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Sound Notes

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  • 1. Sound
  • 2. • Sound are longitudinal waves – Also called pressure waves – created as air pressure oscillates between high and low pressure – On a graph: • Crests show high pressure • Troughs show low pressure
  • 3. • Sound needs a medium to travel through – Needs air molecules to vibrate in order for the energy to get passed along – Space is silent! No air molecules in space!
  • 4. Propagate • All ways waves travel from a single point – Energy decreases with distance from starting point
  • 5. Acoustics • Get dead-spots – Destructive Interference – Areas with no or low sound – Ways to fix 1. Speakers placed in back of auditorium 2. Room built to prevent dead-spots
  • 6. Loudness • The high and low pressure of sound – More energy = louder sound – Sounds too loud can lead to deafness • Unit - decibels (dB) – The decibel scale is a logarithmic scale • Every +10 dB change means twice as loud • Every -10 dB change means half as loud
  • 7. • Some common sounds and their loudness: 10 – 15 dB A quiet whisper, 3 feet away 30 dB A house in the country 65 dB Ordinary conversation, 3 feet away 70 dB City traffic 90 – 95 dB Sustained exposure may result in hearing loss 115 dB Rock concert 94 dB iPod, sound ½ way up 120 dB Pain threshold
  • 8. Pitch • Frequency of sound – Higher frequencies have higher pitches – Lower frequencies have lower pitches – Humans hear frequencies from 20 Hz to 20,000 Hz • Infrasonic – below human hearing – Elephants and submarines use – Communicate long distances • Ultrasonic – above human hearing – Bats use to detect food and objects (echolocation) – Ex) dog whistles
  • 9. • Some common sounds and their pitch:
  • 10. • As we get older, we lose our ability to hear higher frequencies – Most adults cannot hear frequencies over 15,000 Hz – Children can often hear to 20,000 Hz
  • 11. Speed of sound: • 340 m/s (or 660 mph) • Changes with: – Hotter (faster) gases conduct sound faster – Sounds travel faster in solids • because the pressure between particles is higher
  • 12. • If object passes the speed of sound a sonic boom is created Ex) Thunder, bull whips, and bullets • Supersonic - objects travel faster than the speed of sound
  • 13. Doppler Effect • If object is moving relative to an observer, the sound waves are altered – The frequency and wavelength are changed Ex) Passing sirens Ex) Light used in weather radar

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