672 Chapter 22
being possible. A slow leak for allowing the capsule’s rear chamber pressure to follow
weather-induced atmospheric pressure variations determines the low-frequency limit.
The geometry of a measurement microphone’s physical structure is that of a cylinder
with the central axis of the cylinder being perpendicular to the plane that contains the
microphone capsule’s circular diaphragm. This central axis serves as a reference direction
for sound incident on the microphone. Direct sound arrives at 0° relative to this axis while
grazing incidence occurs at 90°, as illustrated in Figure 22.18.
Any measurement microphone should be encased in such a fashion that the microphone’s
physical structure disturbs the sound fi eld in which it is immersed to a minimum degree.
When the microphone capsules are smaller than ½ inch in diameter it is impossible to
incorporate the necessary circuitry and connector in a uniform cylinder having a diameter
equal to that of the capsule. In such instances it is necessary to enclose the circuitry
and connector in a larger cylinder that is joined to the capsule by a smoothly tapered
section matching the larger diameter to the smaller diameter. A notable example of this is
displayed in Figure 22.19.
22.6.1 Measurement Microphone Types
Despite the smoothness of the microphone enclosure, one cannot escape the fact that
at high frequencies the microphone capsule diameter, d , is comparable to the sound
Direct at 0°Grazing at 90°
Figure 22.18 : Illustration of direct and grazing sound incidences.Figure 22.19 : An example of a well-engineered tapered microphone structure.
(Photo courtesy of Alex Khenkin of Earthworks, Inc.)