Linux audio recipes

This is a collection of small techniques for manipulating audio data on the command line. They have been compiled over a number of years from my work on bioacoustic and ecoacoustic datsets, although they should prove useful to anyone who works with audio files.

Most of these should work on other Unix-like systems such as macOS.

Unless you are unusually curious, the best way to use this guide is by seaching (Ctrl+F) in your web browser for what you are trying to achieve.

Tools

The two most common command line tools are Sound eXchange (sox) and FFmpeg, which share some basic functionality. These are widely availabe on Unix-like operating systems (and even Windows). FFmpeg is primarily a tool for video (which is often a useful tool for the acoustician) although it can also be used to manipulate audio.

sox: Sound eXchange

sox Installation

sox installation on Ubunu and other Debian derivatives

sudo apt install sox

sox installation on macOS

First install homebrew.

brew install sox

ffmpeg

ffmpeg installation

ffmpeg installation on Ubunu and other Debian derivatives

sudo apt install ffmpeg

ffmpeg installation on macOS

First install homebrew.

brew install ffmpeg

Getting help

Both sox and ffmpeg come with extensive documentation.

man sox

man ffmpeg

Getting audio file information

sox --info file
soxi file

sox file.mp3 -n stat

Playing audio

The play command is provided by sox.

Play an audio file

play in_file

Play an audio file with n dB of volume reduction

play in_file gain -n

Manipulating channels

Mix stero to mono

sox stereo.wav -c 1 mono.wav
sox stereo.wav mono.wav channels 

ffmpeg -i stero.wav -ac 1 mono.wav

Split stereo to seprate mono

sox in_file.wav out_left.wav remix 1
sox in_file.wav out_right.wav remix 2

Combine two mono files to stereo

sox -M left_file_ right_file_ stereo_file

Combine files by mixing

sox -m in_file_1 in_file_2 in_file_3 out_file

Extract select channels from multichannel file

# Extract channels 2, 4, and 5
sox in_file out_file remix 2 4 5

Manipulating sample rate

Change sample rate of wav file

ffmpeg -i high.wav -ar 44100 low.wav

sox high.wav -r 44100 low.wav
sox high.wav low.wav rate 44100

sox high.wav -r 48k low.wav
sox high.wav low.wav rate 48k

Manipulating bitrate (mp3)

Convert to 256kbps

sox in_file -C 256 output.mp3

Manipulating bit depth

Change bit depth of wav file

ffmpeg -i high.wav -c:a pcm_s16le low.wav

Change bit depth of an entire folder

Source files in raw, output files in wav, convert to 16 bit 44.1kHz Wave file.

for f in raw/*;
do
  ffmpeg -i "${f}" -vn -c:a pcm_s16le -ar 44100 "wav/${f#raw/}.wav";
done

Manipulating file types

Convert file type

ffmpeg in_file.wav out_file.mp3

sox in_file.wav out_file.mp3

Normalisation

Normalisation of amplitude

sox in_file out_file norm

Normalisation of amplitude to -0.1dB

sox in_file out_file norm -0.1

Manipulating amplitude/volume

Increase volume

sox -v 2.0 quiet.wav loud.wav

Decrease volume

sox -v 0.5 loud.wav quiet.wav

Getting sections of a file

Splitting a file

Split a file into n second chunks

sox in_file out_file trim 0 n : newfile : restart

Get first n seconds of a file

sox in_file out_file trim 0 n

Trim n seconds from end of file

sox in_file out_file trim 0 -n

Concatenating files

Concatenating a small number of files

sox in_file_1 in_file_2 in_file_3 out_file

Concatenating large numbers of files

This uses sox to concatenate any matching wave files in a directory.

Note that the wav file format is limited to 4GB due to a 32-bit header.

# Delete temp.wav if it exists
if test -f temp.wav; then
  rm temp.wav
fi

# Delete out.wav if it exists
if test -f out.wav; then
  rm out.wav
fi

# List Wave files and sort by name                                                                                     
ls *.wav | sort -n | while read l;
do     
   echo "$l"                                                                                                                                                          
   if [ ! -f temp.wav ]                                                                                                                                         
   then                                                                                                                                                       
      cp $l temp.wav                                                                                                                                            
   else                                                                                                                                                       
      sox temp.wav $l out.wav
      rm temp.wav                                                                                                                                
      cp out.wav temp.wav                                                                                                                                       
   fi
done

Manipulating playback speed

Increase/decrease playback speed by factor of n

#n can be a decimal value
sox in.wav out.wav speed n

Generating noise

Generate n seconds of white noise

sox -n output.wav synth 1 noise

Generate n seconds of pink noise

sox -n output.wav synth 1 pinknoise

Generating sine waves

Generate n seconds of 440Hz sine tone

sox -n output.wav synth 1 sine 440

Generate an n second swept sine from 2Hz to 20kHz

sox -n swept-sine.wav synth 10 sine 2/20000

Generating a Dirac delta function (Dirac impulse) with tail padding

sox -n -r 48000 dirac.wav synth 1s square pad 0 47999s

Miscellaneous transformations

Reversing audio

sox in_file out_file reverse

Visualisation

Spectrogram

sox in_file -n spectrogram -o out_file.png