physics//wave//sound//ultrasound
Ultrasound is sound above about 20 kHz, the upper limit of human hearing, and it is used to see inside things that light cannot cross: the body in medical scanners, welds and castings in non-destructive testing, and the space behind a car in parking sensors. A piezoelectric crystal does both jobs, turning a voltage pulse into a sound pulse and an incoming echo back into a voltage.
Ultrasound is sound above about 20 kHz, the upper limit of human hearing, and it is used to see inside things that light cannot cross: the body in medical scanners, welds and castings in non-destructive testing, and the space behind a car in parking sensors. A piezoelectric crystal does both jobs, turning a voltage pulse into a sound pulse and an incoming echo back into a voltage.
The working principle is pulse-echo: send a short burst, listen, and time each echo. Every boundary the pulse meets returns part of it, and the round trip gives the depth,
d=c t2.d = \frac{c\,t}{2}.d=2ct.
A medical scanner assumes c=1540c = 1540c=1540 m/s for all soft tissue, so an echo arriving 65 µs after the pulse is drawn 5.0 cm deep. A parking sensor does the same in air at 343 m/s, at tens of kilohertz.
Frequency buys detail and costs depth.
In soft tissue attenuation is roughly 0.5 dB per centimetre per megahertz, so a 5 MHz pulse returning from 10 cm has lost about 50 dB; deep organs are imaged at 3 to 5 MHz, shallow ones at higher frequencies where the shorter wavelength resolves finer structure.
Medical imaging works between about 1 and 18 MHz; non-destructive testing of metals at 2 to 10 MHz; ultrasonic cleaning baths at 20 to 40 kHz, where the aim is the force of the waves rather than an image.
The probe cannot simply touch the skin: a film of air between them reflects almost everything back, a consequence of the acoustic impedance mismatch, so a water-based gel fills the gap.
Bone and gas block the view for the same reason, and bone also absorbs strongly at imaging frequencies. That is why brain imaging with ultrasound needs a way through the skull.
Moving reflectors, red blood cells above all, shift the frequency of their echoes; reading that shift is Doppler ultrasound, explained in Doppler effect.
The assumed 1540 m/s is a calibration choice, not a fact of every tissue: fat is slower, so structures behind a thick layer of fat are drawn slightly deeper than they are.