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Probably irrelevant but still interesting study on ultrasonics

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May 27, 2021
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#1
Something unrelated got me curious about how the auditory nerve was routed in non-primate mammals, and all I really learned on that front is that anatomical illustrations seems to drop sharply in quality when they aren't of humans, but what ended up being more interesting. I found this paper studying the, for lack of a better term, audibility of of an ultrasound during a Transcranial ultrasound stimulation: https://www.sciencedirect.com/science/article/pii/S1935861X20302321

Transcranial Ultrasound Stimulation is the less invasive new procedure that will hopefully take the place of the somewhat more invasive procedure of sticking electrodes into their brain.

So, some import things to point out, this study has nothing to do with audio reproduction, it was performed by neuroscientists studying if a TUS would produce spurious results from the auditory cortex. Four types of tests were performed, with and without the stimulation, and with and without auditory masking. Subject could no longer tell whether the ultrasound was stimulating or not with the auditory masking active.

Important to point out, it was a 500 KHz ultrasound, but it was on a 1 KHz 50% duty cycle. This is where it becomes important that their test isn't really concerning itself with the audibility of ultrasonic frequencies, just if the participant can tell whether the ultrasound is on or not. They actually did do a test to that end, taking a human skull and reproducing the experimental conditions to check for frequencies introduced in the audible band. For example, it's possible that the ultrasound working in that configuration is basically acting like a thermal expansion speaker of sorts. They also mention controlling the rise of temperate of the scalp to <3 degrees centigrade, which I think I might be able to feel if it actually hit that threshold.

So probably not demonstrating any sort of surprise ultrasonic audibility, but I still found it interesting.
 

sergeauckland

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#2
This is where it becomes important that their test isn't really concerning itself with the audibility of ultrasonic frequencies, just if the participant can tell whether the ultrasound is on or not.
When I was younger, I've did several tests for high frequency audibility, and thought that I could reliably hear when a >20kHz tone was switched on and off, even going to 30kHz. As that was pretty unlikely, I looked into what was going on, and found that it wasn't the tone I was hearing, but the residual noise of the generator which was being switched on and off, which fooled me into thinking my hearing was a lot more extended than it was.

It showed me the importance of controls, and accounting for variables. Signal generators specified the harmonic content, not residual noise!

S.
 
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