How To Set a Limiter for Subwoofers

Limiters are intended to protect your speakers from getting fried by too much amplifier power or by clipping your amplifier.  What follows is a method for setting a limiter that should prevent your speakers from being blown under most circumstances.

First, look up the power rating and impedance of your speakers.  For this example I’ll use 1000 watts at 4 ohms.  Using an Ohms Law calculator (that can be found online or downloaded as an app) you can determine that 1000W at 4 ohms requires 63.25 volts. This is the power-handling limit of your speakers in volts.

You will need the following equipment:

Signal Generator or audio player with test tones on cd or mp3
Your mixer/pre-amp

The signal generator can be a CD or MP3 player using a prerecorded sine wave tone or it can be a device that generates sine wave.  There are plenty of sources for either on the internet.

The limiter should have a threshold control, an attack time control, a release time control and an output control.

At this point the amplifier is technically not loaded.  When you connect a speaker load, the amp will most likely clip a little earlier than it will when only connected to a voltmeter.  It may be necessary to lower the output of the limiter by as much as 2 to 3dB to account for the additional current required to run actual loudspeakers.  In general, the better the quality of the amp, the less the level will have to be reduced.

If the reading is still above your limit, lower the threshold until the voltage reading remains at the predetermined limit.  You don’t have to worry as much about clipping if you have more power than you need.  Your limiter is going to be used to prevent the overheating of your coils instead of preventing the clipping of your amplifier.

For subwoofers the attack time should be set between 4ms and 10ms.  4ms attack times should be used for amps that are near or at their limits when producing the required voltage.  Longer attack times can be used when an amplifier can deliver more than the rated power of the speakers.  This allows you to take advantage of the power headroom of the amplifier without heat saturating the woofers and melting the coils.

For peak limiters, a fast release time is preferred.  Usually the release time is set as a multiple of the attack time so a 4ms attack time will call for a release time of 8 to 16ms.  In some cases the release time can simply be set as 2x, 4x the attack time to get the same result.

Some DSPs also give the option of a release rate in dB/s or decibels per second.  Again, for a peak limiter, a faster rate is better.  100dB/s is a faster release rate than 25dB/s.

Some general rules of thumb:

A lower threshold setting with a higher output setting will produce a thicker sound but may lack impact.  This is usually less noticeable and more useful for prerecorded music and helps get the average level of the bass up when there isn’t really enough to keep up with the available tops.

A higher threshold setting and a lower output setting will produce a punchier sound and is generally better for live music where the bass isn’t quite as continuous.

Live music requires more dynamic power because the signals aren’t as compressed as in mastered, prerecorded music.  Prerecorded music doesn’t demand as much dynamic power but it tends to have a higher duty cycle, meaning that the average power going to the speakers is higher through the same period of time.  Live music tends to blow things through clipping amplifiers and massive dynamic transients whereas prerecorded music tends to blow things though heat saturation.  Building a system that can handle either duty requires knowledge of the demands of both as well as the implementation of technologies that can meet the demands of both while protecting the system from the dangers of either.

DJs tend to blow the woofers of live systems.  If you have a live music system that’s got lots of dynamic power, and it isn’t a BASSBOSS system, chances are you have more power than your woofers can take long-term.  For DJ sound or prerecorded music it’s suggested that you adjust your limiters to a lower threshold.  The DJ may end up riding the limiters but your woofers will have a better experience.  You can cut the threshold dramatically and make up a bit with output gain. This will give the illusion of more bass while hopefully keeping the speakers out of trouble. You can also lower the output level of the tops on the crossover by a significant percentage of the cut you made in the subwoofer threshold.  The mastered recordings are far more dense than live music so the perceived level won’t be that different.

Live music tends to blow the highs in DJ systems.  If you plan to use a DJ or club system for live music you will probably find the amplifiers clipping on the peaks because live music is more dynamic.  Besides buying more powerful amplifiers, there’s not much you can do other than to turn down the levels or the limiter thresholds. Lowering the overall level will keep the sound quality up and keep the speakers working but may not keep everyone happy.  Lowering the limiter thresholds will sacrifice the sound quality a bit but that’s a smaller price to pay than blown tweeters or midranges.

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