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.

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.

28 Comments

  1. Sam Alonge

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

    Reply
    • morethanbass

      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.

      Reply
  2. Mando

    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!

    Reply
  3. Mando

    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

    Reply
    • Mando

      I think Release is not same with Decay? right?

      Reply
  4. Mando

    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!

    Reply
    • morethanbass

      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.

      Reply
  5. Adrian Prado

    Hi, I have an amp and it is 800w a 8ohms and a have a pair of Wharfedale Delta 15 speakers (500w RMS each one) here is a link with the description of it.

    My digital processor is a STS 214b and it has a limiter. Now, how do you think do I have to configurate it, I mean the Threshold, Attack, Hold and Release. Thanks for your time.

    Reply
  6. Adrian Prado

    Oh man I forgot some, in the limiter, the attack button is expressed in db/s…

    Reply
  7. morethanbass

    When using dB/s, the larger the number of dB/s, the faster the attack time. To convert to ms, take 3250 and divide by the time in ms. Thus 10ms attack time ~= 325dB/s. 4ms attack time ~= 800dB/s

    Reply
  8. etone

    Great article I set my pa up today to set my limiters but ran in to 2 problems the first one I had to set my limiters to their lowest setting -24db! but the amplifier is still putting out WAY to much power before the limiter even gets enough signal to limit, I wanted to have my amplifiers set full up (so no one can go and turn them up more) and let the limiters do the work, So this leaves me 2 choices reduce the input gain within the dsp unit or reduce the output gain in the dsp unit or both (wont this effect my dynamic range? Also before I go putting my multimeter in to my amp unloaded is this safe for class D and all over power amplifiers ? I was always told never to run a power amp flat out with out a load. Thank you

    Reply
  9. Bay Area Production

    Hello morethanbass,

    Blog is very informative. PA with DBX Driverack PA with pair of QSC KW181 and KW153’s – suggestions on DBX settings for a DJ setup?

    Reply
    • morethanbass

      Hi There,

      I don’t have specific settings to recommend but as far as suggestions go I would try to get the 15″ in the KW153 to align with the 18″ in the KW181 so they can both contribute to the low end of your system. If the KW153 and KW181 were set up to work together by QSC then you should not need any additional processing. The KW181 is specified to have response to 38Hz and the KW153 to 35Hz. From the specs it appears they have similar high-pass filters so I would suggest using them both without additional crossover filters in the dbx. This would give you the best low-end response from the combination. Adding a KW181 to a KW153 in this way should give you a flat response to 125 Hz with a 6dB boost below that. More than 6dB if the gain on the sub is set higher than the top.

      What I’d do with the dbx is put a parametric filter on the inputs at about 40 to 45Hz with a Q of 3 or 4 and boost that by a few more dB. Then I’d set the output limiters for the two boxes. You can set the limiter to fast attack, fast release and threshold to where the speakers don’t clip. (Don’t use overeasy or soft knee.) To this point the signal path should be the same for both the top and sub except perhaps for the limiter settings. If you then wanted to EQ the tops you could do that with their channels’ output EQs.

      Conventional wisdom would suggest rolling the bass out of the tops in order to allow them to get louder but since these tops are specified to go lower than those subs it seems to me you’d be giving up on a large part of why you have such big tops. You will be giving up a bit of level from the tops but the limit is the limit and if it sounds much better at a slightly lower level, in my opinion, it would be worth it. It’s the best thing I can think of short of adding more or better subs, which is always a good idea!

      Let me know how it works out..
      Best,
      David Lee

      Reply
  10. Kabir

    Hi David,

    Rest post and very helpful! I think I’ve got it but before I start I’ve got a couple of questions…

    My amps are the Crown Macrotech amps and don’t have an obvious clip light on them. I think the ODEP lights start to dim on them when they are clipping but I could be wrong. If I know the voltage my speakers can handle should I follow all the instructions above and reduce the threshold on my DSP limiter until the voltage required is met on the multimeter?

    By using this method I’m assuming the speakers will be protected if I set the voltage to the correct level by reducing the threshold on my limiter but how do I know my amp is not clipping?

    Lastly I’m using my amps for pre-recorded music so should I set my gain to maximum and threshold to around +3db (this is because my mixer shows a yellow light for +3db). Then slowly reduce the gain level until the multimeter gives the correct reading?

    Regards
    Kabir

    Reply

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