Sound

1. Sound • Longitudinal wave • requires a medium • (cannot travel in a vacuum)

2. Study of Sound is known as ACOUSTICS

3. Properties of Sound • A sound is a vibration • The vibrating causes the air molecules near the movement to be forced closer. This is called compression • As the vibration moves on, the density and air pressure becomes lower than normal and is called rarefaction • Pressure wave – longitudinal • Frequency = pitch • v = 334 m/s in air at room temperature • Velocity is dependent upon the material

4. Temperature Can affect speed Do molecules move faster or slower as temperature increases? So would sound travel faster or slower as temperature increases?

5. Type of Medium Solid Liquid Gas Waves travel fastest in solids, slowest in gases.

6. Speed of sound • Fastest in solids, slowest in gases. • Air = 340 m/s • water= 1440 m/s • steel = 5000 m/s • Supersonic: faster than the • speed of sound.

7. Speed of sound • Sound travels faster in warm water than in cold water • By measuring the time it takes for sound to travel a known distance through the ocean the average temperature of the water can be calculated = ATOC (acoustic thermometry of ocean climate)

8. Air Helium Hydrogen Water Sea water Iron/Steel Glass Aluminum 343 m/s 1005 m/s 1300 m/s 1440 m/s 1560 m/s ≈5000 m/s ≈ 4500 m/s ≈ 5100 m/s Speeds of Soundat T = 20 C

9. Pitch The highness or lowness of sound. Depends on the frequency of sound waves. High frequency = High pitch Low frequency = Low pitch

10. Intensity • Also called LOUDNESS • Amount of energy • Depends on the amplitude of sound waves. (amplifier) Large Amp. = Loud sound Small Amp. = Soft sound

11. Intensity of Sound • Unit is the “Bel”. Named after Alexander Graham Bell • More commonly used is the decibel (dB) = 0.1 Bel •  (in dB) = 10 log(I/I0) • I is the intensity ( A2) • I0 = 1.0 x 10 -10 W/m2the “threshold of hearing”

12. Jet plane at 30 m Threshold of pain Indoor rock concert Auto interior Street traffic Conversation Whisper Rustle of leaves 140 120 120 75 70 65 1x10-10 1x10-11 Some Intensities (in dB)

13. Reverberation • Sound waves reflecting from hard surfaces • Ex.: Multiple echo resulting from the direct sound AND the reflected sound

14. Reverberation vs Echo Animations courtesy of Paul Hewitt and borrowed from physicsclassroom.com

15. Measurement of loudness Decibels • Near total silence - 0 dB • A whisper - 15 dB • Normal conversation - 60 dB • Lawnmower - 90 dB • Threshold of pain - 120 dB • A rock concert or a jet engine - 120 dB • Gunshot, firecracker - 140 dB

16. Sound is a pressure wave Animations courtesy of Paul Hewitt and borrowed from physicsclassroom.com

17. Ear

18. Human Ear 20Hz--20 000 Hz Infrasonic: below 20 Hz (below our hearing) Ultrasonic Sound: above 20,000Hz (above our hearing)

19. Tuning fork creating a sound wave Animations courtesy of Paul Hewitt and borrowed from physicsclassroom.com

20. Ultrasounds • Ultrasound can be used to create internal images of the human body • Ex. Pregnant woman gets a “picture” of her unborn baby

21. White Noise • An equal mixture of all frequencies of sound • Used to calm stress • Used in office buildings

22. Noise Pollution • Yep! There is such a thing • Causes: • Damage to ear resulting in hearing loss • Stress

23. Dead Spots • Caused by destructive interference • Result – hardly any or no sound • Bad for concert halls – designers be careful!

24. sonar Used to locate underwater objects and distances. ***Reflection**

25. Doppler Effect • Apparent change in frequency (pitch) of a sound from a moving source. • Source moving toward observer: • f’ = f / (1-vs/v) • Source moving away from observer: • f’ = f / (1+vs/v) f = frequency of source, f’=frequency heard by observer, vs = velocity of source, v = velocity of sound

26. Doppler effect Change in pitch due to motion.

29. Doppler effect *Moving towards increases the pitch *Moving away decreases the pitch *Think of sirens zoom

30. Guitar String creating a sound wave Animations courtesy of Paul Hewitt and borrowed from physicsclassroom.com