Script to normalise a WAV file's loudness to specific LUFS using ffmpeg

normalise-loudness.rb

#!/usr/bin/env ruby
#
# Script to normalise an audio file's loudness using ffmpeg
#
# Usage: normalise-loudness.rb  <input.wav> <output.wav>
#
# License: https://unlicense.org/
#
# Requires the 'json' ruby gem:
#
#   gem install json
#

require 'open3'
require 'json'

I="-18"     # Integrated loudness target (LUFS)
TP="-2.0"   # Maximum true peak (dB)
LRA="10"    # Loudness range target (LU)



def loudnorm_analyse(src)
	cmd = [
		'ffmpeg',
		'-i', src,
		'-af', "loudnorm=I=#{I}:TP=#{TP}:LRA=#{LRA}:print_format=json",
		'-f', 'null', '-'
	]

  stdout, stderr, status = Open3.capture3(*cmd)
	raise "ffmpeg error: #{stderr}" unless status.success?

  log, json = stderr.split(/\[Parsed_loudnorm_\d+ @ \w+\]\s*/)
  JSON.parse(json, :symbolize_names => true)
end

def loudnorm_normalise(src, dest, params)
	loudnorm_params = {
		'I' => I,
		'TP' => TP,
		'LRA' => LRA,
		'measured_I' => params[:input_i],
		'measured_LRA' => params[:input_lra],
		'measured_TP' => params[:input_tp],
		'measured_thresh' => params[:input_thresh],
		'offset' => params[:target_offset],
		'linear' => 'true',
		'print_format' => 'summary'
	}.to_a.map {|k,v| "#{k}=#{v}"}.join(':')

	cmd = [
		'ffmpeg',
		'-i', src,
		'-af', "loudnorm=#{loudnorm_params}",
		'-ar', '48k',
		dest
	]

  stdout, stderr, status = Open3.capture3(*cmd)
	raise "ffmpeg error: #{stderr}" unless status.success?

  log, summary = stderr.split(/\[Parsed_loudnorm_\d+ @ \w+\]\s*/)
  return summary
end



if ARGV.size < 1
  $stderr.puts "Usage: normalise-loudness.rb <input.wav> [<output.wav]"
  exit(-1)
end

input_filepath, output_filepath = ARGV
if output_filepath.nil?
  output_filepath = input_filepath.sub(/\.\w+$/, '-normalised.wav')
end

if File.exist?(output_filepath)
  $stderr.puts "ERROR: Output file already exists"
  exit(-1)
end

unless File.exist?(input_filepath)
  $stderr.puts "ERROR: Input file does not exist"
  exit(-1)
end


params = loudnorm_analyse(input_filepath)
result = loudnorm_normalise(input_filepath, output_filepath, params)
puts result