October 16, 2022
Many times I receive questions about this stage. We all have heard of the term “mixing” but many fail to comprehend what is involved in this process and this will be explained here briefly. Firstly, we need to note that under “equalizing”, we do not think about applying EQs to certain signals, but rather performing balancing of the track mix to achieve an optimal harmonic and level balance. This is crucial for the mastering step that follows after the mixing. An unbalanced mix will make mastering difficult and even impossible and can ruin the listening experience.
Balancing the mix refers to the fact that all instruments, tracks and components should be fitting perfectly and cause no peaks or discrepancies that can trigger processing hardware or software to behave erratically and miss the benefits of its purpose. A balanced mix has a good peak-to-average ration (crest factor) and is much simpler to master. To achieve a good balance in mix, we need to go through numerous steps like adding compression, saturation, equalizing, delays, reverbs, and other means, to be able to reach a sound that can “sit” well in the mix. The crest factor or the peak-to-average ratio is depicted in Figure 1. below.
Figure 1. Crest factor of a sine wave
What is in fact a crest factor? Simply stated, crest factor, or sometimes called peak-to-average ratio, is the difference in decibels between the peak and average levels of a signal. In the strictest technical sense, it only applies to steady-state signals like sine, square, saw, or triangle waves.
If we do the decibel math, we see that the crest factor of a steady sine wave is 20*log(1/0.707) = 3dB. So far though, we’re thinking like electrical engineers. Two things happen when we make the transition to audio engineering:
First, most audio level meters show the Root Mean Square (RMS), or average level of a sine wave to be equal to its peak level. This is based on the AES17-2015 standard and has the consequence of giving sine waves a crest factor of 0 dB. Second, we deal with audio signals which, at least in most cases, don’t consist of simple and steady-state tones. However, with a little appropriation and creativity, we can turn the concept into one that’s very useful from an audio mastering perspective and this is why crest factor is a very important indicator for the mastering stage. During mixing, we can refer to the crest factor to identify potential problems that need to be fixed before the song is submitted for mastering either through applying certain mixing techniques or even record the problematic components from the beginning.
Broadly speaking, when a mastering engineer references crest factor, they are comparing some form of average level with the maximum peak level over the same time window. The average measurements used could be R.M.S. (weighted or unweighted), integrated, short-term, or momentary LKFS (Loudness K-Weighted Relative to Full Scale), or something else, while the peak measurements could be sample- or true-peak.
Another important factor of mixing is the actual recording stage. If the music is not properly recorded with properly leveled tracks, clipping and peaks that are converging towards 0dB (or beyond), it will be extremely difficult to mix properly. That is why, many times mixing engineers request the clients to send the material back for recording and engage in further recording sessions to address these issues. If that’s explicitly not possible, then the mixing process can involve drastic techniques such as frequency cutting, clipping of the levels, multiband processing and compression, dynamic processing etc., to be able to alleviate some of these issues. However, in this case, the sound will never be the same quality as when it would have been recorded from the very beginning.