03/04/16

PRINCIPLES OF THE A-B STEREO TECHNIQUE

The A-B Stereo Technique – also known as the Time Difference Stereo – involves two spaced (usually omnidirectional) microphones to record the sound stage.
The microphone spacing introduces small inter-channel differences of the time / phase information contained in the audio signals recorded. These differences occur because of the relative distance from the sound source to each of the microphones. If not on-axis, the acoustic signal from a sound source arrives at one microphone slightly before the other. When reproduced by a 2-channel loudspeaker setup, the timing of the reproduced signals determine the perceived direction.

Besides the localization of sound sources, the signal also contains information on the acoustic space. The time and phase differences recorded acts as stereo cues to enable the listener to "capture the space" of the recording.



Two spaced microphones creating a stereo image
 

Microphone spacing

An important consideration when setting up for A-B stereo recordings is the distance between the two microphones. What the stereo recording should sound like is often a question of taste, which makes it difficult to give exact rules on spacing. It is though, a good idea to keep some important acoustic factors in mind.
Since the stereo width of a recording is frequency-dependent, the deeper the tonal qualities you wish to reproduce in stereo, the wider your microphone spacing should be. Using a recommended microphone spacing of a quarter of the wavelength of the deepest tone and taking into account the human ear's lack of ability to localize frequencies below 150 Hz, leads to an optimal microphone spacing of between 40-60 cm (16-24 in).

Smaller microphone-spacing may apply close to sound sources to prevent the sound image of a particular musical instrument or sound stage from becoming "too wide" and unnatural. Also, it should be noted that increasing the microphone spacing will decrease the system's ability to reproduce the signals from sound sources positioned directly between the microphones.

When collapsed to mono, the AB-recording may suffer from comb-filtering depending on the distance between the microphones and also depending on the sound field. The worst case of coloration is achieved in a direct sound field (dry acoustics / outdoor). The best case is in a completely diffuse sound field (a very reverberant room).
 

Distance between microphones and sound source

The ideal distance from the microphone pair to the sound source not only depends on the type and size of the sound source and on the surroundings in which the recording is to be made, but also on individual taste. The position from where the listener experiences the event – and hence the position from where the microphones record the event – should be chosen with care and feeling.

Musical recordings, such as a full orchestra in a concert hall, are good examples of the importance of correct stereo microphone positioning. Here the microphones would typically be placed above or behind the conductor. Although most instruments project their sound in an upwards direction, the microphones should be placed high enough so that the individual musicians do not shadow each other.

The mix of direct and diffuse sound in a recording is also of crucial importance, so a lot of time can be used establishing the optimum positioning of the microphones. It is here that the versatility of our A-B Stereo Kits comes into play. Using Acoustic Pressure Equalizers for the microphones, the amount of ambiance and the tonal color of the recording can be adjusted without adding any noise. The choice of floor and ceiling mounting of the boom can give you added flexibility when positioning the microphones.

Omni mics in an A-B Stereo setup are often the preferred choice when the distance between the microphone and the sound source is large. This is because omni mics can capture the true low frequencies of the sound source regardless of the distance, while directional microphones are influenced by the proximity effect. Directional microphones will therefore exhibit loss of low frequencies at larger distances. The d:dicate™ 4015A  Wide Cardioid Microphones, however, exhibit by design a richer bass response, making the bass loss at distances much less critical. Therefore, these mics are a good alternative to omnis when some directionality is preferred or needed. The sound color is very similar to our omnis.
 

Suggested microphone kits and accessories

ST2006A Stereo Pair with d:dicate™ 2006A
ST4006A Stereo Pair with d:dicate™ 4006A
SB0400 Stereo Boom, Modular
UA0837 Stereo Boom
APE30RS The Acoustic Pressure, Equalizer, 30 mm Ball for d:dicate™ 4006A
APE40RS The Acoustic Pressure, Equalizer, 30 mm Ball for d:dicate™ 4006A
APE50RS The Acoustic Pressure, Equalizer, 30 mm Ball for d:dicate™ 4006A
UA0777 Nose Cone, for d:dicate™ 4006A
DD0297 Diffuse-Field Grid, Black, for d:dicate™ 4006A

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