In the above screenshot, the x-axis is the input audio level to the compressor while the y-axis is the output audio level of the compressor. At any point of the input level, the output level is the same because there is no compression applied.
Supposing you are working with an untrained vocalist that cannot sing at a consistent volume; in this case, you will need to apply dynamic range compression to even up the vocal level. This is demonstrated clearly in the audio compression tips tutorial on vocals. Below is how this compressor will work (graphically):
vocal compressor using adobe audition
1.) Below -70dB, the compressor will further reduce the input volume. For example at an input level of -85dB, the output level is around -90dB. This will further minimize the noise which is often found at this level.
2.) At -70dB input level, the output level is the same (-70dB).
3.) However starting at -70dB above, the compressor starts boosting the input level. For example, if the input level is at -30dB. The output level is -20dB which is louder than the input.
4.) Finally, it reaches a “limiting” point. For example, if the input level is 0dB (which is the loudest peak), the output of the compressor is only around -10dB. The above dynamic manipulations are typical examples of how a compressor works.
How would you interpret the compressor behavior below?
classic compression
First below -24dB, the compressor will completely ignore the input audio. This means “no compression”. However as soon as the volume will hit -24dB and louder, the compressor will start to apply gain reductions to the input which will then reduce the output level.
For example, at input level of -10dB, the corresponding output level is -15dB based on the graph. Based on the compressor above, the following are its properties:
Threshold= -24dB
Compression ratio = Sample Input level/Sample Output level = -10dB/-15dB or 1:5
Below is an interesting input/output graph for different compression ratios:
compression ratio
You can read here the details on the compression ratio formula.
How about the attack and release? This is discussed in the following tutorial:
How compressor attack and release works- A Beginner Tutorial
Content last updated on June 14, 2012