Installation manual

Community S-Series - Operation and Installation Manual - Page 21
Following is a brief discussion on using limiters to protect loudspeaker systems:
• The limiter should be set so that it provides some measure of gain reduction before
the amplifier(s) begin to clip. If the limiter is set so that it allows the amplifier(s)
to go into hard clipping, it will do little to protect the drivers, except perhaps in the
event of extended microphone feedback.
• If the system is large enough to handle the intended musical style in the size of
room that it’s designed to cover, i.e. if there are enough amplifiers and
loudspeakers to provide the desired SPL (Sound Pressure Level) before the
amplifiers reach clipping, then it will be possible to adjust the protective limiter so
that the amplifiers never can go into clipping, even under abusive conditions. This
will provide an excellent level of protection, without sacrificing headroom and
sound quality.
Conversely, if the system is inadequately sized for the room, or just barely
powerful enough, setting the limiter so that the amplifier(s) never goes into
clipping is probably unrealistic. Such a setting will most likely result in less sound
pressure level than desired, as well as a highly compressed sonic quality. In such
case, you can either upgrade the sound system by adding loudspeakers and
amplifiers, or you can make some intelligent compromises. By carefully adjusting
the attack time, release time and threshold level, you can allow some of the peaks
to get through to the amplifiers while still reducing the potential for damage from
long-term abuse.
• A fast attack time (under 10 ms) will limit most of the peaks in normal program
material. By lengthening the attack time, you can allow short transients to pass
through (like a snare drum), while still reducing longer peaks like vocal and
instrumental crescendos. This will improve the dynamic range, while still providing
a measure of protection.
• Similarly, a long release time will tend to squash the program material, as well as
introduce an audible ramp-up in level as the limiter slowly recovers. By shortening
the release time, you can increase the short-term dynamic range. As long as the
threshold is not set too high, the limiter can still aid in protecting the drivers from
long-term overdrive, but not as thoroughly as it would in an adequately sized
system.
Be cautious of very short attack and release times, as the detector circuits may
start to track the individual cycles of a sustained low-frequency tone, creating a
‘pumping’ effect, almost like that of a tremolo circuit on a guitar amplifier.
It will take some experimentation to derive the best settings for each situation. Maximum
power and voltage ratings are stated on the S-Series Specification Table on Page 9 in this
manual; these may be used as an aid in calibrating limiter settings. Using a true RMS
voltmeter, you can measure the output of your amplifier when driven by a sine wave
generator, and adjust the limiter to prevent the amplifier’s output voltage from exceeding
the stated maximum. Make sure to do this with the loudspeaker disconnected!
Be aware, however, that static voltage settings made by taking measurements derived from
exciting the system with a sine wave generator, or other constant voltage source, will
represent only a part of the picture. The other part is the time constant. If the attack and
decay times are too long, the limiter will not protect against short-term transients; in fact,
it may never engage at all.
In summary, a protective limiter can go a long way towards preserving the lifespan of a
loudspeaker system, but only if the system is sized properly to begin with and the limiter is
carefully adjusted to properly complement the system. Obviously, it’s safer to err on the
conservative side, as the opposite approach may result in driver damage or complete
system failure.