This device is generally known as a shotgun or rifle mic because of its resemblance in appearance to a gun barrel.
Rifle Microphone is a most unusual project; we are going to mount a sensitive microphone behind a series of acoustic wave guides and hope for the best. Actually, our wave guides are aluminum tubes cut to resonate at frequencies near the human voice range. This arrangement also serves to make the unit highly directional as sound other than that arriving at the mouth of the tubes arrives at the mic in an out-of-phase condition and cancels itself out.
With this easily constructed device it is possible (under ideal conditions) to pick up conversations several HUNDRED feet away, in fact, reports of recording conversations up to 30 yards away THROUGH A CLOSED WINDOW have been made…
The device is quite easy to construct: First get together the following materials:
- 56′ of 3/8″ outside diameter aluminum tubing (hardware stores)
- 1 sensitive amplifier with operating paraphenalia
- 1 sensitive microphone element – use either a good condenser or a crystal, as their outputs tend to be a bit higher than their dynamic counterparts.
- 1 funnel
- Assorted small hardware
Cut the tubing into 37 pieces ranging from 1″ through 36″ in one inch divisions. Now bundle them together symmetrically with one end flush (as shown). As you assemble them, blue them together in several places to assure they remain in the state you have put them in (were it only so easy with people…).
Take the funnel and fit it around the flush end of the tube bundle. Being astute enough to read this, I am sure you will notice the tubes do not exactly fit into the funnel. At this realization, take a small hammer and beatt the funnel to nearly as possible. Now fill the remaining spaces with a good rubber based caulking compound.
The microphone element is placed in the neck of the funnel (with a rubber grommet if it is small enough to fit up into the neck – otherwise set it into the other end of the neck before caulking the tubing in place). Glue the mic into place and caulk any air space around it.
Run the microphone wires out the neck of the funnel, glue in place, and also caulk to give it a closed, fairly air invincible seal around the entire unti. If you really feel up to it, fiberglass resin can be used instead of caulking compound.
Run the output wires to your amplifier and place the entire unit on some sort of support, a camera tripod being ideal. (Need I remind you to check matching impedances?). The unit should be aimed at the subject and then panned about for maximum volume (sound does not travel in a straight line, but may vary with wind, etc).
If wind noise overrides your target noise you can place a piece of cheesecloth across the end of the unit. If a particularly difficult noise interferes with your pick-up you can often plug up the tube(s) carrying this noise with a cork.
Good hunting, avoid paranoid people who may feel your rifle mic more than resembles the real thing…