Many compressors also have a Knee parameter, which sets how fast the compressor will begin to compress after the signal reaches the threshold level (see Figure 1.1). A low value (sometimes it’s measured in dB, so 0dB would be the lowest) means that the compression will begin instantly at threshold. A higher setting will gradually ease in the compression, which may sound better on some types of program material where the compression is meant to be less apparent.
When a compressor operates, it actually, decreases the gain of the signal, so there’s another control that allows the signal to be boosted back up to its original level or beyond called Make-Up Gain or Output (see Figure 1.1).
All compressors have a gain-reduction meter to show how much compression is occurring at any given moment (see Figure 1.1). This can look like a normal VU or peak meter, but it reads backwards. In other words, 0 dB means that the signal is below threshold and no compression is taking place, and any travel to the left or down into the minus range shows the amount of compression that’s occurring. For example, a meter that reads -6 dB indicates that there is 6 dB of compression taking place at that time.
Many compressors also have a feature known as a Sidechain (sometimes called a “key” input) which is a separate input into the compressor so that other signal processors can be connected to it (see Figure 1.1). This sidechain can have a number of useful purposes, such as when an EQ is connected to it to make a makeshift de-esser. If only the frequencies in the upper range are boosted, the loud “S” sounds from a vocalist will be attenuated when they exceed the compressor’s threshold.
You can also connect a delay, reverb, or any other signal processor to the sidechain to create some unusual, program-level dependent effects. Because a sidechain isn’t needed for most everyday compressor operations, many manufacturers select not to include sidechain connections on hardware unit, but may include one on the plugin version.
A sidechain, can also be used to “duck” or attenuate another instrument when a new one enters. For instance, if you connected a send of a vocal track into the sidechain of a compressor inserted on a loop track, the loop would lower in volume whenever the vocal entered and then return to its normal level when the vocal stopped. This is what happens at an airport when the music is automatically lowered for a gate announcement and then returns to its normal level when the announcement is finished.
How To Hear the Compression
How many times have you tweaked the parameters of a compressor but you can’t really hear any real changes to the sound?
In most cases the answer is because you need very well-trained ears, which is something really difficult to achieve that even after several years of mixing or producing music.
But there is a trick that can help you to learn the difference between a compressed and an uncompressed sound:
Step 1: Start with the attack time set as slow as possible (probably around 500 ms), the release time set as fast as possible (probably around 0.1 or 0.2 seconds), and the threshold set as high as possible so no compression is triggered.
Step 2: Decrease se the threshold until the meter shows some compression, then turn the attack faster until the sound of the instrument begins to dull, which now means that you are compressing the transient portion of the sound envelope. Stop increasing the attack time at this point and even back it off a little so the sound stays crisp and maintains its clarity.
Step 3: Adjust the release time so that after the initial attack, the volume goes back to at least 90% of the normal level by the next beat. If in doubt, it’s better to have a shorter release than a longer one although you may hear the compressor pumping if it’s set too fast.
Step 4: If you want the compression to sound smooth and controlled, select a lower ratio of around 2:1. If you want to hear the compressor or you want the sound to be punchier, select a compression ratio above 4:1.
Step 5: Bypass the compressor to see whether there’s a level difference. If there is, increase the gain or the output control until the volume is the same as when it’s bypassed.
Step 6: Add the track to the rest of the mix and listen. Make any slight adjustments to the attack and release time as needed.
Remember that the idea of setting the compressor’s attack and release is to make it breathe with the pulse of the song.
The 4 Types of Compressors
The main 4 types of compressors are:
- VCA (voltage–controlled amplifier): it’s an emulation of the old VCA compressors. It perfoms better on the percussive elements of a track, because it is a fast-type of compressor. If you need to preserve the transient of a sound this is best choice.
- Tube: the exact opposite of the VCA. They are a valve-type compressor so they are very slow. Tube compressors are perfect for sounds that don’t have an important transient. They introduce a lot of color, infact it isn’t rare to find someone that uses a Tube type compressor with a ratio of 1:1 just only to give a new form to the sound.
- Opto (optical compressors): a light bulb and a photocell are used as the main components of the compression circuit. The time lag between the bulb and the photocell give it a distinctive attack and release time (like in an LA-2A). They are very neutral compressors.
- FET (field effect transistor): it works with the electrical field as a whole, and gain changes are the result of electrical charges in addition to voltage.
Final Thoughts About Compression
Compression is a key element in most modern productions and having a clear understanding of how it works is important. That way you will be able to achieve the sound that you’re looking for. Remember that a Pro Music Producer knows the result that he wants to achieve before tweaking any knob or pushing any button in his DAW. With that said, practice as much as you can with compression and if you forget something you can always go back to this Article and read it again.
Thanks for reading the article!