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In this first installment of the Foundations of Audio series, author Brian Lee White shows how to improve the sound of a mix with compressors, limiters, gates, de-essers, and other dynamic processors. The course explains the fundamentals of sound waves, and amplitude, explores common compressor controls, and shows how to eliminate unwanted noise using gates and expanders. The course also demonstrates best practices in compression and limiting in a variety of audio applications and covers sculpting the attack and decay of individual notes with transient shapers and applying frequency specific dynamics control with multiband compressors. Exercise files accompany the course and include special Get in the Mix session files.
A typical audio signal, whether it's one individual instrument or a multitrack mix, is usually made up of many quick transients whose amplitude values extend much further than the average level, or body, of the signal. These transieants prevent us from raising the overall level of the signal too high. Because if we did, those transients would exceed zero DBFS, clipping our output converters and causing distortion. Now this presents a dilemma for our human ear, because our ears are tuned to average out loudness over a longer period of time than a millisecond long transient hitting the top of our dynamic range.
So how do we get the average level of our mix up to a comparable level with the rest of the songs in our music collection? By clamping down on those peak transients and raising up the average level of our mix, using a brick wall limiter. Think of a mix going in to a brick wall limiter like a spring being pushed in to a concrete wall. Since the spring can never go further than the wall it is pushing against, the coils will simply move closer together, compressing the total length of that spring in to a smaller space. Let's check out how a brick wall limiter works in a musical context.
Most brick wall limiters are easy to operate and have similar controls. I'm going to use the limiter built in to my DAW for this example, but all the major DAWs come with factory-installed brick wall limiters, in addition to dozens of third party limiter plugins you can purchase. Typically, how we use limiters to maximize the volume of a mix is by placing them as the last insert in the signal chain, usually on the master fader or output of the mixer. This allows the limiter to increase the average level while clamping down on the peaks internally before passing the final signal out to the D to A converter.
In this example I have placed the limiter directly on the stereo example track since I want to avoid limiting the Guide Dialogue track you're listening to. I'll start by first lowering the output control to just under zero dBFS. Say, around .1 dBFS, so that the output from the limiter never exceeds the converter's maximum output. Now I can simply lower the threshold control, which simultaneously increases the gain into our .1 dBFS wall. Let's take a listen. We can see that it's working by looking at the gain reduction meter.
Take a look at the wave form of the muted mixed process track for a visual representation of what the limiter's doing. You don't want to push things too far, or you'll literally flatten your mix and take all the life out of it. Potentially resulting in distortion. Take a listen to what happens if I pulled the threshold down too far. Then, I will slow down the release control, to help reduce distortion in the low frequencies. Sounds nasty, huh? Even if I slow the release control, the distortion of the deep threshold is merely replaced with audible pumping as the limiter breathes in and out, wheezing on the sginal.
Generally I am just going to go a few dB and check my work. Each track and limiter plugin is unique and will be able to handle different amounts of limiting before breaking up. At this point, we don't want to add any plugins after the final brick wall limiter, as any additional level changes to our signal may clip the converter's output. I like to evaluate what my brick wall limiter is doing by pulling down the output ceiling to the same level as the threshold, so I can really hear the limiting, and not get fooled by the additional gain I've added.
One of the biggest mistakes novices make with these things, is they simply grab the threshold, pull it down a whole bunch, and say, awesome, it's so much louder, it must be better. But when we evaluate the processing without any net gain change to the signal, we can hear exactly what the limiter is taking away from our dynamics. Take a listen to as I reduce the output to the threshold value and switch the processing on and off. I will purposely use too much limiting so you can hear how the limiter is effecting the track when monitored without makeup gain.
Notice that if we go too far we will lose all the snap and bite out of our drums and percussion. >> And when the long road is calling me like a smile that comes with the summer breeze. I know somewhere down here, there's a place for you and me. Oh, love, I see you next to the moon, love. I know how much you care, love. But if you stay up there for too long, I know I will be gone.
>> One thing to note is that not all break wall limiters are created equally. Some let you eke out a bit more average level without stealing your punch, while others start sounding pretty bad when you push them. Mixes with less brick wall limiting typically sound better when cranked up, because the transients of the instruments are really allowed to work those speaker cones. Tracks destined for the club or big PA systems will do better with less brick wall limiting in creator peaked average ratio. Tracks destined for further mastering should almost always forgo the final stage of limiting as this process can severely limit, no pun intended, what the mastering engineer is able to accomplish.
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