Why is limiting needed during mastering? What is the key to effective, tasteful limiting? And how much is too much? Don’t limit your skills by snoozing on these tips.
Limiting is the final process in mastering and arguably the most important. Its primary purpose is to make your track as loud as possible without clipping or distortion. While many (if not most) mixes can be improved with some carefully applied equalization or compression, it’s probably safe to say that every track can benefit from limiting – though only if done properly! In this article, we’ll discuss six tips for correctly using limiters in mastering.
As with every other mastering process, understanding the basics is key. Limiters are essentially compressors with extremely high compression ratios. Although limiters and compressors both work by reducing (attenuating) transients and loud signals, the amount of gain reduction applied by a limiter is determined by an output ceiling control set by the user. This sets the ‘limit’ above which the level of the audio passing through cannot be exceeded.
The big bugaboo in mastering is clipping – the squaring off of the waveform that occurs whenever a signal exceeds 0 dB, causing your playback device to output a click or a burst of harsh noise. Obviously this is something you want to avoid! But you also want your master to sound as loud possible, or at least in the same volume range as other commercial recordings. The problem is, if you bring up the overall level of a track, the audio peaks are going to start clipping. A limiter allows you to bring up the level without allowing the peaks to clip.
Modern mastering limiter plugins are extremely precise in catching peaks and won’t allow anything to pass through over their set ceiling, which is why they are sometimes referred to as “peak” or “brick wall” limiters. The good ones do this so well that you can hardly notice it happening if set correctly. The L2 Ultramaximizer and the L3 series of limiter plugins are all designed specifically for mastering. They are renowned for their transparency and their ability to not make the track sound obviously limited or compressed.
Limiters are generally simpler to use than compressors, making it relatively easy to dial in the right setting. They typically offer just three parameters: threshold, release, and output ceiling. (Occasionally an attack control and other parameters will also be provided.)
Threshold determines when limiting begins, while output ceiling specifies how much limiting is applied. When the threshold is low, even relatively low-level signals will undergo gain reduction, while a high threshold will yield a more measured response. Regardless of threshold, lowering the output ceiling always causes more gain reduction. The release control determines how quickly the limiter stops working after the signal drops below the threshold. If it is overly long, you will hear audible pumping, while if it is too short, distortion artifacts may result. Many limiters offer an auto-release option, where the optimum release time for your track is computed based on waveform activity. In many instances this will provide the best results.
If you’ve already applied a fair amount of compression to the stereo buss while mixing, you may not need to use it again during mastering. Just by looking at the waveform, you should able to tell whether there are lots of peaks in the track; if not, you probably don’t need to add any compression. But as with everything audio, let your ears, not your eyes, be your judge. Don’t just use compression for the sake of it – be sure it has a purpose.
As with every other mastering process, when it comes to limiting, less is more. Pushing a limiter too hard usually leads to disappointing results and may even ruin your track altogether.
It’s easy to see why: The more limiting you apply, the more the peaks – typically drums and percussion – get squashed down into the mix. True, this allows you to turn up the overall level, but at the cost of a loss of attack and punchiness. Remember also that limiters are much more aggressive than compressors and are therefore capable of doing more damage to your mix.
The goal should be to apply light limiting that complements the music and allows you to eke out a little more gain, but without sucking out the energy and excitement of the track. Start by setting the output ceiling to -0.2 or -0.1 dBFS (never to 0.0, or intersample peaks will sneak through and cause distortion on consumer playback devices) and a very low threshold of around -0.5 or so. This should result in gain reduction of about 2 to 4 dB, which is really as high as you want to go in most instances. Any more than that and you’ll likely end up with sonic artifacts which aren’t worth the extra level you’re gaining. When set correctly, your limiter should kick in only when things get out of hand yet ensure that no clipping occurs.
In this tutorial you’ll see an example of the L2 Ultramaximizer being used to master your music for online streaming: (The L2 comes into play at 11:26)
Just as there are multiband compressors, modern mastering tools also include multiband limiters such as the L3 Multimaximizer and the L3-16 Multimaximizer. These offer multiple frequency bands (five in the case of the L3 Multimaximizer, a whopping 16 in the case of the L3-16) but do not consist of independent limiters operating separately; instead, they have one central peak limiter which examines the signals of all of the input bands and then calculates optimal attenuation for each, applying automatic gain reduction so that the mixed result is custom-limited to fit the music.
“You set the threshold, which determines how much the overall gain is increased,” explains mastering engineer Yoad Nevo. “The L3 then tries to make the least audible change to each band in order to provide the required gain change. It will automatically limit the most in the bands where the most energy is. This generally should be no more than 2 dB or so, but you can override this by telling it to do more limiting in an adjacent band – for example, you can limit the low end more in order to preserve more of the dynamics in the low-mid band, where the bass and some of the guitars are. The correct use of the L3 adds a great deal more loudness to the track without fundamentally altering the dynamics or balance.”
Some mastering engineers even use multiple limiters. Nevo sometimes starts out with an L2 to boost the level slightly before routing the signal to an L3. “Because the L2 is wideband,” he says, “it applies limiting to the whole frequency range. But the L3 allows me to control the different frequencies in a less dynamic way. Because of the L2, the kick, the snare, and the vocal are all tighter together, so in the L3 I can structure the low frequencies and the midrange a little more finely so that it mainly just limits the kick drum.”
On occasion, Nevo has even been known to add a third limiter to the end of the chain – another L2 – to squeeze another dB or two out of a track.
In this in-depth mastering webinar with Nevo, you can see his L2 and L3 chain in action as part of his entire mastering process:
By now, you’ve probably heard of the so-called loudness wars – the seemingly never-ending battle to master as loud as possible with little regard for the consequences. Our advice: Disengage.
The fact of the matter is that louder is not always better. Yes, the typical response to an increase in loudness is usually positive – we tend to turn the music up when a favorite song comes on – but over time, relentlessly loud music can cause listening fatigue to the point of becoming unbearable. When you start applying limiting, you may feel at first that you have improved things by increasing the apparent loudness, but make sure that you haven’t sucked all the life out of the track in doing so. Which segues perfectly to our final tip…
“Referencing” is a fancy term that just means: keep comparing. Compare your master to the original mix (at equal loudness!) in order to ascertain what you’ve gained (or lost) by using a limiter to reduce peaks and make things louder. Then, once you’ve satisfied yourself that the benefits have outweighed the liabilities, listen to your master on a variety of playback systems to make sure that it translates as universally as possible. Finally, compare it to commercially released recordings of music in a similar genre. (If the music you’re mastering is mellow, there’s no point in comparing it to thrash metal!)
If you’re mastering multiple tracks for the same CD or compilation, this becomes even more critical. “Listen to all the tracks back-to-back, focusing mainly on the vocal,” suggests Nevo. “Be sure to check all tracks on different speakers and at different monitoring levels. If you hear level disparities between the tracks, nudge the levels on your limiter plugin until they are as even as possible.”