✓ Revise Right course
Ultrasound is sound waves with frequencies higher than humans can hear, over 20,000 Hz. Free GCSE AQA Physics notes with key definitions and 15 FAQs.
Ultrasound and Echo Sounding is part of Waves in air, fluids and solids in GCSE AQA Higher Physics.
6 min
Reading time
19
Lessons in this topic
GCSE AQA Higher
Pathway
Reviewed by Revise Right
Content review
Ultrasound is sound waves with frequencies higher than humans can hear, over 20,000 Hz. Free GCSE AQA Physics notes with key definitions and 15 FAQs.
AQA GCSE Physics mock draft, 26–31 Oct. Draft coverage: Physics Paper 1 (Higher tier). Reviewed for an original-practice sitting; check student availability before registering. Marking and results will follow later, after separate approval. Eligible new accounts receive a five-day Pro trial when signup email verification completes. Previous trials keep their original expiry. After expiry, new papers need Pro unless you have an explicitly confirmed free sitting offer for this event. That offer adds no Pro time or AI learning. Saved attempts and released results remain readable. No card or automatic charge.
Use this lesson to stay inside Waves in air, fluids and solids while connecting the detail back to the wider Waves topic.
Read the core explanations from Ultrasound and Echo Sounding before testing yourself from memory.
Imagine trying to see inside your body without cutting it open. Doctors need a way to peek inside safely, and ultrasound gives them exactly that superpower.
Ultrasound waves are sound waves with frequencies higher than humans can hear - above 20,000 Hz. Your ears can detect sounds up to about 20,000 Hz, but ultrasound keeps going much higher, often reaching millions of Hz in medical scanners.
The key property that makes ultrasound so useful is partial reflection. When these high-frequency waves hit a boundary between different materials - like the edge of an organ inside your body - some of the wave bounces back whilst some continues through.
This reflection happens because different materials have different acoustic impedance - essentially how 'resistant' they are to sound waves. The bigger the difference in acoustic impedance between two materials, the stronger the reflection.
The principle of ultrasound imaging relies on precise timing. When an ultrasound pulse hits a boundary inside your body, part of it reflects back to the detector. By measuring how long this takes, we can calculate exactly how far away that boundary is.
Think of it like shouting in a canyon and timing the echo. The longer the delay, the further away the cliff face. Ultrasound works the same way, but with sound waves we cannot hear and distances measured in millimetres.
The ultrasound transducer acts as both transmitter and receiver. It sends out a pulse, then switches to listening mode to detect the returning echoes. Different tissues reflect different amounts of the ultrasound energy back.
Soft tissues like muscle reflect some ultrasound but let most through. Bone reflects almost all of it back strongly. Air reflects nearly everything, which is why gel is needed between the scanner and skin - to eliminate air gaps.
The computer builds up an image by combining thousands of these distance measurements. Stronger reflections appear brighter on the screen, weaker ones appear darker. This creates the familiar black and white ultrasound images.
The fundamental equation for ultrasound distance measurement comes from the basic relationship between speed, distance and time. However, there's a crucial detail that catches many students out.
The ultrasound pulse travels to the boundary and back again - it makes a round trip. This means the total distance travelled is twice the depth we want to find. The equation becomes: distance to boundary = (speed × time) ÷ 2.
Worked example: Calculating Depth Using Ultrasound
An ultrasound pulse takes 0.00013 seconds to return from a boundary inside the body. The speed of ultrasound in body tissue is 1540 m/s. Calculate the depth of the boundary. [3 marks]
Answer: 0.10 m
The speed of ultrasound varies in different materials. In soft tissue it travels at about 1540 m/s, in bone it's faster at around 4000 m/s, and in air it's much slower at 340 m/s. Medical scanners are calibrated for soft tissue speeds.
Ships and submarines use echo sounding to map the ocean floor and detect underwater objects. The principle is identical to medical ultrasound, but the scales are vastly different - we're measuring depths in hundreds of metres rather than centimetres.
A sonar transducer mounted on the ship's hull sends high-frequency sound pulses downwards through the water. These pulses travel at about 1500 m/s in seawater - slightly slower than in body tissue due to the different density and temperature.
When the sound pulse hits the seabed, it reflects back up to the ship. The sonar system measures the time delay and calculates the water depth using the same formula: depth = (speed × time) ÷ 2.
Modern ships use multiple beams to create detailed 3D maps of the ocean floor. This technique revealed underwater mountain ranges, deep ocean trenches, and shipwrecks that were previously unknown.
Echo sounding can also detect schools of fish, submarines, or other objects suspended in the water column. These show up as additional echoes that arrive before the seabed echo, indicating they're at shallower depths.
Medical ultrasound is incredibly versatile and safe. Unlike X-rays, ultrasound uses no ionising radiation, making it perfect for monitoring developing babies during pregnancy. The images show the baby's growth, position, and can detect potential problems early.
Beyond pregnancy scans, ultrasound examines the heart (echocardiography), liver, kidneys, and other internal organs. It can detect tumours, measure blood flow, and guide needle biopsies with millimetre precision.
The safety comes from using non-ionising radiation at relatively low power levels. The sound waves cause tiny vibrations in tissue but don't break chemical bonds like X-rays do. However, heating can occur at high intensities, so exposure times are kept short.
Industrial ultrasound testing checks for cracks and flaws in metal structures like aircraft wings, railway tracks, and oil pipelines. The technique can detect internal defects that would be invisible from the surface, preventing catastrophic failures.
Ultrasound questions often test your understanding of wave reflection and distance calculations. The most common mistake is forgetting to divide by 2 when calculating distances - remember the pulse travels there and back.
When describing how ultrasound works, examiners want you to mention partial reflection at boundaries between different media. Don't just say 'it bounces back' - explain that some reflects and some continues through.
For calculation questions, always show your working clearly. Write the formula first, substitute the values with units, then calculate the answer. Check your answer makes sense - depths inside the body should be centimetres, ocean depths should be metres.
Six-mark questions about ultrasound applications require you to explain both how it works and why it's suitable for the specific use. Compare it to alternatives like X-rays, mentioning safety advantages and limitations.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Ultrasound is sound waves with frequencies higher than humans can hear, over 20,000 Hz. It is used for medical imaging and industrial testing, with medical scanners often reaching millions of Hz.
Ultrasound imaging works by partial reflection: when a pulse meets a boundary between different materials, such as the edge of an organ, some of the wave reflects back while some continues through. The time for each echo gives the distance to the boundary, and the computer builds an image from these measurements.
An ultrasound transducer acts as both transmitter and receiver: it sends out a pulse, then switches to listening mode to detect the returning echoes. A computer combines thousands of distance measurements to build an image.
Acoustic impedance is essentially how resistant a material is to sound waves. The bigger the difference in acoustic impedance between two materials, the stronger the reflection of ultrasound at the boundary.
Echo sounding is a technique using high frequency sound waves to detect objects in deep water and measure water depth by timing reflections. Ships send pulses down at about 1500 m/s in seawater and use depth = (speed × time) ÷ 2.
Gel is used between the ultrasound scanner and the skin to eliminate air gaps, because air reflects nearly all ultrasound. Without the gel, the ultrasound would not get into the body.
Sound pulses travel at about 1500 m/s in seawater, slightly slower than in body tissue, because of the different density and temperature. Echo sounding uses this speed to calculate water depth.
Ultrasound travels at about 1540 m/s in soft tissue, around 4000 m/s in bone and 340 m/s in air. Medical scanners are calibrated for soft tissue speeds.
Sonar detects schools of fish, submarines or other objects in the water as extra echoes that arrive before the seabed echo, showing they are at shallower depths. Depth is found with (speed × time) ÷ 2.
Depth = (speed × time) ÷ 2, because the pulse travels to the boundary and back. A pulse returning after 0.00013 seconds in body tissue at 1540 m/s gives a depth of (1540 × 0.00013) ÷ 2 = 0.10 m.
Ultrasound is used for pregnancy scans because, unlike X-rays, it uses no ionising radiation, making it safe for monitoring developing babies. It shows the baby's growth and position, though exposure times are kept short because heating can occur at high intensities.
Partial reflection is when ultrasound waves meet a boundary between different materials and some of the wave energy reflects back while some continues through. Ultrasound imaging relies on this.
Echo calculations divide by 2 because the pulse travels to the boundary and back again, a round trip, so the total distance is twice the depth. Depth = (speed × time) ÷ 2; forgetting to halve is the most common mistake.
Industrial ultrasound testing checks for cracks and flaws in metal structures like aircraft wings, railway tracks and oil pipelines. It can detect internal defects invisible from the surface, preventing catastrophic failures.
Bone shows up bright on an ultrasound because it reflects almost all the ultrasound back strongly, and stronger reflections appear brighter on the screen. Soft tissues like muscle reflect some ultrasound but let most through.