Microphone practice

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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 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 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".

Conventional stereo recording for loudspeakers

The following microphone techniques can be used to capture the live "soundstage":

• The X-Y technique involves the coincident placement of two directional (cardioid) microphones. When two directional microphones are placed coincidentally, typically at a 90° angle (or greater) to each other (typically with each microphone pointing to a side of the soundstage), a stereo effect is achieved simply through intensity differences between the sound entering each microphone. Due to the lack of time-of-arrival stereo information, the stereo effect in X-Y recordings has less ambience. The main advantage is that the signal is mono-compatible, i.e., the signal is suitable for playback on non-stereo devices such as AM radio. If two bi-directional (figure 8) microphones are used instead of cardioid microphones, this technique is known as a Blumlein pair . Angles for microphones are: 90° - bidirectional, 131° - cardioid, 105° - hypercardioid, 115° - supercardioid.

• The Middle-Side (M-S) array technique is a special case of X-Y and uses a directional cardioid or an omnidirectional pressure microphone (M - middle microphone) and a bidirectional (figure-8) microphone (S - side microphone), placed at a 90° angle to each other with the directional microphone facing the soundstage. 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. The M-S technique with an omnidirectional M microphone is equivalent to X-Y with two cardioids at a 180° angle.

• Near-coincident recording is a variant of the X-Y technique and incorporates interchannel time delay by placing the microphones several inches apart. The ORTF stereo technique of the Office de Radiodiffusion Télévision Française (Radio France), calls for a pair of cardioid microphones placed 17 cm apart at an angle of 110°. In the NOS stereo technique of the Nederlandse Omroep Stichting (Holland Radio), the angle is 90° and the distance is 30 cm. The choice between one and the other depends on the recording angle of the microphone system, not on the distance to and the width of the sound source. This technique leads to a realistic stereo effect and has reasonable mono-compatibility. These interchannel signals have nothing to do with interaural signals which come only from artificial head recordings. Even the spacing of 17 cm has nothing to do with human ear distance. The ORTF and NOS engineers did not think in those terms, because this microphone system was developed for a set of stereo loudspeakers, not for earphones.

• The A-B technique uses two omnidirectional microphones at a moderate distance from each other (20 centimeters up to a few meters). Stereo information consists of large time-of-arrival distances and some sound level differences. With excessively large distances, the stereo image can be perceived as somewhat unnatural, as if the left and right channel are independent sound sources without an even spread from left to right. A-B recordings are not so good for mono playback because the time-of-arrival differences can lead to certain frequency components being canceled out and others being amplified, the so-called comb-filtering effect, but the stereo sound can be really convincing. If wide A-B is used for large orchestras, the center can be filled with another microphone. Then one gets the famous "Decca tree", which has brought us many good sounding recordings.

• The Blumlein shuffler technique uses two microphones spaced around 20 cm (head width), and these are usually, but not necessarily, omnidirectional. A special "Blumlein shuffler" circuit integrates the difference signal, before matrixing it to produce an output in which phase (time delay) information has been converted to amplitude difference. This is a purist technique for providing true stereo from binaural capture, permitting omnidirectional microphones to be used (with their low coloration and flat low-frequency response) for true stereo. It has been little used, probably because of the lack of commercial shufflers. While offering very realistic stereo, it can emphasise low frequencies picked up from the sides unless the shuffler incorporates rolloff in the difference path. A central baffle, in the form of a foam disc suspended between the microphones, provides level separation above 2 kHz where the shuffling has to be phased out.

• The Baffled Omnidirectional technique uses a pair of near-coincident omnidirectional microphones with an absorptive baffle between them and is closely related to binaural technique. Stereo information consists primarily of time-of-arrival differences between the microphones and intensity differences from the baffle. The Jecklin Disk^[1], described by the Swiss radio technician Juerg Jecklin, uses of a 30 cm flat circular sound absorbing baffle arranged vertically with the faces perpendicular to the sound source. Pressure microphones are placed 16.5 cm apart, directly left and right of the disk's center. The KFM Sphere, described by Guenther Theile, consists of two pressure microphones mounted on opposite sides of a 20 cm sphere. The microphones are mounted flush with the surface and arranged with the 0-axis perpendicular to the sound source.

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.^[2] Both systems use the Ideal Cardioid Arrangement (ICA 5, ITU-775 specification), developed by Volker Henkels and Ulf Herrmann.

References

1. ^ Jecklin Disk
2. ^ SPL's Web site