Microphone practice

Main article: Microphone

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".

Various methods of stereo recording

X-Y technique: intensity stereophony

Here there are two directional microphones at the same place, and typically pointing at an angle 90° or more to each other - see also "The Stereophonic Zoom" by Michael Williams. A stereo effect is achieved through differences in sound pressure level between two microphones. Due to the lack of differences in time-of-arrival / 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 most realistic, almost holographic soundstage.

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. 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.

Near-coincident technique: mixed stereophony

For example, the ORTF stereo technique of the Office de Radiodiffusion Télévision Française (i.e., Radio France), calls for a pair of cardioid microphones placed 17 cm apart at a total angle between microphones of 110 degrees which, according to Eberhard Sengpiel, results in a stereophonic pickup-angle of 96°. In the NOS stereo technique of the Nederlandse Omroep Stichting (i.e., Holland Radio), the total angle between microphones is 90 degrees and the distance is 30 cm, so capturing time-of-arrival stereo information as well as level information. It is noteworthy that the spacing of 17 cm has nothing to do with human ear distance. The recorded signals are generally intended for playback over stereo loudspeakers and not for ear phones.

Binaural recording for earphones

Binaural recording is a highly specific attempt to recreate the conditions of human hearing, reproducing the full three-dimensional sound-field with earphones. Most binaural recordings use a 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 with frequency dependent ILD (interaural level difference) and ITD (interaural time difference, max. (Δt) = 630 µs = 0.63 ms) ear signals. A 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 (no speakers), 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. Also, headphones have a frequency response that compensates for the fact that the reflections from the pinnae, head and shoulders strongly affect the frequency spectrum, with the assumption that a recording is taken with a flat frequency spectrum. Introducing the spectral distortion already in the binaural recording results in an unnatural frequency spectrum, even when played through headphones. 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.

Surround microphone techniques

• The Double MS Technique was developed by Chris Wittig and Neil Muncy, and uses a front-facing mid-side microphone pair of direct sound pickup and a rear MS pair facing away from the front. The rear pair is placed at or just beyond the critical distance of the room where the reverberant sound level equals the direct sound level. The matrixed outputs feed front-left, front-right, rear-left, and rear-right speakers.

• The Surround Ambience Microphone Array was developed by Gunther Theile of the Institut für Rundfunktechnik (IRT). Four cardioid microphones are placed 90 degrees to each other and 21 to 25cm apart. No center channel is described.

• The Spider Microphone Array uses a special mike mount with five arms that radiate out from a center point, like a star. At the end of each arm is a condenser microphone aiming outward from the center. Two examples: The Microtech Gefell INA 5 uses five M930 mics in shock mounts. In the SPL Atmos 5.1/ASM 5 Surround Recording System, five Brauner condenser mikes feed a five channel mixing console, which adjusts the mic polar patterns and offers panning, bass management, and surround monitoring. Both systems use the Ideal Cardioid Arrangement (ICA 5, ITU-775 specification), developed by Volker Henkels and Ulf Herrmann.