Shotgun microphones: how they work, and when they are the wrong choice

The shotgun microphone is the visual shorthand for professional sound, and also the most misunderstood tool in the kit. It does not amplify distant sound. It attenuates sound arriving off axis, and only in part of the spectrum.

The interference tube

In front of the capsule sits a slotted tube. Sound arriving from the side enters through several slots at different distances, so those copies arrive out of phase and partially cancel. Sound arriving straight down the tube reaches the capsule in phase and survives.

Cancellation only works when the wavelength is short compared to the tube. A 30 cm tube starts narrowing around 1 kHz. Below that the microphone behaves roughly like a hypercardioid, and at 100 Hz it is close to omnidirectional.

Why indoors is hard

In a room, off axis reflections are not removed, they are filtered. What comes back through the side lobes is a dull, phase shifted version of the voice, which is the hollow boxy sound people blame on the mic. A hypercardioid with no tube usually sounds cleaner in the same space.

Short tube or long tube

• Short shotguns, around 20 to 25 cm, are lighter, less prone to off axis colouration, and fit tight interiors and gimbals.
• Long shotguns, 40 cm and beyond, reject more at high frequency and suit wide exteriors and sports.
• Longer is not more reach. It is more rejection, which increases the ratio of source to surroundings.

Practical settings

Outdoors, use full wind protection, high pass at 80 Hz at the mic when wind is present, and aim at the mouth rather than the chest. Watch the rear lobe: a hypercardioid or shotgun pointing down at an actor is also pointing up at a helicopter, a road, or a generator.

After the shoot

Boom and radio mics usually live together in a poly WAV. Split them into named mono tracks before editing, so you can pick the shotgun where the room is kind to it and the lav where it is not. That is a per line decision, not a per scene one.