The Omnidirectional Microphone

The simplest microphone is the omnidirectional (omni). An omni microphone theoretically picks up sounds from all directions with equal sensitivity.

One side of the diaphragm is exposed to the outer air, but the other side is enclosed by a sealed container. This arrangement causes the microphone to respond to instantaneous changes in air pressure regardless of the direction in which the sound (pressure) waves are travelling. In practice it's never perfect, as sounds coming from behind the microphone are to some extent blocked by the casing and the internal mechanics.

Although such a microphone is very simple to position, it has its pros and cons. Used close to a single sound source (near field) in the absence of noise, an omni probably has the truest response of any microphone. But it's equally sensitive to noise and unwanted sounds in all directions, so it's not ideal for far field use except in the recording studio or similarly quiet surroundings.

Omni microphones are used in environmental sound level meters because directionality must specifically avoided when measuring ambient noise. They're also the basis of hydrophones - microphones specifically designed for use under water. This is for practical reasons as it's possible to enclose an omni microphone in a completely sealed outer case, but a directional microphone can't be sealed or it stops being directional.

An pair of widely spaced omnis - sometimes as much as three feet (1 metre)or more apart - can provide excellent recordings of general ambience. This approach is the basis of the famous Decca Tree, used for decades to record concerts in large halls. It can also serve well for dawn choruses in otherwise quiet locations, but if the spacing is wrong the results are poor, and it's difficult to get right unless you have time to experiment. So in general the wild soundscape recordist should aim for easier setups based on directional microphones.