The terms pickup pattern and polar pattern are often used interchangeably, and for most situations either term will work. However, there is a difference between the two. Both terms refer to the shape of the area to which a microphone is equally sensitive, usually presented as a circular plot, called a polar response curve. (In reality the curve should be a 3-dimensional plot, since sound waves travel in all directions like a sphere. So when you look at such a curve, you need to imagine the pattern is also going around you as well as in front and back of you.) The difference between the two terms is that pickup pattern is a general term that applies to just one frequency, while polar pattern is more precise and indicates the pattern for several frequencies. (See the diagrams.)
Directivity is an indication of the directional characteristic of something—it directionality. It is used to describe the dispersion or reception of loudspeakers, microphones, and antennas. For this discussion, we will be considering only microphones.
A directional microphone is one that has a greater sensitivity from some directions more than others. All microphones are directional except omnidirectional microphones. A unidirectional microphone has greater sensitivity to sounds coming from the front. This includes all directional microphones except the bidirectional microphone. Therefore, microphones are generally broken down into three major categories of microphone directionality: omnidirectional, unidrectional, and bidirectional. Unidirectional can be further refined into five patterns for a total of seven microphone polar patterns: (1) omnidirectional, (2) bi-directional. (3) cardioid, (4) subcardioid, (5) supercardioid, (6) hypercardioid, and (7) lobar.
An omnidirectional microphone has a response pattern with equal sensitivity in all directions. It has an acceptance angle of 360°. Acceptance angle, sometimes called the coverage angle, is the angle between the points on the polar response curve where the level of a sound source has decreased by a specified amount (usually 3 or 6 dB) from the on-axis level. It is used to define the useful working area of the microphone.
A bidirectional microphone pattern (also called a figure-eight or figure-of-eight pattern) has maximum pickup in the front and back with the lowest amounts on the sides. Its acceptance angle is 90°. One important characteristic of bidirectional microphones is that the front and back lobes have opposite polarity. When air pressure goes up in the front the voltage goes up in the front, but down in the back.
A cardioid microphone has maximum pickup in the front, less pickup from the sides, and the lowest pickup from the back. It has an acceptance angle of 131°. It is called “cardioid” because the shape of the pattern resembles that of a valentine-shaped heart.
A subcardioid pattern is similar to a cardioid pattern, but it has a wider response (164° acceptance angle) pattern in the front and less rejection in the rear.
A supercardioid pattern has a narrower pickup pattern than a cardioid, but wider than a hypercardioid. It has an acceptance angle of 116°. The hypercardioid pattern is narrower at 105°.
The narrowest pattern is the lobar pattern. It has an acceptance angle of 90°. The lobar pattern is not familiar to many people, because it is usually called by another name—the shotgun pattern. That is because it is the pattern used in shotgun microphones. These microphones are used to focus on a particular sound source, while trying to isolate nearby sounds.
So why are there so many patterns and why do we care. Choosing the right pattern can help isolate a particular sound source. An omni microphone should be used when you want to capture more of the room sound, the sound reflections and reverberations. On the other hand, when you are recording a singer and his guitar, you can use a bidirectional microphone to capture the guitar and reject the vocal and another bidirectional microphone to do the opposite. A knowledge of pickup patterns is the first step in microphone choice and placement.
Next month we will look at other microphone specifications.
• In 1971, Brewer and Shipley's “One Toke Over The Line” hit No. 10 in the US. When asked during an interview at the time, they said that “toke” had nothing to do with marijuana, but meant token or ticket. As proof, they cited the line, “sittin' downtown in a railway station.” However, many years later, Mike Brewer explained, “One day we were pretty much stoned and all and Tom [Shipley] says, 'Man, Iím one toke over the line tonight.'” He liked the phrase and wrote a song about it. By the way, the steel guitar on that track was played by Jerry Garcia of the Grateful Dead.
• Prior to 1942, most records were released under the name of the band or orchestra, with vocalists getting second billing or no recognition at all. This all changed after a strike by the American Federation of Musicians, after which the vocalists became the stars and the bands were listed as backing musicians.
• Ernest Kador, Jr., whose stage name was Ernie K-Doe, had a No. 1 smash hit with “Mother-in-law” in 1961. However, Dick Clark refused to have him appear on American Bandstand because he felt the song was disrespectful to mother-in-laws.