Sound Wave Notes

Sound waves are longitudinal (compression) waves.

Sound travels through gases, liquids and solids.

Speed of Sound

• Air at 0 °C = 331 m/s
• Air at 20 °C = 343 m/s
• Air at 100 °C = 366 m/s
• Helium = 965 m/s
• Fresh water (25°C) = 1482 m/s
• Sea water (25°C) = 1533 m/s
• Wood = 3828 m/s
• Steel = 5960 m/s
• Stone = 5971 m/s

Pitch

The frequency of a sound wave.

Loudness

The amplitude of the wave.

Doppler effect

The apparent change of sound pitch as a sound source moves towards and away from the observer.

• When getting closer to the sound source the velocity of motion adds to the velocity of the wave.
• The pitch (frequency) is higher.
• When moving away from the sound source the velocity of motion subtracts from the velocity of the wave.
• The pitch (frequency) is lower.

Decibel

Measurement of the power of the sound wave. Sound intensity decreases with the square of the distance from the source.

• Threshold of hearing = 0 dB
• Whisper = 20 dB
• Purring cat = 25 dB
• Normal conversation = 65 dB
• Noisy restaurant = 80 dB
• Power mower = 100 dB
• Threshold of pain = 130 dB
• Jet aircraft take-off = 150 dB