Hmmm, this brings up quite a few questions and a prolonged contemplation on the matter of high-end equipment acting beyond 20kHz...

"The temporal resolution and high-frequency audibility of human hearing are complex issues of both fundamental and practical significance. While the single-tone high- frequency threshold fmax for airborne stimuli is around 18 kHz in individuals with good hearing (Pumphrey, 1950; Hall, 2002), a much higher bandwidth and temporal acuity can play a role in the complete perception of the timbre of the sound. 

Neural processing beyond the cochlea can permit extraction of temporal information at time scales τ shorter than the 1/2πfmax=9 μs that would be nominally expected for a linear system. In binaural localization by interaural time difference, it is well known that differences in arrival times of order 10 μs are distinguishable (Henning, 1974; Nordmark, 1976). Monoaural experiments involving iterated ripple noise (IRN) and inter-pulse gaps have shown similar thresholds in temporal resolution (Krumb- holz, 2003; Leshowitz, 1971). A similar sensitivity for temporal fine structure can be inferred from the discriminability of the virtual pitch of complex tones (Moore et al., 2006; Gockel et al., 2006). It also appears that the cochlea may sense ultrasonic stimulation if the latter manages to reach the cochlea in sufficient intensity, both when presented through the air (Henry and Fast, 1984; Ashihara et al., 2006) but especially when presented through bone conduc- tion (Corso, 1963; Deatherage et al., 1954; Lenhardt et al., 1991; Lenhardt, 1998). It has also been conjectured that such high level ultrasound may possibly change the perception of timbre when superimposed on audible harmonics (Oohashi et al., 1991; Yoshikawa et al., 1995).

Complete article link: here .