Sound Principles:
Limiting

by Jay Frigoletto

When looking to control the dynamic range of your audio, you probably look first to a compressor, and then most likely a limiter. But what is the difference between the two? Which should you choose for a particular application?

Both a compressor and a limiter will reduce the dynamic range of a signal, making the difference between the softest and loudest sounds smaller. A compressor reduces the gain of signals above a set threshold by a ratio that can range from a gentle 2:1 or less, to a moderate 4:1, or possibly a more aggressive 8:1. A 4:1 ratio means that any signal that exceeds the threshold will be attenuated to 1/4 of the excess level. Most compressors give you control over the threshold and a choice of compression ratios. They also give you control over the attack and release times, which correspond to how quickly the gain reduction takes effect after the signal exceeds the threshold, and then returns to normal when the signal falls back below the threshold.

The most significant difference with a limiter is that the ratio is higher, resulting in more aggressive gain reduction. Using a ratio of 10:1 or greater is generally considered to be limiting. Ratios of 20:1 and beyond are not uncommon. Other differences include faster attack and release times, and a higher threshold. What does this mean in practical terms? A limiter is designed to stay out of the way until the signal level gets very close to overload and then clamp down fast and hard to prevent the signal from overloading. It limits the maximum allowable level.

Limiters can be useful on instruments that are very dynamic. Percussion instruments can be played very gently or quite loudly. The initial attack transient can be a very sharp, fast peak. A limiter, with its fast attack time and high ratio, can prevent that peak from overloading and causing clipping distortion. When used in this way, it is often referred to as “peak limiting.” When recording a vocal track, you may insert a limiter just in case the singer unexpectedly belts out that high note. In this case, it’s often called a “safety limiter.”

Individual instruments or tracks in a mix are not the only things that can benefit from limiting. Often it can be applied to the whole stereo mix. Transient peaks like the attack of a snare drum are often much higher than the surrounding instruments in a mix. If you limit those peaks, you can bring up the overall level of the mix without overloading and the accompanying distortion. This is often done in mastering. Aggressive limiting of a mix purely to increase the subjective level is not something you should be doing while mixing. However, using a safety limiter if the tracks are intended for broadcast, or using a limiter or maximizer before making a reference listening copy can be helpful. If a mix is going to have further processing applied, or if it is destined for mastering, leave the final limiting and level maximizing for the mastering engineer. There is no free lunch, and hitting the limiter too hard will eventually result in audible distortion.

Limiters come in both analog and digital versions. One advantage of digital limiters is that the attack and release times can be faster than with analog designs. Also, by applying a small delay before the processing, digital limiters can know what is coming and prevent overshoots. This is known as “look-ahead limiting.” These advantages allow more transparent-sounding limiting of greater levels. The Waves L1, L2, and L3 plug-ins, and L2 hardware are examples of this kind of limiter. In the analog domain, a popular limiter is the Aphex Dominator II. Some compressors have additional limiter sections built in, and some compressors are flexible enough in available ratios, attack, and release times that you can set them up to work as limiters. One of the most expensive analog processors you can buy is the famous Fairchild 670 limiter. It was originally used in mastering for vinyl records to prevent overcutting (one groove cutting into an adjacent groove) or cutting through the master media.

Whether analog or digital, recording or mixing, individual instruments or an entire mix, limiters can prevent overloads, increase apparent volume, and control errant peaks. Used in the extreme, they can also reduce impact or induce distortion. Some provide transparent protection, while others add pleasing color, density, and power. Used alone or in conjunction with other processing, they are essential tools for dynamic range control.