A "shotgun mic" is a type of directional microphone. It is often a condenser microphone and uses an interference tube, which is a tube carved with many slots. The longer the tube, the more directional the microphone will be at low frequencies. The nickname "shotgun" comes from the fact that some model can be relatively long and look somewhat like a shotgun barrel. The pick up pattern of the microphone also resembles the shot of a shotgun.
A shotgun microphone is usually built from a directional microphone with high sensitivity (such as a sensitive small hypercardioid condenser) coupled with an interference tube. The tube uses some clever physics tricks (phase cancellation) to reduce the signal of off-axis sound, thus enhancing the directivity of the microphone.
The use of an interference tube has some side effects :
- consider the threshold frequency f: the tube will be very efficient at filtering frequencies higher than f, but will have no effects at lower frequencies. The longer the tube, the lower this frequency will be. In other words: in order to be very directional, a shotgun microphone needs to be long.
- because of the way interference tubes work, shotgun microphones are very sensitive to reverberation, making them hard to use indoor.