Omnidirectional microphones can pick up sound from virtually any direction. This design can be useful when a device needs to pick up ambient sound or is used in an environment where sound sources are moving and it is not practical to move the microphone along with them. It does have some drawbacks, however, including muddy sound quality in some cases.
The distinctive rounded end is a well-known characteristic of an omnidirectional microphone. The look is created by the bulging mesh installed over the electronic pick-up, which protects the internal electronics and can limit interference like breath noises and pops. Some omnidirectional microphones take this mesh covering one step further, with a cover made from foam used as a protective sleeve over the head of the device. The foam does not prevent sound from entering the mic, and also acts as a shield against wind and explosive breath sounds. The extreme sensitivity of omnidirectional microphones requires meticulous design to keep sound as crisp and clear as possible.
Wireless omnidirectional microphones that transmit signals without the use of a cable are also available. Small wireless mics called lavaliers are usually clipped to the speaker's lapel or blouse and fed to either a concealed wireless transmitter or a channel on an audio mixing board. Omnidirectional lavalier microphones also have the distinctive round shape of their larger counterparts.
The most common uses of an omnidirectional microphone involve groups of singers or instrumentalists. A microphone can be suspended from the ceiling above a choral group or positioned between a vocalist and an accompanying piano or guitar. Solo performers can hold an omnidirectional microphone in various positions and still be amplified.
This equipment can also be useful for meetings and events where there may be multiple speakers but a single microphone, or where it is important to capture sound from several angles. As different speakers add to the conversation or change position, the microphone will still be able to pick up their voices. The alternative is tracking individual speakers with unidirectional mics, which can be time-consuming and expensive, especially for small organizations that don’t have a large budget for sound equipment.
Omnidirectional microphones can be very easy to set up and use, even by inexperienced people who may not have used one before. As a result, someone with little or no experience can usually manage the set up, helping to reduce expenses for an event. This can be helpful at events where attendees may need to use a microphone to speak, like town hall meetings and wedding parties.
The broad pickup abilities of an omnidirectional microphone also make it very usable in environments where wide coverage is needed or where the precise origins of sounds may not yet be known. For example, someone recording wildlife might use an omnidirectional microphone to pick up general sounds and background noises because he or she cannot predict how the subjects might move.
Low-cost options with relatively high quality are also available. This can be useful for organizations concerned about budget, or people starting to learn about sound systems who are not able to invest in expensive specialized equipment. An omnidirectional microphone can be used in a variety of applications, while a more focused unidirectional device is less flexible.
Ideally, an omnidirectional mic would pick up sound in a perfect circle around its center. The laws of physics make this somewhat challenging, however, and in real-world use, this type of microphone cannot pick up sound perfectly from every direction. It can also cut out some high and low frequencies, and sound coming from an extreme angle may not be reliably detected.
The inability to discriminate between wanted and unwanted sounds means that ambient noise can be picked up and amplified. Some performers may want the sounds of an enthusiastic audience to be included in the session, for instance, but others may want these noises blocked out. A unidirectional microphone may be better at keeping background noise out of the recording and amplifying equation.
Another risk with omnidirectional microphones involves the triangle between the microphone, the performer, and the speakers. If an omnidirectional microphone is placed too close to the speakers, it will pick up extraneous noise. This noise is then fed back into the system through the microphone and amplified again. The result is a very unpleasant phenomenon called a feedback loop. Great care must be taken to avoid putting an omnidirectional microphone directly in front of the speakers.
The design of an omnidirectional microphone contrasts with unidirectional microphones, which only pick up sound from a more targeted source. There are several different types of unidirectional mics, each classified by its polar pattern or directionality — the shape created when the sound pickup is mapped on a flat plane. Some options can include a shotgun microphone, which is a highly directional device intended for pointing at a specific point source of sound; and a cardioid, which is named for the heart-like shape of its polar pattern. While multiple unidirectional microphones can offer better sound quality in some cases by capturing specific sources with less background noise, they can be expensive and more difficult to set up correctly.