8
2. Frequency response variation due to treble losses as a result of absorption and
narrowing of the pattern at high frequencies-causing weakening of highs as the
microphone is moved away from the sound source.
3. Rise in treble response on-axis due to diffraction.
4. Loss of treble response off-axis due to phase-loss effect.
5. Variation in ratio of direct to reverberant sound.
6. Tendency of a microphone to favor the nearest sound source due to a combination of
these items, plus the influence of inverse square law.
Other Types of Microphones
For the same ratio of direct to reverberant sound, omni-directional microphones must be
closer to the sound source than cardioid or bi-directional microphones. Microphones
should generally face the sound source head-on or else treble loss due to phase loss will
result. The exception here is for large condenser microphones, which often give the
flattest response at an angle of about 10-20 degrees (off axis), where phase loss and
diffraction effect offset each other somewhat.
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The Sound That Is “More Real than Real”
Ribbon microphones have long been renowned for rich bass. This effect is largely due to
the fact that ribbon microphones generally have excellent bass response to begin with, and
at the same time exhibit an effect known as “proximity effect” or “bass tip-up.”
As illustrated in the following graph, a typical bi-directional ribbon microphone will have
a flat frequency response at a distance of about six feet from the microphone but at
shorter distances the bass response becomes boosted; the effect becomes increasingly
pronounced as the distance between the microphone and the sound source is reduced.
This bass-boosting characteristic can become quite intense and, if desired, can be corrected
by equalization. However, for a multiple microphone setup, the pronounced bass
boosting (due to proximity effect) can be turned to an advantage. If an instrument such as
a trumpet is extremely close miked and the bass is cut to restore flat response, unwanted
low-frequency sounds are cut back by upwards of 20 dB compared to an unequalized
microphone with a flat response; and this discrimination is
independent
of the
microphone’s polar response.
