Microphone

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 all microphones, sound waves are translated into mechanical vibrations[?] in a thin, flexible diaphragm. These vibrations are then converted by various methods into an electrical signal.

Types of Microphone

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.)

Directionality

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.

Microphone techniques

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:

• The collection of extraneous noise. This can be a concern, especially in amplified performances, where audio feedback can be a significant problem. Alternatively, it can be a desired outcome, in situations where ambient noise is useful (hall reverberation, audience reaction.)

• Choice of a signal type: Mono, stereo or multi-channel.

• Type of sound-source: Acoustic instruments produce a very different sound than electric instruments, which are again different from the human voice.

• Processing: If the signal is destined to be heavily processed, or "mixed down", a different type of input may be required.

Basic techniques

There are several classes of microphone placement for recording and amplification.

• In close miking, a directional microphone is placed relatively close to an instrument or sound-source. This serves to eliminate extraneous noise-- including room reverberation-- and is commonly used when attempting to record a number of separate instruments while keeping the signals separate, or when in order to avoid feedback in an amplified performance.

• In ambient or distant miking, a sensitive microphone or microphone is placed at some distance from the sound source. The goal of this technique is to get a broader, natural mix of the sound source or sources, along with reverberation from the room or hall.

Stereo techniques

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.

• The X-Y technique involves the coincident or close placement of two microphones, which may be either directional or omnidirectional. When two directional microphones are placed coincidentally, typically at a 90+ degree angle to each other (typically with each microphone pointing to a side of the sound-stage), a stereo effect is acheived simply through intensity differences of the sound entering each microphone. Variants of this technique exist that incorporate inter-aural time delay by placing the microphones several inches apart. The ORTF technique calls for a pair of cardioid microphones placed 7 inches apart at an angle of 110 degrees.

• The Mid-Side (M-S) technique uses a directional microphone (M) and a bidirectional (figure-8) microphone (S), placed at a 90 degree angle to each other with the directional microphone facing the sound-stage. The outputs of these microphones are mixed in such a way as to generate sum and difference signals between the outputs. The S signal is added to the M for one channel, and is subtracted (by reversing phase and adding) to generate the other channel. M-S has two advantages: when the stereo signal is combined into mono, the signal from the S microphone cancels out entirely, leaving only the mono recording from the directional M microphone; additionally, M-S recordings can be "remixed" after recording to alter or even remove the stereo spread.

• Binaural recording is a highly specific attempt to recreate the conditions of human hearing, reproducing the full three-dimensional sound-field. Most binaural recordings use model of a human head, with microphones placed where the ear canal[?] would be. A sound source is then recorded with all of the stereo and spatial cues produced by the head and human pinnae. Binaural recording is usually only somewhat successful, in addition to being highly inconvenient. For one thing, it tends to work well only when played back directly into the ear canal, via headphones, as other methods of playback add additional spatial cues. Furthermore, as all heads and pinnae are different, a recording from one "pair of ears" will not always sound correct to another person. Finally, as visual cues are generally much more powerful than auditory cues when determining the source of a sound, binaural recordings are not always convincing to listeners.