Ch. 17 – Sound

1. Ch. 17 – Sound

2. Sound waves (like all mechanical waves) need a material to move through • Therefore, there is no sound in space

3. Sound waves are longitudinal The vibration travels when molecules bump into neighboring molecules

4. Sound waves are longitudinal Compressions are regions of high pressure Rarefactions are regions of low pressure

5. Converting the longitudinal wave into an analogous transverse wave

6. High frequency high pitch Frequency is perceived as pitch

7. Dog whistles are ultrasonic Range of human hearing • We can only hear sounds between 20 Hz and 20,000 Hz • Vibrations below 20 Hz are called infrasonic • Above 20,000 Hz are ultrasonic

8. Audible frequencies From Kinetic Books, section 17.2

9. T = + v ( 331 m/s) 1 273 Sound wave speed depends on • Material • Solid  Liquid  Gas • (fast to slow) the disturbance (bump) travels faster if neighboring molecule is close to you (even better if bonded) • Temperature • hotter faster Eqn. 4 – only good for waves IN AIR

10. Speed of sound in various media From Kinetic Books, section 17.4

11. 5 second rule for thunder and lightning! Various speeds of sound speed of sound in various materials, at 24ºC (room temp) Air (0ºC) 331 m/s 740 mph Air 345772 helium 1000 2300 water 1500 3200 seawater 1600 3500 steel 5000 11,300

12. Sound Intensity The sound power per unit area Sound intensity: †

13. As a sound wave travels, it typically spreads out. You perceive the loudness of the sound of a loudspeaker at an outdoor concert differently at a distance of one meter than you do at 100 meters • The intensity of the sound diminishes with distance †

14. Sound Intensity v. Loudness Sound Intensity is objective (a physical characteristic of sound that can be measured in Watts/m2) Sound Level (loudness) is subjective (a physiological sensation that depends on two things, but is measured in decibels) • observer Not everyone will agree that a 80 dB sound is “loud” • frequency • A 3500-Hz sound at 80 dB sounds about twice as loud to most people as a 125-Hz sound at 80 dB; humans are more sensitive to the 3500-Hz frequencies †

15. Sound Level, b A scale for measuring the perceived intensity of sounds Sound level: Recall: log (a/b) = log (a) – log (b) †

16. Loudness measured in decibels • decibels • “deci” (intensity is scaled by factors of 10) • “bel” (in honor of Alexander Graham Bell) • A sound level of 0 dB is the lowest threshold of hearing 10 times more intense than 0 dB 100 times more intense 1,000 times more intense 1,000,000 times more intense • 10 dB = • 20 dB = • 30 dB = • 60 db = †

17. 120 dB 50 dB How loud is “loud”? Source of sound Sound level (dB) Nearby jet airplane 140 Jackhammer 130 Rock concert or siren 120 Vacuum cleaner 70 Normal conversation 50 Mosquito buzzing 40 Whisper 30 Rustle of leaves 10 Threshold of hearing 0 10,000,000 times more power per m2

18. the Doppler effect • The Doppler effect is the perceivedchange in frequency due to motion of the source and/or the listener (why do we emphasize “perceived”? • It is NOT the change in volume

20. It’s a PERCEIVED change in pitch • If a source of sound is moving toward you, you hear a higher frequency than when it is at rest • If a source of sound is moving away from you, you hear a lower frequency than when it is at rest

21. This effect is named for the Austrian physicist who first analyzed it, Christian Doppler (1803-1853) • Doppler’s research concerned light from stars, but his principles apply to sound also

22. Doppler effect → radar guns • When radar waves bounce off a moving object, the FREQUENCY of the reflected radar changes by an amount that depends on how fast the object is moving • The detector senses the amount of frequency shift and translates this into a speed

23. Once you see the cop, he’s already got you! • This is now done with lasers (light waves) which are extremely accurate • They really can tell if you’ve been going just a couple of m.p.h. over the limit!

24. Doppler effect Moving Source Moving Listener … listener moving toward source … listener moving away from source … source moving away from listener … source moving towards listener If both listener and source are moving:

25. Doppler problem 1 QUESTION The 16,000-Hz whine of the jet engines of an aircraft moving with speed 200 m/s is heard at what frequency by the pilot of a second craft trying to overtake the first at a speed of 250 m/s? Take the speed of sound to be 320 m/s. ANSWER

26. When an object is at the speed of sound • It travels as fast as its own sound waves • The sound waves pile up on one another in front of the moving object

27. The first man to break the “sound barrier” in an aircraft was Chuck Yeager • October 14, 1947 • Read the book or see this story in the movie “The Right Stuff” Bell X-1

28. Bow wave = V-shaped disturbance made when object moves faster than wave speedShock wave = cone-shaped disturbance made when object moves faster than speed of sound

29. Object exceeds the speed of its sound waves • Sound waves form a Mach cone • Surface of the cone is the shock wave • Angle of cone narrows at greater speeds

31. Does object have to make sound to break sound barrier? • NO. Once an object moves faster than the speed of sound, it will make sound (“sonic boom”) • Examples: • the crack of a whip or a snapped towel • Fired bullet passing overhead

32. Misconception • The sonic boom is not produced only when the object exceeds the speed of sound • The shock wave (and resulting cracking sound) sweeps continuously behind object “Wow! A sonic boom!” “What was that?!” “Huh?”

33. Exploration • How does the human vocal system produce such amazing sounds? • Feel your throat while making the following sounds • make soft sounds, loud sounds • sing a high note, a low note • say aaah, say eeeee Value of f1 depends on v (speed of wave on string or in tube) and l (length of string or tube)