The invention of a practical microphone was crucial to the early development of the telephone system. Emile Berliner invented the first microphone on March 4, 1877, but the first useful microphone was invented by Alexander Graham Bell. Many early developments in microphone design took place in Bell Laboratories.
In a capacitor microphone (also known as a condenser microphone), the diaphragm acts as one plate of a capacitor, and vibrations produce changes in a voltage maintained across the capacitor plates. Capacitor microphones are expensive and require an external power supply, but give a high-quality sound signal and are used in laboratory and studio recording applications.
In the dynamic microphone a small movable induction coil, positioned in the magnetic field of a permanent magnet, is attached to the diaphragm. When the diaphragm vibrates, the coil moves in the magnetic field, producing a varying current in the coil (See electromagnetic induction). Dynamic microphones are robust and relatively inexpensive, and are used in a wide variety of applications.
In ribbon microphones a thin, corrugated metal ribbon is suspended in a magnetic field: vibration of the ribbon in the magnetic field generates a changing voltage. Ribbon microphones detect sound in a bidirectional pattern: this characteristic is useful in such applications as radio and television interviews, where it cuts out much extraneous sound.
A carbon microphone, formerly used in telephone handsets, is a capsule containing carbon granules pressed between two metal plates. A voltage is applied across the metal plates, causing a current to flow through the carbon. One of the plates, the diaphragm, vibrates in sympathy with incident sound waves, applying a varying pressure to the carbon. The changing pressure deforms the granules, causing the contact area between each pair of adjacent granules to change, and this causes the electrical resistance of the mass of granules to change. Since the voltage across a conductor is proportional to its resistance, the voltage across the capsule varies according to the sound pressure.
A piezo microphone uses the phenomenon of piezoelectricity-- the tendency of some materials to produce a voltage when subjected to pressure-- to convert vibrations into an electrical signal. This type of microphone is often used to mic acoustic instruments for live performance, or to record sounds in unusual environments (underwater, for instance.)
Depending on various aspects of a microphone's construction, it may be nearly equally sensitive to sound coming in all directions (an omnidirectional[?] microphone), or it may be more sensitive to sound coming from a particular direction (a unidirectional microphone). The most common of the unidirectional type is sometimes called a cardioid[?] microphone, because the sensitivity pattern somewhat resembles the shape of a heart; most vocal mikes are cardioid or hyper-cardioid[?] (similar to cardioid but with a tighter area of front sensitivity and a tiny lobe of rear sensitivity.)
Some microphones have more complex sensitivity patterns. Most ribbon microphones are bi-directional[?], receiving sound from both in front and back of the element. This type of response is also known as a figure-8 pattern, because of its shape. Shotgun microphones[?], the most directional form of studio microphone, reserve most of their sensitivity for sounds directly in front of, and to a lesser extent, the rear of the microphone. Shotgun microphones also have small lobes of sensitivity to the left and right. This effect is a result of the microphone design, which generally involves placing the element inside of a tube with slots cut along the side; wave-cancellation eliminates most of the off-axis noise.
A parabolic microphone uses a parabolic reflector to collect and focus sound waves onto a microphone receiver, in much the same way that a parabolic antenna (e.g. satellite dish) does with radio waves. Typical uses of this microphone, which has unusually focused front sensitivity and can pick up sounds from many meters away, include nature recording, eavesdropping[?], law enforcement, and even espionage. Parabolic microphones are not typically used for standard recording applications, because they tend to have poor low-frequency response as a side effect of their design.
There exist a number of well-developed microphone techniques used for miking musical, film, or voice sources. Choice of technique depends on a number of factors, including:
There are several classes of microphone placement for recording and amplification.
There are two essential components that the human ear uses to place objects in a stereo sound-field. These are stereo intensity[?], the relative loudness of sounds entering either ear, and interaural time-delay[?], the slight difference in arrival time at both ears. Additionally, the folds of the pinnae[?] also provide frequency-filtering that can help to place a signal in a 360-degree field of hearing.