Home / IGCSE 0625 / Waves / Ultrasound
IGCSE 0625 / Section 3.4 / Extended
Supplement (Extended) content

Sound you cannot hear.

Above the top note any human can hear lies a whole band of sound still obeying the same physics. Sent out in pulses and timed on its way back, it can map a seabed, find a crack in steel, or picture a baby before it is born.

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The key idea

Ultrasound is sound with a frequency above 20 000 Hz, too high to hear. By sending a pulse and timing the echo, and using distance = speed multiplied by time, it measures depths and images inside objects. The reflected pulse covers twice the depth, so the time is halved to find the depth.

Definition · learn the exact words

Ultrasound is sound with a frequency above 20 000 Hz, beyond the range of human hearing. In pulse-echo methods a pulse is sent out and the time for its reflection to return is measured.

distance = speed × time  (depth = speed × time ÷ 2 for the round trip)

Uses include prenatal scanning, sonar depth finding, and detecting flaws inside metals.

Section 01

Send a pulse, read the echo.

Move the flaw deeper and watch the echo arrive later on the A-scan trace, with the gate marking the target depth.

Stage 1 · Learn

Check ultrasound

Four quick checks. Each correct answer earns XP and lights this skill on your star map.

Quick check+10 XP

Ultrasound is sound with a frequency...

Quick check+10 XP

Which is a use of ultrasound?

Quick check+10 XP

Pulse-echo measurement finds a distance using the relation...

Quick check+10 XP

Ultrasound is preferred over X-rays for imaging a foetus because it...

Section 02

Timing turns into distance.

Because the pulse travels to the boundary and back, the recorded time is for twice the depth.

distance = speed × time
Worked example

An ultrasound pulse sent into a body travels at 1500 m/s and its echo returns after 0.000 20 s. Find the depth of the reflecting boundary.

Step 1 · Round trip distanced₂ = v t = 1500 × 0.000 20 = 0.30 m
Step 2 · Halve for the depthdepth = 0.30 ÷ 2
Step 3 · Answerdepth = 0.15 m
Examiner trap

The echo travels to the boundary and back, so the measured time is for twice the depth. Halve it, or divide the round trip distance by two. Ultrasound is chosen to scan a foetus because, unlike X-rays, it is not ionising and does not damage living cells.

Stage 2 · Exam

Exam-style questions

Unlocks once the four checks above are done. Worth more XP, written in the style of Paper 2.

Finish the four checks above to unlock the exam questions
Exam style+20 XP

An ultrasound pulse travels at 1500 m/s through soft tissue and its echo from a boundary returns after 0.000 10 s. The depth of the boundary is...

Exam style+20 XP

A pulse of ultrasound takes 0.000 50 s to travel through a steel block and reflect back from the far face. If the speed in steel is 6000 m/s, the block is...

Exam style+20 XP

A flaw deep inside a metal casting is moved to a greater depth. The echo on the detector now arrives...

Skill unlocked

Ultrasound, mastered.

Ultrasound completes the sound strand. Keep the chain going.

-Rank -Level -Score -Topics
Go deeper · practice
Six original Cambridge-style questions
Ultrasound as sound above 20 kHz, its uses, and pulse-echo depth from distance = speed times time with the round trip halved. Attempt each, then reveal the worked solution.
Stage 3 · master the unit
Waves challenge
Mixed questions across the whole unit, each one worth XP. Start this only when you feel confident across every topic in the unit, not just ultrasound.
Start the unit challenge →