Sound and ultrasound

How sound is produced

Sound is a longitudinal mechanical wave. It is produced when an object vibrates — for example, the cone of a loudspeaker, vocal cords, or a guitar string. The vibrations push and pull on nearby air particles, creating regions of compression (high pressure) and rarefaction (low pressure) that travel outward.

Sound waves require a medium (solid, liquid or gas). They cannot travel through a vacuum — there are no particles to vibrate.

Frequency, pitch and amplitude

• Frequency (Hz) determines pitch. Higher frequency → higher pitch.
• Amplitude determines loudness. Larger amplitude → louder sound.

Human hearing range

The healthy human ear hears between 20 Hz and 20 000 Hz (20 kHz). The range narrows with age and exposure to loud noise (the upper limit drops first).

• Below 20 Hz → infrasound (felt rather than heard).
• Above 20 kHz → ultrasound.

Reflection of sound — echoes

Sound reflects off hard, smooth surfaces. The reflected wave is called an echo. You can use echoes to measure distance: distance = ½ × speed of sound × time delay (the wave travels there and back, hence ½).

Ultrasound (Higher)

Ultrasound is sound above 20 kHz. Two key uses appear on Edexcel papers:

1. Medical scans (e.g. foetal scan). A probe emits ultrasound pulses. At each tissue boundary, some sound reflects back. A computer measures the time delay and builds an image. Ultrasound is preferred to X-rays for foetuses because it is non-ionising and so does not damage cells.

2. Sonar / depth measurement. A ship sends an ultrasound pulse downward; the time for the echo to return gives the depth. depth = ½ × speed of sound in water × time.

Edexcel exam tip

For "why ultrasound for foetal scans not X-rays?" the mark is for stating ultrasound is non-ionising / does not cause cell damage / does not cause cancer. "Safer" alone is not specific enough for a mark.