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MillimeterShotgun Microphones
Jun 10, 2009 12:00 PM, By George Petersen
Extend your reach—in mono, stereo, or surround.
Neumann RSM 191
Few audio pros are familiar with the name Harry Olson (1901-1982), but he left an indelible mark on the science of audio. A lifelong researcher at RCA, his many accomplishments include codeveloping RCA’s ribbon microphones (such as the famed 44 and 77 models); the RCA Music Synthesizer in 1958; and a 1941 patent for a directional micro¬phone for long-distance pickup of sounds for film, video, broadcast, and sound reinforcement. Olson’s design was somewhat crude, comprising a circular bundle of pipes of varying length placed in front of several mic elements. However, the concept laid the foundation for line microphones, now commonly referred to as shotgun mics.
Various incarnations of ultra¬directional mics followed, eventually becoming the shotgun mics we know today. Essentially, these involve a long tube—acoustical transmission line—placed in front of an omnidirectional mic element. Put very simply, on-axis sounds enter this column relatively unscathed, while side/rear-emanating, off-axis sounds entering through slots along the side of the tube are out of phase with the front signal and are canceled out.
This polar pattern from an Audio-Technica BP4071L shotgun mic demonstrates how directionality increases at higher frequencies.
A touch of reality, please
At least that’s how shotgun mics would work in a perfect world. The reality is that like all directional micro-phones, these units are more directional at higher frequencies and far less so at lower frequencies. Generally, mics with longer interference tubes provide greater directionality at low frequencies. But longer isn’t necessarily better. A longer unit may provide more reach, but an excessively long mic may prove unwieldy for overhead miking from a boom in cramped quarters, such as low-ceiling locations. Likewise, weight may also be an issue—a few extra ounces may quickly seem like pounds for an operator handholding a boom or fishpole for extended periods.
These days, all professional shotgun microphones are condenser models, as dynamic designs began falling out of popularity some years ago. But with the near-universal availability of 48VDC phantom powering, condenser models are the norm. Some pro shotguns also offer the option of being phantom- or battery-powered, which adds versatility, but the onboard battery compartment increases the mic’s size and weight.
Shotgun Mics
Options and add-ons
Features found on some shotgun mics include -10dB pads (great for those loud dialogue sessions) and various offerings in highpass (low-cut) filters. The latter provide a means of tailoring the mic’s response for the situation, especially in boomy environments or for voice-only or dialogue applications. Speaking of LF problems, nearly all pro shotguns ship with a foam windscreen, which is essential in any outdoor setting, and numerous companies (OEM and third-party) offer fur-style and basket-type windscreens that provide even greater protection against wind-borne noise.
Whether the shotgun mic is handheld or boom/camera/stand-mounted, another essential accessory is some kind of shock-mount to absorb mechanical vibrations and handling noise. Again, these are available from both OEM and third-party suppliers, and as most shotgun bodies are about 20mm in diameter, most mounting products are nearly universal. One useful bit I keep in my camera kit is Audio-Technica’s AT8459 swivel-mount mic clamp, which lets me easily aim a shotgun mic in any direction, even when it is camera-mounted. This is ideal in situations where, for example, it’s necessary to keep a camera pointed toward a podium while the mic is kept on-axis to a PA loudspeaker that’s off to the side. Another inexpensive item is a mic cable with a 90-degree XLR-F connector at the mic end, which reduces the overall mic length when it’s boom- or camera-mounted.
Particularly useful for location work, a plug-on wireless transmitter can convert your shotgun into an RF mic, thus eliminating the cable limitations.
Choices, choices, choices
In researching the pro shotgun microphone market, we encountered more than 50 entries (listed here in PDF format or above in JPEG format), with models to suit nearly any application or budget. Among these were seven stereo shotguns and one model with five discrete outputs for surround work.
This article originally appeared in Mix, of which George Petersen is the executive editor.
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