SoundEdexcel IGCSE Physics: Revision notes

Sound waves are longitudinal waves — the particles of the medium vibrate parallel to the direction the wave travels, creating alternating regions of compression (particles pushed close together) and rarefaction (particles spread further apart).

Like all waves, sound waves can be reflected (an echo is a reflected sound wave) and refracted (bent as it passes into a medium where it travels at a different speed).

Sound needs a medium (solid, liquid or gas) to travel through, because it relies on particles vibrating and colliding — it cannot travel through a vacuum.

The human ear can detect sound waves with frequencies between 20 Hz and 20,000 Hz (20 kHz). This is called the range of human hearing.

• Sounds below 20 Hz are called infrasound
• Sounds above 20,000 Hz are called ultrasound

Hearing range varies between individuals and tends to narrow (particularly at higher frequencies) with age.

The speed of sound in air can be measured practically. One method: two people stand a large, measured distance apart. One makes a loud sound (e.g. a clap or starting pistol); the other starts a stopwatch when they see the action and stops it when they hear the sound.

$\text{speed of sound} = \dfrac{\text{distance}}{\text{time}}$

Repeating the measurement and taking an average, and using a large distance, improves accuracy by reducing the effect of human reaction-time error on timing.

A microphone converts the pressure variations of a sound wave into a varying electrical signal. This electrical signal can be displayed as a trace on an oscilloscope screen, showing the wave's shape over time.

The oscilloscope trace can be used to find the frequency of the sound: the time period of one complete wave cycle is read from the screen (using the horizontal time-per-division setting), and frequency is calculated using:

$f = \dfrac{1}{T}$

where $T$ is the time period in seconds.

• Pitch (how high or low a sound sounds) relates to the frequency of vibration of the source — a higher frequency gives a higher pitch
• Loudness (how loud a sound sounds) relates to the amplitude of vibration of the source — a greater amplitude gives a louder sound

On an oscilloscope trace, a higher-pitched sound shows more wave cycles packed into the same time (shorter time period), while a louder sound shows a trace with greater height (larger amplitude).