There are three more types of microphones that I did not cover in the text because they are used very little in radio. However, for those who like to experiment or have special needs, the following provides additional information.

5.1 Surround Sound

With the popularity of 5.1 surround sound for motion pictures and television, how far off is the technology for radio? Closer than you might think. Later, chapter 13 features High-Definition (HD) Radio as the next development in AM and FM broadcasting. HD Radio, as it is called, has the capability to broadcast 5.1 surround sound.

Just as there are stereo microphones, there are 5.1 surround sound microphones. For those producing in 5.1 surround sound, the Soundfield Company offers a unique condenser microphone that employs four microphone capsules, or elements, mounted in a tetrahedral array (tetrahedral describes an equilateral triangle) within the microphone. This array, when combined with a special microphone processor/preamplifier, allows the microphone to utilize any one of a number of different microphone patterns by electronically adjusting individual capsule combinations.

The output of the microphone can produce a left-and-right, front-and-rear, and up-and-down sound field. Soundfield refers to the output of their microphone as a three-dimensional representation of sound. However, all this technology does not come without a price. A Soundfield microphone and processor/preamplifier retails for more than $10,000.

Big Ear

Another unique microphone, the "big ear," is handy for sports production but not often seen in local radio. The microphone is a concave parabolic reflector designed to reflect all the sound that comes into the reflector to one central focal point where a directional or omnidirectional microphone is placed facing into the reflector. Broadcasters use parabolic microphones primarily for sideline sports coverage. The reflector is made of clear plastic so that the operator can see through the center of the reflector to aim it and still see if a 290-pound linebacker headed out of bounds is about to flatten the operator.

The "big ear" is primarily designed to allow people to hear distant voices and sounds, and it is not a microphone that anyone would use for any kind of serious audio or critical recording.

Boundary Microphones

Boundary microphones are pressure-zone microphones that use a unique principal of acoustics to pick up sound. If you stand facing flat up against a hard-surface wall and listen, you will notice that as you move towards the wall, you can hear things you couldn't before. The wall is reflecting sounds toward your ears. Sounds that arrive in phase will be amplified, sounds that arrive out of phase will seemingly disappear, and sounds that are somewhere in between phase will appear to have a "swooshing" quality appearing to come and go.

The wall affects the sound and produces a very uneven frequency-response curve with several deep notches in it where the response drops due to phase cancellation of some sounds. This notched frequency response is called a comb filter effect, since the response curve ends up looking like a hair comb with several of the tines broken out of it.

A boundary, or pressure-zone, microphone uses this acoustic anomaly to its advantage. Research indicates that if a microphone is placed less than a quarter of an inch facing a flat surface, the comb filter effect will not occur below about 18 kHz. Since the average person cannot hear as high as 18 kHz, this creates a unique opportunity to turn hard flat surfaces into very large microphones using reflected sound.

Boundary-effect microphones have electret condenser elements mounted off-center and facing a rectangular, triangular, or a custom-shaped acoustically tuned base plate. When a boundary microphone is placed on a flat surface, the surface reflects sound into the microphone. Boundary microphones have hemispherical or half-omnidirectional patterns since they sit on a flat surface.

Because of their small size, boundary microphones have some interesting uses. A news person could use one for recording speeches or covering conferences. When a boundary microphone is placed on a lectern, the lectern reflects sound into the microphone with very little phase cancellation. For conferences or meetings, a boundary microphone placed in the center of a table will pick up those sitting around the table. Boundary microphones are priced in the $200 to $400 range.

While the cost of many of these microphones may be beyond the reach of a radio station, most are available for rent to use on a special project.