As simple as I can make it -

First you set a "Threshold" - any signal below that level is not affected by the compressor, any signal above that point gets reduced by the amount determined in the next step -

Secondly you will set a "Compression Ratio" - if you set it for instance at a 4:1 ratio that means that for every 4 dB the incoming signal goes above the "Threshold" only 1 dB will pass through to the output.

The benefit is - now you can keep the loud passages from getting out of control, which lets you turn up the quieter parts, knowing you have the compressor watching the peaks don't get too loud.

What gets 'compressed' is the dynamics. It is lessening the difference between the loudest and quietest parts of the track. If you know what I mean....

This is a very general overview I hope that helps.

ya man thanks a lot! I've been curious about what it is and the funny thing is I have been wondering if there was a way I can do exactly what the compressor does.

thanks again man

That's the best elementary description of compression I've yet to see/hear.

As someone who initially avoided compression like the plague for fear of being "destructive" to the audio, it really demonstrates what would have been a great introductory answer.

As someone who has made progress, but still has a lot to learn about the when and why of compression -
In my experience (and that of other relative beginners), compression is the new "black art", much as mastering has been (and still is).

We hear what a powerful tool it is & all the things it can do, but without being shown and *hearing* its uses by a trained professional, it seems to be some magical formula.
How it impacts the actual *sound* often gets lost.

When you're starting out, you just want to get what you hear recorded.
Then you want it to actually sound like you hear, and down the rabbit hole you go.

To those of us here asking more questions than answering, remember there's a reason we use the word "engineer". There's a lot of variables involved, and the final "scientific" judge is the ear. That's why when we ask a question, the answers are often more questions - providing a specific, precise question w/ all relevant information will provide more specific, helpful answers.
I've been guilty of asking the question the wrong way myself.

Thanks to the people of this forum and some kindly, experienced engineer friends, I've learned a lot about the recording/mixing/mastering/all the other detailed sciences involved in creating a "record" - yet I have a mountain more to learn.

Hope this perspective helps beginners understand how to ask the right questions, and veterans to understand the cloud of confusion certain topics (acoustics, compression, etc) present to the less experienced.

Sorry, turned a simple question w/ a simple answer into a long treatise - it just seemed to capture some of the trials/victories I've had in my brief (so far) journey.
When you're just venturing into this world, it's hard to find good help.

Compressors & limiters can actually be used to exaggerate the attack of the sound of numerous instruments. With hardware based units, attack time can be set so that the initial transient passes through before the limiter has time to respond. So while it can tame overall levels it can also accentuates your transients.

Software dynamics processing frequently includes "look ahead" capabilities which, through time delay, allows the gain reduction process to be prepared for the transient before it gets there. I really don't like "look ahead" dynamics processing. It will definitely beat the life out of any transient dynamics. These types of limiters are much more appropriate for broadcast transmitters as opposed to quality recordings. I hate them. They're awful.

Conversely, hardware-based limiters with peak detectors can respond to transients in a more aggressive way than RMS based hardware limiters. Even when you are limiting with RMS limiters, the peaks are still flying through. Your selection largely depends upon the sound you are looking for and want to achieve. I frequently turnoff look ahead in software-based dynamics processing whenever possible. It's the slop that makes a limiter sound good. Not how fast or accurate it may be.

I love compression & lots of it
Ms. Remy Ann David

And there's a great, less basic answer.

Remy - are you speaking specifically of limiting (you only mention compressors in the first line)?

I know where "limiting" kicks in is a subject of debate, but there is a big difference between a 1:5 and 1:12 ratio.

Also, any basic suggestions on RMS vs peak, based on style?
I think that could be a great starting point for experimenting w/ compression.

I'll be looking more closely at how my software comps work (look ahead, specifically). I only have an RNC and a dbx166 for outboard, so a lot of my comp will end up being in the box.

You've made me decide to focus more on dynamics vs level in my mixes - I'll leave the leveling to the mastering engineer, or at worst, deal w/ that later.

Thanks

I guess I missed the last part of his question:

A compressor with the right settings will thicken up your guitar sound and make it fuller sounding and easier to mix, because you don't have the extremes in volumes. A guitar should have dynamics, but using a compressor should make the volume more manageable.

For your drums, a compressor can give them more punch. It can really thicken a kick drum by softening the impact at the top of the spike. You should be able to get a meatier, more full-bodied sound without clipping.

Think of it as someone riding the fader and when the signal coming in gets hotter, it instantly pulls down the fader to a proportional level. When the signal settles back down, the fader goes back up to where it started.

If you have a decent compressor and learn what it can do, it will quickly become your best audio weapon. Then you'll want to keep adding more compressors to get a more professional sound. As far as I'm concerned, I'll take as many compressors as I can get. Just about everything can benefit from some amount of compression. But, I won't have any of them stomping on the signal so hard that it sucks the life out of the track. Dynamics and the air in-between the notes are usually what makes music interesting to me. Just don't go mad with power over-compressing things and you'll be OK.

Good luck.

If you want to discuss "the right settings" we'll continue...

right now I am using logic 8 and there is a compressor channel i can use

should I just use this or is it better to have an "external" compressor (if there is such)

The compressor plug-in in Logic should be perfectly fine.

An external hardware compressor would be a good addition whenever you can afford one. Either inserted into a channel of an external mixer, or inline somewhere, it would allow you to get a better signal into the DAW.

[Personal recommendation] If you do buy a hardware compressor - don't buy the first one you can afford. A lot of the really really cheap ones are practically useless.] Both of the compressors soapfloats referred to in his post ( the RNC and dbx166xl ) would be a good place to set your sights. Both are reasonably priced and very useful.

Some mic-pres may have a compressor function too, so that might be an option. But for now you're off to the races with what you already have.

Have fun!