Microphones and miking techniques - Unidirectional microphones

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This type of microphone has a cardioid polar pattern. The following scheme illustrates how it is built:

Microphones and miking techniques - Internal diagram of a cardioid microphone

Internal diagram of a cardioid microphone

Behind the diaphragm there is an acoustic delay network whose task is to delay sounds coming from behind. Sounds coming from behind stimulate the diaphragm, as we have seen in the case of omnidirectional microphones. However, because of the little lateral holes, the same sound penetrates the posterior part of the microphone and encounters a delay network which escorts the sound down a series of different paths, thus delaying its arrival at the diaphragm. When the delayed sound reaches the diaphragm, it gets inverted in phase in relation to the sound which, thanks to diffraction, has reached the anterior part of the microphone (once again we mustn't forget that we are considering sounds coming from behind). This situation results in the nullification of the two sounds, the external one and the internal one which has been delayed when they both arrive at the diaphragm out of phase. This way the posterior sound is eliminated, or at least drastically attenuated. The same delay network acts upon the frontal sound: a part of it stimulates the diaphragm directly, whilst another part penetrates the lateral holes and after having gone through the delay network arrives at the diaphragm, in phase. This makes sure that the two signals sum up, guaranteeing a faithful reproduction of the frontal signal, which is therefore reinforced.

In the case of condenser microphones , the presence of the posterior plate impedes sound from reaching the frontal diaphragm through the acoustic delay network and therefore a different technique is used. Let's take a look at it.

Another condenser with a posterior diaphragm fixed to it is added. At the output the signal coming from the posterior condenser gets inverted in phase and added to the anterior signal. This allows the posterior sound to be cancelled and the anterior one to be reinforced.

We have seen how the more we try to narrow the cardioid's shape the more we have an insurgence of a posterior lobe. This is due to the fact that the delay network is unable to correctly cancel sounds which come from a direction whose angle in relation to the central direction is too small.


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