@subsection Binaural Beats Using SoX

A binaural beat is an auditory illusion perceived when two different
pure-tone sine waves, both with frequencies lower than 1500 @abbr{Hz,
Hertz}, with less than a 40 @abbr{Hz, Hertz} difference between them,
are presented to a listener dichotically (one through each ear).  For
example, if a 530 @abbr{Hz, Hertz} pure tone is presented to a
subject's right ear, while a 520 @abbr{Hz, Hertz} pure tone is
presented to the subject's left ear, the listener will perceive the
auditory illusion of a third tone, in addition to the two pure-tones
presented to each ear.  The third sound is called a binaural beat, and
in this example would have a perceived pitch correlating to a
frequency of 10 @abbr{Hz, Hertz}, that being the difference between
the 530 @abbr{Hz, Hertz} and 520 @abbr{Hz, Hertz} pure tones presented
to each ear.  For more details, see
@url{https://en.wikipedia.org/wiki/Binaural_beats}.

This module implements a set of user-facing commands for generating
binaural beats.  The commands are organized from high-level commands
that present predefined binaural beats to lower-level commands that
can be used to create new effect sequences.

@subsubsection High-Level Commands For Pre-Defined Binaural Beats

These commands can be called directly to play one of the predefined
binaural beats.  Each of them prompts for a time-scale factor that is
used to determine the duration of the sequence.

@itemize @bullet
@item
@command{sox-rev-up}: A set of binaural beats designed for use
at the start of the day.  Transitions from @emph{Dream} ->
@emph{Think} ->@emph{Act} -> @emph{Focus}.

@item
@command{sox-wind-down}: A set of binaural beats for winding down
at the end of the day.  This can be thought of as the reverse of
@command{sox-rev-up} and the sequence transitions from @emph{Act} ->
@emph{Think} -> @emph{Dream} ->@emph{Sleep}.

@item
@command{sox-turn-down}: Designed for falling asleep.
This sequence starts with a short period of @emph{Dream} before moving
to @emph{Sleep}.

@item
@command{sox-relax}: A variant of the previous sequence,
@code{sox-relax} spends equal time in @emph{Dream} and @emph{Sleep}.

@item
@command{sox-binaural}: Provide a completion-based front-end to
playing any one of the predefined binaural effects (@emph{Delta},
@emph{Theta}, @emph{Alpha}, @emph{Beta}, or @emph{Gamma}.  The
previously defined sequences are built up using these effects.

@item
@command{sox-beats-binaural}: Plays a collection of binaural
beats, prompting for fundamental and beat frequencies for each tone.
The predefined sequences listed earlier were created after first
generating experimental beat-sequences using this command.

@item
@command{sox-slide-binaural}: Prompts for two binaural effects
(see above) and generates a binaural beat that @emph{slides} from the
first effect to the second over a specified duration.

@item
@command{sox-chakras}: Pick amongst one of a predefined set of
sequences designed for @emph{Chakra} meditation.

@item
@command{sox-tone-binaural}: Generate a simple binaural beat
with a single fundamental frequency.

@item
@command{sox-tone-slide-binaural}: Generate a tone that slides
from one binaural beat to another.
@end itemize
