Stereo recording techniques

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Fig. 17

Recordings of larger sound sources, like for orchestras, choirs, etc., involve high demands on the transparency and brilliance of sound, on an even utilization of the whole stereo basis, on a good and high directional resolution and on the reproduced room illusion.
To realize the desired sound impression for the listeners it is very often necessary, to set up several main stereo microphones, or tu use additional support mikes, especially in rooms with unsatisfactory acoustics. Mono or stereo microphones can be used as support microphones.

It is advisable to limit the number of support microphones to as few as necessary.

MONO SUPPORT MICROPHONES

Mono support microphones are suitable for sound sources without audible spatial extent, like single instruments or soloists. The mono signal has to be placed in the desired direction with the help of a panorama-potentiometer (pan-pot) or direction-mixer.
The microphone must be acoustically separated from other te other microphones, to avoid a double reproduction, falsification of directional information or ambiguity. If sufficient separation cannot be realized, then, with the help of a pan-pot or direction mixer, the sound source has to be placed into the direction that corresponds to its real position in the recording room.

STEREO SUPPORT MICROPHONES

If the support microphone has to include a sound source of a certain spatial extent (group of instruments or choir, a piano, organ or harpsichord), an additional stereo microphone will be suitable. Yet its directional allocation has to coincide with that of the main microphone. Correspondingly its basis-width and reproduction direction have to be properly set with the help of the direction mixer.
A basic problem of a stereo support microphone is its poor suppression of sound sources in its back and the diffuse reproduction of sound sources that are outside its recording range. Therefore, in most cases, the use of several mono support microphones would be advisable.

Fig. 18

Fig. 19:
No change of acoustic perception

Fig. 20

Fig. 21:
Basis width=0 (Punctual Perception from the middle)

Fig. 22:
Full Basis, but reverse

Fig. 23:
Shifting to left-half left

Fig. 24:
Shifting to completely left

Fig. 25:
Too big basis

The use of support microphones

As already mentioned above, mono support microphones are recommendable for the recording of sound sources without audible spatial extent, for example a single instrument or a soloist. The mono signal has to be integrated into the sound picture with the help of a panorama potentiometer (PAN-POT) or direction mixer.

Fig. 26

Fig. 27

Switch position	Output left	Output right
L	-10dBu	-dBu
M	-15dBu	-15dBu
R	-dBu	-10dBu

There are four basic arrangements for the use of several stereo support microphones (coincident microphones):

1. In the so-called "staggered arrangement", additional coincident microphones are set up in the symmetry plane of the main microphone (Fig. 28). This requires an adjustment of the represented basis width of the support microphones to the representation of the main microphone in order to avoid a repeated representation of a sound source on the stereo basis. In this case, the basis width of the support microphone has to be set smaller than that of the main microphone due to it's shorter distance to the supported sound source.

Fig. 28:
Coincident support microphone on the symmetry axis of the main microphone.
2 ₁ and 2 ₂ = opening angle of the microphones;
2 '₂ = reproduction angle of support microphone, to be set with direction control.

2. One ore several coincident support microphones are set up outside the symmetry plane of the main microphone (Fig. 29). Main and support microphones have to be brought into correspondence as far as basis width and direction is concerned. As in the preceding example, one has to ensure a sufficient back attenuation of the support microphones, otherwise there will be repeated representations of certain sound sources.

Fig. 29:
Coincident support microphone outside the symmetry plane of main microphone.
2 ₁ and 2 ₂ = opening angle of the microphones,
2 '₂ = reproduction angle (basis width) to be set with direction control,
= direction of support microphone to be set with direction control.

3. Sectorial use of several equal coincident microphones (Fig. 30). The orchestra is devised in sectors; each sector is recorded with it's own microphone; the microphone signals are mixed together respective of the corresponding basis and direction control.

Fig. 30:
Set-up of several equal coincident microphones in sectors.

4. Integration of separated sound sources into the total sound picture (Fig. 31): On the one hand there is often not enough room to arrange all partial sources in the recording range of the main microphone. On the other hand it is useful to arrange all sources around the conductor. The sound signal of a separated source is picked up with its own coincident microphone.

Fig. 31:
Integration of separated sources,
2 ₁ and 2 ₂ = opening angles of coincident microphones.

Common to all these applications is the fact that the better the microphones are separated acoustically, the better the basis and direction control can be handled. As far as the coincident microphone is concerned there is practically no effective possibility of suppressing sound arriving from the back in stereo recordings. Due to the better back attenuation of directive AB-support microphones, such arrangements have proved quite satisfactory.
XY combinations are preferable if large sound sources have to be recorded at a short distance (e.g. wood wind players of an orchestra), whereas MS combinations are better suited for soloist and smaller instrumental groups.

An example of a possible arrangement with several support microphones for a symphony orchestra is shown in fig. 32. A stereo coincident microphone in XY-technique is used as the main microphone. Directive mono microphones (to be set with pan pots) are used in the other instrumental groups since they allow better acoustical separation.

Fig. 32:
Example of microphone arrangement with several support microphones for a symphony orchestra.

Orchestras with sound sources of very different loudness, like rock pop and jazz bands, require far more support microphones than classical symphony orchestras or chamber music groups. During recordings with support microphones for all partial sound sources of the orchestra, the main microphone often serves no purpose and can be dropped. These support microphones are direction and basis-controlled stereo microphones and direction-controlled mono microphones; they are set up in the near soundfield of the instruments and have to record the direct sound of the corresponding instruments, but they have also to be insensitive to the sound of other instruments and to indirect (diffuse) sound.

NOTE: In practice there are many more factors that influence the microphone set-up, (Acoustics of the recording room, arrangement of seats, problems with field of view, loudness balance, musical and artistic requirements, etc.), so that the above mentioned rules have to be adjusted for each individual case.

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

Stereo recording techniques