Omnidirectional, Bi-directional, and Cardioid Microphone (Unidirectional) – You may have heard of these microphone polar patterns, but do you know what they mean?
Well this is certainly not the most interesting topic to read about it. It is, however, an area that requires a significant amount of comprehension. Understanding the makeup of microphones and the way they operate determines the decisions that are made in live situations and in recording sessions. Decisions regarding microphone selection and microphone placement are more effectively made when there is a clear understanding of the actions and reactions of the equipment. If you have an arsenal of microphones, it is important to know the ins and outs of their specifications. Frequency response and SPL handling are obviously two characteristics that help determine which microphone you need for a particular job. Also keep in mind the different microphone polar patterns and how they determine placement in regards to not only the instruments being miked, but also to the other instruments or sounds that are in the immediate area. Understanding the microphone polar patterns helps you choose the right microphone for the job in a more effective way. This leads to better placement execution and more efficient setup time.
Microphone polar patterns are more simply called pickup patterns. These pattern configurations refer to the microphone’s directivity and are contained in the structure of the capsule. The three major patterns are cardioid (unidirectional), bi-directional, and omnidirectional. There are other microphone polar patterns that are variations of the main three, but the discussion of those patterns is saved for another day… but probably not. So here are the main three.
The cardioid polar pattern was given its name due to the shape of its pickup pattern, which resembles a heart. Perhaps the most common of the polar patterns, cardioid is a unidirectional configuration. Microphones that are unidirectional are most sensitive to sounds coming from a single direction. For the cardioid microphone, this position is at the front of the microphone. The sides and rear reject incoming sound. For live vocals, this is a great tool for reducing on-stage monitor feedback. In the recording studio, the cardioid microphone is used for close miking and instrument isolation. It primarily picks up frequencies from the source at which it is aimed, and rejects the bleed of other sounds at the sides and rear of the microphone. However, the characteristics that facilitate isolation are also responsible for what is known as the proximity effect. Without getting too involved in the technical explanation, the proximity effect occurs due to phase and pressure differences. As a cardioid microphone is moved closer to a sound source, there is an increase in the sensitivity to bass frequencies. This may be lead to a boominess quality of sound. This may be useful in situations where a deeper, lower-frequency sound is desired. However, if the boominess becomes unbearable, the use of equalization may be required. Always be cognizant of the proximity effect’s existence, and know how to deal with it. Have you heard of the little dynamic microphone known as the Shure SM57? Umm, of course you have. It is quite possibly the most popular cardioid microphone in the history of the world. Check out some decent budget alternatives.
The second of the three major microphone polar patterns is the bi-directional configuration. As you can probably determine by its name, the bi-directional pattern is sensitive to frequencies arriving to the capsule from two directions. These two entry points occur at the front of the microphone and at the back of the capsule. As you can see from the diagram of the bi-directional pattern, it is symmetrical, unlike the heart-shaped pattern of a cardioid microphone. Because of this, sounds that are rejected at each side arrive to the front and the back of the microphone capsule simultaneously. Because of this, the phase difference that creates rear rejection in a cardioid microphone does not exist in bi-directional models. The lack of phase and pressure differences at the two points means no proximity effect for the bi-directional pattern. If placed strategically, a bi-directional microphone may be a great tool for picking up and rejecting multiple sound sources simultaneously.
The last of the big-three microphone polar patterns is omnidirectional. This pickup pattern is characterized by equal frequency sensitivity from all directions. Due to this lack of rejection, an omnidirectional microphone is not a good choice in situations where isolation is necessary. However, because it allows sounds to enter from all directions, a microphone with an omnidirectional pattern is often chosen as a room mic. Adding the ambience of a room can make a recording sound more natural. This offsets the more unnatural qualities of close-miking with a cardioid microphone. When recording vocals, I use a Radio Shack 33-1070 omnidirectional dynamic microphone in close proximity to an Electrovoice condenser with a unidirectional pattern. This setup exhibits a more round, warm sound than if a cardioid microphone was used alone. And of course, the equal sensitivity of the omnidirectional polar pattern to sounds from all directions means not having to deal with the proximity effect. Read more about recording vocals with an omnidirectional and cardioid microphone simultaneously.
Other microphone polar patterns include subcardioid, hypercardioid, supercardioid, and shotgun. Go check them out if you must.