This month, let’s take a look at some out-of-the-ordinary microphones that are useful in a variety of applications.
Shotgun Microphones
Shotgun mics — so-called due to the long “interference tube” (see Fig. 1) that’s fitted in front of the microphone element — have a narrow pickup pattern which makes them good at rejecting ambient noise and off-axis sound. The mic capsule is generally super- or hypercardioid and is located at the back of the tube, which has a series of vents or slits along the sides.
The interference tube allows on-axis (0 degrees) sounds to pass straight through the length of the tube, but off-axis sounds reach the diaphragm by passing through the slits. As the slits are at different distances from each other as well as from the diaphragm, phase cancellation occurs. The resulting pickup angle depends upon the length of the tube, with pickup ranging from around 70 to 30 degrees.
Obviously, you want to point the tube of a shotgun mic so that the desired sound is on-axis, but you’ll get the best results when the mic is also aimed so that unwanted sound is off-axis. Shotgun mics are really useful in film and video production where you don’t want to see a microphone in the frame and that’s why they’re typically mounted on top of a camera or on a boom pole. They can be sensitive to wind noise, so if you’re using one outdoors be sure to use a windscreen. One thing that shotgun mics can’t do is “zoom in” on a sound, which is where parabolic mics enter the picture…
Parabolic Microphones
If you think that a parabolic microphone (see Fig. 2) looks a lot like a satellite dish antenna, you’re right.
The parabolic reflector (dish) used for a satellite antenna collects radio waves, while the dish of a parabolic mic collects sound waves, directing them toward a single “focal” point which is where the mic capsule is located. The focal point is critical to the effectiveness of a parabolic mic, and most manufacturers will clearly define precisely where the mic capsule should be located. Pickup pattern of the capsule is usually omnidirectional but cardioid capsules can be used with shallow dishes, and the diaphragm should face the center of the dish. Bidirectional, hyper- and supercardioid capsules don’t work very well in a parabolic dish because their ability to reject sound from the sides also means that they reject some of the amplification provided by the dish (yes, amplification. See below). The shape of the dish is critical in providing maximum effectiveness — it must follow the shape of a true parabola.
Parabolic mics are often used in situations where the sound source is low in volume and needs to be boosted in order to be clearly heard, or where a narrow directional pattern is needed to isolate a sound from surrounding noise. When properly deployed, a parabolic microphone has the unique characteristic of being able to zoom in on a sound and gain it up by as much as 30 to 35 dB at 10 kHz. This is a big deal in situations where you can’t be close to the sound source because (1) the level of a sound source is reduced 6 dB every time the distance is doubled and (2) atmospheric attenuation causes high frequencies to be lost as distance from the source increases. At frequencies below around 200 Hz, a parabolic microphone becomes almost omnidirectional due to the fact that longer wavelengths bend around the dish (the bigger the dish, the better the directionality at lower frequencies), but as frequency increases, a parabolic mic becomes highly directional.
Parabolic mics are often seen on the sidelines at football games where they are used to isolate the quarterback call, but they’re also used for surveillance and for capturing sounds in nature such as bird songs.
Boundary Microphones
Any time a microphone is placed near a reflective surface, sound will reach the mic via two paths: direct from source to mic, and via reflections from the surface to the mic. As the reflected path will be longer than the direct path, reflected sound is delayed (see Fig. 3) and when it’s combined with the direct sound, there’s phase cancellation.
This “multipath” of sound reaching the microphone results in comb filtering. We’ve all heard this when you place a handheld vocal mic (or a podium mic with a wide pattern) on a podium. Sound travels from the speaker’s mouth to the mic, but it also reflects off the podium surface to the mic. A boundary mic avoids comb filtering by placing the diaphragm parallel and very close to the reflective surface, in which case the direct and reflected waves reach the diaphragm at the same time Note: podium mics with tight patterns such as hyper-or supercardioid reduce comb filtering by rejecting sound that’s reflected off the podium –ed.
Boundary mics were initially designed for use in public speaking or conference applications, often with omnidirectional or cardioid patterns. When you place a boundary mic on top of a conference table, the resulting pattern becomes half of the capsule’s pattern: hemispherical for an omni capsule or half-cardioid for a cardioid capsule — until wavelength exceeds the dimensions of the surface at which point the mic becomes non-directional.
Somewhere along the line, an engineer tossed a boundary mic inside a kick drum and discovered that the mic captured the attack of the beater on the head quite nicely. Boundary mics of a more recent vintage such as the Shure Beta 91A (see Fig. 4) provide low-frequency extension sufficient that there may not be a need for the outside kick mic that would normally be used to capture the low end.
Pressure Zone Microphones
The Pressure Zone Microphone or PZM™ is a variation of a boundary mic with the capsule arranged in a very specific manner: placed parallel to, and facing the plate. Direct and reflected sounds arrive at the same time and thus in-phase at the slit between the diaphragm and the plate (a.k.a., the pressure zone), so the frequency response is very smooth (see Fig. 5).
When a PZM is mounted on a floor or wall, the boundary becomes a part of the microphone, which extends the low-frequency response. Taped inside the lid of a piano they provide a very natural, uncolored sound — even with the lid closed. In the studio they can be used as room mics by placing them on the wall or floor, and there’s an urban legend that Rush drummer Neil Peart taped one to his chest while recording songs for Moving Pictures.
Lavalier Microphones
Lavalier (“lav”) mics are great for situations where you want a microphone to be unobtrusive. Tie clip mounts are commonly used for lav mics, and in general their frequency response is tailored for placement just above the chest, so while you might think tacking one onto a shirt collar will produce better results, fidelity generally suffers and breath noise increases once the mic is positioned closer than six to eight inches away from the mouth. Handling noise can be an issue with lav mics, so be sure to secure the cable to prevent the mic from moving when the talent moves.
Headworn Microphones
This category includes both headset mics as well as miniature mics designed to be hidden in the hair. I don’t have a great love of headset mics on lead singers because most of them don’t deliver the kind of fidelity you’d want for the application. Regardless, the optimal placement of a headset mic is on the cheek, between the cheekbone and the corner of the mouth — not directly in front of the mouth, where you’ll undoubtedly get tons of popping “Ps” and “Bs.”
Miniature mics (see Fig. 6) can also be hidden in wigs or headwear, the best placement generally being center of the forehead; you can use clips specifically designed for the application to hold the mic and cable in the desired position. If the performer is wearing a wig, you may be able to secure the mic and cable using the same clips that are holding the wig in place. Another option is taping a mic on the cheekbone (no gaff tape, please!), dressing the cable around the ear and taping the cable behind the jaw and on the back of the neck. Be sure to leave enough slack so that the performer can move their head without tugging on the cable.
As far as where you’re going to hide the bodypack for that wireless headworn mic… that’s a subject for another day.
Steve “Woody” La Cerra is the tour manager and front of house engineer for Blue Öyster Cult and Jon Anderson & The Band Geeks.
