Sound & UltrasoundOxford AQA IGCSE Physics: Revision notes

Sound is a longitudinal wave: vibrations in the particles of a medium travel in the same direction as the wave itself, creating alternating regions of compression and rarefaction. Sound needs a medium (solid, liquid or gas) to travel through — it cannot travel through a vacuum, because there are no particles to vibrate.

The human hearing range is approximately 20 Hz to 20 000 Hz. Sounds outside this range cannot be detected by the human ear.

Two properties of a sound wave determine how it is heard:

• Pitch depends on frequency — a higher frequency gives a higher-pitched sound
• Loudness depends on amplitude — a larger amplitude gives a louder sound

These two properties are independent of each other: a sound can be high-pitched and quiet, or low-pitched and loud, depending on its frequency and amplitude separately.

Like other waves, sound can be:

• Reflected — producing echoes when sound bounces off a hard surface back to the listener
• Diffracted — sound waves spread out as they pass through gaps or around obstacles, which is why you can hear someone talking around a corner even though you cannot see them

Ultrasound is acoustic (sound) energy with a frequency above the human hearing range, i.e. above 20 000 Hz.

When ultrasound waves travel through the body (or another material) and meet a boundary between two different media (e.g. tissue and bone), some of the wave is partially reflected back while the rest continues into the next medium. By timing how long it takes for the reflected pulse to return, the distance to the boundary can be calculated using:

s = v × t

where v is the speed of the wave and t is the time taken. Ultrasound equipment displays these reflected pulses as an oscilloscope trace, which can be interpreted to find distances and identify boundaries between different tissues.

Ultrasound has two important medical uses:

• Prenatal scanning — ultrasound scans are used to monitor the development and health of a foetus during pregnancy, without using ionising radiation
• Removal of kidney stones — high-intensity ultrasound waves can be focused to break up kidney stones into small fragments so they can pass out of the body naturally, avoiding the need for surgery