Specifications

EAW Smaart 6 Operation Manual Applications
87
Step 7: Stability Testing
It is vitally important to explore the stability of a sound system that includes one or
more microphones before it goes into service. Otherwise, the engineer may be in the
uncomfortable position of exploring the system’s instability—finding and fighting
feedback frequencies—during a live performance. This is every engineer’s worst night-
mare but can be easily avoided.
Unstable sound systems contain a signal loop, often through the acoustic path, with an
overall gain greater than 1 at one or more frequencies, which causes feedback. A stable
system has a comfortable margin of Gain Before Feedback (GBF) at its intended operating
levels while delivering the required intelligibility and frequency response.
Feedback can damage audio components so exercise caution when testing system sta-
bility. Feedback is particularly dangerous when it builds up very quickly and overdrives
the system, causing overloads and/or clipping. It is a prudent safety precaution to use a
limiter or compressor during stability testing to protect system components. Remember
however that nonlinear devices (e.g., a compressor/limiter) should not be engaged in
the signal path during Frequency Response measurements.
Causes of Instability
Feedback often results from an interaction between off-axis speaker and off-axis micro-
phone response. The biggest problems arise when narrow peaks in the off-axis responses
of both speaker and microphone coincide. These interactions can be very problematic
and are not as easy to control as on-axis responses.
Since feedback frequencies can shift as a performer with a mic moves around, always
perform stability testing at all positions at which a mic will be located during the show.
Since two or more mics can create an unstable situation where just one would not, check
the mics one at a time but also in groups.
Other possible stability problems arise from reverberant room acoustics and too much
reverb employed in the music reinforcement system.
Detecting Instability
A stability problem in a sound system can be easily found by turning up the gain, slowly
and carefully, until feedback starts. If there is no feedback until the gain is increased
well beyond the intended operating level, and the system is free of any ringing (at higher
gain, this becomes feedback) at normal levels, it is stable.
If not, you must improve its stability. The best solution may be electronic, mechanical,
acoustical, educational, or some combination.