What you'll learn
Sound is a wave you can hear, and ultrasound is sound too high for humans to hear — yet ultrasound is used for everything from baby scans to detecting flaws in metal. For AQA GCSE Physics you need to understand how sound waves travel, why they are longitudinal, how the ear detects them, and how ultrasound is used in medicine and industry. This guide covers the nature of sound waves, the range of human hearing, how sound is converted to electrical signals in the ear, and the uses of ultrasound and echo sounding. By the end you should be able to describe sound as a longitudinal wave, explain the limits of human hearing, and explain how ultrasound is used to build up images.
Key terms and definitions
Sound wave — A longitudinal wave caused by vibrations that travels through a medium such as air, water or solids.
Longitudinal wave — A wave in which the vibrations are in the same direction as the wave travels.
Compression — A region of a longitudinal wave where the particles are pushed close together.
Rarefaction — A region of a longitudinal wave where the particles are spread apart.
Frequency — The number of waves passing a point per second, measured in hertz (Hz).
Ultrasound — Sound waves with a frequency above 20,000 Hz, higher than humans can hear.
Echo — A reflected sound wave.
Medium — The material through which a wave travels.
Core concepts
Sound as a longitudinal wave
Sound is produced by vibrations. When an object vibrates, it makes the particles of the surrounding material vibrate too, passing the vibration along as a wave. Sound is a longitudinal wave, which means the particles vibrate in the same direction that the wave travels. This produces regions where particles are squashed together (compressions) and regions where they are spread apart (rarefactions). Because sound needs particles to vibrate, it cannot travel through a vacuum — there are no particles in a vacuum to carry the vibration.
Sound travelling through different materials
Sound travels at different speeds in different materials. It generally travels fastest in solids, slower in liquids, and slowest in gases, because the particles in a solid are closer together and pass on the vibration more quickly. When sound passes from one medium into another, its speed changes, which can cause it to refract. Sound can also be reflected at a boundary, which produces an echo.
The range of human hearing
Humans can hear sounds in the frequency range of about 20 Hz to 20,000 Hz (20 kHz). Sounds below 20 Hz (infrasound) and above 20,000 Hz (ultrasound) cannot be heard by humans. This range varies between people and tends to reduce as people get older, with the ability to hear higher frequencies usually lost first.
How the ear detects sound
Hearing depends on converting sound waves into electrical signals. Sound waves cause the eardrum to vibrate. These vibrations are passed on and eventually converted into electrical signals that travel to the brain, which interprets them as sound. Because human hearing is limited to a particular frequency range, it depends on the parts of the ear being able to vibrate at those frequencies.
What ultrasound is
Ultrasound is sound with a frequency above 20,000 Hz, too high for humans to hear. It behaves like any other sound wave — it can be reflected and refracted — but its high frequency and short wavelength make it useful for imaging and detection. When ultrasound meets a boundary between two different materials, some of it is reflected. By measuring the time taken for the reflection to return, and knowing the speed of the wave, the distance to the boundary can be calculated.
Uses of ultrasound
Ultrasound has several important uses:
- Medical imaging — Ultrasound is used to produce images of the inside of the body, such as scans of an unborn baby. The waves are partly reflected at each boundary between different tissues, and the reflections are used to build up an image. Ultrasound is used instead of X-rays for this because it does not damage cells.
- Industrial testing — Ultrasound can detect flaws and cracks inside metal objects. A reflection from an internal crack returns sooner than a reflection from the far side, revealing the flaw without cutting the object open.
Echo sounding
Echo sounding uses reflected sound (often ultrasound) to measure distances, for example the depth of the sea or the location of shoals of fish. A pulse of sound is sent out, and the time taken for the echo to return is measured. Using distance = speed × time, and remembering the sound travels to the object and back, the distance can be calculated. This is why the time must be halved in the calculation — the measured time is for the round trip.
How ultrasound builds up an image
In a medical scan, a probe sends short pulses of ultrasound into the body. At each boundary between different tissues — for example between muscle and bone, or between fluid and tissue — some of the ultrasound is reflected back to the probe. The machine measures the time each reflection takes to return and uses the speed of sound in the body to work out how deep each boundary is. By repeating this across the whole area and combining the results, a computer builds up a picture of the internal structures. This is how a scan of an unborn baby is produced without any surgery or harmful radiation.
Frequency, wavelength and why ultrasound is useful
Because ultrasound has a very high frequency, it also has a very short wavelength (since wave speed = frequency × wavelength). A short wavelength allows finer detail to be resolved and lets the waves be directed in a narrow beam, which is why ultrasound gives clearer images and more precise flaw detection than lower-frequency sound would. This link between high frequency, short wavelength and useful detail is worth remembering, because exams sometimes ask why ultrasound in particular is chosen for these jobs.
Worked examples
Example 1: Why sound cannot travel through space
Explain why sound cannot travel through a vacuum. Sound is a longitudinal wave that needs particles to vibrate and pass on the vibration. A vacuum contains no particles, so there is nothing to carry the sound, and it cannot travel.
Example 2: Identifying ultrasound
A wave has a frequency of 40,000 Hz. Can a human hear it, and what type of sound is it? A human cannot hear it, because it is above the human hearing range of 20 Hz to 20,000 Hz. It is ultrasound.
Example 3: Echo sounding calculation
A ship sends a pulse of ultrasound to the seabed. The echo returns after 0.6 s. If the speed of sound in water is 1500 m/s, how deep is the sea? First halve the time for the one-way trip: 0.6 ÷ 2 = 0.3 s. Then distance = speed × time = 1500 × 0.3 = 450 m.
Example 4: Why ultrasound is used for scans
Explain why ultrasound rather than X-rays is used to scan an unborn baby. Ultrasound does not damage living cells, whereas X-rays can damage cells and could harm a developing baby. Ultrasound is reflected at boundaries between tissues, so it can still build up an image safely.
Common mistakes and how to avoid them
A very common error is calling sound a transverse wave. Sound is longitudinal — the particles vibrate in the same direction as the wave travels. Learn this firmly.
Students often forget to halve the time in echo-sounding calculations. The measured time is for the sound to travel to the object and back, so you must halve it (or halve the calculated distance) to find the one-way distance.
Another mistake is saying sound travels fastest in gases. It travels fastest in solids, because the particles are closer together and pass the vibration on more quickly.
When explaining the ear, remember the key idea is that vibrations are converted into electrical signals for the brain. Simply saying "the eardrum vibrates" without mentioning the electrical signal is incomplete.
Finally, be precise about ultrasound: it is sound above 20,000 Hz, not simply "very loud" sound. Frequency, not loudness, is what makes it ultrasound.
Exam technique for "Sound waves and ultrasound"
Be ready to state that sound is a longitudinal wave, describe compressions and rarefactions, and explain why sound cannot travel through a vacuum. These are frequent short-answer marks.
Calculations almost always involve distance = speed × time, with the trap that you must halve the time for a reflected pulse. Set your working out clearly and state whether you are finding the one-way or round-trip distance.
For ultrasound uses, name the use (medical scan, flaw detection, echo sounding), and explain that reflections at boundaries are timed to work out distances or build an image. When asked why ultrasound is used for medical scans, always mention that it does not damage cells, unlike X-rays.
Quick revision summary
- Sound is a longitudinal wave caused by vibrations, with compressions and rarefactions; it cannot travel through a vacuum.
- Sound travels fastest in solids, slowest in gases.
- Human hearing ranges from about 20 Hz to 20,000 Hz; the ear converts sound vibrations into electrical signals for the brain.
- Ultrasound is sound above 20,000 Hz; it reflects at boundaries between materials.
- Uses include medical scans (safe, no cell damage), flaw detection in metals, and echo sounding for depth.
- In echo calculations, halve the time because the wave travels to the object and back.