A small omnidirectional microphone element can be a versatile tool. Mics of this type are available from most of the major manufacturers. Their small size allows them to be placed very close to sound sources, even inside of instruments.
The pickup pattern of a small mic element can be modified by the use of boundaries. When a sound wave encounters a boundary, there is a small region near the boundary where the incoming and reflected wave are effectively synchronous in phase due to the very small time offset between them. This “pressure zone” is very tiny at high frequencies and becomes increasingly deeper at low frequencies (long wavelengths).
\If a mic is placed in the pressure zone, then it’s voltage output will be roughly doubled over what it would be in free space, resulting in a 6 dB increase in output level from the mic. If multiple boundaries are used, the increase will be even greater.
This same technique has been successfully used with low frequency loudspeakers for many years, the so-called “ground stacked” subwoofer configuration.
The boundary size must be large enough to reflect the lowest frequency of interest. Wavelengths that are large relative to the boundary size will simply diffract around it rather than be reflected.
The required size can be roughly estimated by making the boundary about one wavelength across at the lowest frequency of interest. For this reason, broadband boundary mics must use a large boundary or a major room surface (floor, wall, ceiling) to achieve the increased output.
A simple but effective way of fabricating a versatile boundary is to use a CD jewel case. Unhinge the cover of the case and reverse it. This creates a smooth “corner” for the mic to be placed in.
The jewel case, used in conjunction with a table top or the floor, can easily be configured to provide two or three boundaries for the mic. Figure 1 shows the magnitude response of each of the boundary configurations pictured in Figures A-C. Both provide a useful directivity increase in the articulation region of the human voice.
The effect can be extended to lower frequencies by increasing boundary size. I recently used two 3-foot by 3-foot plexiglass boundaries to create a corner to mic for a Christmas play. It’s like the CD jewel case, but works for the entire speech range.