Compression
What is Compression?
As the signal goes in, the compressor begins to ‘push down’ on the peak volume levels that pass a certain threshold that you set. Depending on how the compressor reacts to that input sound is controlled by the ratio, attack, release, and various other settings in the plugin.
But we’re not here to go into depth on each of those functions. We’re here to talk about a commonly missed or misused feature of a compressor, Peak and RMS compression.
Peak vs. RMS
Before we can talk about how the compressor changes in relation to the peak and RMS modes, we need to understand what the difference is between ‘peak’ and ‘RMS’ from an audio signaling standpoint.
When we hear “peak” in regards to music production, we’re talking about the loudest moment of a sound. It is the point where the waveform reaches its maximum amplitude.
RMS on the other hand is the square root of the mean squared; Better known as the “Root Mean Square” (RMS). Without getting super mathematical, RMS is almost like the averages of the peaks and valleys of the waveform. But it is not actually the real average.
Notice in the image above of a sine wave the distinction between peak, RMS, and the average amplitudes of the wave. This will give you a better picture of what the compressor is referencing when its either on ‘peak’ or ‘RMS’ mode.
Peak vs. RMS Compression
Now that you understand the differences between peak and RMS of a waveform, you’ll be able to better understand what is happening in your compressor based on which mode (RMS or Peak) you have active.
When you have the ‘peak’ button clicked, the compressor will compress peak audio levels that venture above the set threshold. When the ‘RMS’ button is clicked, the compressor will compress the audio when the root mean square of the waveform ventures above the set threshold.
Pretty simple, right?
Practical Uses for Peak and RMS Compression
In my opinion, I don’t think RMS compression gets used enough. Generally, when compression is being used, it’s ‘peak’ compression that’s being applied. Now, there’s nothing wrong with using peak compression all of the time. However, there will be instances when using peak compression is not as ideal as using RMS compression.
Instances where you want to use ‘peak’ compression are when you want to compress a sound hard and audibly. You intentionally want the loudest parts of a song to get pushed down in a noticeable fashion. One example could be that you have a cool drum loop but you find the kick and snare are peaking too loudly. Using ‘peak’ compression to “press” the kick and snare closer to the other percussion sounds would be a smart choice.
Although sometimes, you may be faced with a particular sound that doesn’t have a lot of transients.
Take a vocal track or orchestral recording for instance. The peak level of these sounds may not change that much (or at all) in your recording. But rather the “density” of the sound does with changing pitches and the addition and/or removal of certain instruments.
This is where RMS compression would be a good idea. The peak level of the recording may not be changing but the RMS definitely is. This is especially happening when a recording ‘seems’ to get louder yet the volume of the audio doesn’t change at all.
Overall, RMS is more user-friendly and provides a smoother compression. RMS compression is great for vocals, guitar, sub-mixes, and mixes.
Summary
In short, peak compressors reduce the volume of peaks that are louder than the point at which you set the threshold. Root mean square (RMS) compressors also reduce volume but only when the RMS value of the sound gets louder than the threshold point you set.
Use peak compression when you want a more noticeable effect like when you want to ‘pull down’ the peak levels of a kick and/or snare in a drum loop. Or you may have a piano recording where a few notes were played a little too loudly. Either or, peak compression would be ideal for those kinds of situations.
Use RMS compression when you want a smoother and not as noticeable compression. Sounds that change in density like orchestras, mix busses, vocals, pianos, guitars, and more are great uses for RMS compression.
Ultimately, it’s up to you when and where you want your sounds to be less dynamic. Use both types in the same chain even. There are not rules so be creative.
