Ultrasound is sound too high in frequency for human ears to detect

1. The boundary is set at $20kHz$, which is the same frequency as $20000Hz$, and any sound above it counts as ultrasound.
   1. That value is the top of what a healthy human ear can pick up, so the number describes your hearing rather than anything that changes in the wave as the frequency creeps past it.
2. Ultrasound is an ordinary sound wave made by a source vibrating very fast, so it needs a medium and travels at the speed of sound in whatever material it is passing through.
3. It is not electromagnetic radiation, it has no place in the electromagnetic spectrum, and it is not ionising.
4. A dog whistle sounding at roughly $30kHz$ is silent to you and perfectly clear to a dog, and a hunting bat finds moths in the dark using ultrasound pulses.
5. In a workshop or a hospital, ultrasound is produced by a transducer, a crystal that an alternating voltage drives into rapid vibration and that also picks up waves arriving back at it.

A pulse of ultrasound is reflected wherever the material it travels through changes

1. All three uses in this article rest on the same measurement, sending a short pulse of ultrasound into a material and timing the reflected pulse that comes back.
   1. Each pulse lasts a tiny fraction of a second, so the transducer has time to fall silent and listen for the return before it fires again.
2. At a boundary between two materials that transmit sound differently, part of the pulse is reflected straight back towards the source and the rest carries on deeper.
3. The greater the mismatch between the two materials, the stronger the reflected pulse, so an air-filled gap inside solid metal sends back a very strong reflection indeed.
4. An electronic timer measures the interval between the outgoing pulse and the returning one, and that time is the only quantity the instrument really reads.

Non-destructive testing finds a crack inside a component without cutting it open

1. A steel casting or a weld can hide a crack or a gas-filled void that nothing on the outside reveals and that would let the part snap under load.
2. The transducer is pressed onto the surface with a smear of gel or oil, which squeezes out the film of air that would reflect the pulse before it ever entered the metal.
3. In a sound component the pulse crosses the whole thickness, reflects from the far surface and comes back, giving a single late echo.
4. If a crack or void sits part way through, it reflects some of the pulse and produces an extra echo that arrives early, ahead of the far-surface echo.
5. The depth of the flaw below the surface follows from the time of that early echo and the speed of sound in the metal, halved because the pulse went down to the flaw and back.
   1. An early echo that returns in half the time of the far-surface echo puts the flaw halfway through the thickness.
6. Sliding the transducer across the surface and noting where the early echo appears and vanishes maps how far the flaw spreads sideways.
7. Railway rails, pipeline welds and aircraft parts are all tested this way where they sit, and a component that passes goes straight back into service.

Medical scanning builds an image from echoes off the boundaries between tissues

1. Pulses of ultrasound are sent into the body from a probe held against the skin with gel spread underneath it.
   1. The gel shuts out the air, because a skin-to-air boundary would reflect almost the whole pulse before it got inside.
2. Different soft tissues such as muscle, fat, blood and fluid transmit sound differently, so part of the pulse is reflected at every boundary between two tissues.
3. Every boundary along the path returns its own echo, and the return time of that echo fixes how deep the boundary lies below the probe.
4. As the probe is swept across the skin, the pattern of return times and echo strengths is assembled into a cross-sectional image on a screen.
5. A developing fetus is scanned with ultrasound rather than X-rays because ultrasound is not ionising, so it avoids any risk of damage to cells that are dividing rapidly.
6. X-rays are also poor at this job because they pass through soft tissue with little absorption, so a boundary between two soft tissues barely shows on the image, while ultrasound reflects from it strongly.
7. One scan can show where the fetus is lying and measure its length, from which its age is estimated.

Sonar turns the return time of a pulse into the depth of the water

1. A ship carries a transducer in its hull that fires a pulse of ultrasound straight down into the sea.
2. The pulse reflects from the sea bed, or from a shoal of fish or a wreck part way down, and returns to a detector on the hull.
3. The speed of sound in water is about $1500m/s$, and it is that value, never the speed in air, that a sonar calculation uses.
4. Putting the measured return time and that speed into the echo calculation gives the depth of water below the ship.
5. A shoal of fish sends its echo back sooner than the sea bed does, so a fishing boat sees the fish and the bottom as two separate marks on one trace.

The pulse travels there and back, so halve the distance you calculate

1. The time an ultrasound instrument records is the time for the whole journey, out to the reflecting surface and back to the detector.

2. Multiplying the wave speed by that time therefore gives the total path length, which is twice the depth or distance you are asked for.

   $$distance=\frac{v\times t}{2}$$

3. In this equation $v$ is the speed of sound in the material the pulse passes through, in $m/s$, and $t$ is the total time from sending the pulse to receiving the echo, in $s$.