With directional microphones, if the sound source is too close to the microphone, emphasis of the bass frequencies occurs. This effect is called proximity effect and is related to the fact that diffraction of the low frequencies occurs.
The diffraction of the low frequencies occurs around the microphone capsule and affects the diaphragm through the delay element. In contrast, sound at higher frequencies has a directional response; these frequencies do not diffract or diffract only slightly. In addition, microphones also react to the speed of sound, which increases disproportionately at low frequencies close to the sound source. The proximity effect disproportionately emphasizes bass frequencies below 200 Hz, causing a more voluminous sound. The overemphasis of the low end depends on the directional characteristic of the microphones. With cardioid microphones, the effect is relatively pronounced; with figure-of-eight microphones, it is disproportionate.
The shorter the distance between the sound source and the microphone, the more low frequencies are boosted, resulting in a dull, unnatural sound. The proximity effect occurs only in directional microphones with pressure gradient receivers at distances of less than 60 cm. Omnidirectional microphones therefore have no proximity effect. Microphones designed exclusively for proximity effect, such as lavalier microphones, either use pressure transducers or high-pass filters to attenuate the low frequencies.