Sound travels through the air in the form of waves. Waves are basically audible disturbances or vibrations of the air. A decibel (db) is the basic unit to measure volume of the power of a wave. Waves also disturb the air in cycle; for example, a wave from the ocean every five minutes or so. Sound waves work the same way, only on a more rapid pace. Frequency measures how fast a wave moves through its cycle. Frequency is measured in hertz---the higher the frequency the higher the sound.
When you mix a track, make sure your listener can hear every element of that track. This may seem like a simple question of where to set the volume fader on each track, but it's not that simple. Each voice or instrument has a dynamic range, meaning that a singer or musician will not perform at one static decibel level during the whole time. A compressor is an effect used to control this problem. It works by reducing the volume of a sound by a certain number of decibels every time the sound gets too loud.
Equalization or EQ shapes the sound of your tracks by boosting, (making louder) or cutting (making softer) certain frequencies. For example, boosting a higher frequency will give you a brighter sound, while cutting the same frequency will make the sound more dull. This is how you "season" your music. For example, you can make a high hat cymbal sound brighter if you boost the frequency around 1000 Hz. If you want to make a bass drum sound deeper you want to boost the frequencies between 60 to 240 Hz.
Cutting frequencies is as important as boosting them. Recordings can sound cluttered when multiple tracks are recorded in the same frequency range. To solve this problem, cut frequencies that are not necessary for a particular track. In the examples above, cut the high frequencies in the bass drum track and the low frequencies in the high hat track.
If you watch a band play, you will never find all of the instruments in a straight line behind the singer. Instead, you might find the piano to the right, the bassist to the left and the drums in the center of the stage. It works the same way in recording. Panning is the process of putting the different parts of a song in the appropriate position in the stereo image. Panning something right makes it sound as if it's coming from the listener's right; panning left makes the recording sound as if it's coming from the listener's left.
When panning, the first thing to remember is what not to pan. The kick drum, the snare drum, the bass or bass instrument make up the foundation of every track and should be, in most cases left in the center--though there are some terrific recordings where the entire drum set was panned hard left or right. Other than that, you can pan things where you want them. You do not have to pan everything 100 percent to get a nice effect. As a general rule stay between 10 and 90 percent left or right.
Picture the band on stage again. Not only would the band not be lined up behind the singer, they also would not be standing shoulder to shoulder in a horizontal line. The singer would be up front, the drums would be in the back and the other musicians would be some where in between them. You want to give the same effect by creating space when mixing. Reverberation or reverb is the best way to accomplish this.
Reverb basically adds an echo to your sound source. Think of it this way: if you were bind-folded and placed in an unidentified space, you could guess where you were in a bathroom or a cave by listening to the echo in the room. In mixing, reverb makes your sound signal appear to be in a certain space by creating the echo you here in that space.
Reverb can add gloss to your sound, but too much of it can make your song sound washed out. Use it sparingly. Two controls that will help clean up your tracks are decay and pre-delay. Decay is how long the echo will last and pre-delay is how long it takes to hear the reverberation after the original source. A shorter decay time and a pre-delay time greater than will instantly clean up tracks. Also remember to use little to no reverb on bass tracks and constant percussion tracks such as high hats.
In the mastering stage, you use the same effect units (compression and EQ) to create a complete album. As with mixing, make sure that no track is louder than the other with compression--unless you are out to achieve a certain effect in the dynamics, such as a crescendo of violins. You also can use EQ to make tracks sound similar to each other.
When mastering, make sure that no frequency overpowers the others. Make sure your mid to bass to tremble ratio sounds good to your ears; also make sure the ratio is fairly constant throughout your CD. Bass frequencies are the hardest to control so make sure to spend some time working in that time. A good mastering program is T-RackS Deluxe. It runs about $400 and gives you the sound of equipment worth five to 10 times as much money.
Make sure your music sound good before mastering. Make sure the vocal is prominent and understandable and all the foundational elements are clear and loud enough.
This is general information about mastering. Take note that mastering is a craft all its own; it is recommended that you let a professional master your project. Mastering prices can range from $50 a track to hundreds of dollars an hour. While this is pricey, mastering is definitely worth it.
Finally, listen to as much recorded music as you can and compare it to your own work. Listen to your mixes in multiple systems to get an accurate representation of your work.