Give It A Boundary
Surface-mount or “boundary effect” mics are designed to effectively reduce interference from the surface on which they are located. If positioned at the junction of two or more surfaces, such as the corner of a room, they reduce interference from each adjacent surface.

In addition, a boundary mic exhibits increased output due to its combining of the direct and reflected sound energy.

To minimize reflection pickup, avoid placing mics near acoustically reflective surfaces. If this is not possible, consider using a surface-mount mic on the primary reflecting surface.

In addition to interference problems, the use of multiple mics creates other potential difficulties.

When delayed, reflected sound (such as a voice “bouncing” off of a lectern surface), it can arrive with the direct sound at the mic, again resulting in comb filtering.

One of these is due to the fact that as the number of active mics in a sound system increases, the overall system gain or volume also increases. This has the effect of increasing feedback problems.

And, of course, each active mic is adding more ambient noise pickup to the system.

A final general rule for mic use: Always use the minimum number of mics. If additional mics are not needed, they may actually degrade the sound system.

If an application can be satisfied with one mic, use only one mic.

BONUS: Not Enough Gain Before Feedback?

• Move microphones closer to sources
• Move loudspeakers farther from microphones
• Move loudspeakers closer to listeners
• Reduce the number of open microphones
• Use directional microphones and loudspeakers
• Eliminate acoustic reflections near microphones
• Reduce room reverberation by acoustic treatment
• Use equalizers to reduce system gain at feedback frequencies

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