What is a Shotgun Microphone?
Shotgun or rifle mics are more properly called ‘Interference Tube’ microphones, and they are often assumed to have magically tight polar patterns that simply don’t exist in reality. Shotgun mics do have their uses, of course, but have to be used intelligently to avoid the significant compromises associated with them.
All shotgun mics employ a standard directional capsule — usually a supercardioid — but with a long, hollow, slotted ‘interference tube’ attached to its front surface. Although this arrangement inherently moves the capsule further away from the sound source — thus making the direct/reverberant ratio slightly worse — the hope is that the tighter directionality (at high frequencies), which reduces the ambient noise, outweighs this disadvantage.
The idea of the interference tube is that the wanted on-axis sound passes straight down the length of the tube to the capsule diaphragm unimpeded, but the unwanted off-axis sound has to reach the diaphragm by entering the side slots. Since this unwanted sound will enter multiple slots, and the distances from those slots to the diaphragm vary, the off-axis sound will arrive at the diaphragm with varying phase relationships and so partially cancel one another out — this is why it is called an ‘interference tube’! Consequently, off-axis sounds are attenuated relative to the on-axis sounds, and hence the polar pattern is narrower towards the front than would be possible with a simple super-cardioid mic on its own.