Spaced pairs: AB and the hole in the middle

An AB pair uses two omnidirectional microphones placed apart. Because they hear almost the same level, direction information comes from the time difference between them, which is the spacing divided by the speed of sound times the sine of the source angle.

The numbers

A source at 90° (directly to one side) is delayed by the full spacing divided by the speed of sound:

Since about 1.5 ms is enough to place a source fully in one loudspeaker, a 60 cm pair reaches full panning with a source about 60° off centre, and everything beyond that is reproduced in one loudspeaker.

Choosing the spacing

A narrow spacing gives a wide recording angle, a wide spacing a narrow one:

• Up to about 50 cm: even a source at the extreme side does not quite reach one loudspeaker (30 cm gets about four fifths of the way, 40 cm about nine tenths), so the whole sound stage is spread across the base.
• 60 cm: a recording angle of about 120°.
• A metre: only about 60° is imaged, and everything outside is crowded into the loudspeakers.

At large spacings the image becomes increasingly unstable, and there is the hole in the middle problem described below.

The hole in the middle

With widely spaced omnis, a source in the centre reaches both microphones at the same time and the same level, so it is placed in the centre. But a source slightly off-centre is placed far away from the centre, because even a small angle produces a large time difference. The result is that sources cluster at the loudspeakers and the middle is empty.

The usual remedy is a third microphone in the centre, fed to both channels at a lower level (the Decca tree does this, see Baffled pairs and the Decca tree).

Why use it anyway?

• Omnis reproduce low frequencies extremely well, with no proximity effect or low-frequency roll-off.
• The images are spacious and natural, with a convincing room impression.
• Omnis are unfussy about placement angle and robust against wind and handling noise.

What to watch

• The mono sum has comb filtering, because the time differences make the two signals partially cancel.
• Spacings above the width of the ensemble make the images unstable.
• Recordings made for small ensembles in large rooms may need a closer spacing than for orchestras in the same room.

Try it

The stereo recording angle tool has presets for AB pairs at 40 and 60 cm. Use delay and distance to convert spacing to time.

Draft: written by an AI assistant, awaiting technical review.