Eli5: what is ultrasound and what makes it “ultra”
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Ultrasound is just sound at very high frequencies – so high our ears can’t hear them. In fact that’s the definition – any sound above 20 kHz is classified as ultrasound. They’re used in medical scans, which are essentially sonar maps. The resolution will be something like the wavelength, so if you want to see an object in your body that’s 1 mm across, you need a wavelength of 1mm. Since the speed of sound in your body is ~1 km/s, that means you need a frequency of 1000 m/s divided by 0.001m, or about 1 MHz. That’s why you need to use ultrasound if you want to take useful scans of your body.
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