is applied to generate a current flow through the microphone. Sound waves strike the microphone diaphragm causing it to vibrate, exerting varying pressure onto the carbon granules. These varying pressure levels translate into varying levels of resistance, which in turn vary the electrical current passing through the microphone. It’s a simple concept resulting in a rugged and reliable, if somewhat noisy, microphone.
Shure and Carbon Mics
Shure’s foray into carbon microphones dates back to 1929, when Shure Brothers, then located at 335 West Madison Street in Chicago, began to transform itself from an AM radio kit parts company to the exclusive distributor for Ellis Electrical Laboratories. In 1932, the business relationship soured between Mr. Shure, then a young man, and Mr. Ellis, considerably older. Shure began manufacturing its own carbon microphones later that year. By the end of the decade, Shure offered four additional types of mics – crystal, condenser, ribbon, and moving-coil dynamic.
The primary applications for carbon mics were public address, broadcast, military operations, and telephony. The rugged mics offered a high output level and their simple construction made them relatively inexpensive to manufacture. However, the background hiss and occasional crackle, the limited frequency response, and the need for a power supply made carbon mics prey to better solutions. They were widely used in World War II and in spite of technological advances, carbon microphones were used in telephones until the 1970s and 1980s. In fact, Shure offered the Model 104C – still used today in communication systems for offshore rig and mining applications – well into the second decade of this century.
Carbon Microphones in the Shure Archives
Since Shure manufactured carbon microphones for over 80 years, there are dozens of artifacts – microphones and microphone parts – in the Shure Archive. Here are a few favorites:
T30-V (Throat Mic)
Microphones like this one (also called laryngophones) were used extensively by pilots because of the deafening noise inside WWII military planes. Carbon transducers, positioned on both sides of the wearer’s throat, converted voice box vibrations into electrical energy. The accompanying instruction sheet (labeled RESTRICTED) warned the user that “the information contained in restricted documents and the essential characteristics of restricted material will not be communicated to the public or to the press but may be given to any known person in the service of the United States and to persons of undoubted loyalty and discretion who are cooperating in Government work.” Similar devices are currently employed by beat-box artists to create percussion sounds.
One of the first microphones manufactured by Shure, the Model 5 carbon mic made its debut in 1933 and was designed for use in Public Address systems. The 1936 Shure catalog touted its “attractive modern design” and chromium plate finish. Model 5 microphones featured Quickway hooks that made it easier to suspend the microphone inside the ring. The ring and hook (U.S. Patent 100,928) were designed by S. N. Shure. While Shure engineers have been awarded hundreds of patents since the founding of the company, the Quickway patent was the only one that Mr. Shure ever personally held. The 1933 retail price of the stylish suspension ring, including the Quickway suspension system and 8 rust-proof springs? $3.50.
The model T-17 was specifically built for the Allied Forces during World War II and was not sold as part of the Shure standard line of products. A page for the 1946 Shure catalog says only this: "The T-17 Hand Held Microphone is the standard and most widely used microphone in World War II. Used in planes, tanks, and for practically all other hand-held requirements." Few historical records exist, but we know that Bakelite, a then-popular synthetic resin, replaced the T-17’s metal exterior in the early 1940s to conserve materials for the war effort.
Listen to a Historic Carbon Microphone
The Past Calls
Carbon microphones are still manufactured today, but they are primarily the domain of hobbyists and audiophiles wrangling vintage parts to create hand-built lo-fi vocal mics. Mark Pirro, bass player for The Polyphonic Spree and founder of Placid Audio, is one of them. He says his customers use them for “the unpredictable nature and grainy lo-fi distortion that carbon elements tend to impart on the signal. They are appropriate anytime they are looking to add an 'element of danger' to the signal or if they are seeking that aerospace/military transmission quality.”
There is a fairly active collectors’ market for both working and non-working carbon mics. Shure 3B mics, for example, evoke the early days of radio broadcasts and can fetch up to $300 on auction sites as decorative items. Making them functional in the 21st century, however, requires the skills of an amateur engineer/radio enthusiast and several external components, including a DC power source, a current limiting resistor, a bypass capacitor and a blocking/matching transformer.
Eventually, carbon mics were replaced by moving-coil dynamic and condenser units.