Two mics, one image
How two microphones, placed with intent, build a sense of width and depth that one never can, and how to keep them from fighting each other.
Where stereo comes from
The image is built from differences
A single microphone hears one point in space. Two microphones, placed with intent, capture something a single capsule never will: a sense of where each sound sits, left to right and front to back. That image is not so much recorded as constructed, and what constructs it is the small differences between what each microphone receives.
Your ears do the same thing. Direction comes from two cues. A sound off to one side reaches the near ear slightly before the far ear, an interaural time difference, and that dominates at low and middle frequencies. The same sound is a little louder at the near ear because the head shadows the far one, an interaural level difference, and that dominates up high. Stereo microphone technique is the craft of feeding a recording the right mix of those two cues.
The physics is simple to anchor. Sound travels about 343 metres per second in air at room temperature, so one millisecond of delay corresponds to roughly 34 centimetres of extra path. Move a microphone, change the timing, and you change where the sound appears to sit.
The technique families
Coincident, near, and spaced
Stereo techniques sort into a few families, and choosing between them is a trade among image width, mono compatibility, and how much of the room you let in.
Coincident pairs place two capsules in almost the same spot, so there is almost no time difference and the image is built from level alone. XY, two cardioids crossed at ninety to a hundred and twenty degrees, gives a tight, dependable centre and the safest mono fold-down there is. It is the quiet workhorse for acoustic guitar, piano, and drum overheads.
Near-coincident techniques add a little spacing. ORTF, capsules seventeen centimetres apart at a hundred and ten degrees, reintroduces small timing differences alongside the level ones, and the result is a wider, more natural image at a modest cost to mono safety. It suits ensembles, choirs, and rooms.
Blumlein, two figure-of-eight capsules crossed at ninety degrees, captures a very wide and lifelike image, including a generous helping of the room behind the mics. In a fine hall it is glorious. In a poor room it tells the truth about the poor room.
Mid-Side deserves its own mention. A forward-facing mid microphone carries the centre, a sideways figure-of-eight carries the width, and the two are decoded as left equals mid plus side, right equals mid minus side. Because width lives entirely in the side channel, you can set the stereo spread after the recording, and muting the side gives perfect mono. That is why broadcast engineers reach for it. The Decca Tree, meanwhile, is less a technique than a system: a left and right pair a metre or two apart with a centre pushed slightly forward, raised over the source to paint the space while close mics add detail on top.
When two mics fight
Phase and the 3:1 rule
The moment more than one microphone captures the same source from different distances, the signals arrive at slightly different times. Combine them and some frequencies reinforce while others cancel, a pattern called comb filtering. It is not a fault, it is arithmetic: a one millisecond offset lines up perfectly at a thousand hertz and is exactly out of step at five hundred, so the tone hollows out in a series of notches.
One field rule keeps this in check when you are miking separate sources that bleed into each other. Keep the distance between any two microphones at least three times the distance from each microphone to its own source. The bleed then arrives far enough down in level that its comb filtering becomes inaudible.
A correlation meter is the quick check. Near plus one, the two channels are closely related and the mix is mono-safe. Drift below zero, and you have phase trouble waiting to collapse the moment someone listens in mono.
Designing depth
Near, far, and the proximity effect
Width is only half of the image. Depth comes from the balance of direct sound to reflected sound. A close microphone hears mostly the source, so it sounds present and intimate. A microphone set further back hears more of the room, so it sounds larger and more distant. Blend the two and you can place a source anywhere from the front of the stage to the back wall, just by changing their ratio.
Close miking carries a side effect worth knowing. Directional microphones lift the low frequencies as the source gets nearer, the proximity effect. Inside ten centimetres it is dramatic, between ten and thirty it is manageable and often musical, and beyond thirty the low end settles to something natural. When a performer moves unpredictably, a microphone designed to suppress the effect keeps the tone steady.
The image is a decision
None of this happens by accident. The width of a recording, the size of its room, the place each instrument sits, all of it is chosen at the moment the microphones go up. Get that right and the mix that follows has somewhere to live.
Explore the divisions
