Limiter

Definition

A limiter is a compressor with a ratio of ∞:1 (or at least ≥ 20:1) and an attack so fast that the output does not exceed the set maximum level (the ceiling). In its brickwall form the applied gain is:

g(t) = min(1, c / max |x(τ)|) for τ within the lookahead window,

then smoothed by a release. The lookahead delays the audio signal by t_la (1–5 ms) relative to the detector: the gain reduction begins before the peak and completes exactly when the peak arrives, following a gradual curve instead of an instantaneous jump that would produce distortion. The release returns the gain to unity; many limiters use a program-dependent release, with a fast component for isolated peaks and a slow one for sustained reductions.

True-peak mode measures peaks on the oversampled signal (×4 or more) and also limits inter-sample peaks. The channel link determines whether the reduction is common to both channels (100%, stable stereo image) or independent (0%, more loudness but a shifting image).

Parameters and units

• Input gain / threshold: how much the signal is pushed against the ceiling (dB).
• Ceiling / output: maximum output level, in dBTP (−1.0 dBTP for streaming).
• Lookahead: 1–5 ms.
• Attack: 0–10 ms in limiters with attack control; a longer one lets part of the transient through to the clipper or the ceiling.
• Release: 10–1000 ms, or automatic.
• Oversampling: ×2–×32.
• Channel link: 50–100%.
• Style / algorithm: transparent, aggressive, for bass, for buses.
• Typical gain reduction in mastering: 1–3 dB on the peaks for dynamic masters, 3–6 dB for loud pop masters.

Musical function

The limiter is the last control of the peaks: it guarantees the master's ceiling and lets you increase loudness by reducing the highest peaks. Used moderately it is transparent; pushed beyond 5–6 dB of reduction it flattens the transients, makes the bass pump against the kick and introduces distortion. In the mix it is used on buses as protection or for density effects; in mastering it defines the final loudness together with the clipper and the compression.

Practical example

Mix at −14.0 LUFS integrated, true peak −4.5 dBTP; target −9 LUFS:

Result: integrated loudness −9.1 LUFS, true peak −1.0 dBTP, PLR from 9.5 to 8.1 dB, gain reduction of 4–6 dB on the kick hits in the chorus. With a fixed release of 5 ms the bass at 50 Hz (period 20 ms) distorts: the gain follows the waveform. Bringing the release to 100 ms makes the distortion disappear.

Common mistakes

• Getting all the loudness from the limiter: beyond 6 dB of reduction the transients flatten; part of the work should be spread across compression and the clipper.
• Release too short on bass-heavy material.
• Ceiling −0.1 dBFS in sample-peak mode for a master destined for streaming.
• Link 0% on material with loud elements on one side: the stereo image sways.
• Limiter on the master bus of the mix delivered to mastering.
• Comparing the limited master with the mix without compensating for loudness.

How to check it

• Limiter meter: gain reduction on the peaks and average; FabFilter Pro-L 2 shows the reduction history and the loudness.
• App analysis: Integrated loudness (BS.1770), True peak (×4, BS.1770), PLR, PSR and Crest factor before and after the limiter. A PLR drop greater than 4 dB indicates heavy limiting; Inter-sample peaks above 0 dBFS must be 0.
• Difference signal: inverting the polarity of the output against the aligned input, you hear what the limiter removes; it should contain only transients, not notes or bass parts.
• Loudness-matched comparison: lower the output of the limited master to the loudness of the mix and compare the transients.