rsvp/noise.sh

## noise.sh
#!/usr/bin/env bash
#              bash 4.1.5(1)     Linux Ubuntu 10.04           Date : 2019-01-02
#
# _______________|  noise : ambient Brown noise generator (cf. white noise).
#
#           Usage:  noise  [minutes=59]  [band-pass freq center=1786]  [wave]
#                          ^minutes can be any positive integer.
#                           Command "noise 1" will display peak-level meter.
#
#    Dependencies:  play (from sox package)

#  Brownian noise, also known as Brown noise or red noise, is the kind of signal
#  noise produced by Brownian motion, hence its alternative name of random walk
#  noise. The term "Brown noise" does not originate from the color, but from
#  Robert Brown who discovered Brownian motion.  Brown noise is produced by
#  integrating white noise.  The sound is a low roar resembling a waterfall or
#  heavy rainfall. We shall filter it through a band-pass, then add effects
#  to mellow the output for ambience.

#  Volume oscillation (amplitude modulation) is used to simulate artificially
#  long ocean waves. Try some of the free online services listed at the end
#  of this file to hear other types of oscillation.
#
#        Our goal is to block out distractions for calm concentration.


#  CHANGE LOG  Get LATEST version from https://gist.github.com/rsvp/1209835
#  2019-01-02  Fix sox warning by adding "-t alsa" appropriately for
#              "play WARN alsa: can't encode 0-bit Unknown or not applicable"
#  2011-09-14  Add volume amplification to compensate for effects previously
#                 applied. Use peak-level meter to avoid clipping.
#  2011-09-13  Add bass and treble tone controls.
#                 at loud volumes, low frequency thumping may be annoying,
#                 so reduce gain on bass. Treble can be used to reduce
#                 harshness from the high frequencies.
#                 Adjust default wave for tremolo to perfectly cycle
#                 within the one-minute sample.
#  2011-09-12  Add end notes on the Cognitive Science aspects.
#  2011-09-11  Repeat use of one-minute segment to cut CPU usage by 95%.
#                 Fix tremolo to give very slow wave oscillation in volume.
#                 (Thanks to xguse for his gist at github.)
#                 Constant volume introduces tension psychologically.
#                 Code posted at https://gist.github.com/1209835
#  2011-09-10  First version based on 2009 article by Tom Swiss, and
#                 subsequent comments. See below for relevant portions.

#           _____ Prelims
set -u
#   ^ unbound (i.e. unassigned) variables shall be errors.
#           Example of default assignment:    arg1=${1:-'foo'}
set -e
#   ^ error checking :: Highly Recommended (caveat:  you can't check $? later).
#
# _______________     ::  BEGIN  Script ::::::::::::::::::::::::::::::::::::::::

minutes=${1:-'59'}
repeats=$(( minutes - 1 ))
center=${2:-'1786'}

wave=${3:-'0.0333333'}
#         ^increase for more volume oscillation, but suggest no higher than 0.20
#          (and no lower than 0.0166667). Its value should consider the 60
#          seconds duration of the repeated sample.

noise='brown'
#     ^your choice: 'white', 'pink', 'brown', 'tpdf'
#     where tpdf stands for Triangular Probability Density Function (cf. dither).
#     N.B. - white and pink noise have higher frequencies than Brown.

len='01:00'
#   ^CONSTANT one minute. (Format for specifying time length is hh:mm:ss.frac)
#     ___ATTN___ We first pre-compute one minute of audio output to file,
#     then playback repeatedly as necessary to satisfy minutes argument.
#     This dramatically cuts CPU usage by 95% after the first minute.


#  For DEBUGGING: "noise 1" shows the peak-level meter; also instant production.
if [ $minutes -eq 1 ] ; then
     progress='--show-progress'
else
     progress='--no-show-progress'
fi


echo " ::  Please stand-by... sox will 'play' $noise noise for $minutes minute(s)."
#          FYI  Channels: 2 @ 32-bit,  Samplerate: 48000Hz.


play $progress  -c 2  --null  -t alsa  synth  $len  ${noise}noise  \
     band -n $center 499               \
     tremolo $wave    43   reverb 19   \
     bass -11              treble -1   \
     vol     14dB                      \
     repeat  $repeats


#  #    Previously published one-line versions misused tremolo:
#  play -c 2 --null synth $len brownnoise band -n 1800 1400 tremolo 500 .1 reverb 50
#  play -c 2 --null synth $len brownnoise band -n 2500 4000 tremolo 20 .1 reverb 50
#  play --null synth $len brownnoise band -n 1200 200 tremolo 20 .1 reverb 20
#  play --null synth $len brownnoise band -n 1200 200 tremolo 20 .1


#            _____ ARGUMENTS explained via "man sox"

#      "-t alsa"  Needed to fix this sox warning (on 2019-01-02):
#           "play WARN alsa: can't encode 0-bit Unknown or not applicable"
#    https://groups.google.com/forum/#!topic/linux.debian.bugs.dist/jCqdwFWPUKk
#    Note: arg placement is critical, e.g. right after "play" will fail.
#
#       -q, --no-show-progress
#           Run in quiet mode when SoX wouldn't otherwise do so; this is
#           the opposite of the -S option.
#
#       -S, --show-progress
#           Display input file format/header information, and processing
#           progress as input file(s) percentage complete, elapsed time,
#           and remaining time (if known; shown in brackets), and the
#           number of samples written to the output file.  Also shown is a
#           peak-level meter, and an indication if clipping has occurred.


#       -c 2
#           Two channels. Without this, the output is not stereo.


#       -n, --null
#           This can be used in place of an input or output filename to
#           specify that a `null file' is to be used.  Note that here,
#           `null file' refers to a SoX-specific mechanism and is not
#           related to any operating-system mechanism with a similar name.
#           Using a null file to input audio is equivalent to using a
#           normal audio file that contains an infinite amount of silence,
#           and as such is not generally useful unless used with an effect
#           that specifies a finite time length (such as trim or synth).


#       synth
#           Some noise options: whitenoise, tpdfnoise, pinknoise, brownnoise.


#       band [-n] center[k] [width[h|k|o|q]]
#           Apply a band-pass filter.  The frequency response drops
#           logarithmically around the center frequency.  The width
#           parameter gives the slope of the drop.  The frequencies at
#           center + width and center - width will be half of their
#           original amplitudes.  band defaults to a mode oriented to
#           pitched audio, i.e. voice, singing, or instrumental music.
#           The -n (for noise) option uses the alternate mode for un-
#           pitched audio (e.g. percussion).  Warning: -n introduces a
#           power-gain of about 11dB in the filter, so beware of output
#           clipping.  band introduces noise in the shape of the filter,
#           i.e. peaking at the center frequency and settling around it
#
#  Consider this for centering the band-pass...
#
#         Freq (Hz)    Octave      Description
#         16 to   32   1st         Human threshold, the lowest pedal
#                                     notes of a pipe organ.
#         32 to  512   2nd to 5th  Rhythm frequencies, where the lower
#                                     and upper bass notes lie.
#        512 to 2048   6th to 7th  Defines human speech intelligibility,
#                                     horn-like or tinny sound quality.
#       2048 to 8192   8th to 9th  Gives presence to speech, where labial
#                                     and fricative sounds lie.
#       8192 to 16384  10th        Brilliance, the sounds of bells and the
#                                     ringing of cymbals. In speech, sound
#                                     of letter "S" (8000-11000 Hz)
#            http://en.wikipedia.org/wiki/Audio_frequency
#
#  Avoid the really low frequencies which will produce disturbing rumble.


#       tremolo speed [depth]
#           Apply a tremolo (low frequency amplitude modulation) effect to
#           the audio.  The tremolo frequency in Hz is given by speed, and
#           the depth as a percentage by depth (default 40). Increasing
#           the depth gives wider range between soft and loud volumes.


#       reverb [-w|--wet-only] [reverberance (50%) [HF-damping (50%)
#           [room-scale (100%) [stereo-depth (100%)
#           [pre-delay (0ms) [wet-gain (0dB)]]]]]]


#       bass|treble gain
#           Boost or cut the bass (lower) or treble (upper) frequencies of
#           the audio using a two-pole shelving filter with a response
#           similar to that of a standard hi-fi's tone-controls.  This is
#           also known as shelving equalisation (EQ).
#           gain gives the gain at 0 Hz (for bass), or whichever is the
#           lower of ∼22 kHz and the Nyquist frequency (for treble).  Its
#           useful range is about -20 (for a large cut) to +20 (for a
#           large boost).  Beware of clipping when using a positive gain.
#
#           When played loud, you may hear thumping bass lines in the
#           case of brownnoise with effects. Reduce annoyance accordingly.


#       vol gain
#           Apply an amplification or an attenuation to the audio signal.
#           Unlike the -v option (which is used for balancing multiple
#           input files as they enter the SoX effects processing chain),
#           vol is an effect like any other so can be applied anywhere,
#           and several times if necessary, during the processing chain.
#
#           The amount to change the volume is given by gain which is
#           interpreted, according to the given type, as follows:
#           if dB, then a power change in dB.  When type is dB, a gain
#           of 0 leaves the volume unchanged, less than 0 decreases it,
#           and greater than 0 increases it.
#           Beware of clipping when the increasing the volume.


#       repeat count
#           Repeat the entire audio count times.  Requires temporary file
#           space to store the audio to be repeated. [But where exactly?]


#  _______________ "white noise" generator with sox [edited for code content]
#                   by Tom Swiss, http://unreasonable.org/node/303
#                   January 2007, updated circa September 2009,
#                   included comments through September 2011
#
#  Sox is "the Swiss army knife of sound processing programs." It includes sound
#  generation capabilties for pure tones and white noise.  "Pink noise" is
#  also in sox's bag of tricks. After a bit of experimentation, I found the
#  following shell script produced agreeable results:
#
#  len='7:00:00'
#  play -t sl - synth $len pinknoise band -n 1200 200 tremolo 20 .1 < /dev/zero
#
#       __________ Comments
#
#  Drew Haven: This beats the heck out of "cat /dev/urandom > /dev/dsp". The band
#  filter is nice to take out the pops.
#
#  gi1242: With recent versions of sox, things are a little simpler:
#       play -n synth 60:00 brownnoise
#  produces brown noise for an hour. (Replace brown with pink/white if you
#  prefer. My baby sleeps best with brown).
#
#  Tom Swiss: "Brown" in "brown noise" means Brownian motion. It's also called
#  red noise. I learned something today, hooray!
#
#  Adrien Beau, 30 January 2011: You can replace the "-t sl -" and "< /dev/zero"
#  parts with the "-n" option, so your sox invocation becomes:
#                  ^= --null (for null file)
#       play -n synth $len pinknoise band -n 1200 200 tremolo 20 .1
#  The brown noise sounds the best in my opinion.
#
#  Dennis Murczak, 5 May 2011: I adapted the line to a "my neighbor is having a
#  party and I need to study" situation:
#       play -c 2 -n synth pinknoise band -n 2500 4000 reverb 20
#  The band pass is centered on human voice frequencies and wide enough to also
#  cover most of the musical frequency range, without producing annoying
#  high-pitched noise. The slight reverb adds a background/ambient quality for
#  less distraction.


exit 0
# _______________ EOS ::  END of Script ::::::::::::::::::::::::::::::::::::::::


#           _____ Free ONLINE alternatives
#
#  Simply Noise for white, pink and brown/red noise generator; uses Flash:
#       http://simplynoise.com   (App is $0.99)
#
#       [Flash consumes about 30 times more than our script in CPU usage!]
#
#  PlayNoise for white, pink, and brown noise generator; uses Javascript/HTML5:
#       http://playnoise.com
#
#  Random.org has 33-second sample audio files containing perfect white noise.
#       The randomness comes from atmospheric noise, which is more natural
#       than the pseudo-random number algorithms. Such files could serve as
#       input to sox for further signal processing.
#       http://www.random.org/audio-noise/


#  _______________ HOW WHITE NOISE WORKS by Saabira Chaudhuri
#                  WSJ 31 Aug 2011
#  http://online.wsj.com/article/SB10001424053111904199404576538274265089288.html
#
#  What people think of as "white noise" may actually be pink noise or brown
#  noise or any number of other colors.  Sound is associated with a color based
#  on where it falls on an audio spectrum of high to low frequencies. White noise
#  contains random sounds across all frequencies and "sounds very much like a
#  hiss because everything is changing in every sample," says Daniel Ellis,
#  associate professor of electrical engineering at Columbia University in New
#  York.  Pink noise, on the other hand, blends some high and lower frequencies,
#  so it sounds like a hiss with a low rumble, he adds. Brown noise shifts to the
#  lower end of the spectrum and sounds like rumbling.
#
#  The most effective noise at blocking out other sounds is white noise because
#  it covers the largest range on the spectrum, says Andrew Catellier, an
#  electronics engineer at the Boulder, Colo.-based National Telecommunications &
#  Information Administration, which publishes a glossary of sounds' color
#  classifications.  Distinguishing noises by their frequencies is a useful tool
#  for scientists and engineers working on practical applications, such as
#  building a cellphone system or an ultrasound machine.  Sound is classified by
#  its audible frequencies and associated with a color based on where it falls on
#  the spectrum of high to low frequencies. White noise is unique in that it's
#  random and includes all frequencies -- akin to how white light has all the
#  colors in the spectrum.
#
#  Calling sounds like rain or thunder white noise is somewhat of a misnomer, but
#  the makers of downloadable apps and sleep machines use the term anyway.  White
#  noise and other soothing sounds, once mainly played on machines to aid
#  nighttime sleep, are increasingly helping make daytime hours more serene.
#  [White noise is a common synthetic noise source used for sound masking by
#  a tinnitus masker.]
#
#  After HeavyDutyApps released an app called Sleep Pillow Ambiance to help
#  people sleep, it quickly realized that many customers used it during the day
#  as well. "The usage varies from people who need help concentrating while
#  working in noisy environments, commuters who need a break from train noise and
#  travelers that need a peaceful environment," says Benny Shaviv, chief
#  executive of the Westchester, N.Y.-based company. The $1.99 app has had more
#  than 1.6 million downloads, says Mr. Shaviv. "By January we were among the Top
#  50 apps in the Healthcare and Fitness category in iTunes."
#
#  Most popular are sounds from nature: rain, wind, waves crashing on the beach
#  and crickets, Mr. Shaviv says. But the app also includes some unexpected
#  sounds, such as cold drink with ice, brushing hair and horse running in field.
#  Thunderstorm is the most popular downloaded noise.
#
#  Developers of these apps say they frequently get requests for new sounds.
#  Steven Jian, co-owner of Simply Noise, has received requests for the sound of
#  passing cars and airport noises. Shaviv of HeavyDutyApps got a request for a
#  sonar noise from a former sailor who served on a submarine.  Todd Moore,
#  founder and CEO of TMSoft, the maker of an app called White Noise, says he
#  created a hair-dryer sound at one woman's request. "She told me that she could
#  not sleep without listening to it and that she had burned [out] six hair
#  dryers over the years."
#
#  Daytime white-noise listeners say the sounds serve two main purposes: to block
#  out distractions and lessen sounds that cause anxiety, such as sirens.
#  "Certain types of noises can be relaxing," says Robert C. Fifer, director of
#  audiology and speech language pathology at the University of Miami. White
#  noise can be used to create a more relaxing working environment, masking
#  sounds and promoting a sense of privacy, he says.
#
#  One small study examined white noise in a classroom environment. The research,
#  led by Goran Soderlund and Sverker Sikström of Stockholm University, looked at
#  51 students at a secondary school in Norway and found that those who normally
#  had difficulty paying attention performed better when white noise was added to
#  the classroom. The findings were published last year in the journal Behavioral
#  and Brain Functions.
#
#  The authors theorized that white noise boosted neural activity, helping the
#  brain work more efficiently. The study predicted that white noise could help
#  children with attention deficit hyperactivity disorder (ADHD) learn to focus
#  on schoolwork better.

#           _____ REFERENCES
#
#  "The effects of background white noise on memory performance
#       in inattentive school children"
#  Göran BW Söderlund1, Sverker Sikström, Jan M Loftesnes and EJ Sonuga-Barke
#  Behavioral and Brain Functions 2010, 6:55 doi:10.1186/1744-9081-6-55
#  Published: 29 September 2010
#  http://www.behavioralandbrainfunctions.com/content/6/1/55/abstract


#  Re: Brown noise, see http://en.wikipedia.org/wiki/Brownian_noise


#  vim: set fileencoding=utf-8 ff=unix tw=78 ai syn=sh :
	#!/usr/bin/env bash
	# bash 4.1.5(1) Linux Ubuntu 10.04 Date : 2019-01-02
	#
	# _______________\| noise : ambient Brown noise generator (cf. white noise).
	#
	# Usage: noise [minutes=59] [band-pass freq center=1786] [wave]
	# ^minutes can be any positive integer.
	# Command "noise 1" will display peak-level meter.
	#
	# Dependencies: play (from sox package)

	# Brownian noise, also known as Brown noise or red noise, is the kind of signal
	# noise produced by Brownian motion, hence its alternative name of random walk
	# noise. The term "Brown noise" does not originate from the color, but from
	# Robert Brown who discovered Brownian motion. Brown noise is produced by
	# integrating white noise. The sound is a low roar resembling a waterfall or
	# heavy rainfall. We shall filter it through a band-pass, then add effects
	# to mellow the output for ambience.

	# Volume oscillation (amplitude modulation) is used to simulate artificially
	# long ocean waves. Try some of the free online services listed at the end
	# of this file to hear other types of oscillation.
	#
	# Our goal is to block out distractions for calm concentration.


	# CHANGE LOG Get LATEST version from https://gist.github.com/rsvp/1209835
	# 2019-01-02 Fix sox warning by adding "-t alsa" appropriately for
	# "play WARN alsa: can't encode 0-bit Unknown or not applicable"
	# 2011-09-14 Add volume amplification to compensate for effects previously
	# applied. Use peak-level meter to avoid clipping.
	# 2011-09-13 Add bass and treble tone controls.
	# at loud volumes, low frequency thumping may be annoying,
	# so reduce gain on bass. Treble can be used to reduce
	# harshness from the high frequencies.
	# Adjust default wave for tremolo to perfectly cycle
	# within the one-minute sample.
	# 2011-09-12 Add end notes on the Cognitive Science aspects.
	# 2011-09-11 Repeat use of one-minute segment to cut CPU usage by 95%.
	# Fix tremolo to give very slow wave oscillation in volume.
	# (Thanks to xguse for his gist at github.)
	# Constant volume introduces tension psychologically.
	# Code posted at https://gist.github.com/1209835
	# 2011-09-10 First version based on 2009 article by Tom Swiss, and
	# subsequent comments. See below for relevant portions.

	# _____ Prelims
	set -u
	# ^ unbound (i.e. unassigned) variables shall be errors.
	# Example of default assignment: arg1=${1:-'foo'}
	set -e
	# ^ error checking :: Highly Recommended (caveat: you can't check $? later).
	#
	# _______________ :: BEGIN Script ::::::::::::::::::::::::::::::::::::::::

	minutes=${1:-'59'}
	repeats=$(( minutes - 1 ))
	center=${2:-'1786'}

	wave=${3:-'0.0333333'}
	# ^increase for more volume oscillation, but suggest no higher than 0.20
	# (and no lower than 0.0166667). Its value should consider the 60
	# seconds duration of the repeated sample.

	noise='brown'
	# ^your choice: 'white', 'pink', 'brown', 'tpdf'
	# where tpdf stands for Triangular Probability Density Function (cf. dither).
	# N.B. - white and pink noise have higher frequencies than Brown.

	len='01:00'
	# ^CONSTANT one minute. (Format for specifying time length is hh:mm:ss.frac)
	# ___ATTN___ We first pre-compute one minute of audio output to file,
	# then playback repeatedly as necessary to satisfy minutes argument.
	# This dramatically cuts CPU usage by 95% after the first minute.


	# For DEBUGGING: "noise 1" shows the peak-level meter; also instant production.
	if [ $minutes -eq 1 ] ; then
	progress='--show-progress'
	else
	progress='--no-show-progress'
	fi


	echo " :: Please stand-by... sox will 'play' $noise noise for $minutes minute(s)."
	# FYI Channels: 2 @ 32-bit, Samplerate: 48000Hz.


	play $progress -c 2 --null -t alsa synth $len ${noise}noise \
	band -n $center 499 \
	tremolo $wave 43 reverb 19 \
	bass -11 treble -1 \
	vol 14dB \
	repeat $repeats


	# # Previously published one-line versions misused tremolo:
	# play -c 2 --null synth $len brownnoise band -n 1800 1400 tremolo 500 .1 reverb 50
	# play -c 2 --null synth $len brownnoise band -n 2500 4000 tremolo 20 .1 reverb 50
	# play --null synth $len brownnoise band -n 1200 200 tremolo 20 .1 reverb 20
	# play --null synth $len brownnoise band -n 1200 200 tremolo 20 .1


	# _____ ARGUMENTS explained via "man sox"

	# "-t alsa" Needed to fix this sox warning (on 2019-01-02):
	# "play WARN alsa: can't encode 0-bit Unknown or not applicable"
	# https://groups.google.com/forum/#!topic/linux.debian.bugs.dist/jCqdwFWPUKk
	# Note: arg placement is critical, e.g. right after "play" will fail.
	#
	# -q, --no-show-progress
	# Run in quiet mode when SoX wouldn't otherwise do so; this is
	# the opposite of the -S option.
	#
	# -S, --show-progress
	# Display input file format/header information, and processing
	# progress as input file(s) percentage complete, elapsed time,
	# and remaining time (if known; shown in brackets), and the
	# number of samples written to the output file. Also shown is a
	# peak-level meter, and an indication if clipping has occurred.


	# -c 2
	# Two channels. Without this, the output is not stereo.


	# -n, --null
	# This can be used in place of an input or output filename to
	# specify that a `null file' is to be used. Note that here,
	# `null file' refers to a SoX-specific mechanism and is not
	# related to any operating-system mechanism with a similar name.
	# Using a null file to input audio is equivalent to using a
	# normal audio file that contains an infinite amount of silence,
	# and as such is not generally useful unless used with an effect
	# that specifies a finite time length (such as trim or synth).


	# synth
	# Some noise options: whitenoise, tpdfnoise, pinknoise, brownnoise.


	# band [-n] center[k] [width[h\|k\|o\|q]]
	# Apply a band-pass filter. The frequency response drops
	# logarithmically around the center frequency. The width
	# parameter gives the slope of the drop. The frequencies at
	# center + width and center - width will be half of their
	# original amplitudes. band defaults to a mode oriented to
	# pitched audio, i.e. voice, singing, or instrumental music.
	# The -n (for noise) option uses the alternate mode for un-
	# pitched audio (e.g. percussion). Warning: -n introduces a
	# power-gain of about 11dB in the filter, so beware of output
	# clipping. band introduces noise in the shape of the filter,
	# i.e. peaking at the center frequency and settling around it
	#
	# Consider this for centering the band-pass...
	#
	# Freq (Hz) Octave Description
	# 16 to 32 1st Human threshold, the lowest pedal
	# notes of a pipe organ.
	# 32 to 512 2nd to 5th Rhythm frequencies, where the lower
	# and upper bass notes lie.
	# 512 to 2048 6th to 7th Defines human speech intelligibility,
	# horn-like or tinny sound quality.
	# 2048 to 8192 8th to 9th Gives presence to speech, where labial
	# and fricative sounds lie.
	# 8192 to 16384 10th Brilliance, the sounds of bells and the
	# ringing of cymbals. In speech, sound
	# of letter "S" (8000-11000 Hz)
	# http://en.wikipedia.org/wiki/Audio_frequency
	#
	# Avoid the really low frequencies which will produce disturbing rumble.


	# tremolo speed [depth]
	# Apply a tremolo (low frequency amplitude modulation) effect to
	# the audio. The tremolo frequency in Hz is given by speed, and
	# the depth as a percentage by depth (default 40). Increasing
	# the depth gives wider range between soft and loud volumes.


	# reverb [-w\|--wet-only] [reverberance (50%) [HF-damping (50%)
	# [room-scale (100%) [stereo-depth (100%)
	# [pre-delay (0ms) [wet-gain (0dB)]]]]]]


	# bass\|treble gain
	# Boost or cut the bass (lower) or treble (upper) frequencies of
	# the audio using a two-pole shelving filter with a response
	# similar to that of a standard hi-fi's tone-controls. This is
	# also known as shelving equalisation (EQ).
	# gain gives the gain at 0 Hz (for bass), or whichever is the
	# lower of ∼22 kHz and the Nyquist frequency (for treble). Its
	# useful range is about -20 (for a large cut) to +20 (for a
	# large boost). Beware of clipping when using a positive gain.
	#
	# When played loud, you may hear thumping bass lines in the
	# case of brownnoise with effects. Reduce annoyance accordingly.


	# vol gain
	# Apply an amplification or an attenuation to the audio signal.
	# Unlike the -v option (which is used for balancing multiple
	# input files as they enter the SoX effects processing chain),
	# vol is an effect like any other so can be applied anywhere,
	# and several times if necessary, during the processing chain.
	#
	# The amount to change the volume is given by gain which is
	# interpreted, according to the given type, as follows:
	# if dB, then a power change in dB. When type is dB, a gain
	# of 0 leaves the volume unchanged, less than 0 decreases it,
	# and greater than 0 increases it.
	# Beware of clipping when the increasing the volume.


	# repeat count
	# Repeat the entire audio count times. Requires temporary file
	# space to store the audio to be repeated. [But where exactly?]



	# _______________ "white noise" generator with sox [edited for code content]
	# by Tom Swiss, http://unreasonable.org/node/303
	# January 2007, updated circa September 2009,
	# included comments through September 2011
	#
	# Sox is "the Swiss army knife of sound processing programs." It includes sound
	# generation capabilties for pure tones and white noise. "Pink noise" is
	# also in sox's bag of tricks. After a bit of experimentation, I found the
	# following shell script produced agreeable results:
	#
	# len='7:00:00'
	# play -t sl - synth $len pinknoise band -n 1200 200 tremolo 20 .1 < /dev/zero
	#
	# __________ Comments
	#
	# Drew Haven: This beats the heck out of "cat /dev/urandom > /dev/dsp". The band
	# filter is nice to take out the pops.
	#
	# gi1242: With recent versions of sox, things are a little simpler:
	# play -n synth 60:00 brownnoise
	# produces brown noise for an hour. (Replace brown with pink/white if you
	# prefer. My baby sleeps best with brown).
	#
	# Tom Swiss: "Brown" in "brown noise" means Brownian motion. It's also called
	# red noise. I learned something today, hooray!
	#
	# Adrien Beau, 30 January 2011: You can replace the "-t sl -" and "< /dev/zero"
	# parts with the "-n" option, so your sox invocation becomes:
	# ^= --null (for null file)
	# play -n synth $len pinknoise band -n 1200 200 tremolo 20 .1
	# The brown noise sounds the best in my opinion.
	#
	# Dennis Murczak, 5 May 2011: I adapted the line to a "my neighbor is having a
	# party and I need to study" situation:
	# play -c 2 -n synth pinknoise band -n 2500 4000 reverb 20
	# The band pass is centered on human voice frequencies and wide enough to also
	# cover most of the musical frequency range, without producing annoying
	# high-pitched noise. The slight reverb adds a background/ambient quality for
	# less distraction.


	exit 0
	# _______________ EOS :: END of Script ::::::::::::::::::::::::::::::::::::::::


	# _____ Free ONLINE alternatives
	#
	# Simply Noise for white, pink and brown/red noise generator; uses Flash:
	# http://simplynoise.com (App is $0.99)
	#
	# [Flash consumes about 30 times more than our script in CPU usage!]
	#
	# PlayNoise for white, pink, and brown noise generator; uses Javascript/HTML5:
	# http://playnoise.com
	#
	# Random.org has 33-second sample audio files containing perfect white noise.
	# The randomness comes from atmospheric noise, which is more natural
	# than the pseudo-random number algorithms. Such files could serve as
	# input to sox for further signal processing.
	# http://www.random.org/audio-noise/


	# _______________ HOW WHITE NOISE WORKS by Saabira Chaudhuri
	# WSJ 31 Aug 2011
	# http://online.wsj.com/article/SB10001424053111904199404576538274265089288.html
	#
	# What people think of as "white noise" may actually be pink noise or brown
	# noise or any number of other colors. Sound is associated with a color based
	# on where it falls on an audio spectrum of high to low frequencies. White noise
	# contains random sounds across all frequencies and "sounds very much like a
	# hiss because everything is changing in every sample," says Daniel Ellis,
	# associate professor of electrical engineering at Columbia University in New
	# York. Pink noise, on the other hand, blends some high and lower frequencies,
	# so it sounds like a hiss with a low rumble, he adds. Brown noise shifts to the
	# lower end of the spectrum and sounds like rumbling.
	#
	# The most effective noise at blocking out other sounds is white noise because
	# it covers the largest range on the spectrum, says Andrew Catellier, an
	# electronics engineer at the Boulder, Colo.-based National Telecommunications &
	# Information Administration, which publishes a glossary of sounds' color
	# classifications. Distinguishing noises by their frequencies is a useful tool
	# for scientists and engineers working on practical applications, such as
	# building a cellphone system or an ultrasound machine. Sound is classified by
	# its audible frequencies and associated with a color based on where it falls on
	# the spectrum of high to low frequencies. White noise is unique in that it's
	# random and includes all frequencies -- akin to how white light has all the
	# colors in the spectrum.
	#
	# Calling sounds like rain or thunder white noise is somewhat of a misnomer, but
	# the makers of downloadable apps and sleep machines use the term anyway. White
	# noise and other soothing sounds, once mainly played on machines to aid
	# nighttime sleep, are increasingly helping make daytime hours more serene.
	# [White noise is a common synthetic noise source used for sound masking by
	# a tinnitus masker.]
	#
	# After HeavyDutyApps released an app called Sleep Pillow Ambiance to help
	# people sleep, it quickly realized that many customers used it during the day
	# as well. "The usage varies from people who need help concentrating while
	# working in noisy environments, commuters who need a break from train noise and
	# travelers that need a peaceful environment," says Benny Shaviv, chief
	# executive of the Westchester, N.Y.-based company. The $1.99 app has had more
	# than 1.6 million downloads, says Mr. Shaviv. "By January we were among the Top
	# 50 apps in the Healthcare and Fitness category in iTunes."
	#
	# Most popular are sounds from nature: rain, wind, waves crashing on the beach
	# and crickets, Mr. Shaviv says. But the app also includes some unexpected
	# sounds, such as cold drink with ice, brushing hair and horse running in field.
	# Thunderstorm is the most popular downloaded noise.
	#
	# Developers of these apps say they frequently get requests for new sounds.
	# Steven Jian, co-owner of Simply Noise, has received requests for the sound of
	# passing cars and airport noises. Shaviv of HeavyDutyApps got a request for a
	# sonar noise from a former sailor who served on a submarine. Todd Moore,
	# founder and CEO of TMSoft, the maker of an app called White Noise, says he
	# created a hair-dryer sound at one woman's request. "She told me that she could
	# not sleep without listening to it and that she had burned [out] six hair
	# dryers over the years."
	#
	# Daytime white-noise listeners say the sounds serve two main purposes: to block
	# out distractions and lessen sounds that cause anxiety, such as sirens.
	# "Certain types of noises can be relaxing," says Robert C. Fifer, director of
	# audiology and speech language pathology at the University of Miami. White
	# noise can be used to create a more relaxing working environment, masking
	# sounds and promoting a sense of privacy, he says.
	#
	# One small study examined white noise in a classroom environment. The research,
	# led by Goran Soderlund and Sverker Sikström of Stockholm University, looked at
	# 51 students at a secondary school in Norway and found that those who normally
	# had difficulty paying attention performed better when white noise was added to
	# the classroom. The findings were published last year in the journal Behavioral
	# and Brain Functions.
	#
	# The authors theorized that white noise boosted neural activity, helping the
	# brain work more efficiently. The study predicted that white noise could help
	# children with attention deficit hyperactivity disorder (ADHD) learn to focus
	# on schoolwork better.

	# _____ REFERENCES
	#
	# "The effects of background white noise on memory performance
	# in inattentive school children"
	# Göran BW Söderlund1, Sverker Sikström, Jan M Loftesnes and EJ Sonuga-Barke
	# Behavioral and Brain Functions 2010, 6:55 doi:10.1186/1744-9081-6-55
	# Published: 29 September 2010
	# http://www.behavioralandbrainfunctions.com/content/6/1/55/abstract


	# Re: Brown noise, see http://en.wikipedia.org/wiki/Brownian_noise


	# vim: set fileencoding=utf-8 ff=unix tw=78 ai syn=sh :