Pressure, gradient, and everything that follows

Two ways to detect sound, and the difference decides everything

A microphone converts air pressure variation into voltage. There are two arrangements, and almost every practical characteristic follows from which one you have.

Pressure microphones have a sealed capsule: the diaphragm is exposed to the air on one side only. It responds to absolute pressure at that point in space. Pressure is a scalar with no direction, so a pressure microphone is omnidirectional: it hears equally from all sides, at least until the capsule itself becomes large compared with the wavelength.

Pressure-gradient microphones are open on both sides. The diaphragm responds to the difference in pressure between front and back, which depends on the direction the sound came from. Sound arriving from the side reaches both faces at once, cancels, and is rejected. This gives figure-of-eight; mixing gradient with a little pressure response gives cardioid and supercardioid.

Everything that follows (proximity effect, off-axis colouration, wind sensitivity, the rear lobe) is a consequence of that choice.

The patterns, and what to use them for

• Omnidirectional. Flat response, no proximity effect, least wind and handling noise, and it hears the room. Excellent when the room is good or when you can be close. This is why lavalier microphones are usually omni: they are 20cm from the mouth, where the direct sound dominates anyway.
• Cardioid. Rejects the rear. The general-purpose choice for a voice in an imperfect room.
• Supercardioid and hypercardioid. Narrower front, but they have a rear lobe: a zone of renewed sensitivity directly behind. Point one at a speaker in a room and you are also recording whatever is behind you. People place monitors and noise sources by cardioid logic and then wonder where the bleed came from.
• Figure-of-eight. Full sensitivity front and back, deep null at 90 degrees. The null is the useful part: it is much deeper than a cardioid’s rear rejection, so a figure-of-eight aimed so its null points at an air-conditioning unit is a powerful tool.
• Shotgun. An interference tube in front of a supercardioid capsule. Sound arriving off-axis enters through slots along the tube at different phases and partly cancels. This only works where the tube length is comparable to the wavelength, so a 30cm tube gives useful directivity above roughly 1kHz and almost none below 500Hz. That is the honest description of a shotgun: directional at high frequencies, broadly cardioid at low ones.

The consequence indoors is important. A shotgun rejects off-axis direct sound but reflections arrive from all directions and at all frequencies, so the tube colours them unevenly. A shotgun in a reflective room often sounds worse than a plain cardioid, hollow and boxy, which is why location sound crews carry both and use the shotgun outdoors.

Proximity effect, which you can use deliberately

Move a pressure-gradient microphone close to a source and the bass rises. Within about 5cm, the boost below 100Hz can be 6 to 10dB.

The mechanism: at a distance, the front-to-back pressure difference comes mostly from the wave’s travel time between the two faces, which is frequency-dependent in a way the capsule design compensates for. Close up, a second effect dominates: the sound’s amplitude itself falls off sharply across those few centimetres because of the inverse square law, and that difference is frequency-independent. The result is a low-frequency lift that grows as you approach.

Two things follow. Omnidirectional microphones have no proximity effect at all, because they have no gradient, which is why a lavalier does not get boomy when it rides up the chest. And the radio announcer’s voice is a cardioid at 5cm with the bass boost fully engaged; it is a technique, not a microphone.

Sensitivity, and why a dynamic needs more gain

A dynamic microphone moves a coil through a magnet: rugged, no power needed, and comparatively insensitive: a Shure SM58 puts out about −54.5 dBV for 1 pascal of sound pressure. A condenser uses a charged capsule and a built-in amplifier, needs 48V phantom power, and typically produces around −37 dBV/Pa, roughly 20dB more.

Practically: a dynamic microphone on a quiet speaker asks for 55 to 65dB of preamp gain, and at that setting the preamp’s own noise becomes audible on cheap interfaces. This is the real reason in-line gain boosters exist and why quiet-voiced podcasters struggle with a budget interface and a dynamic microphone. A condenser at 35 to 45dB of gain asks much less of the preamp.

What no microphone can do

It cannot separate you from your room. The capsule sums everything arriving at that point in space: your voice, the reflection off the desk 3ms later, the fridge, the road. Directionality changes the weighting by some decibels; it does not isolate.

This is the connection to the next lesson: the ratio of direct sound to room sound is set almost entirely by distance, and a more expensive microphone does not change it.

The free path

Nothing in this lesson requires buying anything to learn. Audacity and Ardour, both free, record and let you compare patterns if you have a multi-pattern microphone or two different ones. REW (Room EQ Wizard) is free and will measure a microphone’s response against another one. A phone’s own microphone is an omni pressure capsule, and comparing a phone recording at 10cm with one at 1m demonstrates the distance argument for nothing.

The limitation

Published polar patterns are measured in an anechoic chamber with a sine sweep and shown at a few frequencies. Real patterns vary substantially with frequency: most cardioids are nearly omnidirectional below 200Hz because the capsule is small compared with the wavelength, and become narrow and uneven at high frequencies.

So “cardioid” describes the midrange and misleads about both ends. The practical version: you cannot reject low-frequency noise by pointing a microphone away from it. Rumble, traffic and air handling arrive regardless of aim, and the only remedies are distance, isolation and a high-pass filter.