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So what exactly does it mean to “crossfade audio”? Well before we talk about crossfading, we should probably first talk about just plain old regular fading. What is a fade?
Happily, this is one of those few audio terms that actually sounds like what it means.  Whether you know the term or not, I’m fairly confident that you’ve at least experienced/heard fading going on before.
Pop songs used to end with a fadeout a lot back in the day.  You see it on sheet music all the time. At the end it often says “repeat and fade.”  It just means that instead of an abrupt or definite ending, the song would continue. The volume would just get lower and lower until it was inaudible, at which point it would end.  It fades out.

Fades

Often on TV or movie soundtracks you hear the audio fade in (as opposed to the above example of fading out).  The music (usually) or other audio will start out inaudible and rise in volume gradually until you can hear it at full volume.
This is the same idea for audio items in our recording projects.  We use it as describe above sometimes, especially when producing voice overs or narrations that have background music.
The music frequently fades in before the narrator starts talking.  If the music is not going to continue behind the voice for the duration of the program, as in a podcast or radio show intro perhaps, the music usually fades out just after the narrator starts speaking.
But the most common use of fading when doing multitrack recording projects is simply to allow audio items to start and finish more smoothly.  If the edges of an audio item do not fade in or out at least a little bit, you get clicks and pops due to the sudden stops and starts of the item.

Crossfades

Now we know what fading is. So let’s talk the crossfading. It is when one bit of audio fades out at the exact time as another bit of audio is fading in (see the picture at the beginning of the article).
Let’s say you are recording a vocal track, and you mess up.  What most of us do is wait a second while the program continues recording, and read/sing the part you messed up a second time, and then proceed with the recording.
Things often run more smoothly this way, as opposed to stopping and starting over all the time.  However, doing it like this requires that when you have finished recording, you go back and cut out that messed up part. Crossfades to the rescue!
A common way to deal with mistakes is to simply highlight the screw-up, and cut it out.  This leaves a blank space there (unless you have ripple editing turned on – see our post on THAT here:  https://www.homebrewaudio.com/what-is-ripple-editing), creating 2 audio items, the part before the cut and the part after it.  Now you’ll want to join these two items so you drag the later audio to the left to connect it up like a train section to the end of the earlier audio item (see figure 1).

Crossfades Are About Seamlessness

In order for the audio to sound seamless to the listener, you’ll usually want to slightly overlap the end of the earlier audio and the beginning of the later audio item.  But that will still frequently give you an abrupt and unnatural sounding transition.  So you want the end part of item 1 to be fading out exactly at the same time as the beginning part of audio 2 is fading in.

So most programs, including Reaper (shown in the pictures in this article), default to automatically crossfading any audio items that overlap.  Take a look at Figure 2 where I have connected the same audio items shown in figure 1.  I’ve drawn a box around where the item on the left is fading out as the item on the right is fading in.  The red lines represent the volume of the items.  It sort of looks like an “X.”  I did not have to do anything extra because as soon as the items started overlap, Reaper automatically executed the crossfading.
This kind of crossfading is not done as an effect, where you’d hear something slowly fade in or out pleasantly.  I had to zoom in pretty far to even see the red fade lines. That’s because the fades happen very quickly.  This type of crossfading is used as a tool to help avoid the popping and clicking that frequently accompany the cutting and deleting of audio.

You can edit each fade if you wish. You could change the shape of the curve or make the fade longer or shorter. Do this by hovering your mouse over the edge of the audio item until the “fade” tool icon appears (Figure 3).  Then right-mouse-click to change the shape of the fade. Or simply drag the edge of the item to change the length of the fade.

Items Do Not Have To Be On the Same Track To Crossfade Them

One last note – items do not have to be on the same track to use the crossfade idea.  In Figure 4 I used the same two audio items, but I put the second one on a different track.
They still overlap in time. The end of the first one still fades out just as the beginning of the second one is fading in.  The only difference is that they are on different tracks.
So now you know what crossfading means. And you know how you can use it to smooth out the transitions between audio items. Crossfading is a good tool to give you better audio recordings.