Unlike other effects, compression seems to be the hardest for me to recognize as there's no much of notable change when it's applied, so how do you determine if your drums need to be compressed or not? Would you compress them sperately or the whole mix at once? Is there any case where you compress a sound and not a drum (like a piano for example)? Would there be any channel which doesnt need to be compressed?
When I was learning about compression, I found the answer in its name. "Compression," "Compressor," it is a unit that compresses sound waves. They are used to dynamically control levels within your mix. As far as compressing your drums, they likely need compression if they haven't already been processed (like a pre-made loop etc.). To understand a compressor, it helps to look at one on your DAW, apply it, and see what happens when you adjust the threshold, attack, release, knee, and makeup gain parameters. Play with the threshold parameter first, look at what happens to the needle at the two extremes, listen to the result, and analyze your sound to be able to get the best sound you can.
Compressing separately and the whole mix at once is common practice. The idea of parallel processing is, in essence, how the greatest mix engineers make their songs sound so pristine (along with a whole SLEW of other things). I think the way you're wondering about compressing different channels is good, but you should approach it from a listeners standpoint. Use your ear to determine which channels' dynamic levels need straightening out. Don't just throw compressor inserts around willy nilly, but use them generously because more often than not, something likely needs compression to glue it together with the rest of the channels in the mix.
What is compression?
Imagine you are next to a stereo and its speakers
You are listening to a song. The song is at a nice volume, but suddenly in the chorus the volume is too loud, it hurts your ears.
To remedy this you use the stereo's volume knob to turn the volume down until it's at a pleasant volume again.
Then the chorus part ends, but since you lowered the volume and the non-chorus parts of the song are already at a lower volume, now the volume is too low.
To fix this you use the stereo's volume knob to increase the volume, until it's at a pleasant volume again.
Congratulations, you are a compressor! A compressor does exactly this, but automatically, according to how we tell it to behave.
We as humans are lazy, inaccurate, and slow. At least compared to machines. So instead of doing all this volume control by hand, we use compressors to do it for us, in a way that is more precise and replicable. So you can see compressors as an intelligent volume knob. You can be very specific about how this volume knob behaves, but that's a subject for another question.
Compression seems to be the hardest for me to recognize as there's no much of notable change when it's applied.
There are several reasons (probably infinitely many?) why compression can be hard to recognize.
- There's not much compression going on.
- There's not much amount of compression going on.
- Compression on performers is supposed to sound like the performer is controlling the volume, not like an effect (think of autotune for correction vs autotune for effect).
- You don't know the original dry audio, so how are you trying to notice compression? What are you comparing the compressed audio to?
- Untrained ears don't know where or what to look for.
- Some mixing engineers have made of compression an art, and they became very good at making it sound organic, subtle.
But there are just as many reasons (intinitely many?) why compression can be easy to identify. In that sense, I disagree with the notion that "compression is the hardest to recognize". I don't think there's an effect or process that is intrinsically harder or easier to recognize than others, it all depends on the amount applied, which is always variable.
Compression is often the less understood, and if you don't understand compression it's hard to know what to look for, hence hard to identify. But those are different things.
Compression is always hard to recognize
Compression isn't always hard to recognize, and it's not always expected to be hard to recognize.
- One example is electronic music, where it is often used to make "breathing" effects.
- Are your ears trained? Do you use compressors often? Maybe you are not looking in the right places, at the right times. Maybe compression is more noticeable than you think it is.
So how do you determine if your drums need to be compressed or not?
Experience, trained ears, study (like any other engineering tool, really).
Compressors can achieve a lot of stuff though volume control, so you first need to be able to identify issues with recordings or performances, and know the traditions and conventions of the style you are mixing or mastering for. Some few examples:
- If the drums have parts that are too loud, but there are parts that are perfectly fine. Compressors can leave the soft parts untouched, while reducing the volume of the loudest parts.
- If the drums have some parts too quiet, and parts too loud, you can use compressors to make both parts sound more similar in volume (also known as reducing the dynamic range).
- You can make drums have more punch and accentuate attack too by not compressing them, and suddenly compressing them, in a very short amount of time, then mixing them with the original dry signal.
- In general, you use compressors when you identify a mistake or other areas of opportunity or improvement related to volume and sometimes tone and timbre.
- Performance mistakes are not always involved, a lot of times it is just the sound vision of the producers, engineers, genre, style, listeners, expectations, etc. In that sense compressors are just another sound design tool, often used as makeup, but used as effect just as often.
Would you compress them sperately or the whole mix at once?
You can do either, or both. Both mixing engineers (who work with the tracks separately) and mastering engineers (who work with already mixed songs) use compressors extensively, so most commercial releases do both.
But it depends on the audio, and what you are trying to achieve. Sometimes only the tracks are compressed, sometimes only the whole mix, sometimes only one track, sometimes nothing is compressed. There's no rule.
Is there any case where you compress a sound and not a drum (like a piano for example)?
Yes, you can compress whatever you want, whenever you want. There's an infinite list of cases and justifications. Perhaps the pianist was too loud, and the drummer wasn't.
Compressors can be used, and often are, for any instrument, any audio. There are compressors tuned for specific situations though, but that's for another question.
Would there be any channel which doesnt need to be compressed?
Yes, compression is not always used, or needed. Some jazz and classical stuff has very soft compression applied, or sometimes even no compression at all! (though there's "no compression" compression, similar to "no makeup" makeup.)
Compressors are just tools. Sometimes they are needed, sometimes they are not.
Compressor inserts work by reducing gain after a signal passes a set threshold, using compressed-to-original ratio.
The onset of gain reduction can be fine tuned with an attack ramp, and the duration of the reduction can be fine tuned with the release time.
They can be set to react to peaks, or root mean square amplitude (more akin to VU meters and loudness).
Compressors can be used as a correctional tool, and also creatively.
The ratio of gain reduction can often be turned up to infinity, which transforms the compressor into a limiter (depending on attack, etc.)
There is also a "knee" control which fine tunes the threshold point (interpolates the onset over a range).
A "compressor" is a family of processors which includes upward and downard compressors, and upward and downward expanders, depending on gain and threshold being positive or negative.
Compressing a track usually makes softer parts louder and louder parts softer, and that will usually bring out the spectral characteristic of a track within a mix (the loudness range being more compact will change perception of spectral characteristics).
The only thing you should be wary of is unnecessary compression.