Installation manual

Community S-Series - Operation and Installation Manual - Page 20
USING PROTECTIVE LIMITERS
Although S-Series loudspeaker systems are well protected against potentially abusive
operating conditions by their internal DYNA-TECH protection circuits, the use of an active,
outboard limiter can add an extra measure of insurance for long-term reliability.
Additionally, an active outboard limiter can be adjusted to provide a subtler degree of driver
protection, precisely tailored to each user’s specific needs in terms of musical styles and
operating conditions.
A limiter is a device that detects the level of the signal that it’s receiving over a given
interval or “time constant.” When the input level crosses a certain threshold, for longer
than a certain time period, the limiter introduces some degree of gain reduction. In turn,
the gain reduction reduces the dynamic range of the program material that reaches the
amplifier.
Limiters, and their close cousin Compressors, are used in music recording and sound
reinforcement to create certain effects such as perpetual sustain of guitar and bass notes.
They’re also widely used to reduce or eliminate peak levels from vocalists, percussionists,
and other dynamic sources, as well as often being employed to avoid overdriving amplifiers
and speakers.
Limiters that are suitable for protecting loudspeaker systems are available as stand-alone
products, as well as often being included as a function of many DSP based loudspeaker
controllers or ‘loudspeaker management systems.’
Limiters typically allow adjustment of some or all of the following parameters:
• Threshold
• Input Level
• Output Level (frequently called ‘makeup gain’)
• Attack Time
• Release Time
• Compression Ratio
Virtually all limiters allow the user to set the threshold, or level, at which point gain
reduction will occur. Most models also permit the user to adjust the limit ‘ratio.’ Ratio
refers to the magnitude of gain reduction. At a 2:1 ratio, every 2 dB increase in input level
above the threshold will result in a 1 dB increase in output. At a 10:1 ratio, every 10 dB
increase in level above the threshold will also result in a 1 dB increase in output. From
these two examples, you can readily see how the Ratio control settings will affect the
dynamic range.
Incidentally, the term ‘limiting’ generally refers to ratios of 10:1 or higher (some say 20:1),
whereas ‘compression’ is the term used for lower ratios, such as 2:1 or 4:1. In fact, the
only distinction between ‘compression’ and ‘limiting’ is the ratio of gain reduction.
Many limiters will allow the user to set the time constant that determine the length of the
transients that are permitted to pass before the limiting function engages (this is called
Attack), as well as the time span in which the limiter will ‘recover,’ or return to a state of
non-limiting (called Release).
When used to protect a loudspeaker system, the limiter should be set so that gain reduction
engages at some point before damage occurs to the drivers. If the threshold is set too low,
the resultant sound quality will be ‘squashed.’ In other words, the dynamic range of the
program material will be reduced to a degree that is audibly unpleasant. Conversely, if the
threshold is set too high, the limiter will not engage early enough to protect the
loudspeaker system from damage.
It is very difficult to suggest exact limiter settings, because the ideal setting for one system
and one style of music may be undesirable for another system or another style of music.
Coupled with this, the dynamic response characteristics of the amplifier(s) used in the
system also play a large role in determining proper limiter adjustments. Amplifiers differ
from one make and model to another in their dynamic headroom capability, sometimes
greatly, and also in how they behave when they clip (see section on Amplifier Clipping on
page 30 for more information on driver damage caused by clipping.) Because of this, we
can offer only general guidelines on how to set up your system.