Notes 21.1 – Properties of Sound

Pitch – how the human brain hears and interprets sounds of different frequencies. Higher pitch = higher frequency Humans can hear frequencies between 20 Hz and 20,000 Hz.

Tuning forks produce single pitch (frequency) sounds. Most other objects create sounds that are a combination of frequencies. Acoustics – the science and technology of sound.

The loudness of a sound is measured in decibels (dB). Louder sound waves have greater amplitudes and more energy. The decibel scale shows how some sounds compare.

The speed of a sound depends on the medium. Speeds of Sound The speed of a sound depends on the medium. Speed of sound in air at 0°C is about 330 m/s (750mph). Typically, sound travels faster in liquids, fastest in solids. As the air gets warmer, the speed increases. Subsonic – moving slower than sound Supersonic moving faster than sound Material Sound Speed (m/s) Air at freezing 330 Air at 20°C 344 Helium 965 Water 1530 Wood 2000 Steel 5940

Sonic boom – a build up of sound waves from an object traveling faster than sound.

Doppler Effect – the shift in the frequency of a wave as the wave source moves. If an object is moving toward you, the sound waves get closer together (higher frequency or pitch). If the object is moving away, the waves spread out (lower frequency, pitch)