Sound Wave Notes

1. Sound waves are longitudinal (compression) waves.
2. Sound travels through gases, liquids and solids.
3. Speed of Sound
   1. Air at 0 °C = 331 m/s
   2. Air at 20 °C = 343 m/s
   3. Air at 100 °C = 366 m/s
   4. Helium = 965 m/s
   5. Fresh water (25°C) = 1482 m/s
   6. Sea water (25°C) = 1533 m/s
   7. Wood = 3828 m/s
   8. Steel = 5960 m/s
   9. Stone = 5971 m/s
4. Velocity = wavelength x frequency
   1. The musical note middle C has a frequency = 262 Hz. The velocity of sound in air is 343 m/s.
   2. wavelength = 343 m/s / 262 Hz = 1.31 meters
5. Pitch - The frequency of a sound wave.
6. Loudness - The amplitude of the wave.
7. Doppler effect - The apparent change of sound pitch as a sound source moves towards and away from the observer.
   1. When getting closer to the sound source the velocity of motion adds to the velocity of the wave.
   2. The pitch (frequency) is higher.
   3. When moving away from the sound source the velocity of motion subtracts from the velocity of the wave.
   4. The pitch (frequency) is lower.
8. Decibel - Measurement of the power of the sound wave. Sound intensity decreases with the square of the distance from the source.
   1. Threshold of hearing = 0 dB
   2. Whisper = 20 dB
   3. Purring cat = 25 dB
   4. Normal conversation = 65 dB
   5. Noisy restaurant = 80 dB
   6. Power mower = 100 dB
   7. Threshold of pain = 130 dB
   8. Jet aircraft take-off = 150 dB