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.
Apparently the high frequency alone did not effect brain activity unless coupled to audible or "low frequency" sounds.
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:
"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."
Alpha brainwaves, which were seen when both high and low frequencies were combined, correlate with being "relaxed and alert". Interesting findings.
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"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."
Alpha brainwaves, which were seen when both high and low frequencies were combined, correlate with being "relaxed and alert". Interesting findings.