<P> A unidirectional microphone is primarily sensitive to sounds from only one direction . The diagram above illustrates a number of these patterns . The microphone faces upwards in each diagram . The sound intensity for a particular frequency is plotted for angles radially from 0 to 360 ° . (Professional diagrams show these scales and include multiple plots at different frequencies . The diagrams given here provide only an overview of typical pattern shapes, and their names .) </P> <P> The most common unidirectional microphone is a cardioid microphone, so named because the sensitivity pattern is "heart - shaped", i.e. a cardioid . The cardioid family of microphones are commonly used as vocal or speech microphones, since they are good at rejecting sounds from other directions . In three dimensions, the cardioid is shaped like an apple centred around the microphone which is the "stem" of the apple . The cardioid response reduces pickup from the side and rear, helping to avoid feedback from the monitors . Since these directional transducer microphones achieve their patterns by sensing pressure gradient, putting them very close to the sound source (at distances of a few centimeters) results in a bass boost due to the increased gradient . This is known as the proximity effect . The SM58 has been the most commonly used microphone for live vocals for more than 50 years demonstrating the importance and popularity of cardioid mics . </P> <P> A cardioid microphone is effectively a superposition of an omnidirectional and a figure - 8 microphone; for sound waves coming from the back, the negative signal from the figure - 8 cancels the positive signal from the omnidirectional element, whereas for sound waves coming from the front, the two add to each other . A hyper - cardioid microphone is similar, but with a slightly larger figure - 8 contribution leading to a tighter area of front sensitivity and a smaller lobe of rear sensitivity . A super-cardioid microphone is similar to a hyper - cardioid, except there is more front pickup and less rear pickup . While any pattern between omni and figure 8 is possible by adjusting their mix, common definitions state that a hypercardioid is produced by combining them at a 3: 1 ratio, producing nulls at 109.5 °, while supercardioid is produced with about a 5: 3 ratio, with nulls at 126.9 ° . The sub-cardioid microphone has no null points . It is produced with about 7: 3 ratio with 3 - 10 dB level between the front and back pickup . </P> <P> "Figure 8" or bi-directional microphones receive sound equally from both the front and back of the element . Most ribbon microphones are of this pattern . In principle they do not respond to sound pressure at all, only to the change in pressure between front and back; since sound arriving from the side reaches front and back equally there is no difference in pressure and therefore no sensitivity to sound from that direction . In more mathematical terms, while omnidirectional microphones are scalar transducers responding to pressure from any direction, bi-directional microphones are vector transducers responding to the gradient along an axis normal to the plane of the diaphragm . This also has the effect of inverting the output polarity for sounds arriving from the back side . </P>

What is the function of the microphone in a telephone