The Microphone - James West

Electret Microphone Co-Inventor

Ninety percent of microphones used today are based on the ingenuity of James Edward West, an African-American inventor born in 1931 in Prince Edwards County, VA, and Gehard Sessler from Rosenfeld, Germany, also born in 1931. If you’ve ever talked on the telephone, you’ve probably used their invention.

Dr. James E. West and Gerhard Sessler, developed the mic (officially known as the Electroacoustic Transducer Electret Microphone) while with Bell Laboratories, and they received a patent for it in 1962. The acoustical technologies employed became widely used for many reasons including high performance, acoustical accuracy and reliability. It is also small, lightweight and cost effective.

 

West started at Bell labs as an intern and joined them full-time in 1957 after graduating from Temple University. As the co-inventor of their microphone, James West has received numerous awards and honors including a Fellow of IEEE, Industrial Research Institute's 1998 Achievement Award, 1995 Inventor of the Year from the State of New Jersey and induction in the Inventors Hall of Fame in 1999. James E. West holds 47 US patents and more than 200 foreign patents from his 40-year career with Bell Laboratories.

During his career, West also involved himself with programs designed to encourage minorities to take more of a role in the sciences. In the 1970's, he was a member of the Association of Black Laboratories Employees (ABLE) at Bell Labs that influenced management to fund the Summer Research Program (SRP) and Cooperate Research Fellowship Program (CRFP) – programs that helped more than 500 non-white students graduate with degrees in science, engineering and mathematics.

James Edward West now works with Johns Hopkins University as a research professor.

 

How a Microphone Works

In order to speak to larger groups of people, there has long been a desire to increase the volume of the spoken word. This desire led to the development of the microphone.

 In 1665, the English physicist Robert Hooke was the first to experiment with a medium other than air with the invention of the "lovers' telephone." 

The first microphone that enabled proper voice telephony was the (loose-contact) carbon microphone. 

The carbon microphone is the direct prototype of today's microphones.

 

The carbon microphone functions as a transducer. A transducer is a device that translates one form of energy into another. In this case, it converts sound to an electrical audio signal. It consists of two metal plates separated by granules of carbon. One plate is very thin and faces toward the speaking person, acting as a diaphragm. Sound waves striking the diaphragm cause it to vibrate, exerting a varying pressure on the granules, which in turn changes the electrical resistance between the plates. Higher pressure lowers the resistance as the granules are pushed closer together. A steady direct current is passed between the plates through the granules. The varying resistance results in a modulation of the current that reproduces the varying pressure of the sound wave.  At the other end of the transmission, the process is reversed transforming the electric signal into sound waves that a person can hear.

Carbon microphones were widely used in telephone systems until the 1980s. Their low cost and technical capacity were well suited for telephony. For plain old wire based telephone service (POTS), carbon-microphone based telephones can still be used without modification.

The first (loose-contact) carbon microphone was independently developed by David Edward Hughes in England and Emile Berliner and Thomas Edison in the US. Although Edison was awarded the first patent in mid-1877, Hughes had demonstrated his working device in front of many witnesses some years earlier, and most historians credit him with its invention.

 

Hughes' device used loosely packed carbon granules - the varying pressure exerted on the granules by the diaphragm from the acoustic waves caused the resistance of the carbon to vary proportionally, allowing a relatively accurate electrical reproduction of the sound signal. Hughes also coined the word microphone. He demonstrated his apparatus to the Royal Society by magnifying the sound of insects scratching through a sound box. Contrary to Edison, Hughes decided not to take out a patent; instead, he made his invention a gift to the world.

In 1916, Bell Labs developed the next breakthrough with the first condenser microphone. A condenser microphone is made up of two plates: a front plate and a back plate. The front plate is known as a diaphragm and is made of a very light material. As acoustic energy hits the diaphragm it vibrates, thus reducing and increasing the distance between the two plates.

When the plates are close, capacitance, or the change of current, increases and when the plates are farther apart, it decreases. The plates are powered with a fixed charge supplied by an exterior power source. West and Sessler made their contribution by addressing the requirement for the exterior power source represented by the battery in the diagram above.

 

Condenser microphones span the range from telephone transmitters through inexpensive karaoke microphones to high-fidelity recording microphones. They generally produce a high-quality audio signal and are now the popular choice in laboratory and recording studio applications. The suitability of this technology is due to the very small mass that must be moved by the incident sound wave. They do however require a power source. Power is necessary for establishing the capacitor plate voltage, and is also needed to power the microphone electronics.

 

The desire to reduce the power needs of a capacitor microphone led to the development of the electret microphone by Sessler and West. In 1962, they solved the problem of externally provided power source required by condenser microphones through use of a permanently charged material called an electret. An electret is a ferroelectric material that has been permanently electrically charged or polarized. The charge is embedded in an electret by alignment of the static charges in the material, much the way a magnet is made by aligning the magnetic elements  in a piece of iron.

Magnetism refers to physical phenomena arising from the force caused by magnets, objects that produce electrical fields that attract or repel other objects.

 

All materials experience magnetism, some more strongly than others. Permanent magnets, made from materials such as iron, experience the strongest effects, known as ferromagnetism. With rare exception, this is the only form of magnetism strong enough to be felt by people.

Iron Magnet                                                          Iron Atom

Magnetic fields are generated by rotating electric charges. Electrons all have a property of angular momentum, or spin. Most electrons tend to form pairs in which one of them is “spin up” and the other is “spin down.” However, some atoms contain one or more unpaired electrons whose spin can produce a directional magnetic field. The direction of their spin determines the direction of the magnetic field. When a significant majority of unpaired electrons are aligned with their spins in the same direction, they combine to produce a magnetic field that is strong enough to be felt.

The Earth itself is a giant magnet. The planet gets its magnetic field from circulating electric currents within the molten metallic core. A compass points north because the small magnetic needle in it is suspended so that it can spin freely inside its casing to align itself with the planet's magnetic field.

 

Permanent magnets are the result of ferromagnetism. The prefix “ferro” refers to iron because permanent magnetism was first observed in a form of natural iron ore called magnetite, Fe3O4. Pieces of magnetite can be found scattered on or near the surface of the earth, and occasionally, one will be magnetized. These naturally occurring magnets are called lodestones. “We still are not certain as to their origin, but most scientists believe that lodestone is magnetite that has been hit by lightning.”

 

People soon learned that they could magnetize an iron needle by stroking it with a lodestone, causing a majority of the unpaired electrons in the needle to line up in one direction. Around A.D. 1000, the Chinese discovered that a magnet floating in a bowl of water always lined up in the north-south direction. The magnetic compass thus became a tremendous aid to navigation, particularly during the day and at night when the stars were hidden by clouds.

 

Other metals besides iron have been found to have ferromagnetic properties. These include nickel (Ni), cobalt (Co), and some rare earth metals such as samarium (Sm), or neodymium (Nd), which are used to make super-strong permanent magnets.

 

A coil around a magnet can also be made to move in a pattern of varying frequency and amplitude to induce a current in a coil. This is the basis for a number of devices, most notably, the microphone. Sound causes a diaphragm to move in an out with the varying pressure waves. If the diaphragm is connected to a movable magnetic coil around a magnetic core, it will produce a varying current that is analogous to the sound waves. This electrical signal can then be amplified, recorded or transmitted as desired.

 

When this modulated electrical signal is applied to a coil, it produces an oscillating magnetic field, which causes the coil to move in and out over a magnetic core in that same pattern. The coil is then attached to a movable speaker cone so it can reproduce audible sound waves in the air. The first practical application for the microphone and speaker was the telephone, patented by Alexander Graham Bell in 1876

 

The applications of electromagnets are nearly countless. They can be used by a giant crane to lift junk cars at a scrap yard is also used to align microscopic magnetic particles on a computer hard disk drive to store binary data. New applications are being developed every day.

 

In the electret microphone, thin sheets of polymer electret film are metal-coated on one side to form the membrane of the movable plate capacitor that converts sound to electrical signals with high fidelity.

Due to their good performance and ease of manufacture, hence low cost, the vast majority of microphones made today are electret microphones; annual production estimates are over one billion units. Nearly all cell-phone, computer, PDA and headset microphones are electret types. They are used in many applications, from high-quality recording and lavalier (head set) use to built-in microphones in small sound recording devices and telephones.

 

The electret-condenser capsule operates in the same way as the externally polarized condenser, except that the required polarizing charge is held permanently within the diaphragm or backplate. No form of external powering is needed in order to charge the diaphragm. However, the high impedance output of the capsule still requires an impedance-changing amplifier, therefore an internal battery supply or phantom power supply is needed.

 

The simplest type of electret-condenser microphone is the charged-diaphragm type, using an electret foil diaphragm as a compromise between good electret and good mechanical properties. A electret is typically a plastic film, such as mylar, of approximately 0.0002" (.005 mm) which is spattered with a conductive metal such as gold or nickel. This film is then heated and changed with a high dc potential. A well-designed electret will retain its charge for a period of ten years. Further, it is predicted that it will take a period of from thirty to one hundred years before the capsule sensitivity would drop by 3dB.

For any questions, please contact David Trotman :

415-298-8979

1519 O'farrell St. San Francisco, CA 94115