Directional microphones suppress noise from certain directions while retaining good sensitivity to sound in one direction. The microphone can be used to transmit sound to the sides of the diaphragm from two separate input ports (open ports of the microphone sound tube), and the sensitivity of the microphone to the sound direction is achieved by reaching the direction of the sound on both sides of the diaphragm. The implementation of microphone directionality depends on two delays:
1. External time delay
It refers to the time it takes for the sound outside the hearing aid to pass from one input port to the other, and is approximately equal to the distance between the two ports separated by the speed of sound near the head. (The effective speed of the sound is lower than the speed near the surface of the skull because the sound waves are scattered around the head in two directions)
2. Internal time delay
Because the rear port includes an acoustic damping or resistor, combined with the opening on the back of the diaphragm, it can pass most of the frequency without clipping for amplification, but with some delay for all filters.
The sound coming from the back reaches the front port later than the latter port, however, when the front low pass filter is reached, the sound entering the back port is delayed. If the internal and external delays are equal, the sound from behind will reach both sides of the diaphragm and there is no pressure on the diaphragm. This way the microphone is not sensitive to sound coming from behind. If the internal delay is less than the external delay, the microphone will be insensitive to sound from other directions.
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