Waves and sound · System
Ultrasound at work
Nothing about ultrasound is exotic. It is ordinary sound at a frequency our ears happen to stop short of — and every job it does is one of the two things any wave can do: deliver energy, or bring back information.
Start here
The weld looks perfect. That is the problem.
A steel weld on a bridge girder has been ground smooth and painted. From the outside it is flawless. Somewhere inside it there may be a crack a couple of millimetres across, and if there is, it matters enormously. Nobody is going to cut the girder open to find out.
How would you find out what is inside a solid block of steel, and where?
Sound goes into steel perfectly well, and it reflects wherever it meets a boundary between one material and another — and a crack is a boundary, steel against air. Time the echo, use the speed of sound in steel, and halve it. The whole method is the echo you already know, at a frequency chosen so that something two millimetres across is big enough to reflect it.
Ultrasound is sound above the top of the human range, so above about 20 000 Hz. Nothing else about it is unusual. It is made by something vibrating, it needs a material to travel through, it moves at that material's speed of sound, and it reflects wherever it meets a boundary between one material and another. Every one of those is the ordinary behaviour of sound, and every one of them is what the applications rely on.
What ultrasound is useful for splits neatly in two. Sometimes what is wanted is the energy the wave carries: enough of it, concentrated in a small space, will shake dirt off a surface or warm tissue several centimetres under the skin. Sometimes what is wanted is the information it brings back: send in a short pulse, time what returns, and the timing tells you where the boundaries are. A microphone does the information job at ordinary frequencies, turning the pattern in the air into a matching pattern of electricity.
At the bench · a probe on a block, and a screen showing two pips
Send a pulse in. Time what comes out.
Change a control to begin
A probe pressed to the top of a block sends a short pulse of ultrasound straight down and listens. The screen shows the moment it left and the moment it came back. Set the material, and set how deep the reflector is.
Commit first. The same reflector sits 100 mm down in a block of steel and in a block of water. Which echo comes back sooner?
What the block is
100 mm
Depth of the reflector
—
below the probe face
Speed in this material
—
—
Total path, down and back
—
Time between the pips
—
The figure
Carrying energy, or carrying information
Energy · cleaning bath
The wave's energy makes microscopic bubbles form and collapse against every surface in the tank, scrubbing dirt out of places no brush reaches. Jewellery, spectacles, engine parts and surgical instruments.
about 40 000 Hz
Energy · physiotherapy
The energy is absorbed a few centimetres into the tissue and warms it there, which is hard to do from the surface. Used on strained muscles and stiff joints, under the direction of a physiotherapist.
about 1 000 000 to 3 000 000 Hz
Information · medical scan
Every boundary inside the body sends part of the pulse back. Timing each echo gives its depth, and thousands of them together are assembled into a picture. No cutting, and no ionising radiation.
about 2 000 000 to 15 000 000 Hz
Information · microphone
The arriving wave sets a diaphragm vibrating, and its movement becomes a changing voltage with the same pattern in it. The information in the sound is now in a wire, and can be stored, sent or amplified.
ordinary audible sound, about 20 to 20 000 Hz
The energy uses want as much of the wave delivered into one place as possible. The information uses want as little disturbance as possible and care only about what comes back, and when. Same physics, opposite priorities — which is why a scanner runs at a tiny fraction of a cleaning bath's power.
Key fact
Ultrasound is sound above the top of the human range, above about 20 000 Hz, and it behaves like any other sound: it needs a material, travels at that material's speed of sound, and reflects at a boundary. It is used either for the energy it carries — ultrasonic cleaning and physiotherapy — or for the information it brings back, where the time an echo takes gives the depth of whatever sent it. A microphone does the same information job for audible sound, turning it into a matching electrical signal.
Think again
“Ultrasound is a special kind of wave that can get through solids where ordinary sound cannot.”
Ordinary sound gets through solids extremely well — better than through air, in fact, because the particles are closer together and more strongly linked. Put your ear against a wall and you will hear the room next door more clearly than through the doorway. Ultrasound is not a different kind of wave at all: it is the same longitudinal pressure wave, just at a frequency above the top of our hearing. What the high frequency buys is a short wavelength, and a short wavelength reflects off small things. That is why a scanner can pick out a two-millimetre crack and a shout cannot.
“A scan works by shining ultrasound through you and seeing what comes out the other side.”
Almost nothing comes out of the other side, and the machine is not looking there anyway. The probe sends a pulse and then listens with the same face it sent from, timing every echo that returns from a boundary inside. That is also why the operator puts gel between the probe and the skin: a thin layer of air between two solids reflects almost the entire pulse straight back off the surface, and the machine would see the skin and nothing else. The gel removes the air and lets the pulse into the body at all.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Calculate
A probe on a steel block sends a pulse down and the echo returns 0.060 ms after it left. Sound travels at about 5000 m/s in steel. How deep is the reflector?
Rung 2 · The one that catches people
A student says ultrasound is used inside metal and inside bodies because ordinary sound cannot travel through solids. Which statement is right?
Rung 3 · Explain
Explain how an engineer uses a probe and a timer to find a crack hidden inside a steel girder, and how they work out how deep it is.
Rung 4 · Take it somewhere new
Before a scan the operator squeezes a cold gel onto the skin and presses the probe into it. Explain why the scan would not work with a thin layer of air between the probe and the skin.
Key note
Ultrasound is sound above about 20 000 Hz, and it obeys all the ordinary rules of sound: it needs a material, it travels at that material's speed of sound, and it reflects at a boundary between materials. It is used for the energy it carries, in ultrasonic cleaning baths and in physiotherapy, and for the information it brings back, where timing an echo gives the depth of whatever reflected it. The pulse travels down and back, so the depth is half the total path. A microphone does the information job for audible sound, turning the pattern in the air into a matching electrical signal.
Going further
Why the frequency has to be so high is a question about wavelength. A wave reflects usefully off something roughly its own size or larger, and at 5 MHz in soft tissue the wavelength is about 0.3 mm, so features a fraction of a millimetre across show up. Drop to an audible 5000 Hz and the wavelength in the same tissue is about 300 mm — the width of the whole abdomen — and the pulse would sail past everything inside without noticing it. Detail costs frequency, which is why a scanner looking deep into a body uses a lower frequency than one looking at something just under the skin: high frequencies are absorbed faster and do not reach as far, so every scan is a trade between how deep and how sharp.
The same trade turns up in nature. A bat hunting in the open uses a lower call that reaches further; the same bat closing on a moth switches to a higher, shorter call and fires it many times a second, giving up range for detail exactly when it needs the detail.
Before this lesson
Connects to
At GCSE this becomes
- Ultrasound imaging and echo sounding calculations, reflection and transmission at a boundary, and how ultrasound compares with X-rays for medical imaging.
Where to next
Ask Mr Badmus AI
Got an echo time and a material, and want the depth?
If a scan or a course of treatment has been mentioned to you and you are anxious about it, asking what it is for and what will happen is always reasonable. You can talk to a doctor, a school nurse or any adult you trust. Childline is free, confidential and open at any hour, on 0800 1111, and you do not have to give your name.
The bench is a teaching model. The speeds are round values for one ordinary sample of each material at about 20 degrees Celsius, and they shift with alloy, temperature and, in a real inspection, with the kind of wave the probe launches; published figures for steel run from about 5000 to about 5900 m/s. The block is drawn to scale in depth from 0 to 200 mm, and the timing trace is to scale over a fixed 0.30 millisecond window; the pulse is treated as leaving at an instant and reflecting cleanly off one boundary, whereas a real trace also shows the far face of the block, edge reflections and noise. How much of a pulse a boundary sends back depends on the two materials either side of it and is not modelled here. Physiotherapy and medical scanning are described, not instructed, and both are carried out by trained practitioners.
Lesson content © MrBadmusAI.