2. SOUND

3. Longitudinal Wave Travels through some medium Cannot travel through a vacuum How does vibrating drum produce sound? Skin moving up presses air molecules close together and creates high pressure – compression Skin moving down spreads molecules out causing low pressure - rarefaction

4. Speed of Sound

5. The elasticity of steel is greater than air, so we would expect the velocity of sound in steel to be ________ than air A) Less than B) Greater than C) The same as D) Not enough information

6. Some Speeds Air (20°)343 m/s Air (0°)331 m/s Helium1005 m/s Hydrogen1300 m/s Water1440 m/s Seawater1560 m/s Iron or Steel5000 m/s

7. Speed In Air Speed in air also depends on temperature v = 331 + (.60) T

8. What is the speed of sound in air at 45° C?

9. Example A general rule of finding the distance of a lightning strike is that there is “one mile for every 5 seconds before the thunder is heard”. Is this true at about 20°C? 1 mile = 1610m

10. Characteristics of Sound Loudness – intensity of sound wave Loudness is determined by the amplitude of the wave

11. Pitch Tone of the sound Depends on the frequency Higher frequency, higher pitch low high

12. A sound with a frequency of 150 Hz is ___________ in pitch of a 300 Hz sound.

13. Range of Sound Audible range – for humans, we can hear tones between 20 Hz and 20000 Hz Infrasonic – tones lower than 20 Hz Ultrasonic – tones higher than 20000 Hz

14. A sound of 35000 Hz is __________

15. Graphing Sound Longitudinal waves can be hard to show graphically Can be represented by plotting pressure vs. time

16. Intensity Loudness Energy transported by a wave per unit time across an area Proportional to square of amplitude Intensity = Power/Area = W/m 2

17. Human Hearing Can detect 10 -12 W/m 2 to 1 W/m 2 Threshold of Hearing – minimum intensity we can hear Threshold of Pain – above this sound is painful

18. True or False People can hear sounds of 10 -14 W/m 2

19. Log Scale Since intensity increase by factor of 10, log scale can be used Unit of scale is called the decibel (dB) – symbol   = 10 log ( I/ I 0 ) I – intensity of the sound I 0 – threshold of hearing

20. For Comparison gentle breeze10 dB soft whisper20 dB quiet library40 dB conversation60 dB machine shop100 dB rock band110 dB threshold of pain120 dB jet plane140 dB rocket launch180 dB

21. Example What are the decibels of sounds with intensities of 5.0 x 10 -6 W/m 2 ?

22. What is the intensity level for a sound of 20 dB? A) 1 x 10 -8 B) 1 x 10 -9 C) 1 x 10 -10 D) 1 x 10 -11