Now, for the basic two rules:

Rule #1: There are no rules. If you want to use a compressor/limiter of any type, shape and size in your music, then go ahead and use it.

Rule #2: When in doubt, don't use it!

How can you tell when you have enough compression?

Discussing sound in print is like describing colors to a blind person, but let me try. Here's a simplistic example.... supposing there are two sonic qualities of music, one called punchy, the other smooth.. Let's say that some music sounds better punchy, other music sounds better smooth. Let's also assume for this example that you can achieve punchy or smooth sound through different amounts and types of compression, or not using compression at all.
In general, try to avoid overall compression in the mix stage if: you're mixing punchy music (the type of music that needs punch), perhaps using some individual compression on certain instruments or singers---and the mix already sounds punchy (good) to you. you're mixing smooth music, and your mix already sounds smooth. you play a well-recorded CD of similar music, and your CD in the making already sounds good (or better than) the CD in the player. your music already seems to accomplish the sound you are looking for.

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Technical reasons to avoid overall compression on your album:

Save decisions on overall compression and individual tune equalization for an expert CD mastering house because:

1. The mastering house will have a more appropriate compressor with the proper attack, ratio, and release times exactly right for your music. If you mixed to digital tape, they will probably use a 24-bit digital compressor for the purpose.
2. They will likely be more experienced than you about the compromises, advantages and disadvantages of applying overall compression.
3. The mastering house can program that compressor with precision, adjusting it optimally for each tune in question. You're working out of context (without having the perspective of the entire album) by attempting these sorts of decisions during mixing.
4. The mastering house will be able to monitor your "CD in the making" using a calibrated monitoring system so that they know exactly how loud your "CD in the making" is compared to other CDs of similar music. For more information on loudness, see my article Level Practices in Digital Audio.
5. A good mastering house will be able to do all of this in a non-destructive, non-cumulative manner. In other words, after making a reference CD, they will be able to undo anything you are unhappy with, whether it be compression, EQ or levels. Whereas, most digital audio editing stations can only perform destructive EQ or compression, only with 16-bit wordlength, with a consequent loss of resolution as long internal words are either dithered (resulting in a veil if further processed), rounded (slightly better than truncated), or truncated to 16 bit. For further information, see my article The Secrets of Dither.
6. For the same technical reasons, it is not a good idea to use a digital compressor (or any digital processor) on your material before sending it for mastering. If you do feel the need to insert one of these boxes, for example, to give a demo CD to your client, be sure to also make a non-processed version to prepare for the mastering house. It is likely that the mastering house will have a fresher-sounding, more effective approach at polishing your material, and it's self-defeating if they have to try to undo what was done.
7. If you apply overall compression to your music, and your choice of compressor was wrong (e.g., the compressor you chose caused subtle pumping or breathing, loss of transients, loss of life or liveliness, etc. These are typical symptoms of "compressor misuse" on tapes I have received), the mastering house will have a difficult or impossible time attempting to undo the damage. As I've mentioned, mastering is like whittling soap; it is hard to undo compression. However, I do have some tricks up my sleeve (grin) that can restore some life to squashed tapes.

Dynamic Range--The Ups and Downs of Music

Before we can study the art of compression, we must learn to appreciate the power of music's dynamic range. How does music grab our interest? For short periods (about the length of a "single" played on the radio or in the disco), power and loudness can grab our initial attention. But at home, variety of dynamics maintains our interest for long periods of time. Good music written for a long-term musical experience contains a judicious mixture of variety and similarity in dynamics. A production which is relentlessly loud (or relentless in its sameness) can become boring very fast. At the age of 10, I learned the lesson of Franz Joseph Haydn's Surprise Symphony, the first composition to teach the importance of dynamic contrast. Musical genres that depend on constant sameness become old very fast. Disco died because it became boring, and I'm convinced that overcompression (which eliminates dynamic contrast) contributed to its death, by creating a continuously loud, boring dynamic.

Films provide an ideal framework to study the creative use of dynamic range. The public is not consciously aware of the effect of sound, but it plays a role in a film's success. I think the movie The Fugitive succeeded because of its drama, but despite an aggressive, compressed, fatiguing sound mix. From the beginning bus ride, with its super-hot dialog and effects, all the crashes were constantly loud and overstated, completely destroying the impact of the big train crash. I can hear the director shouting, "more more more" to the mix engineers. Haven't they heard of the term "suspense"? In contrast, the sound mix of '97's biggest movie, Titanic, is a masterpiece of natural dynamic range. The dialog and effects at the beginning of the movie are played at natural levels, truly enhancing the beauty, drama and suspense for the big thrills at the end. Kudos to director James Cameron and the Skywalker Sound mix team for their restraint and incredible use of dynamic range. That's where the excitement lies for me.

Compressors as Tools To Manipulate Dynamic Range

Compression is a tool; when used by skilled hands, it has produced some of the most beautiful recordings in the world. A lot of music genres are based on the sound of compression, from Disco to Rap to Heavy Metal. And a skilled engineer may intentionally use creative compression to paint a mix and form new special effects; this intended distortion has been used in every style of modern music. This is analogous to the work of the greatest visual artists; many painters are quite capable of producing a natural-looking landscape, but have abandoned that medium to create abstractions which at first glance look like the fingerpaint work of a six-year-old. But a skilled observer realizes what the master artist is communicating. The keys here are intent and skill. Too often, in music, unskilled compression can squash sound, remove the life, vigor and impact, and replace it with boring mush. Many engineers don't know what uncompressed, natural-sounding audio sounds like. It actually takes more work and skill to make a natural-sounding recording than an artificial one. In audio as in the visual arts, first learn to paint naturally; then and only then can you truly understand the art of creating distortion. Learn where compression is useful, and where it does a disservice to the music. A compressed production may sound good on a boombox, but when reproduced on a high-fidelity system, it can sound overbearing and ultimately lifeless. That's why we may need to separately mix "single" and album cuts.

Compressors are commonly used in recording (tracking), mixdown, and in mastering. Everyone has his own style of working with compressors and there are no rules. However, before you make your rules, start by working without any compressors! This learning process will teach you to make better-sounding music later on; the compressor becomes a tool to handle problems, not a crutch or substitute for good recording techniques. First, learn about the natural dynamics and impact of musical instruments, then begin to alter them with compressors (which can include using compression to create special effects). Every 5 years or so, give yourself a reality check...try making a recording or mix with little or no compression. You'll rediscover what I call the microdynamics of music. It's a real challenge, but a refresher course may point out that less compression will buy you a more open, more musical sound than you've previously been getting.

Watch For These Compression-related Pitfalls

Tracking. When tracking vocalists (who have a habit of belting now and then), a well-adjusted compressor can sound reasonably transparent, and most engineers agree the cure is better than the disease. But watch out for a "closed-in" sound, "clamping down" when the vocalist gets loud, or loss of clarity or transparency. Compare IN versus BYPASS before committing to tape. Match levels to make a fair comparison. If you notice too much degradation, maybe it's time to consider a different compressor or change the settings you are using. The sound should be open and clear...remember that no amount of equalization in the mixdown can substitute for capturing a clear sound quality in the first place. This is true for all the lead instruments, including trumpets and electric guitars. If possible, put the uncompressed sound on a spare track---it may save your life. If there's any "rule," most engineers would agree to save the decision on drum and percussion compression until mixing. There are always exceptions---every piece of music is unique. Just remember, you cannot undo the damage of overcompression, so be careful about compression during tracking.

Mixdown. There are two possible places to apply compressors during mixdown. The first is on individual instruments or stereo pairs; the other is on the console mix buss.

For individual tracks: The same precautions apply to the use of compressors in mixing as in tracking. Start fresh each time---free yourself of preconceptions. Although you compressed the bass on 9 out of the last 10 albums, maybe this time you won't need a compressor. Each musician is an individual. In general, the better the bass player, the less compression you will need to use, and the greater the chance that compression will "choke up" his sound. Get to know the sound of your instrumentalists. What is your mixing philosophy? Are you trying to capture the sound of your instrumentalists or intentionally creating a new sound? While more and more music is created in the control room, it's good practice to know the real sound of instruments; learn how to capture natural sound before moving into the abstract.