For a run-of-the-mill ½ inch microphone the short answer is nothing.
However the long answer is a bit more involved.
Basically if the frequency range is below 16 kHz and an accuracy of ±2dB is acceptable then there is no real difference between the types.
So what is the difference between these types of microphone?
Well, first you have to understand the different types of sound field involved.
In theory a free field is a sound field where the sound waves are free to expand outwards forever from the source. That is, we assume that there are no reflections or reverbarations. In practice, we would consider an anechoic chamber to be a free field or an outdoor measurement, provided we measured at a sufficient distance from the ground.
A diffuse field or a random incidence field, is a sound field where the sound waves arrive equally from all directions. Another way to think of a diffuse field is that you have sounds coming from different directions in succession with no time inbetween their arrival. For example, in a diffuse field the sound waves that arrive at a person’s ears are so completely different that the brain finds it impossible to work out where the sound came from.
A pressure field is a sound field where the sound pressure has the same magnitude and phase at any position in the sound field.
It is important to understand that all of these microphone types are fundamentally the same. They are transducers that are designed to sense pressure levels in air. The differences between them are in the designs of the microphone heads. Each type of microphone will normally be supplied with a calibration data sheet that will show the frequency response against sound pressure. Comparing the frequency response against sound pressure for the three microphone types will show the differences clearly.
The differences between the three types of microphone generally occur at higher frequencies as we stated previously, below about 16 kHz the response from each will be similar. This is partly to do with the physical size of the microphone with respect to the wavelength of the sound being measured.
Free field microphones are designed to compensate for the effect of the microphone itself in the field. So you measure the sound as though the microphone was not there. They are designed this way as, of course, the presence of the microphone will affect the propagation of the sound wave. A free field microphone should be pointed towards the sound source at a 0° angle of incidence.
Diffuse field microphones are designed to respond in a uniform manner to any signal arriving on its measuring surface from any angle. Generally, these tend to be oriented at about 75° to the direction of the sound wave propagation in a free field.
Pressure microphones are designed to respond to a uniform frequency response to the sound level itself. When used in a free field a pressure microphone should be mounted at 90° to the direction of the sound wave propagation, effectively the sound passes the measuring face of the microphone.
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