Polar patterns

The directions the mic picks up from.Ideal shapes; real mics match them best in the mids. Each mic's polar plot shows its measured pattern.

Reach

In a room, the echoes come from all directions, while the source is in one. A directional mic hears the source fully but less of the room, so it can stand farther away for the same balance of the two. How far, compared to an omni:

• Wide cardioid: 1.4×
• Cardioid and figure-8: 1.7×
• Supercardioid: 1.9×
• Hypercardioid: 2×

How a capsule gets its pattern

Pressure (omni): the capsule is sealed at the back, so the diaphragm feels only the air pressure in front of it. Pressure has no direction: sound from anywhere pushes it the same way.

Pressure gradient (figure-8): the diaphragm is open on both sides and moves with the difference between them. Sound from the side reaches both faces at once, so nothing moves. Sound from behind moves it the other way round (reversed polarity). A ribbon works like this.

Both at once (cardioid and its relatives): the back is open too, but through a longer, damped path: an acoustic delay. Sound from behind arrives at both faces at the same moment and cancels out; sound from the front doesn't. Together this works as a mix of omni and figure-8:

The share of figure-8 in the mix sets the pattern: wide cardioid 30 %, cardioid 50 %, supercardioid 63 %, hypercardioid 75 %.

Switchable patterns (U 87 Ai, C414, sE4400, Schoeps MK 5): two cardioid diaphragms back to back. The front one alone: cardioid. Both, added: omni. Both, one reversed: figure-8. Mixes in between give the rest.

The proximity effect

Why directional mics get boomy up close, and omnis don't. A gradient capsule moves with the difference between the pressure at its front and at its back.

Far away, the back gets the sound as strong as the front, only a moment later. That delay is the only difference, and in the bass, with its long, slow waves, it's tiny. So the mic is built to boost its own bass, which evens this out.

Up close, the sound spreads out and weakens as it goes. The front, being closer, gets it noticeably stronger than the back: a second difference, the same at every frequency. In the bass it outweighs the tiny delay difference, and the mic's built-in bass boost lifts it too. So the closer the source, the more bass.

The closer the source, the higher up it starts: for a cardioid, around 550 Hz at 5 cm, 270 Hz at 10 cm, 90 Hz at 30 cm. A figure-8, all gradient, gets about 6 dB more; an omni, all pressure, none.

Wind, pops, and handling noise

Omnis handle them much better. Wind or the puff of a “p” isn't a sound wave coming from afar but air rushing against the capsule: a large pressure difference over a very short distance, which is what a gradient capsule responds to (the proximity effect, taken to the extreme). The rear openings let it in from behind too. And to make up for its weak bass, a gradient diaphragm is loosely suspended, so bumps and cable knocks move it easily. An omni's diaphragm is sealed at the back and held taut: it feels only the pressure itself.

In practice

• Patterns change with frequency. Omnis narrow at the top, as the capsule's own body shades sound from behind; large diaphragms narrow more (the mic types).
• Aim the nulls: a cardioid rejects what's straight behind it; a super- or hypercardioid what's behind to the sides, where stage monitors go. A figure-8 ignores the sides: two singers facing each other, or the side mic in M/S stereo.