The Distortion Product Otoacoustic Emissions (DPOAE) challenge the concept of the sound as externally constructed entity and place the listener as both a recipient and an agent of the auditory process. The listener cease to be indexically oriented towards an external sound and navigates towards physiological conditions of audition.

In 1856, Hermann von Helmholtz was the first to identify sum and difference tones as products of auditory distortion. In earlier experiments - dated back to 1748 and 1754 respectively - the organist, Georg Sorge (1703–1778), and the violinist, Giuseppe Tartini (1692–1770)  observed the phenomena of "a third tone" which can be attest to the same phenomena (For a history of combination tones before Helmholtz, see 'The Theory of Beats and Combination Tones, 1700-1863 by Carlton Maley Jr, V) 

 

Helmholtz used two prolonged tones with harmonic interval of less than an octave played at high volume. When two tones - a lower and a higher - sounded together, a third (combination) tone appeared. The observed tone had the frequency equal to difference of the two primary tones -     e.g. the combination of 100 and 250 Hz played at a strong intensity created a
tone of 150 Hz. The combination tone is not emitted externally, but it is clearly, objectively perceived by the listener. On the note regarding objectivity, in 'Experimental Music Since 1970' Jennie Gottschalk poses an interesting question: "If the sound happens exclusively within the hearing mechanism, is it truly objective?". Perhaps "it could be called physically subjective, yet mentally objective".

 

An initial explanation for these phenomena was that high intensity levels forced the linear mechanics of the physical auditory system into a nonlinear region. The nonlinearity thus was thought to be located in the middle ear or in the basilar membrane [Helhmholtz, 2013]. A shift in paradigm were findings of Thomas Gold and David T. Kemp which proved that ear rather than being a passive, should be considered as an active system, and that parts of the inner ear - specifically, the outer hair cells of the basilar membrane - act as an active amplification system. Nowadays the phenomenon of auditory distortion form an area of research on otoacoustic emissions, where the “distortion” is defined as a positive feedback mechanism within the cochlea called cochlear amplifier - a sort of "a tiny loudspeaker in the ear" [Ashmore, 2010]. In the field of medical practice the otoacoustic emission phenomena has been used, among other purposes, to test hearing in infants (see: https://www.ncbi.nlm.nih.gov/pubmed/12764789).     
 
Jonathan Kirk and Christopher Haworth have provided an extensive list of examples from 20th-century music where the auditory distortion product had been explored as an intentional material strategy. 

 

Maryanne Amacher contributed greatly to research on auditory distortion and their creative application into sound art. Her compositions such as 'Head Rhythm 1" and "Playing Thing 2" utilised the Triadex Muse - a digital sequencer instrument designed and build by Edward Fredkin and Marvin Minsky at the Massachusetts Institute of Technology (MIT). By generating fast-peaced interlocking patterns of short sine tones at very high volumes Amacher achieved a presence of distinct separate musical stream from the auditory distortion. In the liner notes to 'Sound Characters (Making of the Third Ear)' a CD released by Tzadik, Amacher gives a description of the experience of these tones: 

When played at the right sound level, which is quite high and exciting, the tones in this music will cause your ears to act as neurophonic instruments that emit sounds that will seem to be issuing directly from your head ... [my audiences] discover they are producing a tonal dimension of the music which interacts melodically, rhythmically, and spatially with the tones in the room. Tones “dance” in the immediate space of their body, around them like a sonic wrap, cascade inside ears, and out to space in front of their eyes ... Do not be alarmed! Your ears are not behaving strange or being damaged! ... These virtual tones are a
natural and very real physical aspect of auditory perception, similar to the fusing of two images resulting in a third three dimensional image in binocular perception ... I want to release this
music which is produced by the listener ...

Maryanne Amacher had laid foundation for systematic exploration of distortion product otoacoustic emission, and engagement with their novel musical properties. Recent examples of creative exploration of phenomena include '2x3 channels' by Florian Hecker (2009), Christopher Haworth's  'Correlation Number One' (2010), Marcus Schmickler's 'Fortuna Ribbon' (2015)\footnote{\url{https://vimeo.com/channels/1137214/126234679}}, Thomas Ankersmit 'Otolith' (2015)\footnote{listen to an excerpt here: \url{https://soundcloud.com/weerzin/live-at-ctm-2015-excerpt}}, and Florian Hecker's 'FAVN' (2017).

 

The work of '/si:v/' continues and extends this tradition. The application of DPOAE used in the work of has been based on the Quadratic ( f2 – f1) and Cubic (2 f1 – f2) Difference algorithms proposed in \citep{kendall2014sound}\footnote{}. The authors have provided a set of synthesis techniques by which the DPOAE's can be used in a determined and creative way with control over dynamic parameters of tone such as vibrato, tremolo, spectra and spatial location. An implementation of side-band sample and hold modulation developed for the purpose of the composition of '/si:v/' can be heard clearly in ... minute of the work. The technique generates phrases of patterns cascading up and down the spectrum  which produce a distinct auditory distortion product with shifting spatial location. A demonstration sound sample can be heard here:       
Additionally, the Dynamic Sinusoidal Synthesis (DDS) model described by \citet[p. 13]{kendall2014sound} has been used to generate material for section ... of the work. In DSS the amplitude and pitch of the QDT spectrum is a result of the analysis process performed on a complex and dynamically changing in time model signal (samples generated with the New Pulsar Generator (nuPg) described above). The working of sound material and form completes only at the site of the unique psychology and physiology of the listener; '/si:v/' attempts to treat the "ear as instrument" \citep{haworth2012ear}.