Microphone Technology 64722.3 Nature of Response and Directional Characteristics .............................................
Pressure microphones are those where only one side of the diaphragm is exposed to
the actuating sound fi eld. Such devices are basically insensitive to the direction of
the arriving sound as long as the wavelength is large compared with the diaphragm
circumference. At high frequencies when the wavelength becomes comparable to or
even less than the diaphragm circumference, two directional effects become evident. For
sound directly incident on the exposed face of the diaphragm, the partial refl ection of the
pressure waveform at the diaphragm surface increases the acoustic pressure amplitude
over that which would exist in an undisturbed sound fi eld. For sound incident from the
rear of the exposed face of the diaphragm, the active face of the diaphragm is in the
shadow of the microphone’s housing structure and experiences a pressure less than that
of the undisturbed sound fi eld. This front-to-back discrimination can only be avoided by
employing physically small microphone structures. This is the reason why measurement
microphones often have capsules of ¼ inch diameter or even less.
A controlled directional response can be obtained by employing a sensing diaphragm,
both faces of which are exposed to the sound fi eld of interest. Such diaphragms
experience a driving force that depends on the spatial rate of change of pressure rather
than on the pressure itself. Consider the situation shown in Figure 22.3.
Figure 22.3 is a bare bones illustration of a diaphragm stripped of details of the
transducing mechanism. Both sides of the diaphragm are exposed to a sound wave that
Figure 22.3 : Compliantly mounted diaphragm with both sides exposed to a sound fi eld.