Carbon Microphones

What Are Carbon Microphones?

Thomas Edison officially patented the carbon microphone in 1876. Nevertheless, most people credit the invention to either Emile Berliner in the United States or David Edward Hughes in England, who both happened to invent it independently around the same time. The advantage of carbon microphones over other microphone designs is their high output, low impedance, and affordability. The biggest drawback in standard applications has to be a very limited frequency response. Carbon microphones eventually became less popular as dynamic, and the later invented condenser microphones became more affordable.

Usage

Before the proliferation of vacuum tube amplifiers in the 1920s, carbon microphones were the only practical means of obtaining high-level audio signals. They were widely applicable in telephone systems until the 1980s, while other applications utilized different microphone designs much earlier. Their low cost, inherently high output, and frequency response characteristics were perfect for telephony. For plain old telephone service (POTS), carbon-microphone-based telephones exist without modification. Carbon microphones, usually modified telephone transmitters, were characteristic of early AM radio broadcasting systems. However, their limited frequency response, as well as a relatively high noise level, led to their abandonment of those applications by the late 1920s. People continued to use them for low-end public address and military and amateur radio applications for some decades afterward.

Construction

The carbon type of microphone was developed quite early. The construction of a typical carbon microphone includes a small cup that is packed with pulverized carbon and enclosed at one end by a brass disk called the button. The button is coupled to a circular metal diaphragm — the button and the black plate at the rear of the cylinder form the connection terminals. A battery provides an activating voltage across the carbon.

Transferring Sound

When sound strikes the diaphragm, the carbon granulates in the button vibrate, becoming alternately more and less dense as the diaphragm moves. The electrical resistance of the carbon thereby fluctuates and converts the battery voltage into a corresponding fluctuating current that is an electrical representation of the sound. The transformer stops the current. It also serves to isolate the low impedance of the element from that of the input to block the battery DC from the output.

Problems

Carbon microphones don't have excellent sonic characteristics. Furthermore, they are quite inexpensive and rugged. For this reason, people still use them in utility sound applications. One example would be that the standard telephone microphone element has long been carbon-type. However, dynamic microphones also exist in many newer phones. Carbon microphones can lose some efficiency and become noisy if the granules in the bottom become compacted. However, just tapping the element against a hard surface usually cures the problem.

Additional Resources & Source Texts

• Carbon Microphone