Handbook for Sound Engineers

534 Chapter 16

2. Capsule with the diaphragm and counter elec-
   trode(s).


3. Rear inlet.


4. Electronic circuit.

The directional characteristics are based on two

different principles:

1. In the low-frequency range, tube microphones be-
   have as first-order directional receivers. The tube
   in front of the capsule can be considered as an
   acoustic element with a compliance due to the en-
   closed air volume and a resistance determined by
   the lateral holes or slits of the tube. The rear inlet is
   designed as an acoustic low-pass filter to achieve
   the phase shift for the desired polar pattern (nor-
   mally cardioid or supercardioid).


2. In the high-frequency range, the acoustical proper-
   ties of the interference tube determine the polar
   patterns. The transition frequency between the two
   different directional characteristics depends on the
   length of the tube and is given by

(16-26)

where,

fo is the transition frequency,

c is the velocity of sound in air in feet per second or

meters per second,

L is the length of the interference tube in feet or

meters.

Referring to Fig. 16-94, if the tube is exposed to a

planar sound wave, every lateral inlet is the starting point

of a new wave traveling inside the tube toward the

capsule as well as towards the frontal inlet. Apart from

the case of frontal sound incidence, every particular wave

covers a different distance to the capsule and, therefore,

arrives at a different time. Fig. 16-94 shows the delay

Figure 16-91. By using a microphone as shown in Fig.
16-89 and superimposing two cardioid patterns (top row),
directional response patterns (bottom row) can be
obtained. Courtesy Neumann USA.

Figure 16-92. A supercardioid interference tube micro-
phone. Courtesy DPA Microphones, Inc.

Figure 16-93. Directional characteristics and frequency
response of a DPA 4017 microphone. Courtesy DPA
Microphones, Inc.

A. Directional characteristics.

B. Frequency response of switching filters.

C. On and off axis response measured at 60 cm (23.6 in).

Figure 16-94. Schematic of an interference tube micro-

phone.

fo c

2 L

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