Apparently the high frequency alone did not effect brain activity unless coupled to audible or "low frequency" sounds.<p>I wonder if the two frequencies interact to form beat frequencies or complex wave superpositions in the audible range. That would jive with this description of the Japanese researchers' instrument:<p>"Traditional gamelan music of Bali Island, Indonesia, a natural sound source containing the richest amount of high frequencies with a conspicuously fluctuating structure, was chosen as the sound source for all experiments."<p>Alpha brainwaves, which were seen when both high and low frequencies were combined, correlate with being "relaxed and alert". Interesting findings.
I'm not sure how related this is, but I used to get bad headaches from the ultrasonic sensors on my self-driving RC car project. I always wondered if others were affected, and how that impacted other ultrasonic-based projects.