Select the Right Pickup Pattern for PA or Recording Sound
In a perfect world, sound recordists and PA systems would have subjects in acoustically treated rooms without extraneous noise, loud stage monitors and boomy rooms.
Unfortunately, they often have to contend with these, and understanding the different microphone directional patterns and how they pick up sound is key to getting the best results from any particular sound reinforcement or recording situation. The principal type of microphone pickup patterns are as follows.
Omnidirectional microphones pick up sound equally from all around. They do not favour any direction at all – in theory a singer would sound just as good signing behind the microphone as in front of it. This type of microphone is usually the easiest to engineer and therefore often the cheapest for a particular quality of construction.
Their lack of directional discrimination is both a strength and a weakness, depending on the situation. For stage and PA applications they can easily lead to feedback since although the stage monitors and main speakers usually lie behind the microphone and the vocalist in front, they cannot separate out the two.
However, omnis excel at recording meetings for instance – one placed in the middle of a meeting room will pick up all the speakers, without favouring one side of the meeting or the other.
The cardioid microphone has a heart-shaped pickup pattern, with most of the sensitivity to the front of the microphone and much less sensitivity to the back. This is a common pickup pattern for stage microphones such as the venerable Shure SM58.
Cardioid microphones discriminate against noises from behind the mic, and this is often a very useful characteristic that is easy to understand. Putting the wanted sound in front of the mic and the unwanted sound behind it – any situation where it is desirable to record a single sound source in a less than ideal environment is a candidate for using a cardioid.
These are very commonly used for music recording, because of the need to discriminate against the noise of stage monitors and the spill of sound of the other musicians who will have their own microphones to form part of the overall mix.
A more directional variation on the cardioid with less pickup to the sides.
Figure of Eight (Bidirectional) Microphone
This use to be a classic pattern from the ribbon dynamic microphone which was a high-quality microphone in early broadcasting. It has good pickup from the front and back, but little sensitivity either side.
Its classic application was radio interview recording, where the microphone was placed between the interviewer and interviewee, and unwanted noises could be minimised by keeping them at right angles to the line between the two participants. This is less common that the other two patterns.
How Directionality is Shown in Specifications
Microphone directionality is usually shown in the specs as a polar graph of sensitivity to sound. This is a bird’s eye view from the ceiling looking down on a microphone facing forward. In the directions where the plot is furthest from the mic the microphone is most sensitive, and where it is closest the mic is least sensitive.
Omnis are therefore an even circular pattern around the microphone, whereas cardioids have a heart-shaped pattern furthest out at the front of the microphone and almost touching it at the low-sensitivity area at the back.