Usually, high-quality speech recordings require the sound source to
be fairly close to the microphone's diaphragm. This may trigger a so-called
proximity effect. Proximity effect is the increase in the low-frequency
sensitivity of a microphone when the sound source is close to it. This
is particularly true of cardioid, directional microphones. To counter
that, most high-end directional microphones use a low-frequency roll-off
filter to restore the response to its flat, natural balance. Some microphones
have a user-selectable switch to control the filter. The proximity effect
may be responsible for speech spectra showing emphasis in the low-frequency
range, around the first and second harmonics.