FFmpeg loudnorm filter - dual pass loudness normalization example - http://k.ylo.ph/2016/04/04/loudnorm.html

loudnorm.rb

#!/usr/bin/env ruby
require 'optparse'
require 'json'

metadata = {}
TARGET_I = "-16"
TARGET_TP = "-1.5"
TARGET_LRA = "11"
SAMPLE_RATE = "44100"
BITRATE = "128k"

summary = nil
optparse = OptionParser.new do |opts|
  summary = opts

  opts.banner = "Normalize audio loudness."
  opts.banner = "Usage: loudnorm.rb [options] INFILE OUTFILE"

  opts.on('-h', '--help', 'Display help') do
    puts opts
    exit
  end
  opts.on('--title TITLE', String, "Write title to output metadata") do |t|
    metadata[:title] = t
  end
  opts.on('--artist ARTIST', String, "Write artist to output metadata") do |a|
    metadata[:artist] = a
  end
  opts.on('--album ALBUM', String, "Write album to output metadata") do |a|
    metadata[:album] = a
  end
end.parse!

if ARGV.length != 2
  puts summary
  exit
end
infile = ARGV.first
outfile = ARGV.last

# Run the first pass.

ff_args = [
  "ffmpeg",
  "-hide_banner",
  "-i",
  infile,
  "-af",
  "loudnorm=I=%s:TP=%s:LRA=%s:print_format=json" % [TARGET_I, TARGET_TP, TARGET_LRA],
  "-f",
  "null",
  "-"
]

first_pass_out = IO.popen(ff_args, err: [:child, :out])
measured = JSON.parse(first_pass_out.readlines.last(12).join)
first_pass_out.close

loudnorm_string =
  "loudnorm=I=%s:TP=%s:LRA=%s:measured_I=%s:measured_TP=%s:measured_LRA=%s:measured_thresh=%s:offset=%s:linear=true:print_format=summary" %
  [TARGET_I, TARGET_TP, TARGET_LRA, measured['input_i'], measured['input_tp'], measured['input_lra'], measured['input_thresh'], measured['target_offset']]

ff_args = [
  "ffmpeg",
  "-y",
  "-hide_banner",
  "-i",
  infile,
  "-af",
  loudnorm_string,
  "-ar",
  SAMPLE_RATE,
  "-b:a",
  BITRATE,
  "-ac",
  "1",
  "-write_xing",
  "0"
]

if metadata[:title]
  ff_args << "-metadata"
  ff_args << "title=%s" % metadata[:title]
end
if metadata[:artist]
  ff_args << "-metadata"
  ff_args << "artist=%s" % metadata[:artist]
end
if metadata[:album]
  ff_args << "-metadata"
  ff_args << "album=%s" % metadata[:album]
end

ff_args << outfile
exec(*ff_args)