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8.2 - DIRECTIVITY

8.2.8 - HIGHLY DIRECTIONAL MICROPHONE SYSTEMS

As we have seen in section 8.2.4 (The Mathematical Model for Directivity), with first-order directivity microphones, the lowest directivity factor possible is about 0.25 . However in many situations we need to able to pick up sound at some distance from the sound source without too much of the surrounding ambient noise. Outside broadcasts of many sporting events need to add realism to the sound commentary by pin-pointing individual sounds such as the racket or net impact - or players movements in a tennis match - or perhaps the kick of a football etc. Recording of bird song and other animal noises also need a much smaller directivity index or tighter directivity pattern to pick up a reasonable sound.

The supercardioid or hypercardioid directivities are sufficient for many applications but they represent the limit of the directivity factor for 1st order directivity patterns. The directivity factor can however be improved with one of the following acoustic systems associated with the basic 1st order directivity microphone:

• an interference tube to produce what is normally called a rifle mic
• a reflector in the form of a parabola

A third system using an acoustic lens has been tried but has proved too cumbersome to be operationally viable.