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When I listen to and play around with a number of effects other than compression (reverb, delay, etc), I have no problem hearing how altering them changes the sound of the music.

With compression it's not as apparent, though. I know that the purpose of compression is to bring down (or in some cases bring up) the different audio levels in a piece according to the sound I desire. However, when I'm just playing around with it, it's still hard to hear how it changes the sound of the music.

Can you explain to me the effect that changing the compression of a song has?

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    What kind(s) of track(s) are you applying compression to? It's a lot harder to hear compression on distorted electric guitar and synthesizers. Clean, natural sounds react more to compression. Most compression is actually designed to not be obvious. Try extreme settings. – Todd Wilcox Mar 6 '17 at 14:31
  • In case it's helpful, here's an example of compression on a piano: youtu.be/yQKiDI4yZKQ?t=1m39s - the compressor bringing down the peaks and bringing up the quiet bit means the piano doesn't die away as fast as it "naturally" should. As a pianist, this is very unsettling, but I hear it a lot in pop/dance. Apparently "Lady Madonna" by The Beatles also has very strong compression on the piano to give this unnaturally sustained sound (but I couldn't find a version online that's not a cover). – cloudfeet Mar 6 '17 at 16:47
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    There's also something called "side-chain compression" which is often used for artistic effect in dance music, where the peaks of one track (e.g. the drums) cause a dip in other tracks (example). As people have said below, you also need to have the ratio set to something other than 1, and the limit/threshold low enough that the compressor is actually active. – cloudfeet Mar 6 '17 at 16:52
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Compression isn't as obvious when you're just playing with it, compared to reverb, delay etc, as you've already found. It really comes into its own when used in a working situation; for instance, it's very common to put quite a lot of compression on a lead vocal track, and in fact if you don't do that, the vocals will sound rather odd, compared to what you're used to hearing from your general music listening. Using compression allows the singer go from soft to really letting rip, so you get the impression of extreme emotion, but within a dynamic range that the equipment can handle. This was more of an issue in the vinyl days, of course, but it has become so much part of what we think recorded music 'sounds like' that its use has become ubiquitous.

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With compression turned on, you may well not hear any differences. It's designed to keep volume to within certain levels, at the top and bottom. So, if what you're playing doesn't exceed the top level that's set, or sound quieter than the lowest threshold set, it effectively has nothing to do, and that's just what it does - nothing.

When the thresholds are set much tighter, that's when you'll hear it kicking in better.

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Compressors are interesting because part of their goal is to be subtle. The basic idea of compressors is control over volume. Many instruments pick up different colors of sound when they are played loud, especially the human voice. Musicians often want to take advantage of this palette of colors in their music, but there's a challenge. If you have multiple players in a band, they have to pay attention to their relative volumes. Likewise, if you are playing a song which has louds and softs, they all have to fit in a dynamic range. This can be particularly difficult if music is to be recorded and played in noisy areas later, an issue well known by anyone who listens to classical music in a car.

The implementation is simple. When the volume reaches a threshold, circuitry begins to "limit" it by decreasing the volume. This happens relatively quickly (on the order of a few ms, so on the order of a few cycles of a 1kHz tone), the actual rate being defined by an input on the compressor called the "attack." Likewise there is a corresponding "release" which says how fast the compressor returns to normal volumes.

One of the nice things about compression is that it retains much of the underlying sound color. This gives you control over the dynamics while giving the musician control over the tone. However, it isn't perfect. The effect is similar to multiplying the signal by a "volume" channel, which means in frequency space the effect is a convolution. This smears the frequencies in the input a bit. If you're listening to try to detect the sound of a compressor, this smearing is the audible effect. Depending on your settings it may be a major or a minor player in the final sound.

One of the most common things that is compressed is a voice. This has two effects. One is that it gives the musician access to a larger volume range, and thus the unique sounds that come from the human voice as it changes volume, but frees them from having too much actual dynamic range change. The other is that that smearing effect makes the vocals feel a bit fuller, like reverb does. Whether you want this depends on your music. Most rock bands will compress the vocals, while an opera singer may seek a compression-free sound so that the clarity of their voice shines through.

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Compressors are very broad subject. There are various compressors with different abilities (limiters, sustainers dynamic compressors). Getting used to noticing a compression takes some time. Also your rig may contain a lot of natural compression already (humbuckers, amplifier settings).

At first set attack to min (if you have one). Sustain to max and level according to taste. Play gently and aggressively with and without compression.

There are also side effects of compression. Some frequency bands feel more compressed by others so it changes sound. There are also compressors that try to compensate that.

The studio compression tries to be as invisible as possible. The mild compression will emphasize dynamic playing instead of killing it.

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