Sox: silencing the first 100msec of audio file then fading in

Question

With sox, I need to silence (ie. mute) the first 100msec (or 4800 samples) of a multichannel audio file, then commence a rapid fade-in over 40 msecs. I've not been able to work out how to do it. Do fade-ins have to be linear with sox or are there other options?

Suggestions please?

Answer 1

Muting and fading within the audio stream are not directly supported, but in your case, you can do it in these steps:

1. Remove the first part of the audio with trim 0.1. (You could write trim 4800s as well, if your sample rate is 48000 Hz.)
2. Do the fade-in on what is now the start of the audio (0.10–0.14 of the original audio), with fade 0.04.
3. Prepend the audio with silence to make up for what you removed, with pad 0.1.

So, the complete command is:

sox infile outfile trim 0.1 fade 0.04 pad 0.1

By default, fade uses a logarithmic fade curve starting at –100 dB. However, you can choose a different curve by specifying a letter directly after fade, as in fade t 0.04. There are t for linear, q for quarter-sine, h for half-sine, p for parabolic, as well as l for logarithmic (the default, but in case you want to make it explicit).