QuickTip: Sweetening Sound Effects

There are a few rules you have to follow when editing audio.

One of the most important is that you should always cut sound clips on a 'Zero Crossing'.  What's a zero crossing? -- well, digital sound is just a series of numbers, CD sound for example is stored as numbers ranging from -127 to +127.  The number Zero is the same as silence.  The further away from zero the number values get, the louder the sound.

In most programmes, audio waveforms are drawn with Zero along the middle of the track, with +127 at the top and -127 at the bottom (assuming you're using CD quality sound).  If you zoom right into the waveform, you'll see that the sound wave goes up and down passing through Zero over and over again.  In the screenshot from Final Cut Pro 7 below,  Zero happens to be along the dark purple line and a single frame of video is highlighted in black:

Each point where the waveform passes through Zero is a 'Zero Crossing' and this is where you should ideally cut the audio.

But there are a couple of problems.  Firstly, FCP 7 only lets you make cuts exactly on video frames.  Chances are there won't be a zero crossing exactly at the beginning or end of the frame.  The next problem is that wherever you have two sound clips together that don't both start and end at Zero, you'll get a nasty 'click' on the sound track where they join.  So how to fix this?

The easiest and most effective way is simply to add a subtle 'Fade-In' and 'Fade-Out' effect to every single Sound FX clip.  That way you can guarantee that every clip will start and end on Zero and there will be no chance of getting a glitch on the sound track.

You could use audio transitions from the Effects menu to add the Fade-In and Fade-Out but there is another way which is worth knowing about.  Make sure you have clip overlays turned on and put a Fade-In and Fade-Out on one clip like this.

Next select the clip and go to Edit > Copy

Now select all of the other sound effects clips by pressing A for the arrow tool and drawing a box around them in the timeline.  When they are all selected, go to Edit > Paste Attributes

The Paste Attributes window will open up.  We only want to copy and paste the 'Levels' keyframes we put on the first clip.  We also want to make sure that the keyframes are squashed or stretched to the length of the other clips otherwise they might drop off the end of the clip.  Make sure "Scale Attribute Times" is ticked and then tick "Levels" as the only attribute we want to paste:

Click OK and you should end up with Fade-In and Fade-Out on every Sound FX clip with no more worries about bad clicking on the audio track.