Microphone techniques

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.
 * Situational circumstances: Sometimes a microphone should not be visible, or having a microphone nearby is not appropriate. In scenes for a movie the microphone may be held above the pictureframe, just out of sight. In this way there is always a certain distance between the actor and the microphone.
 * Processing: If the signal is destined to be heavily processed, or "mixed down", a different type of input may be required.
 * The use of a windshield as well as a pop shield, designed to reduce vocal plosives.

Basic techniques
There are several classes of microphone placement for recording and amplification.
 * In close miking, a microphone is placed relatively close to an instrument or sound source. This serves to reduce 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 trying to avoid feedback in an amplified performance.
 * In ambient or distant miking, a microphone — typically a sensitive one — 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 ambient sound, including reverberation from the room or hall.

Stereo recording techniques
There are two features of sound that the human brain uses to place objects in the stereo sound-field between the loudspeakers. These are the relative level (or loudness) difference between the two channels Δ L, and the time-delay difference in arrival times for the same sound in each channel Δ t. The "interaural" signals (binaural ILD and ITD) at the ears are not the stereo microphone signals which are coming from the loudspeakers, and are called "interchannel" signals (Δ L and Δ t). These signals are normally not mixed. Loudspeaker signals are different from the sound arriving at the ear. See the section "Binaural recording for earphones".

X-Y technique: intensity stereophony
Here there are two directional microphones at the same place, and typically placed at 90° or more to each other.

A stereo effect is achieved through differences in sound pressure level between two microphones. Due to the lack of differences in time-of-arrival and phase-ambiguities, the sonic characteristic of X-Y recordings is generally less "spacy" and has less depth compared to recordings employing an AB-setup.

When the microphones are bidirectional and placed facing +-45° with respect to the sound source, the X-Y-setup is called a Blumlein Pair. The sonic image produced by this configuration is considered by many authorities to create a realistic, almost holographic soundstage.

X-Y and Blumlein recordings offer the best compatibility with mono reproduction, barring the M/S technique.

A stereo microphone integrates two microphones in one unit to produce a stereophonic signal. A stereo microphone is often used for broadcast applications or field recording where it would be impractical to configure two separate condenser microphones in a classic X-Y configuration. Some such microphones have an adjustable angle of coverage between the two channels.

See also Acoustic intensity.

A-B technique: time-of-arrival stereophony
This uses two parallel omnidirectional microphones some distance apart, so capturing time-of-arrival stereo information as well as some level (amplitude) difference information, especially if employed in close proximity to the sound source(s). At a distance of about 50 cm (0.5 m) the time delay for a signal reaching first one and then the other microphone from the side is approximately 1.5 msec (1 to 2 msec). According to Eberhard Sengpiel, this is enough to locate the sound source exactly at the speaker on the respective side, resulting in a stereophonic pickup angle of 180°. If you increase the distance between the microphones you effectively decrease the pickup angle. At 70 cm distance it is about equivalent to the pickup angle of the near-coincident ORTF setup.

M/S technique: Mid/Side stereophony
This coincident technique employs a bidirectional microphone facing sideways and a cardioid(of any characteristic; Alan Blumlein described the usage of an omnidirectional transducer in his original patent) at an angle of 90° facing the sound source. One mic is physically inverted over the other, so they share the same distance. The left and right channels are produced through a simple matrix: Left = Mid + Side, Right = Mid - Side (the polarity-reversed side-signal). This configuration produces a completely mono-compatible signal, the width of which can be manipulated after the recording has taken place, which makes it especially useful for the usage on film-based projects. Although many times the M signal is represented as a omnidirectional microphone, in effect only cardioid microphones should be used as only this way a convincing stereo image can be created.