The dynamic compressor seems to deal with compression. To compress something is to
squash and reduce that something. The compressor in audacity enlargens amplitude.
It should be called a DECOMPRESSOR. Bad semantics.

I want to compress, decrease volume insections of a sound file.
Can somebody instruct me how to achieve this ?
Comments are much appreciated.

Toronto Canada


A “dynamic compressor” squashes / reduces the “dynamic range”.
The “dynamic range” is the range (variation) between quiet sounds and loud sounds.

With the default settings it makes quiet sounds louder. Thus it is reducing the “dynamic range” by reducing the difference between quiet sounds and loud sounds.

A “decompressor” is more commonly called a “dynamic range expander”, or simply an “expander”.
An expander increases the dynamic range - that is it either makes quiet sounds quieter, or loud sounds louder, or both.

The Compressor in Audacity is a bit unusual in that it “compresses upward” - that is, it makes quiet sounds louder rather than making loud sounds quieter. However, this is the exact equivalent to “compressing downward” and then “amplifying”.

Note that “amplifying” can also be “upward” or “downward”.
Amplifying is known technically as changing the gain. Increasing the gain makes waveforms bigger (louder) and reducing the gain makes waveforms smaller (quieter).
Reducing the gain is commonly known as “attenuation”.

So the upshot is:
If you want to “compress downward”, you can do it by applying the Audacity Compressor effect, and then apply the Amplify effect with a negative amount of amplification (negative gain).

Personally I would prefer that Audacity shipped a more conventional compressor effect that compressed downward by default.

The Audacity Compressor has other options too. See:
I generally prefer to use the “based on peaks” option with “Make-up gain” off.

Not sure if this will help. If you want to change the “entire volume” of a section of audio, hold your mouse at the beginning of the section and drag to highlight (like you would do with a text file ) , then use the amplify tool in effects to increase or decrease volume.

There is also the Envelope Tool for manually varying the amplitude:
I find this tool a bit awkward, but it can sometimes give better results than “automated” processes.

Thank you for your quick respose to my novice questions.
I will try to learn these techniques.

I once signed on the the VLC media player forum.
That community was very disinterested in ’ novice ’ learners.

I dropped at least 6 questions into the forum community,
which boasted 22,000 members. Zero reply to every question.

Thanks to you guys !
Toronto Canada

Hmm, if your using a Mac, you could try the Apple AU Dynamic Processor…

If you try the compressor with default settings on a recording that is way too low in recording level, it acts as a sort of auto-gain. So I understand the semantic thing…

Could it be renamed to “Dynamic compander” to avoid semantic issues? :laughing:

Personally I have a strong preference that the Audacity Compressor should be a more typical / classic effect.

I sympathise with Jimwick who clearly has an idea about what a compressor should do, but was confused by the fact that the Audacity compressor appears to do the opposite of what is expected. We only ship one dynamic compressor with Audacity, so in my opinion it should be a “classic” compressor that meets the expectations of a “standard” compressor.

All dynamic compressors have individual “character”. It’s one of those effects (like “reverb”) that has infinite variety and subtle difference in the algorithm can have huge impact on the subjective results. This is on the line where technology and art meet. The algorithm for the Audacity compressor is actually very good, and is capable of producing a professional and sweet sounding effect, but I think it could be a lot easier to use. No doubt the interface will be improved at some time, but there are a limited number of hours in the day for those contributing to the development.

Hang on. The very page you point to makes clear that the default (tied to the RMS choice) is downwards compression. Of course make-up to 0 dB is on by default.

Or are you alluding to yet another subtlety in the effect that the Manual skates over? :confused:


Yes, makeup gain is on by default, so the net effect is that it compresses “upward”.
As I wrote in an earlier post, upward compression is the “exact equivalent to compressing downward and then amplifying”, which is exactly what Audacity is doing with the default settings.

I think many would expect that the peak/RMS choice is not tied to compression direction, but are you arguing that make-up gain should be off by default? The problem would then be that naive users would say “Help, my waveform has vanished”.


The waveform is hardly likely to “vanish” unless they also push the Threshold down to an extremely low level, but why does the threshold go so low anyway?

Why also aren’t their smaller ratios than 1.5 to 1? That’s a real blunt tool.

1.5 to 1 is a very small amount of compression, especially if you set the Threshold quite high. With a compression ratio of 1.5:1 and a high threshold, the effect will be so subtle that you can’t hear the difference.

I’m gonna go with user error then, because I was working with an orchestral track with very wide dynamic range and I could not for the life of me get the built-in Compressor tool to avoid being over-aggressive at raising volume on the quiet parts. I didn’t try very hard though, just ended up using Chris Capel’s 1.2.6 plugin instead.

(I’m crossposting I know, but I really think it’s relevant to both thread. Sorry mods.)

A good tip for using any dynamic range effect (compressors, limiters, et al), amplify (or normalize) up to 0 dB before you use the effect. Why? Because it makes it much easier to see and hear what the effect is doing. For example, if you normalize to 0 dB and then use the compressor with the “Threshold” set to -3 dB, then it is that top 3 dB that is compressed.

Having said that, I do agree to the extent that I think that the range of the compression ratio should start at 1:1.