The humble microphone has been with us for well over a century, and has come a long way from its crude beginnings. However, identifying the inventor of the microphone is not exactly an easy task, and depends very much on the definition used.

 

The beginnings of the microphone

 

A microphone is a device for converting acoustic power into electric power that has essentially similar wave characteristics. Microphones convert sound waves into electrical voltages, which are eventually converted back into sound waves through speakers. They were first used with early telephones and then radio transmitters.

 

Etymologically, its function is easily explained. Thus, "microphone" comes from the Greek words "micro" meaning "small," and "phon" meaning "sound." The term first appeared in a dictionary in 1683 as "an instrument by which small sounds are intensified." This was in reference to acoustical hearing devices, such as the ear trumpets and megaphones of that era.

 

Microphone development as we know it today was distilled from telephone technology, whose roots go back to the mid-1800s. The word "microphone" was first coined by Wheatstone around 1827, and was used to describe a purely acoustic device, like a stethoscope, which he had developed to amplify weak sounds.

 

However, microphones as we know them started with the first articulate telephone transmitter, developed by several inventors. One of the first was the German physicist Johann Philipp Reis (1834 - 1874), whose design for a sound transmitter used a metallic strip resting on a membrane with a metal point contact completing an electric circuit.

 

The next recorded attempt was that of Elisha Gray (1835 - 1901), an American inventor and one of the founders of what became the Western Electric Company. His invention was called a “liquid transmitter” and featured a diaphragm, attached to a movable conductive rod, immersed in an acidic solution. A second, fixed rod, alongside the first continued the circuit through the solution, with a battery connecting the two. Sound pressure variations through the diaphragm caused the separation between the two rods to vary in proportion to the sound, producing corresponding changes in the electric resistance through the cell and, therefore, the amount of current flowing around the circuit.

 

In 1876, Alexander Graham Bell employed a very similar transmitter design for the first transmission of intelligible speech over a rudimentary telephone system. However, the true inventor of the telephone was originally disputed, as Bell filed his original patent application for the telephone on the same day that Elisha Gray also applied for a caveat (a means of protecting an idea in advance of a full patent application), announcing his intention to claim the same invention. Nevertheless, as history records it, Bell was the winner in this situation, having filed his application several hours before Gray's caveat.

 

The carbon microphone

 

In 1876, Emile Berliner was the person to invent the first microphone that was used as a voice transmitter, known today as a telephone. At the U.S. Centennial Exposition, Emile Berliner had seen a Bell Company telephone demonstrated, and was inspired to find ways to improve the newly invented telephone. The Bell Telephone Company was impressed with what the inventor came up with and bought Berliner's microphone patent for $50,000.

 

The poor quality of the “liquid transmitters” introduced by Gray and Bell prompted a number of inventors to pursue alternative avenues of design, and David Edward Hughes (1831 - 1900) was one such man. He wasn't a no-name, as he had already been granted a patent in 1855 for a type-printing telegraph instrument, which enjoyed great success both in America and Europe. By 1878, he had designed a new kind of microphone, using carbon granules loosely packed in an enclosed space.

 

In response to varying pressure from a sound diaphragm, the electrical resistance through the carbon granules changed proportionally. Although the performance of this kind of microphone is poor by today's standards (inherently noisy with high distortion), at the time, it was a significant step forwards and the enabling technology for voice telephony.

 

Hughes' work was perfected by Thomas Alva Edison (1847 -1931), well known for refining the carbon granule microphone, which resulted in the carbon-button transmitter in 1886. This consisted of a cavity filled with granules of carbonized anthracite coal confined between two electrodes, one of which was attached to a thin iron diaphragm.

 

Edison's transmitter was simple and cheap to manufacture, but also very efficient and durable, becoming the basis for the telephone transmitters used in millions of telephones around the world, for the most part of the last century.

 

It wasn't long after their invention that the carbon microphones had already started to be perfected, this process being stimulated by the advent of the electrical disc recording and radio broadcasting, in the early 1920s.

 

One of the best known microphones of that time bore an octagonal design, often seen in photographs of the early broadcasting stations – the Marconi-Reisz “transverse-current” carbon microphone. This particular type was invented by a young employee of the German company Reisz, Georg Neumann, who later went on to manufacture microphones under his own name.

 

The capacitor microphone

 

Nevertheless, the reign of the carbon microphone was set not to last, because of the instability problems of the carbon granules used in their making. This was what triggered extensive research and brought new alternatives. One avenue was the piezoelectric (crystal) transducer, based on fundamental research by the Curies in the previous century.

 

Thee transmitters usually used quartz or Rochelle salt crystals, but the sound quality was not particularly good. Today, piezoelectric foils in contact microphones use specialized ceramics with very respectable results.

 

The first capacitor microphone (and associated impedance converter/amplifier set) was developed by EC. Wente in 1917, based on work at Bell Laboratories in America. This was a laboratory sound intensity measurement tool and it wasn't until the early 1920s that precision stretched-diaphragm condenser microphones started to be manufactured for recording and broadcast applications.

 

The thermionic valve (invented in 1907 by Lee de Forest) was a key factor in this, as capacitor microphones require impedance conversion, impossible to achieve in any other practical way. In order to satisfy the high-quality microphone requirements of the rapidly growing radio broadcast and recording industries, Western Electric introduced the 394 condenser microphone. Subsequently, RCA came out with the 4AA condenser mic. With the introduction of condenser microphones, the problems of signal-to-noise ratio and frequency response associated with the carbon microphones, then in general use, were overcome.

 

Condenser microphones were employed, to a limited extent, in the BBC, from 1926, but they had a reputation of being “temperamental,” due to their susceptibility to moisture, causing “frying noises.”

 

The electromagnetic microphone

 

Electromagnetic microphones (moving coil, moving iron and ribbons) were relatively late on the scene, because permanent magnets were very weak and only electromagnets could create sufficient flux densities. As a consequence, the first moving coil microphones were very large and required power supplies.

 

The Marconi-Sykes, the first design to become popular, used a thin, flat, annular coil of aluminum, as both diaphragm and motive coil, suspended on cotton wool. The magnetic field was created by a large electromagnet consuming around 4A from an 8V battery.

 

Alain Blumlein, an employee of the Colombia Graphophone Company (later to become EMI), brought his contribution to the development of the moving coil microphone, using a diaphragm made from a laminate of balsa wood (impregnated with celluloid) and thin sheets of aluminum foil.

 

An anodized aluminum motive coil was riveted to the diaphragm and, in his first tests, the electromagnet was powered by batteries borrowed from the cars of several of his colleagues. His first HB1A microphone (named after its two main inventors, Blumlein and Holman) was tested in 1930, by comparing it with the Western Electric Condenser Transmitter (CT) microphone, the best standard of the day.

 

After numerous revisions, including a screw tensioning system to adjust the diaphragm resonance, the result (the HB1B) was widely used in the EMI recording studios and by the new BBC television station at Alexandra Palace when it opened in 1936.

 

The ribbon microphone

 

The first ribbon microphone also appeared around 1930, and is believed to have been developed by Harry Olson, based on a modified ribbon loudspeaker (invented by E. Gerlach in 1924). Even though early designs were excessively large, the ribbon microphone stood out from the very beginning, thanks to its sound quality comparable to the condenser microphones of the time and the fact that it wasn't susceptible to moisture.

 

Perhaps the two most famous microphones to be commercialized were developed by Harry F. Olson at RCA: the velocity ribbon microphone series and the unidirectional ribbon microphone series. These vintage microphones are still in great demand up to this date.

 

When Olson developed the velocity mic, it was a large step forward in microphone technology, as it was the first high quality directional microphone. The effective solid angle of sound reproduction for the figure-eight velocity mic is one-third that of the omni directional mic. This means a reduction of 5 dB on the effective sound pickup of reverberation and other unwanted sounds. The directional properties of the velocity microphone were found to be useful in reducing effects of reverberation and increasing the intelligibility of reproduced speech.

 

The next logical step was the development of the unidirectional or cardioid pattern. Olson's 77A, which was introduced in 1933, consists of mini- and bidirectional capsules whose outputs are combined so that they yield the cardioid pattern. The cardioid pattern also affords the same effective angle of sound reception as the figure-eight, and hence, the same advantages with the addition of a front-to-back rejection characteristic.

 

Developments

 

The ribbon microphone represented a true advance as far as mics were concerned, and such an invention couldn't be let go too easily. This is why several people and companies tried to gradually perfect the microphone, coming with various additions that have improved the quality of these devices over the years.

 

Western Electric and ST&C were the first to go down this path, employing a ribbon capsule (pressure gradient, figure-of-eight response) and a separate moving coil capsule (pressure operated, omni response) in the same unit. The diaphragms of the two capsules were in close proximity, and their outputs combined electrically in series to produce a cardioid polar response.

 

Later, a more practical way to create a cardioid response emerged: a single transducer with a rear phase-shifting acoustic network. This scheme was adopted by the likes of Western Electric, Shure and Electrovoice in America, as well as Neumann, AKG, ST&C and others in Europe.

 

The interference tube (shotgun) microphones appeared as a result of film and television industries being developed, and the need for greater directionality in microphones. However, although this type of mics is highly used today, directionality at low frequencies remains a problem, unless the interference tube is extremely long. Nevertheless, Audio Technica found a way to overcome this flaw, namely, by the combination of digital signal processing techniques with a multiple capsule array.

 

The miniaturization of conventional condenser microphones had to wait until the Field Effect Transistor became available (with its extremely high input gate impedance) to replace valve impedance converters. Other attempts at miniaturization focused on the close integration of transducer and amplifying circuit, with many attempts dating back to the 1950s.

 

Innovations

 

The current interest in high-sample rate digital systems have encouraged manufacturers to market microphones designed to take advantage of this new fidelity. Such marketers are Sony, Sennheiser and Earthworks.

 

One of the most important innovations in microphone design was the Soundfield capsule, originally conceived and developed in the 1970s to originate ambisonic surround sound material. The Soundfield microphone comprises four sub-cardioid capacitor capsules in a tetrahedral array, producing “A-format” signals. These are combined electronically (with compensation for the physical separation between capsules) to produce “B-format” signals, which are the basis of “UHJ Ambisonic” encoded material.

 

These signals represent the outputs of four (perfect) virtual microphones consisting of three mutually perpendicular figure-of-eight elements – left/ right (X), front/back (Y) and up/down (Z), plus an omnidirectional component (W). These can be thought of as three-dimensional extensions to Blumlein's original MS configuration.

 

An Ambisonics decoder calculates which combinations of B-format signals to route to which loudspeakers (“D-format” signals), given information about their number and approximate location, and how to process them to create incredibly stable and accurate surround sound images.

 

In recent years, some of the more radical approaches to microphone design have included detecting the movement, in response to sound pressure variations, of charged particles – a system analogous to the ionic loudspeaker. Another idea is the laser-velocity transducer where a vibrating reflective surface is scanned by a low power laser, the resulting Doppler shift conveying the audio signal. Moreover, over the last decade, research into “optical microphones” has started to bear fruit.

 

Optical A/D microphones are currently being developed. On the horizon, we see the technologies of fiber optics, lasers and interferometers applied to the electrical and digital transduction of acoustical phenomena. Once the past has been clearly laid out before us, the future is easy to imagine. Many inventions of the future will be stolen from early predecessors, as it has always been done. All you have to do is sit and wait!