3.10 - Sound Waves & Hearing

Sound is a longitudinal wave that needs a medium

• A sound wave is produced by a vibrating object, which passes vibrations into the particles of the surrounding material.
• Sound is a longitudinal wave: the particles vibrate back and forth along the same direction the wave travels.
• As they vibrate, particles bunch into regions of high pressure called compressions, and spread apart into regions of low pressure called rarefactions.
• The wave is this travelling pattern of compressions and rarefactions. The particles vibrate about fixed positions - it's the wave, not the material, that travels.

Sound needs a medium

• Because it is carried by vibrating particles, sound needs a medium - a solid, a liquid, or a gas.
• Sound cannot travel through a vacuum (such as space) because there are no particles to pass the vibrations on.

The range of human hearing

• Humans can detect sound waves with frequencies between about 20 Hz and 20,000 Hz (20 kHz).
• Sounds below 20 Hz or above 20,000 Hz fall outside this range and cannot normally be heard.
• As a person ages, the upper frequency limit falls.

Reflection of sound and echoes

• Sound waves reflect off hard, flat surfaces. A reflected sound wave is called an echo.

• Sound obeys the law of reflection: the angle of incidence equals the angle of reflection, measured from the normal.

• Soft surfaces (such as carpets and curtains) absorb sound rather than reflect it.

• An echo is heard after the original sound because the reflected wave has to travel further to reach the ear.

Loudness and pitch as wave properties of sound

• Loudness depends on amplitude - A larger amplitude carries more energy, producing a louder sound.
• Pitch depends on frequency - A higher frequency produces a higher-pitched sound; a lower frequency produces a lower-pitched sound.

Displaying sound on an oscilloscope

A microphone converts sound waves into electrical signals, and an oscilloscope displays the signal as a trace on a screen.

• A louder sound produces a trace with a larger amplitude.
• A higher-pitched sound produces a trace with more complete cycles across the screen.