Stereo techniques: AB, Blumlein, Mid/Side, close mic, room mic and spot mic.

So far, recording has used a single microphone. Moving to several capsules raises two new questions: how to distribute the sound between the two channels and how far to place each microphone from the source. Answer them badly and you get a wide stereo image that falls apart in mono, or a stack of layers that sound like several rooms at once.

This chapter covers AB, Blumlein and Mid/Side techniques, and the three recording distances combined within one session: Close mic, Room mic and Spot mic.

Four stereo techniques and three recording distances

Stereo recording is determined by two parameters: how much difference there is between the two channels and what kind of difference it is. If the capsules share a position, the difference is in level and the image is very stable. If they are spaced apart, a time difference appears too, adding width but making the mono sum more complicated.

AB and Blumlein work with spacing; Mid/Side works with polar pattern and allows width adjustment afterwards. Alongside them, working distances—Close mic, Room mic and Spot mic—determine how much detail and room sound each channel contains, and work equally with mono recording or any of the three techniques above.

The rule tying everything together is simple: the further apart you place microphones on the same source, the more care you need with phase. The guide to phase and cancellations and the guide to series, parallel and Mid/Side routing show how to correct and use that behaviour once the signal has been recorded.

AB and Blumlein: space between capsules

The AB technique, also called a spaced pair, places two microphones at a chosen distance, typically between thirty centimetres and several metres, both pointing towards the source. It produces the widest soundstage of the three: the time difference between channels is large, and the brain interprets it as space. It is a usual choice for choirs, ensembles, strings and room recording.

Its limitation is mono compatibility. With separated capsules, the same waves reach the two channels with different phase offsets; when summed, part of the spectrum cancels and the sound becomes thin. The classic defence is the 3:1 rule: the distance between microphones should be at least three times each microphone’s distance from the source. Yamaha includes it in its stereo-technique guide, and it works as a quick setup criterion.

Blumlein uses two figure-of-8 microphones crossed at 90 degrees, with their capsules as close together as possible. They pick up sound from the front and rear, so the image is very open and the room sound feels natural and enveloping, with reasonably good mono summation. The price is that they also pick up whatever is behind the microphones: walls, street noise and the equipment itself. In a decent room it is a beautiful technique; in a noisy one, it is a room microphone in disguise.

Mid/Side: width adjustable in the mix

Mid/Side abandons the idea of two identical channels and combines two different roles. A directional microphone points towards the source and records the Mid channel: the central information. A figure-of-8 microphone faces perpendicular to it, towards the sides, and records the Side channel: what sounds off-axis.

The Side signal is duplicated, phase-inverted and sent to one channel while the original goes to the other; this reconstructs stereo. The advantage is that the ratio between Mid and Side can be changed in the mix: more Side gives a wider image; less Side gives a more concentrated image, without rerecording. That is why Mid/Side is used both in studios and for ambience recording, and why it is compatible with any playback, including mono when handled properly.

The trade-off is that decoding has to be done correctly. If the figure-of-8 microphone picks up a direct source, or if the Side channel is processed without respecting the structure, the image shifts or loses body.

Close mic, room mic and spot mic

The three distances answer different questions and often coexist in the same recording.

Close mic is very near the main source and defines the sound: a voice, the edge of a cymbal, a kick drum head or piano detail. It adds presence and isolation and reduces the room sound entering the take. Its risk is excessive proximity: uncontrolled bass, plosives and a very flat image if it is the only recording.

Room mic is further away, in the room, and captures sound after it has reflected. It adds cohesion, depth and the character of the space; with drums, it is what makes the kit sound like a drum kit rather than five independent pieces. Its risk is the opposite: it introduces background noise, air-conditioning noise and reflections from a room that may be untreated.

Spot mic is close to a particular element needing its own attention: a snare, scraper, tom, a particular part of a piano or a guitar’s wood. It emphasises detail or compensates for an area the main recording does not cover well. Because it is so close, its level and character are usually very different, and you need to plan its contribution to the mix from the moment you position it.

Combining recordings without disrupting phase

When several layers coexist, compatibility depends on three decisions:

1. Proportional distances. Apply the 3:1 rule between microphones recording the same source; if that is not possible, change the angle before the distance.
2. Check in mono. Listen to the mono sum as you position microphones: if the sound becomes thin, there is cancellation to resolve before recording the good take.
3. Watch polarity. Invert one layer’s polarity and check which of the two positions sounds fuller; the correct choice is always the one that adds body.
4. Clear roles. Define which layer leads and which support it. If all three compete for the centre, the recording sounds dense even if each microphone is perfect on its own.

Three starting setups that work

Rather than memorising techniques, it is useful to have three tested setups that solve the situations that occur again and again.

Open grand piano. An XY pair over the opening beneath the lid gives a centred, stable image, with the pedal and soundboard well located. If width is the goal, add a more distant AB pair capturing the room and adjust the balance between detail and space in the mix; if you want a very wide image with later control of width, Mid/Side in the same position is the most flexible route. A spot mic near the hammers adds definition in passages where the right hand stands out.

Acoustic guitar and small ensemble. A restrained stereo recording is usually enough here: ORTF or XY at a metre and a half already provides an image without complications. If you choose AB, respect the 3:1 rule and check the mono sum before accepting the takes, because a sustained chord is the best way to reveal cancellation.

Room and ambience. To capture the space as a sound in its own right, Blumlein or a spaced AB pair are natural options, provided the room is worthwhile. Recording fifteen seconds of room silence first—the sound of the empty room—saves serious problems when a take needs editing or extending.

Each of these setups is revisited for particular sources in the guide to recording voices, strings and ambience with multiple microphones, where this chapter’s techniques are applied to specific instruments and spaces.

Next step

With recording techniques sorted, apply them source by source: recording vocals, guitar, piano, drums and percussion in the studio.