Inaudible High-Frequency Sounds Affect Brain Activity: Hypersonic Effect
Here is an interesting study from Japan that challenges the notion that inaudible sounds do not affect the acoustic perception of audible sounds.
Note: The authors use "HFC" to denote high frequency components of sounds, above the audible range, below 22kHz. "LFC" denotes low frequency components of sounds, below the audible range, above 22kHz.
"Despite the fact that nonstationary HFCs were not perceived as sounds by themselves, we demonstrated that the presentation of sounds that contained a considerable amount of nonstationary HFCs (i.e., FRS) significantly enhanced the power of the spontaneous EEG activity of alpha range when compared with the same sound lacking HFCs (i.e., HCS). In parallel experiments employing exactly the same stimulus and methods, PET rCBF measurement revealed that FRS activated the deep-lying brain structures, including the brain stem and thalamus, compared with HCS. In addition, subjective evaluation by questionnaire revealed that FRS intensified the subjects' pleasure to a significantly greater extent than HCS did. We conclude, therefore, that inaudible high-frequency sounds with a nonstationary structure may cause non-negligible effects on the human brain when coexisting with audible low-frequency sounds. We term this phenomenon the “hypersonic effect” and the sounds introducing this effect the “hypersonic sound.” We do not think that the hypersonic effect is specific to the sound material used in the present study because we previously confirmed, by EEG analysis, that the same effect can be introduced by different sound sources containing a significant amount of nonstationary HFCs (e.g., Oohashi et al. 1994)."