Handbook for Sound Engineers

Microphones 555

transfer function of the headphone, so the sound pres-
sures presented at the entrance of the listener’s ear
canals will duplicate those at the entrance of the head’s
ear canals.
Diffuse-field equalization is suitable for situations
where the source is at a distance from the head. For
recordings close to a sound source or in a confined
space, such as the passenger compartment of an auto-
mobile, another equalization called independent of
direction (ID) is preferred. The equalization is internal
in the head in Fig. 16-138.
Signals PHr(t) and PHI(t) from the heads can be
recorded and used directly for loudspeaker playback,
analysis, or playback through headphones. As a
recording tool this method can surpasse many other
recording techniques intended for loudspeaker repro-
duction. The full benefits of spatial imaging can be

heard and enjoyed with earphone playback as well as

with high quality loudspeakers.

The heads are constructed of rugged fiberglass. The

microphones can be calibrated by removal of the

detachable outer ears and applying a pistonphone.

Preamplifiers on the microphones provide polarization

and have balanced transformerless line drivers. A record

processor and modular unit construction provides dc

power to the dummy head and act as the interface

between the head and the recording medium or analysis

equipment. The combination of low noise electronics

and good overload range permits full use of the 135 dB

dynamic range of the head microphones and 145 dB

with the 10 dB attenuator switched in.

For headphone playback, a reproduce unit provides

an equalized signal for the headphones that produces

earcanal entrance sound signals that correspond to those

at the corresponding location on the artificial head.

An important parameter to consider in any head

microphone recording system is the dynamic range

available at this head signal output. For example, the

canal resonance can produce a sound pressure that may

exceed the maximum allowed on some ear canal-

mounted microphones.

16.8.2 In the Ear Recording Microphones^18

In-the-Ear (ITE™ recording and Pinna Acoustic

Response (PAR™ playback represent a new-old

approach to the recording of two channels of spatial

images with full fidelity and their playback over two

channels, Fig. 16-138. It is important that the loud-

speakers are in signal synchronization and that they be

placed at an angle so that the listener position is free of

early reflections.

Low noise, wide frequency, and dynamic range

probe microphones employing soft silicone probes are

placed in the pressure zone of the eardrum of live

listeners. This microphone system allows recording

with or without equalization to compensate for the ear

canal resonances while leaving the high-frequency

comb filter spatial clues unaltered. The playback system

consists of synchronized loudspeaker systems spaced

approximately equal distances from the listener in the

pattern shown in Fig. 16-139. Both left loudspeakers are

in parallel, and both right loudspeakers are in paralle.

However, the front and back loudspeakers are on indi-

vidual volume controls. This is to allow balancing

side-to-side and to adjust the front-to-back relative

levels for each individual listener. The two front loud-

Figure 16-137. Georg Neumann KU 100 dummy head.
Courtesy Georg Neumann GmbH.

Figure 16-138. Block diagram of a dummy head binaural
stereo microphone system.

Dummy

head

Dummy head

free-field

equalizer

Dummy head

free-field

equalizer

Pr(t)

PHR(t)

PHl(t)

Pl(t)

Record processor Reproduce unit

Headphone

free-field

equalizer

Headphone

free-field

equalizer

Listener

Pr(t)

Pl(t)