Audio compressor controls: The threshold control
This is Part 2 of our mini-series covering the controls of the compressor. You can find Part 1, about the ratio control, here...
I'm going to assume that all of the controls are set to values that would be useful for a vocal recording, with a ratio of 4:1 which isn't extreme, but should make the compression effect easy to hear.
Bear in mind that some compressors don't have a threshold control. The threshold is fixed and you have to adjust the input level instead. This is an old-fashioned way of working, but since many plug-in compressors emulate vintage equipment it has come back into a favor to a certain extent. This will be covered in a separate article on the topic.
What the threshold control does
The threshold control sets the level above which compression will take place. So if the signal stays below the threshold level it will not be compressed. Only when the signal rises above the threshold will it be compressed.
So low-level signals remain unchanged. High-level signals are reduced in level. This is exactly what you should expect from a compressor.
Highest threshold level
Typically the threshold of a plug-in compressor will be calibrated in dBFS, which means decibels relative to full-scale, which is the highest level possible in an audio file. The DAW can handle much higher levels internally, but a maximum threshold level of 0 dBFS will be able to deal with just about all circumstances.
A hardware compressor will have a threshold level calibrated in dBu, which is the signal voltage relative to 0.775 volts. A maximum threshold level of +10 or +20 dBu will cover all normally expected input levels.
Lowest threshold level
Moving on, the lowest threshold level of a plug-in compressor can be something like -60 dBFS. Setting the threshold this low will allow just about all of the signal to be compressed because it will hardly ever fall below this level. A hardware compressor might typically go down to -20 or -24 dBu. It would be uncommon to need the threshold to be lower than this. Bear in mind that the hardware compressor's maximum threshold level is higher.
In typical use
In typical use you would want the compressor to bring down the higher levels of the signal and leave the lower levels unchanged. So you might set a plug-in compressor's threshold to -20 dBFS. This means that when the signal level is below -20 dBFS there is no compression. When the signal rises above -20 dBFS, then the compression effect kicks in.
A lower threshold leads to more compression. So if more compression is what you want, either you can increase the ratio, or lower the threshold. They don't have precisely the same effect so you must judge it by ear.
It's as simple as that! But the key to artistically-effective compression is to balance carefully the ratio and threshold controls. They both offer either more compression or less compression, but they do it in different ways. Close listening, as always, should be your guide.