I always find it interesting discussing perceived loudness and the use of limiters with my students. In the audio community generally there is a lack of awareness with regards to the effects of peak reduction and its change to the tonality and degradation of mastered material. Some of this stems from a lack of understand of equal loudness but also from the manufactures of digital limiting plug-ins, which are the first contact for a lot of young engineers.
Most, if not all, work on the basis of turning up the program material, just making it louder, and are not focused on equal loudness - effective peak reduction relative to the original source. The waves ‘L’ series is one of the exceptions to this having the ability to link the reduction/output controls. However, this is not its standard mode or default where gain reduction and output are not linked.
Another factor is the misunderstanding of the term ‘limiter’. It was originally only associated with analogue limiting. This is not brickwall as with many digital limiters. Analogue limiters, with the exception of a few modern additions, cannot ‘look ahead’ and/or do not have an ultra fast attack time, 1/15ms at best. The term limit means high ratio gain reduction and an aggressive slope/knee in practical terms.
I still find it amazing that there are so many audio articles discussing loudness and explaining how to make tracks louder using digital (brickwall) limiting. There are some interesting points of practice in some of these articles, compensating for the effects of the limiter (mainly the dulling), but I’ve never thought of a digital (brickwall) limiter as part of the mastering process. I look at them as part of the output format restrictions in the same way we would think about dither, except limiting is often a forced addition and not always needed. The exception is with some types of heavily compressed music such as Rock or some Dance genres where the even level (smooth translation) of the drum peaks add to the style and overall musical effect.
The ‘Loudness Wars’ have made digital limiting a necessary addition on digital delivery formats in most commercial music, not because of the audio community striving to excel in their sound but to be perceptively louder than another release. Or as I like to say, it’s the ‘Can you make it louder than my mate’s band’ effect!
If we ask ourselves what we are trying to achieve in the mastering process we should be answering by making a track ‘fit for purpose’ in relation to the output format. This could be CD, Vinyl, DVD-A, SACD, Digital Download and so on. If we’re concerned with dynamic range we should be concerned with it in its final consumer format. This being the case, why is a brickwall digital limiter in the mastering process? If we are going to an analogue format such as vinyl the answer should be we’re not, as there is no ‘headroom’ clip maximum as with digital. So why should we be concerned with them at all?
Basically we should be mainly concerned with the average level and not the peak in the mastering process. The limiter should be there in a digital system to catch the peaks because we have a digital maximum. The perceived loudness of a track doesn’t come from the direct reduction to the peak transient with a digital limiter (which will only dull the sound) it comes from good frequency balance and use of effective compression to even the rms of the audio and the effective use of downwards compression/classic limiting (high ratio gain reduction with a fast attack). The digital limiter is then used to even out the peak material if necessary, dependant on genre as previously stated.
What makes a track sound ‘Mastered’ is not a brickwall limiter squashing the living daylights of a track but good attention to frequency balance and the average weighting of the rms. In smoothing out the average rms to enhance the listening experience the quieter sections of a song will cut though in noisy playback environments and, equally, the louder sections won’t override this average. Both things, if left uncorrected, would cause the listener to reach for the volume control. If we just ‘limit’ a track to make it perceptively louder we still have the same issues with the rms, hence the track will still sound unmastered but possible perceptively as loud a ‘commercial’ track in some sections. Though obviously if we make an equal loudness comparison back to the original it would show the track has been dynamically hyper-compressed/dulled and does not sound as good as the source. Hence we haven’t met our basic mastering goal of making the track sound better!
There is a rational to using digital limiting when we are trying to even out the peaks to achieve the effect of a smooth sounding mid/high end, especially with drum peaks. This is often the case in Rock or some Dance music genres. But again we should ask ourselves what the limiter is actually doing to the sound. We spend hours and hours using compression in a mix to put the attack (definition) in to our mix to achieve the clarity we require. If we just apply a limiter surely all we are doing to remove all this good work by removing this definition!
If we take any limiter and reduce the peak detail of a mix more than a few db (over 3- 4) and make an equal loudness comparison (see previous blog posts for explanation) we will hear the dulling effect of this limiting. Different limiters will sound more or less intrusive than others but they will all dull the tone, the more the reduction the more the dulling to the point where the depth of field and general definition starts to be reduce. This is the basic error a lot of people make in trying to master tracks. If we want to make our tracks ‘loud’ we need to look at frequency balance (tone) and average track rms.
A good master also always comes from a good mix, we can improve and correct mix errors but if the mix is dynamically and tonally poor in constancy it’s never going to sound loud without intrusive correctional measures (heavy multiband work). However, it can be argued if the master sounds better than a poor mix we’ve still done our job.
This basic error in the application of limiting is based on all you have to do is turn it up! If it’s louder it will sound better, we know this is not the case in reality. If we don’t make an equal loudness comparison we cannot correctly hear the effect of the limiting process being applied. Always compare back to the pre-limit mastering and original source to make sure we’re not losing anything from either.
I use classic analogue limiting (high ratio gain reduction) everyday in commercial mastering and mixing but I’m wholly aware of its effect on the sound on the source material and how to compensate if necessary. I use the digital brickwall limiter as an application after the main mastering process relative to the output format/genre.
Many of my clients make releases on vinyl and CD. The mastering for vinyl will always be more dynamic by the nature of the output format. This should be the consideration and not just making a track perceptively loud. Make your track sound right, even and then think about its output format and required perceived loudness.
John Paul Braddock