Sound Waves and Ultrasound — AQA GCSE Physics (Separate)
Sound is a longitudinal wave. Ultrasound has uses in medicine and industry.
Sound waves
Sound travels as a longitudinal wave — the particles vibrate parallel to the direction of energy transfer, creating compressions and rarefactions. Sound needs a medium (solid, liquid or gas) and cannot travel through a vacuum.
- Sound travels fastest in solids (particles closest together) and slowest in gases.
- The normal range of human hearing is about 20 Hz to 20 000 Hz (20 kHz).
- Sound waves can be reflected (echoes), refracted and absorbed.
Hearing
When sound waves reach the ear, they cause the eardrum and other parts to vibrate, which is interpreted as sound. Human hearing is limited to the 20 Hz–20 kHz range because of the way the ear converts vibrations.
Ultrasound
Ultrasound is sound with a frequency above 20 000 Hz (above the range of human hearing). When ultrasound meets a boundary between two media, part of it is reflected. By timing the reflected pulses, distances and images can be worked out.
Uses of ultrasound:
- Medical imaging (e.g. scanning a fetus) — non-ionising, so safer than X-rays.
- Industrial detection of flaws in materials.
- Cleaning and measuring distances (echo sounding/sonar).
Exam tips
- Sound is a longitudinal wave that needs a medium (no sound in a vacuum).
- Human hearing range: 20 Hz – 20 kHz.
- Ultrasound is above 20 kHz; it is reflected at boundaries.
- Ultrasound uses: medical imaging (non-ionising), flaw detection, sonar.