Some useful tips about sox rec

I’ve spent a lot of time experimenting with SOX to do VOX and have gotten it to work reasonably well. I’ve been using Audacity to view the resultant wave form, and have settled on the following SOX command…

rec snd.wav silence 1 .5 2.85% 1 1.0 3.0% vad gain -n : newfile : restart

This will:

wait until it hears activity above the threshold for a half second, then start recording (silence 1 .5 2.85%)
stop recording when audible activity falls to zero for one second (... 1 1.0 3.0%)
trim off any initial silence up to voice detection (vad)
normalize the gain (gain -n)
store the result into a new file (snd001.wav, snd002.wav)
restart the process

Getting the “silence” numbers correct involved a lot of trial and error, and will depend on ambient noise as well as the sensitivity of your microphone. I’m using the microphone in the Logitech QuickCam IM on a Raspberry Pi through USB.

On a side note, this whole thing complains with the following…

rec FAIL formats: can’t open input `default’: snd_pcm_open error: No such file or directory

… until I created this variable in the environment:

export AUDIODEV=hw:1,0

Again - this involved a lot of experimentation with the values for “silence”, and it WILL need some tweaking for your environment.