How To Set a Limiter for Subwoofers

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

Crossover

Limiter

Amplifier

Voltmeter

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.

The signal flow should be:

source mixer/pre-amp crossover (LF out) limiter subwoofer amplifier voltmeter

In a DSP the crossover and limiter will be in one unit but the procedure is the same.

Connect the voltmeter to the output terminals of the amplifier. Do not connect the speaker(s).

Turn your amp’s attenuators to maximum output, adjust the threshold of the limiter to maximum and set the output gain to unity or 0dB.  Set the ratio to the highest setting, usually indicated as a high ratio like 20:1 or as much as infinity:1.

Set the attack time to the minimum setting such as 0.1ms.

Set the release time to minimum also, usually about 1ms.  Some may be labeled as a multiple of the attack time so a low number here is also preferred.

Pick a frequency near the center of the operating band of the speaker you’re protecting. For a subwoofer, 50Hz would be a good choice.  Play that frequency on your source device and bring up the gain on the mixer.  You should be able to see the output voltage of the amplifier rise on the voltmeter.  Bring the level on the mixer up until the clipping indicators come on.  At this point the output of the amplifier should be above the voltage you determined to be their maximum.  In this example the amplifier should be able to produce over 63.25 volts.  If the amplifiers clip indicators come on before the voltage reading gets to your maximum, your amplifier is underpowered for your speakers, but you still need the limiter!

Leave the signal level up to where the clipping indicators stay on.  Lower the Threshold control until the clip lights go off.  Check the voltage reading on the voltmeter.  If the reading is below your predetermined limit, you’re almost done.

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.

In a BASSBOSS system everything is done for you.  Massive power is available, massive power handling is provided, all the protections are in place and all the parts are matched and balanced to work together in harmony.

Production companies are being called upon to provide sound for DJ/producers and prerecorded music acts more every day.  Having a system that can handle the demands of anything you throw at it, effortlessly and flawlessly, is infinitely preferable to having a system that crashes and burns when the latest music genre is thrust upon it without warning!  Delicate is not synonymous with detailed.  A system that is detailed can also be robust and powerful.  A massively loud, effective subwoofer system doesn’t have to be brutish or inaccurate.  It isn’t necessary to sacrifice nuance in order to get reliability.   Or to get output capacity.  Yes, when you push the levels to their maximum there will be some loss of detail, but when you have a system with a maximum that’s well above your needs, it’s not working as hard and is therefore even more likely to be able to reproduce the nuance since it’s not working hard.

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17 responses to “How To Set a Limiter for Subwoofers

  1. David, Is it preferred to have release times be the same for Low, Mid, and Hi , limiter output settings ? SAM / S.A. Concert Sound

    • Hi Sam,

      The short answer is no. The release times should be appropriate to the frequency bands. You’re better off with shorter attack and release times for higher frequencies. This helps make the limiting action less perceptible.

  2. Hello, i used your guide and is very useful. My required voltage was about 63 volts, and my amp deliver about 70 volts (clipping amp), i lowered the threshold until my reading was about 59 volts (no more clipping) I used 50hz test tone. But sometimes when i play music (recorded music, iTunes music) the amp clipping starts. Should i pay attention to this? i measured the voltage with speakers connected and the volts don’t go over my speaker limit (63 volts) they stay around 30-40. Why my am is still clipping?

    Thanks!

    • Hi Mando,

      The amp may be clipping because the transient peaks are getting through the limiter before it clamps down. In other words, because the attack time on the limiter is set long enough to allow brief peaks that will trigger the clip light on your amp to come on before the limiter reduces the level. If the sound quality is not compromised then you can allow this to happen.

      In many cases, amplifier manufacturers will have the clip light illuminate for longer than the time the amp is clipping in order to allow you a better chance to see the light and do something about it. This helps you to avoid blowing tweeters but is less useful for protecting woofers. To put it another way, the attack time on your full-range amp’s clip light is shorter than the attack time on your set-for-subwoofers limiter.

      If the limiter is doing its job, the clipping will be very brief and the woofers will not be at risk. You can try reducing the attack time on the limiter but, depending on the limiter, the result may sound worse than a tiny bit of clipping. A 10ms attack time will provide a more dynamic sound than a 5ms attack time but the 10ms attack time may allow the transients to briefly clip the amp and trigger the clip lights even though your woofers are very likely able to survive a 10ms peak. Ultimately your woofers can probably live with the clip light coming on occasionally and you can think of it as an indicator that you shouldn’t turn up any more once you start seeing it. Of course, if you hear audible distortion, turn the levels down.

      • Glad to hear this!

        My woofer is working now on 44hz – 200hz (settings from crossover) i set limiter attack to be on 16ms, hold on 100 and release on 260ms. Do you think should i change this values to a betters ones? my sub is 15″ 500w (RMS) 8ohm / 2000w (PEAK).

        My amp is 590w 8ohm.

        Thank you for helping me out!

      • You might try setting the attack time at 10ms, the hold to 50ms or less and the release to something very near the attack time. I want the limiter to “let go” just a bit slower than it “grabbed on” so between 10 and 20ms would be about right. If your limiter specifies a release time in milliseconds then it should be much shorter than 260.

        When a limiter has a “hold” setting then the release setting is often a speed setting, not a time setting. The release may be specified in dB/s which means Decibels per Second, which is a way of describing release speed, or how fast the limiter lets go of the signal and allows it to rise back to full level. If that’s the case with your limiter, a higher number would be faster.

        With long hold and release times you kill dynamics, or worse, you hear the limiter “breathing” because its action is out of time with the dynamics of the music.

  3. My limiter is in milliseconds.
    Those are the specs

    Threshold: –20 … +15 dB
    Attack time: 1 … 100 ms
    Holding time: 0 … 100 ms
    Decay time: 10 … 1000 ms

    • Limiter settings are based on time, amplitude and speed of action. All frequencies relate to time. Higher frequencies have shorter rise times and require faster attack times. Lower frequencies require longer attack times because if your attack time is too short, the lower frequency doesn’t have time to get through before the limiter reduces the level, so the limiter ends up squashing the low-end out of your system. It’s a balancing game.

      44Hz requires just about 23ms for a full wavelength cycle to get through the limiter. 200Hz, the other end of your operating range, requires only 5ms for a full cycle to get through the limiter. The 200Hz may clip your amp but the 44Hz will stress your woofers more. 10ms is nicely between the two so that’s about where I’d put the attack time.
      Hold time is something I like to keep to a minimum for a limiter. Since you can go as low as 1ms, I’d try that first.
      Decay time is how fast the limiter allows the signal to return to an un-attenuated state. I’d go with 10 to 20ms. If the limiter holds too long or takes a long time to release then the signal may still be reduced when it’s un-necessary so the limiter will not be tracking and reacting to the dynamics of the music.

      You want a limiter to attenuate the signal just enough to prevent audible clipping and damaging drivers but you don’t want it reducing the levels when those peak signals aren’t present because then you miss the details in the signals that aren’t the biggest ones.

  4. Hi again,

    Another question, my amp can deliver about 910w in 4 ohms (per channel), and i have 2 pairs of subs, each pair has 2x500w rms (1000w total in 4ohms per pair).

    I know my amp is underpowered for each of my pair, but i can use it if i use the limiter correctly and i don’t put my amp in clip? can i drive my subs safe?

    What is your advice for protecting my amp and also my subs.

    Big thanks!

    • Yes, you can run your subs absolutely safely if you follow the instructions above. Set the limiter so the amp doesn’t clip and you’l be fine.

      The reason under-powering is considered dangerous is because most of the time people will tend to clip their amps trying to get greater output and the clipping tends to burn voice coils. The only consequence of under-powering your subs, provided you are not clipping your amp, is that they will not reach their absolute maximum output. In your case you’re only slightly under-powered so you’ll still be able to get very nearly as loud as they can go.

      And your amp will be fine, too. Almost any amp on the market is capable of running at 4-ohms per channel so it doesn’t sound like you have anything to worry about there. Your amp is not at risk because it can’t produce as much power as your speakers can handle. Clipping the amp is likely to damage speakers but not likely to damage the amp. What damages amps are things like shorted speaker wires, too many speakers on a channel, voltage spikes and sags or overheating. If you avoid those, your amp should be fine.

    • Hi Adrian,

      If you go through the steps for setting the limiter outlined above you will have most of the work done. Because these are full-range speakers, not subwoofers, you will need to configure a little differently. When limiting a full-range signal, the limiter will respond to the highest signal peak, which is often the bass, and so it may cause the highs to be attenuated when the bass hits.

      Because of this you will want to minimize the engagement of the limiter, so keep the threshold a little higher and reduce the gain to prevent clipping. The idea is to use the limiter to catch the peaks and prevent clipping but have it engage as little as possible so as to retain as much of the dynamics as possible.

      Try setting your threshold at 0dB and output at 0dB with 4ms attack, 10ms hold and 10ms release. When setting the limiter according to the instructions outlined above, if the amp exceeds your voltage limit or clips with these settings then reduce the output to -1dB followed by reducing the threshold to -1dB. Continue to reduce both the output and threshold equal amounts until your amp isn’t clipping or exceeding your voltage limit. (You may have to go above 0dB with some amps but keep the two levels equal.) After that, try listening to music at high levels to evaluate whether the limiter is affecting the audio quality. Ultimately the limiter should improve sound quality at high levels by preventing overdrive, distortion and potential damage. If it prevents your speakers from getting quite as loud as they would without the limiter, but doesn’t sound bad, then it’s doing its job.

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