The makeup gain parameter refers to a gain control at the output of a compressor. Compressors reduce the level of the loudest signals, so after implementing compression, you usually end up with a quieter signal than the original.

You can use makeup gain to makeup for, or recover, the amount of level that you have lost by using the compressor. To see this in action, setup a compressor with 2dB of gain reduction, and no makeup gain.

With the compressor on, you’ll notice a reduction in the level overall.

[mix, uncompressed, then compressed]

Now, increase the makeup gain by the same amount that your gain reduction meter shows.

[mix, compressed with makeup gain]

Turn the compressor off and on.

[mix, bypassing compression]

You should notice the way the compressor is changing the sound, but not hear any change in apparent level coming from the speakers.

[mix]

Some compressors offer an auto-makeup gain feature. This automatically applies makeup gain that matches the amount of gain reduction in your compressor.