Microphone
A microphone, colloquially called a mic, or mike, is a transducer that converts sound into an electrical signal. Microphones are used in telecommunication, sound recording, broadcasting, and consumer electronics, including telephones, hearing aids, and mobile devices.
Several types of microphone are used today, which employ different methods to convert the air pressure variations of a sound wave to an electrical signal. The most common are the dynamic microphone, which uses a coil of wire suspended in a magnetic field; the condenser microphone, which uses the vibrating diaphragm as a capacitor plate; and the contact microphone, which uses a crystal of piezoelectric material. Microphones typically need to be connected to a preamplifier before the signal can be recorded or reproduced.
History
To speak to larger groups of people, a need arose to increase the volume of the human voice. The earliest devices used to achieve this were acoustic megaphones. Some of the first examples, from fifth-century-BC Greece, were theater masks with horn-shaped mouth openings that acoustically amplified the voice of actors in amphitheaters. Between 1664 and 1685, the English physicist Robert Hooke was the first to experiment with a medium other than air with the invention of an early telephone made of stretched wire with a cup attached at each end. Today, this is known as a tin-can telephone.In 1856, Italian inventor Antonio Meucci developed a dynamic microphone based on the generation of electric current by moving a coil of wire to various depths in a magnetic field. This method of modulation was also a lasting method for the technology of the telephone. Speaking of his device, Meucci wrote in 1857, "It consists of a vibrating diaphragm and an electrified magnet with a spiral wire that wraps around it. The vibrating diaphragm alters the current of the magnet. These alterations of current, transmitted to the other end of the wire, create analogous vibrations of the receiving diaphragm and reproduce the word."
In 1861, German inventor Johann Philipp Reis built an early sound transmitter that used a metallic strip attached to a vibrating membrane that would produce intermittent current. Better results were achieved in 1876 with the "liquid transmitter" design in early telephones from Alexander Graham Bell and Elisha Gray – the diaphragm was attached to a conductive rod in an acid solution. These systems, however, gave a very poor sound quality.
File:David Edward Hughes.jpg|thumb|upright|David Edward Hughes invented a carbon microphone in the 1870s.
The first microphone that enabled proper voice telephony was the carbon microphone. This was independently developed by David 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. The Berliner microphone found commercial success through the use by Alexander Graham Bell for his telephone and Berliner became employed by Bell. The carbon microphone was critical in the development of telephony, broadcasting and the recording industries. Thomas Edison refined the carbon microphone into his carbon-button transmitter of 1886. This microphone was employed at the first radio broadcast, a performance at the New York Metropolitan Opera House in 1910.
File:Bogart Bacall AFRS.jpg|thumb|left|Humphrey Bogart, Jack Brown, and Lauren Bacall with RCA Varacoustic MI-6203 ribbon microphones broadcast to troops overseas during World War II.
In 1916, E.C. Wente of Western Electric developed the next breakthrough with the first condenser microphone. In 1923, the first practical moving coil microphone was built. The Marconi-Sykes magnetophone, developed by Captain H. J. Round, became the standard for BBC studios in London. This was improved in 1930 by Alan Blumlein and Herbert Holman who released the HB1A and was the best standard of the day.
Also in 1923, the ribbon microphone was introduced, another electromagnetic type, believed to have been the work of Harry F. Olson, who applied the concept used in a ribbon speaker to making a microphone. Over the years these microphones were developed by several companies, most notably RCA that made large advances in pattern control, to give the microphone directionality.
The introduction of the Neumann U 47 in 1949 was a turning point for microphone technology. It was the first studio condenser microphone to use a large dual-diaphragm capsule with switchable pickup patterns and a vacuum tube amplifier, setting a new standard for high-fidelity, warm, and detailed vocal and instrument recording. With television and film technology booming there was a demand for high-fidelity microphones and greater directionality. Electro-Voice responded with their Academy Award-winning shotgun microphone in 1963.
The Shure SM57 of 1965 revolutionized the recording of instruments and amplified live music. The SM57 used the Unidyne III capsule to deliver clear and distortion-free sound. Its compact and rugged design allowed it to be put close to drums and amplifiers. It has become one of the top selling microphones in history.
Varieties
Microphones are categorized by their transducer principle and by their directional characteristics. Sometimes other characteristics such as diaphragm size, intended use or orientation of the principal sound input to the principal axis of the microphone are used to describe the microphone.Condenser
The condenser microphone, invented at Western Electric in 1916 by E. C. Wente, is also called a capacitor microphone or electrostatic microphone—capacitors were historically called condensers. The diaphragm acts as one plate of a capacitor, and audio vibrations produce changes in the distance between the plates. Because the capacitance of the plates is inversely proportional to the distance between them, the vibrations produce changes in capacitance. These changes in capacitance are used to measure the audio signal. The assembly of fixed and movable plates is called an element or capsule.Condenser microphones span the range from telephone mouthpieces 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 inherent suitability of this technology is due to the very small mass that must be moved by the incident sound wave compared to other microphone types that require the sound wave to do more work.
Condenser microphones require a power source, provided either via microphone inputs on equipment as phantom power or from a small battery. Power is necessary for establishing the capacitor plate voltage and is also needed to power the microphone electronics. Condenser microphones are also available with two diaphragms that can be electrically connected to provide a range of polar patterns, such as cardioid, omnidirectional, and figure-eight. It is also possible to vary the pattern continuously with some microphones, for example, the Røde NT2000 or CAD M179.
There are two main categories of condenser microphones, depending on the method of extracting the audio signal from the transducer: DC-biased microphones, and radio frequency or high frequency condenser microphones.
DC-biased condenser
With a DC-biased condenser microphone, the plates are biased with a fixed charge. The voltage maintained across the capacitor plates changes with the vibrations in the air, according to the capacitance equation, where Q = charge in coulombs, C = capacitance in farads and V = potential difference in volts. A nearly constant charge is maintained on the capacitor. As the capacitance changes, the charge across the capacitor does change very slightly, but at audible frequencies it is sensibly constant. The capacitance of the capsule and the value of the bias resistor form a filter that is high-pass for the audio signal, and low-pass for the bias voltage. Note that the time constant of an RC circuit equals the product of the resistance and capacitance.Within the time frame of the capacitance change, the charge is practically constant and the voltage across the capacitor changes instantaneously to reflect the change in capacitance. The voltage across the capacitor varies above and below the bias voltage. The voltage difference between the bias and the capacitor is seen across the series resistor. The voltage across the resistor is amplified for performance or recording. In most cases, the electronics in the microphone itself contribute no voltage gain as the voltage differential is quite significant, up to several volts for high sound levels.