1. Sound Chapter 15

2. Topics for Sound Sound wave propertiesSound wave properties Speed of soundSpeed of sound EchoesEchoes BeatsBeats Doppler shiftDoppler shift ResonanceResonance Anatomy of EarAnatomy of Ear

3. Sound Wave Properties

4. Sound Waves are Longitudinal Waves The air molecules shown below are either compressed together, or spread apart. This creates alternating high and low pressure.

5. Frequency The frequency of a sound wave (or any wave) is the number of complete vibrations per second. The frequency of sound determines its pitch.

6. The higher the frequency, the higher the pitch

7. Wavelength Wavelength is the distance between two high pressures, or two low pressures. This property is dependent on the velocity of the sound and it’s frequency. Wavelength and frequency are inversely related. Short wavelength (high frequency) results in a high pitch.

8. Frequency and the human ear A young person can hear pitches with frequencies from about 20 Hz to 20000 Hz. (most sensitive to frequencies between 1000 and 5000 Hz). As we grow older, our hearing range shrinks, especially at the high frequency end. By age 60, most people can hear nothing above 8000 Hz. Sound waves with frequencies below 20 Hz are called infrasonic. Sound waves with frequencies above 20000 Hz are called ultrasonic.

9. The Amplitude of a Sound Wave Determines its loudness or softness

10. Velocity of Sound

11. The velocity of sound depends on the medium it travels throughthe medium it travels through the temperature of the mediumthe temperature of the medium

12. Sound travels faster in liquids than in air (4 times faster in water than in air)Sound travels faster in liquids than in air (4 times faster in water than in air) Sound travels faster in solids than in liquids (11 times faster in iron than in air)Sound travels faster in solids than in liquids (11 times faster in iron than in air) Sound does not travel through a vacuum ( there is no air in a vacuum so sound has no medium to travel through)Sound does not travel through a vacuum ( there is no air in a vacuum so sound has no medium to travel through) The speed depends on the elasticity and density of the medium.The speed depends on the elasticity and density of the medium.

13. In air at room temperature, sound travels at 343m/s (~766 mph)In air at room temperature, sound travels at 343m/s (~766 mph) v = 331 m/s + (0.6)Tv = 331 m/s + (0.6)T –v: velocity of sound in air –T: temperature of air in o C As temperature increases, the velocity of sound increases Effects of Temperature

14. Relationship between velocity, frequency, and wavelength V = fV = f V = velocity of soundV = velocity of sound = wavelength of sound = wavelength of sound f = frequency of soundf = frequency of sound

15. Echoes: REFLECTION

16. Echoes are the result of the reflection of sound Sound waves leave a source, travel a distance, and bounce back to the origin.

17. Things that use echoes... Bats Dolphins/ Whales Submarines Ultra sound Sonar

18. REFRACTION OF WAVES

20. Refraction of Sound as the sound wave transmits into the warmer air at lower levels, they change direction, much like light passing through a prism

22. DIFFRACTION: THE BENDING OF WAVES THROUGH A SMALL OPENING

23. BENDING OF A WAVE

24. Sound waves move out like this: http://www.kettering.edu/~drussell/Demos/doppler/doppler.html

25. But when they move, the front of the wave gets bunched up (smaller wavelength) and the back of the wave starts to expand (larger wavelength): http://www.kettering.edu/~drussell/Demos/doppler/doppler.html

26. C B A Observer C hears a high pitch (high frequency) Observer B hears the correct pitch (no change in frequency) Observer A hears a low pitch (lower frequency) http://www.kettering.edu/~drussell/Demos/doppler/doppler.html

27. When the source goes faster, the wave fronts in the front of the source start to bunch up closer and closer together, until...

28. The object actually starts to go faster than the speed of sound. A sonic boom is then created. http://www.kettering.edu/~drussell/Demos/doppler/doppler.html

29. Doppler Effect The doppler effect is a change in the apparent frequency due to the motion of the source or the receiver. Example: As an ambulance with sirens approaches, the pitch seems higher. As the object moves by the pitch drops.

30. Police use the Doppler Shift when measuring your speed with radar A frequency is sent out of the radar gun The sound wave hits the speeding car The frequency is changed by the car moving away from the radar and bouncing back The amount the frequency changes determines how fast you are going The faster you are going, the more the frequency is changed.

31. Equation that describes the doppler effect. f d = f s (v + v d ) (v - v s ) f s is the actual frequency being emitted f d is the perceived frequency as the source approaches or recedes v d is (+) if the observer moves toward the source v d is (-) if the observer moves away from the source v s is (+) if the source moves toward the observer v s is (-) if the source moves away from the observer

32. Example Sitting at Six Flags one afternoon, Mark finds himself beneath the path of the airplanes leaving Hartsfield International Airport. What frequency will Mark hear as a jet, whose engines emit sounds at a frequency of 1000 Hz, flies toward him at a speed of 100 m/sec? (temp is 10 o C)

33. Solution v = 331 + (0.6)T v = 331 + (0.6)(10) v = 337 m/s f d = f s (v + v d )f = ? (v – v s )f s = 1000 Hz v d = 0 m/s v s = 100 m/s

34. Solution f= 1000 (331 + 0) (331 – 100) f = 1430 m/s

35. SOUND INTENSITY: THE LOUDNESS OF SOUND

36. Sound Intensity The intensity of a sound is the amount of energy transported past a given area in a unit of time. Intensity = power/area The greater the amplitude, the greater the rate at which energy is transported-the more intense the sound Intensity is inversely related to the square of the distance. As distance increases, the intensity decreases.

37. Threshold of Hearing The human ear is sensitive to variations in pressure waves, that is, the amplitude of sound waves. The ear can detect wave amplitudes of 2x10 -5 Pa up to 20 Pa. The amplitudes of these waves are measured on a logarithmic scale called sound level. Sound level is measured in decibels (dB).

38. DECIBEL MEASURES THE LOUDNESS OF SOUND RELATES TO THE AMPLITUDE OF THE WAVE EVERY INCREASE OF 10dB HAS 10x GREATER AMPLITUDE

39. Source of SoundLevel (dB)Increase over Threshold Threshold0 dB0 Normal Breathing10 dB10 Whisper20 dB100 Normal Conversation60 dB10 6 Busy street traffic70 dB10 7 Vacuum cleaner80 dB10 8 Average factory90 dB10 9 IPod at maximum level100 dB10 Threshold of pain120 dB10 12 Jet engine at 30 m140 dB10 14 Perforation of eardrum160 dB10 16

40. A SOUND 10 TIMES AS INTENSE IS PERCEIVED AS BEING ONLY TWICE AS LOUD

41. NOISE POLLUTION · Prolonged exposure to noise greater than 85-90 dB may cause hearing loss · Brief exposures to noise sources of 100-130 dB can cause hearing loss · A single exposure to a level of 140 dB or higher can cause hearing loss

42. Hours Per Day Noise Level (dB) 8 90 495 2*100* 1105 0.5110 EXPOSURE TO LOUD NOISE

43. Reducing Sound Intensity Cotton earplugs reduce sound intensity by approximately 10 dB. Special earplugs reduce intensity by 25 to 45 dB. Sound proof materials weakens the pressure fluctuations either by absorbing or reflecting the sound waves. When the sound waves are absorbed by soft materials, the energy is converted into thermal energy.

44. Resonance

45. Natural Frequency Nearly all objects when hit or disturbed will vibrate. Each object vibrates at a particular frequency or set of frequencies. This frequency is called the natural frequency. If the amplitude is large enough and if the natural frequency is within the range of 20-20000 Hz, then the object will produce an audible sound.

46. Timbre Timbre is the quality of the sound that is produced. If a single frequency is produced, the tone is pure (example: a flute) If a set of frequencies is produced, but related mathematically by whole-number ratios, it produces a richer tone (example: a tuba) If multiple frequencies are produced that are not related mathematically, the sound produced is described as noise (example: a pencil)

47. Factors Affecting Natural Frequency Properties of the medium Modification in the wavelength that is produced (length of string, column of air in instrument, etc.) Temperature of the air

48. Resonance Resonance occurs when one object vibrates at the same natural frequency of a second object, forcing that second object into vibrational motion. Example: pushing a swing Resonance is the cause of sound production in musical instruments. Energy is transferred thereby increasing the amplitude (volume) of the sound. http://www.pbs.org/wgbh/nova/bridge/meetsusp.html

49. Types of Resonance Resonance takes place in both closed pipe resonators and open pipe resonators. Resonance is achieved when there is a standing wave produced in the tube. Closed pipe resonators –open end of tube is anti-node –closed end of tube is node Open pipe resonators –both ends are open –both ends are anti-nodes

50. Closed pipe resonator

51. Harmonics of Closed Pipe Resonance The shortest column of air that can have a pressure anti-node at the closed end and a pressure node at the open end is ¼ wavelength long. This is called the fundamental frequency or first harmonic. As the frequency is increased, additional resonance lengths are found at ½ wavelength intervals. The frequency that corresponds to ¾ wavelength is called the 3 rd harmonic, 5/4 wavelength is called the 5 th harmonic, etc.

52. Open pipe resonator

53. Harmonics of Open Pipe Resonance The shortest column of air that can have nodes (or antinodes) at both ends is ½ wavelength long. This is called the fundamental frequency or first harmonic. As the frequency is increased, additional resonance lengths are found at ½ wavelength intervals. The frequency that corresponds to a full wavelength is the second harmonic, 3/2 wavelength is the third harmonic, etc.

54. Problems 1. Matt is playing a toy flute, causing resonating waves in a open-end air column. The speed of sound through the air column is 336 m/s. The length of the air column is 30.0 cm. Calculate the frequency of the first, second, and third harmonics.

55. Solution 1.L = λ/2 2 x L = λ 2 x.30 =.60 m v = f λ 336 = f (.60) f = 560 Hz. (first harmonic) 2 nd harmonic = 560 + 560 = 1120 Hz. 3 rd harmonic = 1120 + 560 = 1680 Hz

56. Problem 2. Tommy and the Test Tubes have a concert this weekend. The lead instrumentalist uses a test tube (closed end air column) with a 17.2 cm air column. The speed of sound in the test tube is 340 m/s. Find the frequency of the first harmonic played by this instrument.

57. Solution 2. L = λ/4 4 x L = λ 4 x.172 =.688 m v = f λ 340 = f (.688) f = 494 Hz

58. Beats

59. A beat occurs when sound waves of two different (but very much alike) frequencies are played next to each other. The result is constructive and destructive interference at regular intervals.

60. This oscillation of wave amplitude is called a beat. The frequency of a beat is the magnitude of difference between the frequencies of the two waves, f= ‌ f A – f B ‌ See example problem 10 on p. 367.

61. Anatomy of the Ear

63. Pinna Sound starts at the Pinna

64. auditory canal Then goes through the auditory canal

65. Tympanic Membrane The sound waves will then vibrate the Tympanic Membrane (eardrum) which is made of a thin layer of skin.

66. Malleus (hammer) Incus (anvil)Stapes (stirrup) The tympanic membrane will then vibrate three tiny bones : the Malleus (hammer), the Incus (anvil), and the Stapes (stirrup)

67. Cochlea The stapes will then vibrate the Cochlea

68. Inside look of the Cochlea The stapes vibrates the cochleaThe stapes vibrates the cochlea The frequency of the vibrations will stimulate particular hairs inside the cochleaThe frequency of the vibrations will stimulate particular hairs inside the cochlea The intensity at which these little hairs are vibrated will determine how loud the sound is.The intensity at which these little hairs are vibrated will determine how loud the sound is. The auditory nerve will then send this signal to the brain.The auditory nerve will then send this signal to the brain. 