Why does compressor add gain to things below threshold (Make-up Gain is not selected)

I never fully understand the compressor. I’ve read a million pages on each setting. I want to ask a question about the graph you see when changing settings.

When I am setting the parameters of Audacity’s Compressor, I notice that, as I change the compression ratio, the graph changes for audio BELOW the threshold as well. It appears that positive gain is being added to things below the threshold in larger amounts the more I increase the “ratio.” Why? And how much in relation to the ratio? Why is there gain at all if it’s below the threshold? I don’t see anything ever stating or explaining this fact.

I’ve seen audacity use different explanations depending on which manual you’re reading. One provided in another forum stated, “(Noise Floor) Sets the signal level below which the compressor holds the gain constant while waiting for the signal level to come back up above the set level… This setting isn’t a threshold below which compression is not applied; move the slider to extreme left (or below the actual noise floor) to allow gain changes during pauses.” This gives me a clue, but it still doesn’t explain the formula or reasoning for gain below the threshold. Why is there any gain below threshold?

*Please bear in mind that this fact does not change when you select or unselect the “make-up gain” setting or the Noise Floor setting. BONUS POINTS if anyone can explain that conundrum as well.


Just a heads-up: there will be a new, different, compressor in the upcoming 3.6.0

It will also have the advantage that you can run it it real-time in the effects stack.


This is due to a limitation in how the graph display is calculated. The graph assumes that “makeup gain” is being applied, and it makes this assumption regardless of whether makeup gain is actually applied or not.

Without makeup gain, we should see the line above the threshold angle further down as the ratio increases (and no change to the line below the threshold).

When makeup gain is applied, then the entire line is shifted upwards until the top right end of the line reaches 0 dB.

When using compression based on Peak level (“Compress based on peaks” selected), the compressor always applies makeup gain (it is an “upward compressor”). When compression is based on RMS level (“Compress based on peaks” NOT selected), then makeup gain can be toggled on/off using the “Make-up gain for 0 dB after compressing” checkbox. (Thanks SrgtSunflower for the correction).

Yes this is all a bit confusing, and Audacity has long needed a better compressor. The next Audacity release is scheduled to have a new compressor to replace the current effect.

So, the graph assumes “make-up gain.” Can’t say I’m too surprised the graph is flawed.

I had a feeling this issue was more about the language audacity uses. Given how many changes Audacity has had over the years, it’s crazy that wouldn’t be improved. Thank you for pointing me in the right direction.

*** I think we got it backwards though. According to the Audacity wiki (and my own tests), “When using RMS, the compressor uses “downward” compression,” and “When using peak values, “upwards” compression is applied.” Of course the graph mentioned does not change that preview based on which one you select; it reacts the same, showing upward compression or “make-up gain” regardless of your choice.

This was really helpful. I knew I wasn’t crazy! Haha. So:

  1. The graph stinks and doesn’t change with each settings as you might expect.
  2. Compression based on peaks will apply upward compression to audio below your chosen threshold, even if you don’t select “make-up gain.”

Wow. Thank you, folks!

Oops, sorry, “I” got it backwards. :blush:

Compression based on Peak level is always upward compression.
I’ll correct my previous post.