The receiver principle of microphones describes how they respond to sound from different directions. There are two main types.
Pressure Receiver
A pressure receiver is a microphone that responds to sound pressure and thus operates in an omnidirectional manner. This means it has the same sensitivity in all directions, as pressure is a scalar quantity, which is independent of direction. However, in practice, at high frequencies, directional effects can arise due to interference, diffraction, and reflection because the size of the microphone falls within the range of sound wavelengths.
Pressure receivers are optimized depending on their application to adjust the frequency response for specific environments, such as free field, diffuse field, or pressure field. This is achieved through special acoustic networks or electrical filtering. Pressure receivers also have a small opening to accommodate changes in air pressure, which does not affect the audio range.
Pressure Gradient Receiver
A pressure gradient receiver is a microphone that responds to the pressure difference between the front and back of the diaphragm. This pressure difference arises due to the varying sound propagation times on both sides of the diaphragm. The diaphragm is exposed to sound from both sides, resulting in identical signals when sound arrives from the front and back, but with inverted polarity for rear sound.
This design creates a polar pattern in the shape of a figure-eight, where sound from 90-degree angles (the sides) does not elicit a response. The frequency response rises to a certain frequency, above which interference or cancellation may occur. A pressure gradient receiver is particularly sensitive to low frequencies in the near field, which enhances the so-called proximity effect, while pressure receivers do not exhibit this effect.
By combining these principles, additional polar patterns are created, such as the cardioid (sensitive to sound from the front, insensitive to sound from the back) and the supercardioid or hypercardioid, which offer a greater directional response.
