Microphone practice


Microphone practice

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 sound very different from 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.

Contents

Basic techniques

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

  • In close micing, 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. Close micing often affects the frequency response of the microphone, especially for directional mics which exhibit bass boost from the proximity effect.
  • In ambient or distant micing, 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.

Multi-track recording

Often each instrument or vocalist is miked separately, with one or more microphones recording to separate channels (tracks). At a later stage, the channels are combined ('mixed-down') to two channels for stereo or more for surround sound. The artists need not perform in the same place at the same time, and individual tracks (or sections of tracks) can be re-recorded to correct errors. Generally effects such as reverberation are added to each recorded channel, and different levels sent to left and right final channels to position the artist in the stereo sound-stage. Microphones may also be used to record the overall effect, or just the effect of the performance room.

This permits greater control over the final sound, but recording two channels (stereo recording) is simpler and cheaper, and can give a sound that is more natural.

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 article "Binaural recording for earphones".

Various methods of stereo recording

X-Y technique: intensity stereophony

XY Stereo

Here there are two directional microphones at the same place, and typically placed at 90° or more to each other.[1] 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 "spacey" and has less depth compared to recordings employing an AB setup.

Blumlein Stereo

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.

A further refinement of the Blumlein Pair was developed by EMI in 1958, who called it "Stereosonic". They added a little in-phase crosstalk above 700 Hz to better align the mid and treble phantom sources with the bass ones. [2]

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 close 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 ms (1 to 2 ms). If the distance is increased between the microphones it effectively decreases 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

Mid-Side Stereo

This coincident technique employs a bidirectional microphone facing sideways and a cardioid (generally a variety of cardioid, although 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 and, if the Mid and Side signals are recorded (rather than the matrixed Left and Right), the stereo width can be manipulated after the recording has taken place. This makes it especially useful for film-based projects.

There is some controversy as to whether MS micing technique can create translation issues when used with matrix encoded cinema surround formats such as Dolby SR LtRt, which relies on phase relationships between left and right channels of the stereo recording in order to decode surround information.

Choosing a technique

If a stereo signal is to be reproduced in mono, out-of-phase parts of the signal will cancel, which may cause the unwanted reduction or loss of some parts of the signal. This can be an important factor in choosing which technique to use.

  • Since the A-B techniques use phase differences to give the stereo image, they are the least compatible with mono.
  • In the X-Y techniques, the microphones would ideally be in exactly the same place, which is not possible – if they are slightly separated left to right, there may be some loss of high frequencies when played back in mono, so they are often separated vertically. This only causes problems with sound from above or below the height of the microphones.
  • The M/S technique is ideal for mono compatibility, since summing Left+Right just gives the Mid signal back.

The equipment for the techniques also varies from the bulky to the small and convenient. A-B techniques generally use two separate microphone units, often mounted on a bar to define the separation. X-Y microphone capsules can be mounted in one unit, or even on the top of a handheld digital recorder. Since M/S setups can give a variable soundstage width, they are often used in small 'pencil microphones' to mount on video cameras, matching a zoom lens.

References

  1. ^ Michael Williams. "The Stereophonic Zoom" (PDF). Rycote Microphone Windshields Ltd. http://www.rycote.com/images/uploads/The_Stereophonic_Zoom.pdf. 
  2. ^ Eargle, John (2004). The Microphone Book (2 ed.). Focal Press. p. 170. ISBN 0240519612. http://books.google.co.uk/books?id=w8kXMVKOsY0C&pg=PA170. 

External links


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