The frequency response that is published by the manufacturer of a microphone is a very theoretical graph. Under real circumstances the frequency response changes noticeably depending on the mic placement and the room. It is possible to calculate the influence two main effects:
Proximity Effect: Directional mics (Fig. of Eight, all types of Cardioids) show a bass boost, when the sound source is getting closer to the mic. The more directional the mic, the more intense the effect. Omni-directional mics do not show this effect.
Loss of high frequencies in the diffuse field: Real-world omni-directional mics are getting more directional in the high frequencies. High frequencies from the back are attenuated, making the sound darker. If the sound would come from the front only (the direct sound), this would not be an issue. In the diffuse field–in a highly reverberated room–sound is arriving at the mic from all directions at equal levels. This means that darker sounds also contribute equally to the final signal. The result is an overall darker sound compared to the direct signal.
That is the reason for the existence of diffuse-field corrected omnis: they emphasize the high frequencies to balance the loss.
There is also a loss in the high frequencies because of the atmosphere, but this effect is much smaller and ignored here.
Mic-Angle
Plain Frequency response
Distance between Sound and Microphone
Reverberation Radius
Become a Patron!