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What is an acoustic labyrinth?

There are basically two ways to design a microphone. The first is where one side of the diaphragm is open to the air, and the other side is completely sealed. The second is to have both sides open to the air. (I suppose you might also consider having both sides sealed as a third type, but this won't pick up much sound!)...

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Perhaps an acoustic labyrinth is where the acoustic minotaur hangs out?

OK, terrible joke - now the serious stuff. There are basically two ways to design a microphone. The first is where one side of the diaphragm is open to the air, and the other side is completely sealed. The second is to have both sides open to the air. (I suppose you might also consider having both sides sealed as a third type, but this won't pick up much sound!).

The type where only one side is open is known as the 'pressure microphone' as it is sensitive to tiny variations in the pressure of the surrounding air, caused by sound waves. Pressure is a so-called scalar quantity and has no direction. Pressure 'presses' equally in all directions, therefore this microphone is omnidirectional.

The type where both sides are open is known as a 'pressure gradient microphone' and is sensitive to the difference in pressure between the front and rear of the diaphragm. Pressure gradient is a vector quantity and does possess direction. Therefore this microphone is directional, and logically enough it is equally sensitive to the front and to the rear, but not sensitive at all at the sides.

So where do we get the cardioid microphone from, which is sensitive to the front, fairly sensitive at the sides, but totally insensitive at the back? The answer is to create a sort of halfway house between the pressure microphone and the pressure gradient microphone. This is done by allowing one side of the diaphragm to be completely open, the other side is partially open through an acoustic labyrinth that controls access to the rear. This is technically difficult to accomplish and early cardioid microphones had poor performance compared to omnidirectionals and figure-of-eights. However, microphone manufacturers have made impressive achievements and now there is little about a good quality cardioid microphone to criticize.

The acoustic labyrinth also features in the hypercardioid microphone.

By David Mellor Wednesday October 4, 2006
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