The compressor formula assuming a De-Esser behaviour would be:
For Input Levels > Threshold:
Outputlevel = (Inputlevel – Threshold)/Ratio + Threshold
If you are interested to find out the relationships between the input and output levels based on a given threshold and compression ratio, download this Excel sheet.
For example, this a typical compressor behavior with the following parameters:
Threshold: -25dB
Ratio= 2:1
Compressor plot
As you can see, the output level reduces if the input is louder than -25dB. This attenuation in volume is due to compression. But below -25dB, the output level is unchanged (or the same with the input).
Now you have a clear idea about the basic parameters, incorrect adjustment of attack and release times can also result to incorrect compression. Based on the definition defined in the previous section; attack times is : “the time it takes for the compressor to complete the gain reduction based on the compression ratio”. Ideally, based on the square wave audio above, the attack times should be less than 8ms to effectively compressed the wave which has a frequency of 62.5Hz. In addition, the natural release time should also be less than 8ms to bring back to the normal level once the wave does not need compression. You can picture it out below ( the light gray color are the compressed output):
Compressed wave
As you can see in the above compressed square wave, you will notice that if the attack and release times are both set less than 8ms, you will get a naturally compressed wave for a frequency of 62.5Hz. Based on the above analysis, you can deduce:
Attack time= 0.5 * (1/f) = where f is the frequency in hertz
Release time has the same formula= 0.5(1/f)
A. Typical attack and release times for bass frequencies/low frequency band: (10Hz to 200Hz)
For the lowest possible frequency:
Attack time= 0.5 (1/10Hz)= 50ms
Release time = 0.5(1/10Hz) = 50ms
For highest possible bass: Attack time and release times= 0.5(1/200Hz) = 2.5ms
Recommendations: In multiband compressor you can tweak the attack and release times for low/bass frequency band starting from 2.5ms up to 50ms. If you put some safety factor of 2 in the above attack and release times. You can have up to 50ms x 2= 100ms for bass. Although in most cases, below are the most popular attack and release time setting for bass compression in mixing:
Attack time: 40ms
Release time: 80ms
In mastering since the bass spectrum consist of a mixdown of several bass instruments,(not just one frequency element such as illustrated in the example above) you can assign a higher attack and release times which is around 5 times.The actual setting depends on the audio material being mixed.
Attack time: 300ms
Release time : 900ms
B. Typical attack and release times for mid frequencies ( 250Hz to 5000Hz)
This is where the vocal and mid range frequencies are found. Vocals are highly transient and requires fast attack and release times. In this type of frequencies, I would like to use a high safety factor such as 10. The release times could not be actually the same with the attack times since these frequencies are the combination of melodies other instruments:
Recommendations:
Attack times= 2 ms to 20 ms
Release times = 20ms to 200ms
In mastering using multiband compressor, the same concept applies:
Attack times: 100ms
Release time: 300ms
C: Typical attack and release times for high frequencies > 8000Hz
This requires very fast attack and release times
Attack/Release times= 0.5* 1/f= 0.5 * 1/8000= 0.1ms
In practical applications such as in mastering you can use:
Attack times: 25ms
Release times: 50ms
Content last updated on October 11, 2012