I've recently inherited a shotgun mic that seems to be in pretty good condition. However, I never do any kind of video or broadcast work, so I can't see myself using it for its intended purpose. I'm loath to get rid of something if I can make use of it, so are there any uses for a shotgun mic in the studio?
James Gately, via email
SOS Technical Editor Hugh Robjohns replies: You can always find a use for a decent mic in a studio, but shotgun — or rifle — mics aren't the easiest to use because their particular blend of properties don't really work well in enclosed spaces.
The shotgun mic gets its name from the long slotted tube — the 'interference tube' — affixed in front of a (usually) hypercardioid capsule. The idea of the tube is to enhance the rejection of off‑axis sound sources, and thus make the polar pattern more directional, although it relies on on‑axis sounds not being picked up off‑axis (and vice versa), and that means it doesn't work so well in an enclosed and reverberant space.
In normal use, the sound wavefront from an on‑axis source travels down the length of the tube unheeded, to strike the capsule diaphragm in the usual way, and so generates the expected output. However, sound wavefronts from an off‑axis sound source enter the tube through the side slots. The numerous different path lengths from each slot to the capsule itself mean that multiple off‑axis sound waves actually arrive at the diaphragm at the same time and with a multitude of different relative phase shifts. Consequently, this multiplicity of sound waves partially cancel one another out, and so sound sources to the sides of the microphone are attenuated relative to those directly in front. The polar pattern essentially becomes elongated and narrower in the forward axis, and the microphone is said to have more 'reach' or 'suck'.
Sadly, though, there's no such thing as a free lunch, and in this case the down side is that the interference‑tube phase cancellation varies dramatically with frequency (because the phase‑cancellation effects relate to signal wavelength as a proportion of the interference‑tube slot distances). If you examine the polar plot at different frequencies of a real interference‑tube microphone, you'll see that it resembles a squashed spider: deep nulls and sharp peaks in the polar pattern appear all around the sides and rear of the mic. What this means, in practice, is that off‑axis sounds are captured with a great deal of frequency coloration, and if they move relative to the mic, they will be heard with a distinctly phasey quality.
So while it might seem that a shotgun mic could afford greater separation in a studio context, in reality the severe off‑axis colouration makes the benefit rather less advantageous, the strongly coloured spill doing more damage than good and making it almost impossible to get a sweet‑sounding mix.
Shotgun mics really only provide useful advantage out of doors (or in very large and well‑damped enclosed spaces), and where no other, better‑sounding alternative is viable. My advice would be to sell the mic to someone who is involved with film, video or external sound effects work, and use the funds to buy something more useful for your studio applications!