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Introduction

To enable the recording and/or transmission of sound events, the acoustical signals must be transduced into electrical signals, recorded (transmitted) in this form and finally reconverted into acoustical signals.
There are different terms according to the number of electrical transmission channels used: monophony in the case of one channel; stereophony in the case of two channels; quadrophony in the case of four channels. The number of transmission channels corresponds to the number of possible mutually independent acoustical signals in recording and replay.
Stereophonic recording (transmission) can be done either in "room-related stereophony" or "head-related stereophony" (dummy-head stereophony).


1. Head-related stereophony (Dummy-head stereophony)

Head-related stereophony is a simple and logical technique: Technically speaking the sound field which exist at the location of the two ears of a listener in the recording room is recorded by an artificial head, a so-called dummy head, and is reproduced directly at the ears of a listener in the replay room with the help of earphones.

Dummy head
Fig. 1: Dummy head

The dummy head simulates the human head in acoustic sense as far as possible. In place of ear drums the dummy head has suitable microphones whose directional characteristics and other physical properties correspond to the properties of human ears (Fig. 1). Characteristic to and advantageous only for this technique is the fact, that all sound incidence directions, including above and behind, as well as distances, can be exactly reproduced.

The precise natural representation of the room sound (initial reflections and reverberation) is possible only with this technique. The transmission of all sound source directions with only two channels is possible, since the dummy head linearly distorts the signals dependent on the direction of sound incidence, corresponding to the linear distortion of the ear-signals for various directions of sound incidence.

On the other hand we still have the disadvantage that the sound picture, played back by earphones, is always related to the listener's head and not to the listening room, so that a sound source always follows every movement of the listener's head.
A further disadvantage is that optimal reproduction is only possible with head phones (Fig. 2); replay arrangements with two or four loudspeakers have the substantial disadvantage that the listening area is extremely small.

Reproduction of dummy head recordings with headphones
Fig. 2

Sennheiser has constructed another type of dummy head, using two miniature electret microphones at the ends of a support (in the form of a U) so that they can be hung into the external ears of a human head or of a dummy head, just in front of the eardrums. Thus the recorded sound is the one entering the auditory canal (Fig. 3).

Sennheiser dummy head
Fig. 3

Sennheiser thinks that by placing the microphones at the level of the eardrum, the sound information has to pass the auditory canal once during recording and a second time during listening via headphones. That is one time to many; so the sound has to be picked up at the entry of the auditory canal.
The microphones with their special support are thus separated from the head. One can wear them also oneself during the recording, what by no means is very practical.

The microphones used are omnidirectional with a hump of 3 dB at about 10.000 Hz to compensate the high frequency attenuation of the head.

Dummy-head recording
Fig. 4
Dummy-head recording (transmission)

 

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PDF Stereo Recording Techniques (570 KB)
Download this document in Acrobat format for printing and offline viewing.
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BackNextUp Stereo recording techniques