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Microphones: History & Development

 The microphone, (coined by Sir Charles Wheatstone in 1827), in its simplest form consists of a membrane that vibrates as sound waves hit its surface. The membrane is attached to a coil of wire that floats around a small magnet. As the coil vibrates back and forth, in response to the sound waves hitting the membrane's surface, an electric current is induced in the coil that is proportional to the sound wave. This electric current is passed to an amplifier that will greatly boost the input signal and send this on to the speakers in a much louder sound wave. In essence the speaker works on the same principal as the microphone, but while the microphone membrane is small, the speaker membrane is much larger, and capable of moving a larger mass of air which through amplification translates into an larger version of the input signal. A microphone by definition has two meanings: it is an instrument for intensifying weak sounds. And it is a device for transforming sound waves into electrical impulses.

 

 

In the mid-1920 the development of the condenser microphone and the electronic vacuum tube amplifier paved the way for sound on film recording, the first high quality, wide range condenser microphone was developed by E. E. Wente at Bell Labs as a measurement standard in the late 1910s. 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. A condenser microphone is essentially a capacitor, made of two plates separated by an thin insulator. One plate is charged negative and the other positive. When one plate of the capacitor moves in response to sound waves this movement influences the electric charge on the second plate and these changes are amplified to create a measurable signal.

 

 

 The omni directional dynamic microphone was developed by Wente and Thuras in the late 1920s, and introduced as the Western Electric 618-A. Actually, the original dynamic or moving coil techniques were patented by Ernst Siemens in 1874. He even specified the diaphragm to be the frustrum, (portion), of a cone, which was used in many other inventions. Nonetheless, the 618-A was the first practical dynamic microphone. A dynamic microphone takes advantage of electromagnet effects. In a dynamic microphone, the diaphragm moves either a magnet or a coil when sound waves hit the diaphragm, and this movement creates a small current.
  Because of the simplicity of the 618-A, as compared to the condenser microphone and amplifier, the omni-dynamic mic proved to be more practical for many applications, although the 618-A was considered to be an omni-directional mic, the microphone becomes very directional in the high-frequency range.
  Western Electric developed a dynamic mic in the late 1930s that was omni directional to 15 kHz. Called the 630A, it was better known as the Eight-Ball mic, because it resembled an eight-ball right off a pool table.

 

 Even though there were several advancements in microphone technology the early models had not proved themselves reliable enough for broadcast work. But, in 1929 and 1930, at NBC's installation of new audio facilities for its Chicago Civic Opera House broadcasts, the 18 carbon microphones were replaced by three parabolic sound reflectors used in conjunction with a condenser microphone. This was the first application of highly directional microphone techniques. NBC subsequently used these parabolic dishes in both the Philadelphia and New York Metropolitan Opera Houses and in its Time Square studios. In 1939, Mason and Marshall of Bell Labs reported on a design of a tubular microphone which used a single element and acoustical tubes of varying lengths to achieve a highly directional pickup pattern. This tubular design paved the way for the shotgun mics we are familiar with today.

 

 

The Olson / RCA Legacy

 

  Perhaps the two most famous microphones to be commercialized were developed by Harry F. Olson at RCA. They were the 44A, B, BX velocity ribbon microphone series (1930-1940), and the 77A, B, C, D, and DX unidirectional ribbon microphone series (1931-1937). These vintage microphones are still in great demand; the 44BX could be found in many NBC studios, and the 77DX is currently used on NBC's "Tonight show" and on "Late Night with David Letterman."

77A, 77D, 77DX

 When Olson developed the velocity mic it was a large step forward in microphone technology; 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.

 

Sound Reinforcement

 

While RCA's microphone developments efforts were concentrated more towards broadcasting/recording applications, Western Electric, in 1939, introduced the 639A unidirectional microphone with sound reinforcement applications in mind. The 639A consists of a ribbon velocity element and a dynamic pressure element, whose outputs are combined so that they yield a cardioid pattern. Marshall and Harry of Bell Labs reported in 1941 that due to the reproducing characteristics of monaural sound systems in use, directivity needed to be supplied by the microphone in order to produce a more natural balance of direct-to-reverberant sound. They further stated, "This (feedback in a reinforcement system) is merely a special case of extraneous noise, and its effect can generally be reduced by directivity in the microphone."

 

Marshall and Harry implemented two field tests on the 639A mic, one of a broadcast of a 30-piece orchestra and another of a sound reinforcement system. In the studio, the 639A allowed for an ideal acoustical location of the mic and it was commented that the bass reproduction was much clearer than with other mics. Marshall and Harry attributed the clarity of bass to the suppression of reverberant bass energy pickup, where the studio's acoustical treatment was deficient. The 639A was installed at the House of Representatives, in Washington, D.C., where feedback conditions were so severe that other types of microphones had proven inadequate in providing sufficient reinforcement. The 639B has a six-position switch that yields omni, cardioid, several types of hypercardioid, and figure-eight patterns. In the House of Representatives, the hypercardioid afforded an increase of 5dB in system gain -- wherein was the difference between success and failure of the entire installation.

 In the late 1930s, Benjamin Bauer of Shure Brothers developed a new cardioid dynamic microphone that used a single element and acoustic means to achieve its directional pattern. The Shure Unidyne made it debut in 1941 and was a turning point for microphone design, manufacture, and reinforcement applications.

 

Crystal Microphones

 

In 1880, Jacques and Pierre Curie discovered the piezoelectric effect. Piezoelectric crystals were first used by Langevin in 1917, in connection with his research efforts in underwater acoustics using ultrasonic transducers. In 1919, using Rochelle Salt, Alexander Nicolson first demonstrated a variety of piezoelectric devices, including loudspeakers, phonograph pickups, and microphones.
  Problems in manufacturing crystals with uniformity and the necessary shapes prevented the commercial production of any of these devices. Almost 10 years later, C.B. Sawyer and C.H. Tower developed processes to manufacturer uniform complex-shaped piezo crystals. This led the way for many piezoelectric or crystal transducers, as they were first called.

 The crystal microphones work on the principal that certain crystals change their electrical properties as they change shape . By attaching a diaphragm to a crystal, the crystal will create a signal when sound waves hit the diaphragm and cause the crystal to deflect.
  The Argonne 57 is an example of a crystal microphone.

 

 

Electrets

 

Work on the electret condenser microphones dates back to as early as 1928. These microphones used permanently polarized wax plates. Eventually, microphones with wax electrets were offered commercially by Bogen (1938 to 1940) under the name No-Voltage Velotron. The first large scale application of electric transducers was during WWII, when wax-electret microphones were used in Japanese field equipment. The wax-electrets, however, did not catch on due to their instability and very small capacitance which complicates the mic-preamp design. From 1948 through the early 1960s, work continued in electret microphone technology, turning up materials such as acrylics, ethyl cellulose, polystyrene, vinyl polymers, and ceramic electrets.

 In 1962 and 1965 electret microphones in which the diaphragm was composed of a metalized thin foil of Mylar or Teflon, respectively, which has been converted into an electret were proposed. Finally, in 1968, Sony brought out the finest electret condenser microphone. Later, around 1971, Primo Company Ltd. introduced an electret mic with a monolithic IC preamp. Foil-electrets are manufactured in countless numbers; the Japanese production of electrets alone is estimated to exceed 20 million units per year.

 Electret microphones are a variant of condenser microphones that mostly utilize a permanently charged diaphragm over a conductive metal back-plate. They are especially responsive to the range of sounds from the lower mids to the highest frequencies, but aren't very good for bass

 

 

The Future

 

 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. Those who worked in the labs in the 1800s and early 1900s left us with a long list of inventions to be implemented with modern materials, and new electronic and manufacturing technologies. These new devices can be categorized into mechanical and non-mechanical transducer systems. Diaphragms made of materials yet to be patented will use various modulation and sampling techniques to convert their motion into data. 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.

 "There is room for improvement in even the best microphones. Possibly an understanding of the limitations can be had by considering that, for perfection, a microphone should possess no inertia, and should produce an output directly proportional to the air pressure applied". Although this quote is from the book, Public Address Systems, by James R. Cameron published in 1935, it still holds true today.

 This article was used with the kind permission of Lloyd Microphone Classic, ( http://lloydmicrophoneclassics.com/mic_history.html), using information from Testa Communication and the Author Jesse Klapholzjesse@klapholz.com) from the September 1986 issue ofSound & Communications magazine. http://www.soundandcommunications.com. Thanks !

 

 

Types of Microphones

 All modern microphones are trying to accomplish this same principal of taking a fluctuating pressure wave of air and converting it into a proportinately fluctuating electrical signal. There are basically five different ways used to accomplish this conversion process.

Carbon microphones - These are the oldest and simplest microphones invented by David Edward Hughes in 1878. This microphone uses carbon dust that has a thin metal or plastic diaphragm on one side. As sound waves hit the diaphragm, the dust is compressed which changes its resistance. By running an electric current through the carbon, the changing resistance changes the amount of current that flows.

Dynamic microphones - A dynamic microphone takes advantage of electromagnet effects. In a dynamic microphone, the diaphragm moves either a magnet or a coil when sound waves hit the diaphragm, and this movement creates a small current. Dynamic microphones were developed during the 1930s and allowed for greater sensitivity and clearer sound reproduction.

Ribbon microphones - The ribbon microphone was invented in 1942 for radio broadcasting. In a ribbon microphone, a thin ribbon is suspended in a magnetic field. Sound waves vibrate the ribbon and this changes the current flowing through it.

Condenser microphones - A condenser microphone is essentially a capacitor, made of two plates separated by an thin insulator. One plate is charged negative and the other positive. When one plate of the capacitor moves in response to sound waves this movement influences the electric charge on the second plate and these changes are amplified to create a measurable signal. Condenser microphones usually need a small battery to provide a voltage across the capacitor. Condenser microphone were developed in the 1970s and like dynamic mics allowed for greater sensitivity and clearer sound reproduction.

Crystal microphones - Certain crystals change their electrical properties as they change shape . By attaching a diaphragm to a crystal, the crystal will create a signal when sound waves hit the diaphragm and cause the crystal to deflect.

 There are also variations on each of the microphone types that pertain to the frequency responses of the microphone. A flat response to frequency treats all incoming frequencies the same while one having a "Presence Peak" will favor a certain frequency over others, and will add more definition to that favored frequency.

 An important factor in choosing a microphone is the microphones "Polar Pattern", this pattern is basically the area of the mic that is most receptive to the sound waves being directed at it. This "Pickup Pattern" is the major determining factor when choosing a mic for a particular purpose, such as vocal, instrument,and so forth.
The polar patterns of the different types of microphones and the general use of these types is listed below;

Omnidirectional, - Has all-around pickup which includes the pickup of room reverberation. Omnidirectionals offer little isolation unless you mic close to your source, have a low sensitivity to pops, (explosive breath sounds), and have no up-close bass boost, (proximity effect) . Proximity effect is an emphasis on the low frequency components of any source that is very close to the diaphragm.

Unidirectional, (cardioid, supercardioid, hypercardioid) - Has a selective pickup and rejects room acoustics, (reverb due to reflected sound), and background noise. Unidirectionals offer good isolation, and therefore, good separation on recorded audio tracks.They have a broad angle of pickup for sources in front of the mic with maximun rejection of sound behind the mic.

Supercardioid, - Offer maximum difference between front hemisphere and rear hemisphere pickup, (good for stage usage). SuperCardioids have more isolation than the cardioid and less reverb due to room acoustics.

Hypercardioid, - Has maximum side rejection in a unidirectional mic, offering maximum isolation and maximum rejection of reverberation, leakage, feedback, and background noise.

Bidirectional, - Has nearly equal front and rear pickup, with rejection of sound coming from the side, (good for that two part harmony). These are also good for stereo effect by using two Bidirectionals crossing at 90°.

A comparison of Polar Patterns .

 

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