SciTech

How Things Work: Microphones

Credit: Photo illustration courtesy of Rusty Sherrif via Flickr Credit: Photo illustration courtesy of Rusty Sherrif via Flickr

In September of 1919, President Woodrow Wilson spoke to a crowd of nearly 50,000 at San Diego’s Balboa Stadium in favor of his proposed League of Nations. In addition to the excitement surrounding a visit from the president, the Journal of San Diego History reported that those in attendance also witnessed a major triumph in technological history: It was the first time the president had used an electric device to capture his voice, and after amplification, make his voice heard to a large live audience. That device was a microphone.

The microphone was invented years before Wilson’s speech, by German-born American inventor Emile Berliner in 1876, according to the journal Technology and Culture. Wilson’s speech is just one illustration of how microphones were first used. Today, microphones have countless applications: telephones, hearing aids, radio, television, and motion picture productions. Over the years, the microphone has changed in shape and form. However, it has retained the same identity at its core: A microphone is a transducer, which converts one form of energy into another. In the case of the microphone, sound is converted into electrical signal.

Sound exists as a result of air vibrations and changes in air pressure, and there are several ways that microphones capture those changes and turn them into electric currents. Two of the most common forms are “dynamic” and “condenser” microphones.

A dynamic microphone operates through a concept called electromagnetic induction. Put simply, sound waves travel past a small metal plate called a diaphragm, which is attached to a coil of wire surrounded by a magnet. When the coil vibrates within the magnetic field in response to sound, electric current is produced, thus converting sound energy into electric energy.

Condenser microphones operate on different principles. In these microphones, sound waves cause two electrically-charged metal plates to vibrate. These two metal plates are collectively known as a capacitor. The microphone’s diaphragm acts as one of the plates of the capacitor, and when the distance between the two plates changes as they vibrate in response to sound, electric current is produced.

One key difference between condenser and dynamic microphones is that the use of a capacitor in a condenser microphone requires a source of power, whether it is a battery inside the microphone or some external power source. A dynamic microphone, on the other hand, can operate without any source of electricity.

There are several other types of microphones, such as the ribbon microphone — which produces electrical current when a metal ribbon vibrates in response to sound within a magnetic field — and the electret microphone — which is a type of condenser microphone that is commonly used in devices such as cell phones and computers.

One of the first microphones, the carbon button microphone, consists of two metal plates that are separated by compacted carbon granules. When sound enters the microphone, the vibrations induce a change in pressure between the granules. An electric current traveling through the granules changes in response to the change in pressure.

Microphones have come a long way since their creation in the late 19th century. Today, microphones can capture sound from specific directions relative to the direction the microphone is pointed. For example, omnidirectional microphones capture sound in all directions, while “shotgun” microphones focus on one certain direction and are relatively insensitive to other directions.

Despite these additional features and improvements in microphone sensitivity, the inner workings have remained the same and enable society to communicate through sound, and preserve those sounds over time.