Stereo Miking

Stereophonic: Greek for “solid sound”

Three ways of creating stereo using two speakers …

Coincident

When using coincident pairs, the mics will need to be calibrated …

Point the mics the same way
Put both faders on unity
Pan hard left and right
Phase-reverse one mic
Switch monitoring to mono
Get someone to speak into the mics from about 2 feet away and at mid-height
Adjust the gains so the monitoring level is as quiet as possible
Next, kill the phase reverse and monitor mono-ing
Get someone to walk around the mics while talking. If the central image moves, the mics will not be suitable
Finally, check the left-right boundaries

Known as AB by the BBC and other European broadcasters; be careful!
12 to 16 dB shift between channels is enough for a perceived hard pan
Spot mics can be added to an overall stereo pair; may need to be delayed (one millisecond per foot)

The polarity of the S lobe facing left should be the same as the polarity of the M mic, otherwise the image will be reversed.
Stereo image separately controllable

Developed for Ambisonic recording; true surround
One mic produces four virtual outputs, like one M and three S’s …
1. W — omnidirectional
2. X — bidirectional, left-right
3. Y — bidirectional, front-back
4. Z — bidirectional, up-down
Can be recorded in B-format to manipulate relative levels on playback

Used in systems like QSound.

Dr Harvey Fletcher’s ‘wall of sound’ (1930’s); up to 80 mics in a row in front of an orchestra, with corresponding speakers in identical positions in a listening room
Managed to reduce it to three mics (like a Decca Tree) and three speakers (like the front of a 5.1 system)
Spaced stereo suffers from comb filtering when summed to mono
Most people do use stereo, so stereo-only listening can be achieved
Directional mics use the pressure-gradient principle (which compensates for the inadequacies of the pressure gradient by making the diaphragm resonate at low frequencies), which results in a restricted low-frequency pickup
Omnis have a much smoother LF pickup, but can’t be used in a coincident pair
Playback through speakers is compromised, because the time differences between mics is mixed with the natural time differences we hear when listening to two speakers. This confuses the stereo imaging, although the impression is normally spacious and avoids the analytical imaging of coincident pairs

Normally called an AB pair
Spaced between 1/2 and 1/3 of the width of the sound stage
The 3:1 rule states that the mics should be three times further apart from each other than their distance from the sound source
The problem is a ‘hole’ in the middle of the sound stage; moving the mics can cause phase problems, but is the only way to fix it
Directional mics can also be used in a spaced array

A two-mic, spaced pair technique which only works when monitored through headphones
Aims to mimic the hemispherical pickup patterns of out ears, as well as the spacing and shadowing provided by the head
Placing omni mics either side of someone’s head works pretty well!
A Jecklin Disc mimics the head, with holes for the mics about 15—18cm apart, 25—30cm in diameter and covered in absorbent padding
More accurate results can be achieved by using a dummy head, which can also include dummy pinnae!

Three omnis in a triangular pattern, the central one forward of the others
Rough dimensions: rear mics about 140cm apart, with the central mic about 75cm in front
Outer mics can be angled to the edges of the sound stage to maximise their HF pickup
There is some danger of comb filtering, but the results can be great
The central mic picks up sound before the outriggers, meaning that the sound stage builds from the centre and then expands to the sides. We are not aware of this, but the effect works nicely.