Copyright 2017 by William Karl Thomas

The one media device that has not changed too dramatically since its invention is that instrument which translates sound waves into electrical impulses, making possible the transmission of audio over wires, optical fibers, lasers, and radio waves, and the recording of audio in mechanical, optical, and magnetic mediums for posterity; i.e., the microphone.

There are five basic types of microphones and each has its virtues and liabilities, its virtues making it particularly useful for specific sound applications. Sound waves are compressed waves of air created by a vibrating surface, such as a violin string or the vocal chords in our larynx. All five types of microphones depend on a diaphragm to capture sound waves. The dictionary gives four definitions for a diaphragm from "the effective aperture of the lens in a camera" to "a thin contraceptive cap fitting over the cervix" to provide birth control. The one that applies to microphones is "a taut flexible membrane in mechanical or acoustic systems." In four microphone designs the membrane is a thin metal or plastic disc, and, in the fifth one, the membrane is a corrugated ribbon of metal foil.

The first microphone capable of coherent speech, which is still used today for some applications, was invented in England in the early 1870's by David Edward Hughes, even if business savvy Thomas Edison got a patent on it in 1877, as he did with much technology he himself did not actually invent. That microphone consisted of a metal diaphragm with a tiny metal cup attached to it, but separated by a non-conductive washer. Inside the cup were loose microscopic granules of carbon which would bounce around when sound waves vibrated the diaphragm. A small current applied to wires, one attached to the diaphragm and one to the metal cup, would be modulated (modified or made variable) by the varying resistence to the current produced by the bouncing carbon granules.

This 'carbon microphone' was used for the first radio broadcast, a performance at the New York Metropolitan Opera House in 1910, and for the first sound movie, Lee deForest's Phonofilm demonstration of his new 'sound on film' process. To create the optical sound track, the carbon microphone was attached in series to a battery and to a small light bulb whose beam was focused on the film's optical sound track through a narrow slit. Sound caused the microphone's bouncing carbon granules to vary the current to the bulb, and the bright and dim variations of light were recorded on the film's optical sound track as bars with different shades of light and dark, known as a 'variable density' optical sound track. Later optical sound tracks used different methods to accommodate different types of microphones capable of handling a wider range of audio frequencies, or 'higher fidelity' (hi-fi) as we know it today. But the carbon microphone was used in telephones into the latter half of the twentieth century, and is still used today in high ambient sound situations, such as airplane cockpits and around big machinery in factories, because carbon microphones only respond to a sound source close to the microphone and frequencies mostly between 200 to 800 cycles per second, which is the principal range of most human verbal communication.

A microphone's specifications are expressed in a graph in which the vertical increments represent volume in decibels (dB) both above and below the threshold of human hearing, and the horizontal increments represent the frequencies in cycles per second (cps). Most of us know dogs can hear higher frequencies than we can, and zoologists know elephants can hear much lower frequencies than us. So everything above the horizontal centerline is what we might hear, and everything below it is what dogs and elephants and other creatures might be hearing. Humans hear, at best, from 20 to 20,000cps. An ideal microphone will have a chart that rises above the centerline before 20cps, maintains a nice 'flat line' around 10 or 15dB until it passes 20,000cps before it drops off, but no microphone is that ideal and almost all show a wobbly curved line that slopes into human hearing range around 60cps and drops below hearing around 16,000cps. That's not bad when you consider an average healthy twenty year old usually starts hearing about 100cps and can't hear much over 12,000cps, and most senior citizens hear a lot less. A musician's trained ear might, with concentration, hear more, but most microphones used for music or professional recording exceed the spectrum of most humans.

Carbon microphones have a specifications chart that rises into human hearing around 200cps and drops below around 800cps. In the search for a microphone with a wider frequency spectrum, E.C. Wente of Western Electric developed the condenser microphone in 1916. Without getting too technical, electronic circuitry uses two basic building blocks; 'resisters' which are poor but stable electrical conductors that resist the flow of electrical current to a limited degree, and 'condensers' which kinda relay an electrical current from one circuit to another without making actual physical contact. Physically, resisters are made of carbon and the greater volume of carbon in the resister increases its resistence (sometimes called 'impedance'), as measured in 'ohms' (the name of the guy who invented them). Physically, condensers (sometimes called 'capacitors') are two wires attached to two sheets of thin metal that are very close to each other without ever touching because they are separated by a layer or thin plastic or oil, which is called a 'dielectric.' If a carbon microphone modulates a current by using a diaphragm to vibrate some carbon, Mr. Wente figured why can't a diaphragm be one of the two plates of a condenser and modulate the current it relays to its other plate. His condenser microphone had one heavy rigid immovable disc for one plate at the back of the housing, and a thin same size disc diaphragm for the second plate, the two extremely close together, and a direct current (DC) applied to the wires attached to them. Sound waves hitting the diaphragm would move it back and forth, modulating the DC current. Despite the fact that they had much better 'frequency response' (some of today's condenser microphones claim a relatively 'flat' response around 40-18,000cps), early condenser microphones were fragile and batteries for them were unreliable and relatively unavailable, so condenser microphones fell out of popular use for decades.

The search for a more rugged microphone not dependent on unreliable batteries coincided with the invention of speakers. Early radios could only be heard through headphones, and those headphones employed diaphragms that were modulated by electro magnets located very close behind them, a principal known as 'variable reluctance.' A modulated current from the radio circuitry applied to the coil of the electro magnet could vibrate the diaphragm enough to serve as a headphone, but not produce enough sound to be heard more than a few inches away. The knowledge that a wire coil moving in the magnetic field of a permanent magnet produces an electric current in the coil had been around almost 200 years, and led to the invention of electric motors and generators in the early 1800's. In 1924 Chester W. Rice and Edward W. Kellogg patented the modern speaker based on the 'moving coil' or 'dynamic' principal. It was, essentially, a diaphragm with a coil attached to it, and the coil was surrounded by a permanent magnet. Audio modulated current applied to the coil would attract and repel the diaphragm from the magnetic field, creating sound waves. The diaphragm was a large cone of stiff paper, easier to move back and forth because it was lightweight paper, and louder because it was larger and the cone projected the sound forward sufficiently to be heard by a larger audience.

This design could also work in reverse. Sound moving the paper cone back and forth within the magnetic field would produce current in the attached coil, thereby acting as a 'dynamic microphone.' Many low cost intercoms and similar two way communication devices use the speaker as a microphone, switching the circuitry back and forth for sending or receiving sound. Quality dynamic microphones employ a more compact and rugged metal or plastic disc diaphragm. Their virtues are ruggedness, a wide frequency spectrum comparable to condenser microphones, no batteries required, and they have a low input impedance which allows their tiny current to travel over longer wires. From 50 to 500 ohms is considered 'low impedance.' Dynamic microphones are the most widely used microphone design worldwide.

One speaker design that didn't last long had a corrugated stiff paper ribbon instead of a cone, and, in 1923, Harry F. Olson reverse engineered a ribbon speaker to make a 'dynamic ribbon microphone.' Replacing the corrugated paper with a corrugated ribbon of metal foil which moved in the magnetic field of a permanent magnet, ribbon microphones could produce the widest frequency spectrum of all dynamic microphones, but they were not as rugged. The ribbon was vulnerable to strong winds and physical trauma, but they are still highly regarded for studio work and musical performance.

Which brings us to the fifth and final widely used microphone, the piezoelectric or 'crystal microphone.' When vacuum tube (radio tube) circuitry became inexpensive enough to employ in consumer items like phonographs, the most expensive part was the dynamic phonograph pickup. It was discovered that some crystals produce an electric current when stressed. Attaching the phonograph needle to one end of a tiny crystal and securing the other immovable end produced a tiny modulated electric current when sound vibrated the diaphragm, and that current could be fed into the high impedance input of tube amplifiers, eliminating the high cost of a dynamic pickup and the impedance matching transformer it required to be compatible with tube electronics. Although their frequency spectrum was from 100-12,000cps at best, their low cost made them popular. Crystal microphones use a metal or plastic diaphragm with a tiny center post on the back which presses against one end of the crystal, modulating the tiny current the crystal produces. Their low cost and compatibility with the high impedance input of tube electronics made them popular in tape recorders and other consumer audio products. From 10,000 to 50,000 ohms is considered high impedance. Their high end versions were popular with 'ham' radio operators who regard their more limited frequency spectrum as 'crisp' and ideal for verbal transmissions.

There are other newer types of microphones; fiber optical, laser, liquid, and micro-mechanical to name a few. They are mostly associated with microchip technology and unusual special applications, and not likely to be found in most professional audio recording environments.

Another aspect of the five basic designs mentioned is how they are housed, which can determine their 'pickup pattern.' The two commonest patterns are 'omnidirectional,' such as the common 'stick mike' which picks up sound from all directions, and the more selective 'cardiod mike' which picks up sound in a heart shaped pattern where the microphone is located in the juncture at the top of the heart and with its front facing the interior of the heart, hopefully suppressing the sound from the back of the microphone. The simplest way to do this is to put it in a housing with sound damping material at the back of the housing. The complex way of doing it is to have two dynamic microphones facing opposite directions with their impedance matching transformers wired so the microphone facing the rear of the housing has its signal cancelled out of the combined modulated signal the microphone produces. This expensive version is called a 'sound cancelling dynamic microphone.' Both types of microphones may have 'wind screens,' usually a ball shaped foam custom fitted cover for the microphone which will protect it from gusts of wind, speaker's breath, and moisture from the speaker's breath.

There are many types of microphone stands, the commonest being a floor stand, a table stand, a boom attached to either floor or table stands, and a fishpole. Fishpoles are usually handheld, but sometimes mounted on a 'preambulator,' which is an articulated boom mounted on a vertical stand with wheels that a sound grip pushes around to follow a performer. Most professional microphones in the United States have a 5/8" female thread with 27 threads per inch (tpi) known as the Unified Special thread (UNS) which is compatible with most of the above stands.

Here are a variety of specialized microphones:

  • Studio mikes: Highest quality dynamic, condenser, or ribbon mikes used in a studio protected environment for musical or film set recording. Often used in multiples for different instruments or vocalists whose signals are mixed by a sound engineer.
  • Wireless mikes: usually dynamic microphones input to miniature FM radio transmitters used in association with compatible FM receivers, often concealed on film or stage actors during performance, or by law enforcement for surveillance.
  • Shotgun mikes: usually dynamic microphones house in specially designed long tubular housings that attempt to cancel all sound except for a narrow angle at the front of the tubing, often used for nature filming or surveillance.
  • Lapel mikes: Miniaturized usually dynamic microphones mounted on a clip for quick easy attachment to clothing of talk show guests or documentary interviews.
  • Concealed mikes: Miniaturized dynamic or crystal microphones hidden in watches, jewelry, books, smoke alarms, and other concealed housings, and almost always used for surveillance.
  • Fishpole mike: Usually a dynamic mike on the end of a long pole handheld by a sound grip above film actor's head to provide maximum ratio of actor's voice to ambient sound without the microphone being seen by the camera.
  • Throat mike: Carbon mike strapped to pilot's throat or helmet strap to provide maximum ratio of voice to ambient sound of jet engines.
  • Contact mike: Miniaturized dynamic or crystal microphone physically mounted on a musical instrument to provide maximum ratio of specific instrument sound to ambient and other instrument sounds.

Vintage Media Equipment stocks almost all of the above basic microphone designs and accessories for them, including some associated electronics such as tape recorders, mixers, headsets, and more. We can also advise you how vintage equipment can best serve your needs and save you money. Check out our Audio Video page and query us about anything you see that interests you.


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