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How the attack time of the compressor actually work


The attack time is not the time the compressor waits?

attack time compressor wrongI often hear people describing the attack time of compressors in the completely wrong way. I hear this everywhere, from home studio enthusiasts to professional studio owners and music educators. Maybe it's just bad communication but I thought that it was worth looking into. 

The misconception of the attack time

The attack time of the compressor is actually not what a lot of people think it is. The general idea is that the attack time is the time the compressor ”waits” (after the threshold is passed) until the set attack time is up, then it shuts on like a switch. The function would in this case work equivalent to a delay and should rather be called something like ”hold time” or ”waiting time”.

This kind of functionality would probably not be that musical or transparent and would probably sound glitchy and stuttering. The knee setting would be the only way to control the smoothness of the compression. Maybe you could argue that the ratio control also alter the smoothness but that's not really what we're talking about here.

The way the attack time actually works

attack time compressor rightJust like the name actually sugests, the attack time is the time the compressor takes to attack. Just like any other types of attacks you can attack something fast or slow. The attack time therefore equals the time it takes to reach the set compression (from zero to full compression), post threshold. This creates a more smooth experience than if it would have just turned on. The way to measure attack time is not standardized and can differ between different compressors (digital and analog). Some manufacturers call it something else (like just fast/slow) to stay away from the confusion.

So how does the attack time actually work?
Lets say you've got a compressor with a ratio of 2:1 and an attack time of 100ms. If you give it an audio signal of 2dB above the threshold the compressor will start to react immediately with no delay (independently of attack time setting) and (in a slope) reach the point of 1dB of compression after 100ms has passed.

If you take the time it takes to attack away from the attack it is no longer an attack. It just is or isn’t. On or off. 

The logic

Effects like the delay often have a "time" or "delay time" knob that alter the time until the delayed "reply" appear. I think that it would be weird to call that function "attack time" and I haven't seen any delays that call that function "attack time" even though it works just as it is suggested that the attack time works in a compressor. Aaaaaaand vice versa... 

The attack time is the time the compressor takes to reach full compression (set by the ratio) after it passes the set threshold. 

Confrontation

I recently saw a video by a big plug in producer where they stated that:
"Attack time, the amount of time the compressor waits to start working”
(From the youtube video ”Compressor attack | iZotope Pro Audio Essentials”).

I mailed iZotope and asked about it and got this response:
”iZotope compressors handling of Attack is the same as any other manufacturer’s. To be more exact, Attack is the amount of time the compressor takes to fully compress the signal based on the compressor’s other parameters (threshold, ratio, etc.)”.

Reasons of confusion

It seems that this misconception of the attack time sometimes could just be a result of poor descriptions or maybe just a bad choice of words. In this case I wouldn't use the word "wait" to describe the attack time. This really confused me into believing that they thought that their own compressor worked in a way that it (and other) compressors doesn't work.  

compressor ratio

Pictures like the one above I believe could also confuse people. It describe the functionality of the ratio in a compressor and though it is a great infographic you have to keep in mind that this only applies to the ratio. Therefore this could be a wrongfully depicted infographic of the complete compressor functionality including the attack time. If the attack time was present it would have made a smoother transition after the threshold.

attack time compression

The picture (above) shows the wrong and the right functionality of the attack time set to 100ms with a ratio of 2:1. 

Don’t take my word for it

Universal audio:
"Attack time… refers to the time it takes for the signal to become fully compressed after exceeding the threshold level."

Waves: (about their ssl g comp)
”Attack controls the quickness of the compressor’s response to changes”

SSL: (About Duende native bus comp)
”The attack time is the time taken for the compressor to carry its gain reduction when it is presented with a level above it's threshold”

Elysia: (about the Mpressor)
”Attack: the transient response of the compressor. It determines the time the mpressor needs to reach 10 dB of gain reduction”

Bob Katz: (from his book "mastering audio")
”Attack time is the time it takes for a compressor to implement full gain”

iZotope:
"Attack time, the amount of time the compressor waits to start working”

Wikipedia (About ”Dynamic range compression”)
The attack is the period when the compressor is decreasing gain in response to increased level at the input to reach the gain determined by the ratio.

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