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+#!/usr/bin/env bash
+#              bash 4.1.5(1)     Linux Ubuntu 10.04           Date : 2011-09-11
+#
+# _______________|  noise : ambient Brown noise generator (cf. white noise).
+#
+#           Usage:  noise  [minutes=59]  [band-pass freq center=1786]  [wave]
+#                          ^minutes can be any non-zero integer.
+#
+#    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.
+
+#  [/] - could increase volume oscillation (amplitude modulation) to simulate 
+#           waves or breathing pattern -- but this could be too relaxing.
+#           Hear the free online services listed at the end of file.
+#           Our goal here is calm concentration in a noisy environment.
+
+
+#  CHANGE LOG  Code posted at https://gist.github.com/1209835
+#
+#  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.
+#  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:-'0.02'}
+#         ^increase for more volume oscillation, but suggest no higher than 0.20
+
+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.
+
+
+echo " ::  Please stand-by... sox will 'play' $noise noise for $minutes minutes."
+
+
+play --no-show-progress -c 2 --null synth $len  ${noise}noise  band -n $center 499 \
+     tremolo $wave 37  reverb 19  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"
+
+#       -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, gives a 
+#                                horn-like or tinny quality to sound.
+#  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, the sound of 
+#                                the letter "S" (8000-11000 Hz)
+#        Source:  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)]]]]]]
+
+
+#       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   $0.99 app available
+#  [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
+
+
+#           _____ References
+#  Re: Brown noise, see http://en.wikipedia.org/wiki/Brownian_noise
+