FIG. 2: Equal-power crossfades frequently work best for seamless musical transitions.
Types of Fades
There are three types of fades: fade-ins, in which the signal grows in amplitude from silence to its nominal level over a period of time; fade-outs, in which the signal gradually drops to silence; and crossfades, which are a combination of the first two. Crossfades smooth over edit points by fading out the old material and fading in the new simultaneously. Crossfades can last from a few milliseconds to several seconds and require sufficient material on either side of the edit point to work with. Crossfades can be used to segue smoothly from one song to the next, to fix mistakes in musical performances by allowing notes to be replaced seamlessly, and to create strange new sounds by grafting different elements into a unified whole.
Fades and crossfades have amplitude curves, which specify the rate of attenuation over the length of the fade. The simplest example is a linear curve (which is actually a straight line), in which the rate of attenuation stays constant over the length of the fade. Linear fades are perfectly useful, but they are far from being the only option. Logarithmic fades start quickly, then slowly taper off toward the end. Exponential fades start slowly, then move quickly toward the end. S-type fades start quickly, slow down toward the middle, then speed up again toward the end. Each of these fades has a subtly different effect and is useful in various situations.
Many DAWs allow you to create custom fades by adjusting these basic fade shapes or by drawing your own free-hand fades. When creating crossfades, you can specify the shape of the fade-out curve and fade-in curve separately, although symmetrical shapes are commonly used. Linear (or equal-gain) crossfade curves are often specified as the default crossfade type. In my experience, though, equal-gain crossfades often leave the impression of a hole or dip in amplitude at the edit seam. Logarithmic (or equal-power) crossfade curves don't exhibit that characteristic, because the in and out regions are both being played at higher amplitudes around the edit (see Fig. 2). This is the curve I use as my default curve.
Many songs end by fading out over a vamp or repeating chorus. Pay special attention to these fades, because they play an important role in the listener's final impression of the song. Start by thinking about where you want the fade to end. I tend to like fades that end at a musical boundary, such as the end of a chord progression. Another effective technique is to fade gradually over the length of a chorus, then end the fade after the beginning of the next chorus. This creates the impression that the song is continuing on and on.
When creating song fade-outs, use a mouse or control surface to create fader automation; don't create a computer-generated fade region. The manual fade has a more human, musical character, making the fade feel like part of the performance.