Sound Balancing

• what is meant by balance
• what internal balance is
• what synthetic balance is
• how to place coincident pairs of microphones
• the advantages and disadvantages of coincident mics
• what a "filler" mic is
• when to use manual dynamic compression
• how wave reinforcement/cancellation effects mic positioning
• how to place spaced mics
• the advantages and disadvantages of spaced mics

Introduction

Balance is:

• the relative intensities of the separate components that make up a total mix, or
• the relative proportion of direct to indirect sound.

Music balancing can be divided into two categories:

• reproducing a sound that already exists,
  i.e. recording music that is already (internally) balanced
• creating a sound from separate elements, i.e. a synthetic balance.

Examples of groupings producing internally balanced music:

• Symphony orchestras
• Quartets (eg. string, wind, brass)
• Choirs
• Brass bands
• Pipe bands (bagpipies)
• Recitals (eg. soloist with piano accompaniment).

For internally balanced ensembles a single coincident pair of mics. may be sufficient for a stereo recording. Coincident mics. are so called because they are positioned as close to each other as is physically possible.

Directional mics. must be used, eg. AKG 414.
Omni directional microphones would result in near mono sound.
If the acoustic is less than ideal two coincident pairs of mics. may be required.
Alternatively, a single coincident pair may be used with additional, single, "spot" mics.
There is an alternative system of placing mics. to capture an internally balanced sound. This is known as the "spaced mic. system". For the moment let's consider the coincident pair system.

Coincident Pair system

1. If the acoustic is too dry, artificial reverberation can be added, (at the time of the recording) but a more natural sound is achieved by placing additional mics. to pick up the reflected sound.
2. If the hall is too reverberent, (or too noisy) move the mics. closer to the source or sources of sound. This will probably cause an imbalance, (a particular section, instrument or voice will be favoured) so add extra mics. to restore the balance.

Coincident Pair, Initial positioning:

• For most instruments the best sound quality comes not only from in front but also above. Mics. should therefore generally, be placed well above head height.
  A good starting position for placing a coincident pair is about 3 metres behind and 3 metres above the conductor's head. A second coincident pair should be placed farther back and higher.
  The sound from the two pairs of mics. can then be compared and ..
  • the pair that gives the best blend of sound without losing clarity is left where it is while..
  • the other pair is moved and a further comparison is made.
  The process should be repeated, each time moving the pair of mics. that gives the poorer result, until no improvement is noticed.

  The main pair of mics. should be made secure and the second pair can be used as space mics., (to pick up reverberation) if necessary. This pair may also function as a back-up should the main pair fail for any reason.

  "Filler" mics. should then be added for any section of the ensemble that lacks clarity. Remember though, the extra mics. are added to help recreate the sound heard by the audience in the hall - not to change the internally balanced sound of the ensemble. This also applies to any soloist mics. with the possible exception of very quiet instruments (eg. classical guitar, harpsichord) playing with a full orchestra. Although the audience is usually unaware, sound reinforcement is often applied to these instruments in public concerts. An increase in the level of their sound, relative to the orchestra, is therefore possibly justifiable in a recording also.

  The dynamic range of a large orchestra can be as much as 100 dBs. The dynamic range may have to be reduced during recording. If the level of the recorded signal is too high, distortion will occur.

  When recording "light" or "popular" music compression is carried out with a compressor / limiter. For the type of music commonly described as "serious" or "classical", compressor / limiters are not desirable. A compressor / limiter tends to iron out subtleties of dynamic expression essential to the performance. When recording this kind of music, dynamic compression is best achieved manually.
  Manual compression involves anticipating loud or quiet passages and easing faders down or up to keep within the maximum and minimum levels required by the system. To anticipate level changes, the sound engineer needs to be familiar with the music. It is worthwhile learning to read a full music score.

Sound waves reflected from the walls and ceiling of a hall can give unwanted reinforcement or cancellation of waves. It is wise therefore to avoid placing mics. on the central line of a symmetrical hall or the axis of a domed ceiling.

Placing a pair of coincident microphones

Advantages of the Coincident Pair system

• Positional definition can be outstanding.
• The image is stable, (stereo image does not wander) due to the proximity of the mics.
  to each other.
• Few mics are required
  • with less chance of operational errors and
  • lower costs if hiring mics.
  The two capsules may even be contained in the body of one mic.

• It may be necessary to work a long way from a large ensemble so the proportion of indirect to direct sound may be unacceptable.
• Mics. may have a non-linear response off-axis. This can lead to a "muddy" central sound.

When working about 4 metres from a large ensemble, some sources will be closer to a coincident pair than others. The closer sources will be prominent in the mix. This defect is exacerbated by the "inverse square law" effect. If one source is twice as close to the mics. it will sound 4 times as loud. The spaced mic. system was evolved to overcome this problem.

Spaced microphone system

3 central mics act as a stereo system; they should be faded up and down together.
Outer mics. reinforce the wings of the ensemble.
If the mics. are too widely spaced a series of mono sounds will result;
if too close, the sound will be thick / muddy.
Each mic. should be panned to give a sound position corresponding to the physical location of the sound source.
Omnidirectional mics. may be used; they give a rich sound. Since they are rather directional at higher frequencies even omni. mics. should be angled towards the area they cover.

Advantages of the spaced mic. system

• Using omni mics there is a better pick-up of reverberation
  so it is possible to work closer to the source,
  (giving greater clarity) without a too intimate sound.
• Subtle changes can be made to mic. levels to suit each occasion.

Disadvantages of the spaced mic. system

• The clutter of extra mics.

Remember, the extra mics. are added to reproduce the sound which the conductor has developed not to change the perspectives of the instruments within the ensemble, (which is internally balanced).
It is good practice to invite the conductor / leader into the control room to listen to the balance during rehersal.

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More about sound balancing
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