This article is all about limiters—when to use them, how to use them, and how to train your ears to tell if they’re mangling your original sound source. But first, we need to define what a limiter actually is.
Scour the internet and you’ll see a host of definitions when it comes to limiters. Most agree on the specs: it’s essentially a compressor with a ratio of more than 10:1, though I’ve seen 8:1, 20:1, 100:1, and ∞:1 bandied about.
I’d like to argue that a limiter is defined as much by its purpose as its specs, and here’s why:
We compress for many reasons. We might want to shape the feel of an instrument or shrink its dynamic range into a more manageable space. We may compress for color as well.
In this piece you’ll learn:
A limiter, however, is usually used for one reason: to catch the loudest moments of a source, bringing them down in a way that a) protects against unwanted distortion, and b) maintains the integrity of the mix’s overall balance/color.
To put a button on it (or in this case, four buttons, all in): Your UA 1176 compressor is capable of reaching ratios over 10:1. But would you put it on your master bus? Probably not, as it would color the tonality of the whole mix.
Questionable side effects
People often use limiters to increase the overall level or loudness of an instrument (or more frequently, groups of instruments). But is this the best use of them?
That’s up for debate. Some engineers prefer not to juice the input gains or thresholds of their limiters, choosing instead to treat the signal as much as possible before it hits the limiter. They use gain boosts, EQ, and compression to achieve the proper tone and desired level before utilizing the limiter to catch peaks.
Other engineers rely on the limiter’s ability to drive an input signal against the output ceiling. Others still might run two limiters in series, boosting only a little into the ceiling with each one.
As long as it sounds good, any approach is valid; the trick is to make sure you know it sounds good. It’s easy to be fooled in the moment.
When to use limiters
You may find yourself using limiters on peaky instruments that otherwise sound fine (metal drums come to mind) when mastering. Vocals, which can also spike suddenly, are subjected to limiting here and there as well (Nectar Pro sports a switchable limiter).
Still, the majority of limiter-talk tends to center on the master bus, and this makes sense: Limiters are ubiquitous at the end of the mastering chain.
Think of the limiter as a bouncer, standing just outside the door of a club, keeping harsh digital overs outside of the proceedings, and doing so with the force of a brick wall, which brings us to…
We often call such processors “brickwall limiters,” as their implementation keeps the signal from breaking through a predetermined ceiling (maybe we should call them “brick-ceiling limiters”). Usually the desired ceiling is somewhere between -1 and -0.3 dBFS—the digital ceiling, beyond which you get distortion.
These limiters operate with infinitely high ratios, always implementing a delay in the signal in order to see any peaks coming down the pike. Sometimes you can tweak how far the limiter peers into the distance with a dedicated lookahead parameter. Sometimes this parameter is off-limits to the user.
Whichever kind of limiter you have, know that it may not protect against clipping, and your metering might not show the clipping unless it’s designed to display these ‘true peaks’.
Some digital limiters aren’t configured, out of the box, for catching the peaks that might occur between digital samples (intersample peaks). For that, you can switch to a mode specifically programmed to catch intersample peaks (commonly referred to as “ISP” or “True Peak”).
True peak operation
Within the last few years, true-peak technology has become de rigueur for any modern limiter to compete—and many limiters now offer it as a selectable option.
There is, of course, a more controversial point to make: yes, many mastering engineers use true-peak limiters—but plenty of engineers dislike ISP limiters, claiming their sound affects the material in a deleterious, adverse way.
Some would rather lower the output ceiling to ensure nothing got past 0 dBFS than submit a louder master with a true-peak limiter deployed. Others don’t even care about true-peak distortion at all.
Indeed, take your favorite tunes from the last five years and put them through iZotope RX. Click on Waveform Stats. I can guarantee that many of your favorite tunes easily clip past 0 dBTP.
Personally, I go on a case-by-case basis, depending on the needs of the client and the needs of the song. On a recent EP I mastered for the artist Peach Face, one tune didn’t take to ISP limiting so well, while another was improved by the addition of the ISP setting on Ozone.
Whether or not you want to use true-peak limiting is up to you or your mastering engineer. Strictly speaking, I can tell you this: use true peak whenever you want to make absolutely sure there is no clipping when your audio is run through a D/A converter.
Limiters can come with attack and release controls, but not all of them do. As a matter of fact, their GUIs can appear vastly different, depending on the brand. Some limiters have threshold sliders dictating the level at which gain reduction begins; these sliders also affect the corresponding output gain, and as such, the signal seems louder the more you pull these sliders down. Other limiters ditch the threshold control altogether, showing you only input gain; these increase the level—and gain reduction—the more you push them up into the digital ceiling.
The mechanics of your limiter may vary, but the overall tools are the same: a parameter dictating your highest possible level in dBFS (commonly called the “ceiling”), a knob/slider that determines the amount of gain reduction—one that is similar to a compressor’s threshold (quite often, this parameter makes the signal feel “louder”) and a time constant of some kind; this is usually a release control, though sometimes attack is offered as well.
Some limiters offer stereo linking controls, the manipulation of which can have an effect on the width of your mix. Left totally unlinked, both channels will be limited independently, causing each channel or the other to dip in level on its own. This can create an interesting sense of width, because each side of the stereo image is reacting differently to the limiter, drawing the ear in different directions. When pushed too far, unlinked limiting can cause the stereo image to wander in distracting, unpleasant ways.
Conversely, you can often opt to link the left and right channels, in which case the louder one, regardless of its stereo placement, will trigger gain reduction across the board. This creates a more uniform stereo image, as you aren’t hearing those variances in the left and right channels.
Often degrees of linking are provided, usually in percentages. Your ears will ultimately be the judge of what sounds best here.
Ozone Pro’s Maximizer, which handles the limiting duties in iZotope’s heralded mastering suite, offers two stereo controls, one that focuses on the transient portion of the signal, and another that centers on sustained material over time. You’ll also note their Transient Emphasis slider, which handles transient preservation before the limiting stage.
To better hear how these work, try implementing the exercise we’ll outline in just a moment.
The rhythm of algorithms
These days, limiters often employ selectable algorithms to better suit your individual music, some of which deploy sophisticated multiband technology under the hood. Take some of Ozone’s venerated IRC algorithms, for example.
IRC stands for “Intelligent Release Control,” and to some extent, IRC will preserve the dynamics and overall clarity of your mix. Now, with so many different IRC settings—four overall, with separate “styles'' in IRC III and IRC IV—it can be tough to know which is best for the music at hand. Luckily, most companies offer manuals to explain their algorithms, and iZotope is no exception, providing a detailed explanation of IRC in their documentation.
Where does multiband come in? That would be IRC IV, which uses a multitude of bands, rather than three or four, as you would’ve seen in the conventional, multiband limiters of yesteryear. IRC IV’s bands are split up in a psychoacoustic way to better serve the material at hand, resulting in a more transparent and natural effect.
All of this is well and good, but how do you actually use a limiter to your advantage? After all, if set wrong, limiters can introduce their own distortion, as well as unpleasant side effects on the groove.
Here’s a step-by-step guide for tuning your ears to the sonic specificities a limiter imparts on your mix. I usually go through some iteration of this process every time I demo a newly-released limiter.
1. Link the limiter’s input and output as you tweak
Many limiters allow you to link the input gain or threshold control with the output ceiling, so that as you push one, the other comes down in level. This way, your ears won’t be fooled by a satisfying jump in loudness, and you can better judge the moment you’ve gone too far.
Ozone Pro has a linking feature that pairs the threshold slider to the ceiling. Turn that on as you pull the threshold down. At some point, you’ll hear audible distortion. Stop here and pull back until you don’t hear the distortion anymore.
Now you’re ready for step 2.
2. A/B between gain-matched bypass and limited signal
Compare the limited signal with its bypassed variant, carefully noting the differences in timbre. Does the track feel narrower with the limiter on? Are the transients pillowy, softer, or otherwise altered? Does the groove of the whole piece seem different? Note your generalized findings, and write them down if that helps you (it helps me with internalization).
3. Set up a delta test
This is where things get fun—and depending on the limiter, a bit tricky. We’re defining “delta” here as the real-time difference between your limited mix and bypassed version. It’s similar to soloing the esses of a de-esser (where you’d only hear the ess), or the noise output of a de-noiser (where you’d only hear what it’s removing).
Isolating and soloing these artifacts will call attention to them. Hearing them apart from the mix will tune your ears to what’s missing from your limited material once you switch it back on.
It’s a bit like training yourself to hear the effects of MP3 encoding using Ozone’s codec simulator: Once you hear what the encoding takes out of the original mix, those weird, lossy qualities of MP3s reveal themselves. The same general principle underscores this exercise.
Some limiters offer specific delta parameters, but not all do. Luckily, you can make your own with some clever routing, multiple instances of a limiter, and a plug-in that flips polarity.
Please note: different DAWs handle latency in different ways, so you may need to experiment for your own purpose in order to create a true A minus B listening test. Luckily, the workflow is roughly the same in Logic Pro X and Pro Tools, two of the more common DAWs.
First, kill the output of your source track, selecting “no output” (or whatever variant of that terminology your DAW provides) in the output section. Next, route the source to two auxiliary channels on two sends. Duplicate the limiter to both auxiliary channels, and bypass the limiter on your source track. On the second channel, flip the limiter itself into bypass.
Logic users take note: Do not use your DAW’s global bypass, but rather, the plug-in’s own bypass parameter.
Next, load any utility plug-in after the limiter that flips the phase of both the left and right channel.
Logic users, I cannot stress this enough: it’s essential for you to have both limiters instantiated for the trick to work, or else latency will be an issue, and you won’t hear what you’re supposed to hear—the sporadic limiting of your mix, usually on the kick and snare drums, when the limiter is audibly pushing down on signal.
A final note on delta tests: there are plug-ins that can actually set up delta tests for you, so you don’t have to go through all this routing yourself. Some of them are exceedingly cheap as well.
By way of example, here’s a snippet of music:
Limited Piece of Music
And here’s it’s delta/difference signal:
Limited Music's Delta Signal
4. Go back to the A/B comparison
That’s right: Go back to step two and compare the limited mix to its bypassed counterpart. Now that you’ve identified, heard, and internalized what the limiter is taking away, you should be able to hear the differences between the two more clearly. Start moving parameters around, and you’ll find you’re hearing what they’re doing; less guesswork will go into tweaking the limiter’s time-constants and stereo interdependence options, as your ear has tuned itself to what the limiter is taking away.
It’s Your Turn to Experiment with Limiters
Limiters are getting better and better all the time. Some are quite revolutionary—Ozone Pro, for instance, employs an assistant which intelligently sets your limiter, and goes farther by placing a dynamic equalizer before the limiter which helps achieve an even more transparent operation.
The larger point is this: As limiters are changing all the time, it’s important to understand their basic functions, and also, to find ways of testing limiters you may want to purchase in the future. It is my hope I have provided you with the necessary tools to do so in this article. May you walk away with unlimited knowledge on a limited subject.
Sorry, had to.