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What is the no-go area around a crossed pair of figure-of-eight microphones?

A crossed pair of figure-of-eight microphones is sensitive to the front, sensitive to the rear, but elsewhere there is a 'no go' area? Why is this?

It is common to record in stereo using a coincident crossed pair of figure-of-eight microphones.

'Coincident' means that the diaphragms of the mics are as close together as possible. 'Crossed' means that one mic points left, the other points right, separated by 90 degrees.

'Pair' means there are two of them :-)

A coincident crossed pair always works, if you can find the right position for the mics, and captures a good clear recording.

When figure-of-eight pattern mics are used for the crossed pair there is an interesting feature - since the mics are equally sensitive to the front and to the rear, the rear is effectively another 'front'.

This is commonly exploited in drama recording where a group of actors can divide themselves between front and rear, and in the resulting recording they will all seem to be located together.

However there are two 'no-go' areas around a crossed pair of figure-of-eights.

Sound that approaches the mics from the sides has the unfortunate effect of striking the front of the diaphragm of one mic, and the rear of the diaphragm of the other. So the two diaphragms are pushed and pulled in opposite directions.

In other words, the signals are out of phase, or inverted, with respect to each other.

When this is played through loudspeakers or headphones, the net result will be one eardrum being pushed inwards while the other is sucked outwards, and vice versa.

There is no sound in nature that does this, so the human hearing system has difficulty to interpret what it is hearing. It sounds distinctly odd.

So the moral of this story is never to place sound sources in either of the two quadrants to the sides of a crossed pair of figure-of-eight mics.

This is fine for direct sound sources where you have control. But you have little control over reflections reaching the sides of the pair.

Out of phase reverb is a characteristic of crossed figure-of-eights and perhaps accounts for the fact that although in theory this configuration should produce perfect stereo sound, in practice it doesn't quite live up to that ideal.

By David Mellor Wednesday June 21, 2006