Hypersonic effect

The hypersonic effect is a term coined to describe the phenomenon reported in some scientific studies, which demonstrate that although humans cannot consciously hear sounds at very high frequency (above around 20 kHz), the presence or absence of those frequencies has a measurable effect on their psychological reaction.

Evidence
In research published in 2000 in the Journal of Neurophysiology, researchers described double-blind experiments in which subjects were played music, sometimes containing high-frequency components (HFCs) above 25 kHz and sometimes not. The subjects could not consciously tell the difference, but when played music with the HFCs they showed differences measured in two ways:


 * EEG monitoring of their brain activity showed statistically significant enhancement in alpha-wave activity
 * The subjects preferred the music with the HFCs

It is a common understanding in psychoacoustics that the ear cannot respond to sounds at such high frequency, so one question that this research raised was: does the hypersonic effect occur via the "ordinary" route of sound travelling through the air passage in the ear, or in some other way? A peer-reviewed study in 2006 seemed to confirm the second of these options, by testing the different effect of HFCs when presented via loudspeakers or via headphones. The hypersonic effect did not occur when the HFCs were presented via earphones.

The 2006 study also investigated the comfortable listening level of music with and without HFCs, an alternative way of measuring subject response to the sound. The CLL for the music with HFCs was higher than that for the music without HFCs - this provides a quantitative way to demonstrate general listener preference for the music with HFCs.