Trying to make her ultrasound setup quieter, Xu shortened the pulse repetition period to 20 kilohertz, which meant each burst had to last only a few microseconds instead of building up to full power like a ringing bell. Expecting this to weaken the effect, she instead tried firing these ultra-short bursts at the highest pressure she could reach, repeated rapidly, on a piece of pig heart tissue sitting in a water tank. Within about a minute she saw what looked like smoke coming off the tissue and a hole appeared straight through it, a result she repeated three times before concluding it was real.
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The original ultrasound frequencies she worked with were in the 700 kilohertz to 5 megahertz range, far above audible sound, but the repeated bursts themselves fell within human hearing range.
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Her own instinct was that a shorter pulse should mean less power, since ultrasound waves, like a bell, need time to ring up to full amplitude.
- 03
She used a personal rule that an effect had to repeat three times before she trusted it was real, which led her to call her advisor Charles Cain into the lab.
384 words · auto-generated from the episode video
12:30And our hearing range for human beings is from 20 Hertz to 20 kilohertz. Uh, and, uh, but ultrasound therapy is like, I do bursts of ultrasound waves and use different power so that I can see if they can do anything. And the burst of this ultra, you know, burst of ultrasound that will repeat them. And then the repeating of this ultrasound wave burst are within our hearing range and they are powerful. So, so they're loud. So my lab mates are like, you're too loud, not only you're failing and you're loud. And I was like, okay. Oh, you know, I, I was desperate, you know, I was trying
13:10everything. So, uh, I thought two things. One is as engineer, uh, you know, if a hammer doesn't work, I try a bigger hammer. So I just increase the power. Secondly is, uh, uh, if he thinks I'm too loud, then I will make it quieter by repeating those bursts beyond our hearing range, 20 kilohertz. Ah, okay. So, so the ultrasound is like really fast. And I'm using, you know, like, uh, beyond, you know, uh, uh, I was using a frequency that's 700 kilohertz, five megahertz and those were high. But the bursts of the waves were within our hearing range. So in order to make it quieter, I made it so that it's 20 kilohertz.
13:54But in order to do that, which means I have to make those bursts really, really short. So 20 kilohertz, meaning that I'm repeating every 50 microseconds. So my bursts have to be a few microseconds, which should fit enough. But which is a stupid thing to do because like ringing a bell, it takes time to ring up to the full power. Ultrasound is the same way. You need to have enough of the length in order for the ultrasound wave to reach its high power. So I'm basically cutting my ultrasound wave pressure way down by doing this short microsecond ultrasound pulse, while I think I should go, you know, bigger
14:37hammer, right? But I was like, okay, I will humor him because nothing worked anyway. And so I tried a microseconds ultrasound pulse at as high a power or pressure as I possibly could get. And I repeat them really fast.