Switching tracks suddenly in your audio/video projects can be really jarring for the audience. Crossfades can help make natural-sounding transitions between audio tracks, and you can really take advantage of them if you know a little about how sound works.
What is a Crossfade?
Odds are you know what a fade is, though maybe not by name. When an audio track starts with silence and the volume rises up out of nowhere, that’s called a “fade-in.” When a track slowly lowers its volume until it’s nothing but silence, it’s called a “fade-out.” How “sharp” a fade is directly involves how much volume is lost or gained over what period of time. Sharper fades happen quickly while duller or steadier fades take a long time. This is what a fade-out looks like visually:
A fade-in looks similar.
A crossfade is essentially doing both simultaneously to two separate tracks. The first track slowly fades out and the second fades in, but instead of there being silence in between, it happens concurrently. It usually sounds like you opened the door to another room with different music, then went through it and closed the door behind you.
Why is it Useful?
Many songs use fading techniques to great effect when starting or ending, or at particular parts in the song. The same goes for video; it’s useful to be able to fade in to a crowd’s cheer, or fade out of an original audio source in favor of narration of some kind. Crossfading is valuable because it allows these changes to happen quickly without being jarring, without introducing silence, and while sounding smooth and more natural. DJs often use this technique while matching beats of two different songs to produce a continuum of sound, while editors often use crossfades (as well as fade-ins and fade-outs) to make the introductions of different sound components sound less “sudden” and more natural.
You can group crossfades into three vague categories, and each sounds very different from the others.
Mid: Each track is linearly faded. You can download a short sample track here: mid crossfade. Visually, it looks like this:
You can see that the volume increase/decrease looks steady. Depending on the volume of the original tracks, this sounds more or less even.
High: The faded-out track starts fading slowly, then picks up speed as time goes on. The faded-in track, on the other hand, picks up volume fairly quickly and that increase of volume slows down over a period of time. You can download a short sample track here: high crossfade. Visually, it looks like this:
The volume changes look like bulges here, with the effect that the both tracks have a high volume through the duration of the fade, and there is still some abruptness to it.
Low: The faded-out track drops its volume pretty quickly and the speed of this drop slows down over a period of time. The fade-in track starts gaining volume slowly but it picks up speed as time goes on. You can download a short sample track here: low crossfade. Visually, it looks like this:
The changes here look like concave cuts. For the duration of the fade, both tracks have a reduced volume until the original track is almost completely gone. The effect feels almost like a lull (but lacking complete silence) and the volume then builds up quickly again after, almost like a swoop.
When the two tracks crossover, their volume builds. For mid-level crossfades, at halfway through the transition the volume of each track is half. Low-level crossfades are below half volume halfway through the transition, and high-level crossfades are above half-volume halfway through the transition.
Sound Differences in Crossfades
Sound is measured in Bels, or more commonly, as fraction of that unit: decibels. Human hearing is very sensitive to acute changes in sound. Just like how we can hear very low frequencies (like 20 Hz) and very high frequencies (like 20,000 Hz), we can hear very soft sound and very loud sounds. In fact, our ears have a sensitivity from 1 to 130 decibels, which is to say that the loudest sound you can hear is roughly 10 trillion times loader than the softest sound you can hear! As such, what appears to be a “linear” change in volume is actually logarithmic. In crossfades, if you want to mess with the rate of change of volume, you need to more aggressively change it. It helps to see things visually.
Linear Crossfades in Audacity
In Audacity, it’s easy to add linear crossfades. Align the two tracks you want to crossfade in the timeline, either by editing or by using the time shift tool. When you’re lined up, select a portion of the track you want to fade out. Go to Effect > Cross Fade Out.
Then, in the next track, select the portion you want to fade in. Go to Effect > Cross Fade In.
You can delete the rest of the first track if you’re done with it. Be careful with the track that you’re fading in, though, as deleting it will move it back to the beginning. You can either use the time shift tool to bring it back to where it needs to be, or better yet, just convert the first part of the track to silence.
Crossfading High or Low
Making high or low crossfades isn’t automated in Audacity. One way to easily do it requires the use of the Envelope Tool.
The Envelope Tool will allow you to change the volume of any track without actually changing the amplitude. As the actual sound wave isn’t changed, the source file will remain untouched. You can add multiple points to further shape the necessary change. After selecting this tool, click on your track, and drag to change the level of volume.
Each click will add a new handle in the form of a white dot that you can move. Just manually shape the curve to about what we described above. Of course, your track may have different needs. If your track’s volume itself changes, then you can compensate or ignore it. As always, do what sounds right.
Crossfading has a variety of uses for both audio and video editing. Depending on what type of crossfade you do, you’ll be able to achieve a variety of effects for different purposes. Now that you know how crossfades work, you can choose which one works best in your projects. After all, it’s all about choice isn’t it?
Note: The music used in the sample tracks is by Talvin Singh; “Traveller” and “Butterfly” from the album OK
Other articles in the Audio Editing series:
The How-To Geek Guide to Audio Editing: the Basics The How-To Geek Guide to Audio Editing: Basic Noise Removal How To Add MP3 Support to Audacity The How-To Geek Guide to Audio Editing: Cutting, Trimming & Arranging
