1. Chapter 26
Sound
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2. Origin of Sound
All sounds are produced by the vibrations of material objects
For example in a piano, violin, or guitar a sound wave is produced by vibrating strings

3. Sound is made when something vibrates.
Origin of Sound
Sound is made when something vibrates.
The vibration disturbs the air around it.
This makes changes in air pressure.
These changes in air pressure move through the air as sound waves.

4. Sound Waves Alternating areas of high & low pressure in the air
ALL sound is carried through matter as sound waves
Sound waves move out in ALL directions from a vibrating object

5. Nature of Waves
Waves (Def.) – A wave is a disturbance that transfers energy.
Medium – Substance or region through which a wave is transmitted.
Speed of Waves – Depends on the properties of the medium.

6. Properties of Waves  or T
pressure
horizontal axis could be:
space: representing
snapshot in time
time: representing
sequence at a par-
ticular point in space
Wavelength () is measured from crest-to-crest
or trough-to-trough, or upswing to upswing, etc.
For traveling waves (sound, light, water), there is a speed (c)
Frequency (f) refers to how many cycles pass by per second
measured in Hertz, or Hz: cycles per second
associated with this is period: T = 1/f
These three are closely related:
f = c

7. Longitudinal vs. Transverse Waves
Sound is a longitudinal wave, meaning that the motion of particles is along the direction of propagation
Transverse waves—water waves, light—have things moving perpendicular to the direction of propagation

8. Why is Sound Longitudinal?
Waves in air can’t really be transverse, because the atoms/molecules are not bound to each other
can’t pull a (momentarily) neighboring molecule sideways
only if a “rubber band” connected the molecules would this work
fancy way of saying this: gases can’t support shear loads
Air molecules can really only bump into one another
Imagine people in a crowded train station with hands in pockets
pushing into crowd would send a wave of compression into the crowd in the direction of push (longitudinal)
jerking people back and forth (sideways, over several meters) would not propagate into the crowd
but if everyone held hands (bonds), this transverse motion would propagate into crowd

9. Questions A wave in which the particles of the medium move at right
angles to the direction of the wave is a __________.
Transverse wave
High pitch is produced by sounds that have__________.
High frequency

10. Speed of Sound
Sound speed in air is related to the frantic motions of molecules as they jostle and collide
since air has a lot of empty space, the communication that a wave is coming through has to be carried by the motion of particles
for air, this motion is about 500 m/s, but only about 350 m/s directed in any particular direction
Solids have faster sound speeds because atoms are hooked up by “springs” (bonds)
don’t have to rely on atoms to traverse gap
spring compression can (and does) travel faster than actual atom motion

11. Examples of Sound Speeds
Medium
sound speed (m/s)
air (20C)
343
water
1497
gold
3240
brick
3650
wood
3800–4600
glass
5100
steel
5790
aluminum
6420

12. Components of Sound Pitch (how high or low) Loudness (volume)
Timbre (tone color)

13. Vibration
Back and forth movement of molecules of matter
For example,

14. Does high density material tend to cause sound to go faster or slower?
Is low temperature cause sound to go faster or slower?
Is highly elastic material cause sound to go faster or slower?

15. The Physics of Sound

16. Pitch … is the "highness" or "lowness" of a tone.
Pitch corresponds to frequency.

17. Pitch The vibration patterns of some sounds are repetitive.
Vibration patterns are also called waveforms.
Each repetition of a waveform is called a cycle.
We can hear frequencies between 20 hertz or cycles (vibrations) per second (low pitches)to 20 kilohertz, i.e. 20,000 Hz (high pitches).

18. Pitch Continued Infrasonic frequencies < 20 Hz Ultrasonic
Human hearing range
frequencies between 20 Hz and 20,000 Hz

19. When the frequency of a sound doubles we say that the pitch goes up an octave.
We can hear a range of pitches of about ten octaves.
Many animals can make sounds and hear frequencies that are beyond what we can hear.

20. Could you hear a pitch of 19 Hz?

21. Sound in Air
...a longitudinal wave in air caused by a vibrating object.
Demo: slinky

22. The Ear Sound is carried to our ears through vibrating air molecules.
Our ears take in sound waves & turn them into signals that go to our brains.
Sound waves move through 3 parts of the ear; outer ear, middle ear, & inner ear.
Middle Ear

23. The Ear
The outer ear gathers sound waves, passing them through the ear canal to a tough membrane called the eardrum
The vibrating eardrum passes the sound to three tiny bones in the middle ear – the hammer, anvil, and stirrup – which amplify the sound wave

24. The Ear
The stirrup vibrates and transfers the sound to a membrane in the oval window, then on to the inner ear’s cochlea, a spiral-shaped structure that contains hair cells
As the hair cells in the cochlea vibrate, nerve impulses are sent through the auditory nerve to the brain

25. The Ear
The sound waves cause pressure changes against our ear drum sending nerve impulses to our brain.

26. Speakers: Inverse Eardrums
Speakers vibrate and push on the air
pushing out creates compression
pulling back creates rarefaction
Speaker must execute complex motion according to desired waveform

27. What type of wave would moving a rope attached to a wall simulate?
Question
What type of wave would moving a rope attached to a wall simulate?

28. Sound in Air Continued Sound requires a medium. solid, liquid or gas
Demo: Bell in a evacuated Bell Jar
Sound waves have compression and rarefaction regions.

29. Noise Measuring Equipment
Sound Level Meters (SLM)
Continuous on-mobile sources
Noise Dosimeters
Mobile/variable
noise sources

30. Questions
Would sound travel faster on a train track or in a pool?

31. Compression
Where molecules are being pressed together as the sound waves move through matter
For example,
a wave travels through the springs just like sound waves travel through the air
the places where the springs are close together are like compressions in the air.

32. Rarefaction
The change in direction of a wave as it crossed the boundary between two media in which the wave travels at different speeds

33. Questions Refraction occurs because waves__________.
Move at different speeds in different mediums
In rarefaction, the molecules of the medium are _____.
Spaced apart

34. The Physics of Sound

35. The Physics of Sound

36. Are the air molecules more spread out in compression or rarefraction?

37. Media that Transmits Sound
Most sounds you hear are transmitted through air.
Solids and liquids are generally good conductor of sound— much better than air
Sounds cannot travel through a vacuum

38. Speed of Sound in Air
Speed of sound = 340 meters/second or 760 miles/hour
Sound travels faster in hot, humid climates
Speed of sound depends on elasticity

39. Speed of Sound in Air Speed of sound = (331.5 + 0.6T) m/s
So at higher temperatures the speed of sound is faster because of the faster moving molecules. At 0 degrees the speed in air is m/s. This speed increases with the temperature at about 0.6 m/s per degree Celsius.
Speed of sound = ( T) m/s
T = temperature in Celsius

40. Speed of Sound and Refraction
Sound travels faster in warm air. Why?

41. Late one summer night ( T = 20 degrees Celsius), lightning is seen from an approaching storm and five seconds later, thunder sound is heard. How far away is the storm?
Speed of sound = ((331.5) + 0.6(20))m/s
Speed of sound = m/s
V = d/t
343.5 m/s = d / 5s
D = m

42. Wavelength & Frequency
Wavelength is the distance between one part of a wave and the same part of the next wave
Frequency is the number of waves moving past a point in one second

44. Can you predict how far a storm is if you here thunder 8 seconds away?
340m/s x 8 s = 2720 m

45. Sound Intensity and Loudness
Intensity of Sound refers to the amplitude of the pressure variations in the sound wave

46. Loudness
The physiological sensation directly related to the sound intensity
Measured in bels
(10 bels = 1 decibels)

47. As the sound spreads out from its source, the concentration of power becomes less.
As the distance from the source increases the amount of power is spread over a greater area.
The amount of power per square meter is called the intensity of the sound.

48. Loudness To create vibrations energy is used.
The greater amount of energy used the louder the sound.
The strength of the changes in air pressure made by the vibrating object determines loudness.

49. Loudness
A sound of 10 decibels is or 101 or 10 times as intense as 0 decibels.
20 decibels is 102 or 100 times the intensity 0 decibels.

50. Source of Sound Loudness (db)
Threshold of Hearing 0
Conversation
Ear Damage Begins 85
Amplified Music
Jet Airplane at 30 meters 140

51. Common Sound Intensities

52. Hearing Protection Devices and Their Noise Reduction Ratings
EAR Foam Plugs (NRR = 29 dB)
EAR CARBOFLEX (NRR= 20 dB)
Moldex PURAFIT Foam Plugs
(NRR=30 dB)

53. The human perception of intensity is known as _____.
Loudness
Sound waves are _______________ waves.
Compressional
The speed of sound depends on?
Temperature
Density
Elasticity
Reflection of a wave occurs when the wave______.
Strikes a boundary and bounces back
The pitch of a sound depends on its__________.
Frequency

54. Humans do not perceive sound intensity linearly.
For us to perceive a sound as twice as loud its intensity must be ten times greater.
The perceived intensity level of sound is measured in a logarithmic scale using a unit called the decibel (dB) 2

55. From the perspective of the logarithmic scale the threshold of pain is
1,000,000,000,000
times as great as the threshold of hearing.

56. What is the loudest setting should set their ipod at to maintain a healthy ear?
85 decibels

57. Timber
is the specific property of sound that enables us to determine the difference between a piano and a harp.

58. The predominant pitch is called the fundamental frequency.
An extremely broad variety of tone colors exist because most sounds that we perceive as pitch actually contain many frequencies.
The predominant pitch is called the fundamental frequency.

59. Although we would perceive a string vibrating as a whole,
it actually vibrates in a pattern that at first appears to be erratic producing many different overtone pitches. What results are particular tone colors or timbres of instruments and voices.

60. The other frequencies which occur in a mathematical series are called the harmonic or overtone series.
When C1 is the fundamental the following pitches represent its first fifteen successive overtones.

61. Forced Vibrations
…the setting up of vibrations in an object by a vibrating force.
Examples of Forced Vibration:
A tuning fork touching a wood surface
Sounding boards for stringed instruments
Matching tuning fork boxes

63. Natural Frequency
…the frequency at which an elastic object naturally tends to vibrate.
At this frequency, a minimum energy is required to produce a forced vibration.
The natural frequency of a body depends on its elasticity and its shape.

64. What factor besides shape affects natural frequency?

65. Natural Frequency Examples
Dropping Aluminum Rods
Ringing Small and Large Bells
Xylophone
Rubbing a Wine Glass
Mass on a Spring

66. 38. What is the speed of a wave if the frequency is 300 Hz
and the wavelength is 150m?
S = λf
S = (150)(300) =
45000 m/s
39. What is the wavelength of a 350 Hz wave traveling at
500m/s?
S = λf
λ = 1.4 meters
40. How many cycles per second would be characteristic of a
30 m wave traveling at 300 m/s?
S = λf
10 cycles/sec (Hz)

67. All Shapes of Waveforms
Different Instruments have different waveforms
a: glockenspiel
b: soft piano
c: loud piano
d: trumpet
Our ears are sensitive to the detailed shape of waveforms!
More waveforms:
e: french horn
f: clarinet
g: violin

68. Section 3 – Music
Music – sounds that are deliberately used in a regular pattern
Natural frequency – frequency at which the material vibrates
Resonance – the ability of a medium to vibrate by absorbing energy at its own natural frequency

69. Resonance
…is the result of forced vibrations in a body when the applied frequency matches the natural frequency of the body.
The resulting vibration has a high amplitude and can destroy the body that is vibrating.

70. Examples of Resonance breaking a wine glass using sound
mass on a spring at resonance
a singing rod caused by forced vibration
a tuning fork exciting a guitar string
In 1940, the Tacoma Narrows Bridge was destroyed by wind-generated resonance.

71. 1. Frequency times wavelength equals_________.
2. What is the number of complete wave cycles per unit time?
3. A ____________ is a region in the medium in which
molecules are crowded together.
4. The ability of an object to vibrate by absorbing energy at its
natural frequency is called _________.

72. Sound Interference
Overlapping crests of a wave will result in an increased amplitude.
Overlapping a crest and a trough results in a decrease in amplitude.

73. Beats
Beats - the periodic variation in loudness of two sounds played together
The beat frequency is equal to the difference in the frequency of the two sounds.

74. Acoustics... ...the study of sound properties.
When a sound wave strikes a surface it can be.…
(a) reflected.
(b) transmitted.
(c) absorbed.
(d) all of these. 

75. Reflection of Sound e.g. an echo
Reverberation - re-echoed sound, multiple reflections of sound waves from walls
Compare reflections from a hard wall with that from a carpet wall.
Demo: Whip

76. Refraction of Sound Refraction - the bending of a wave
Sound waves bend toward cooler air.
Desert and Lake Example

77. Radio Broadcasts AM - Amplitude Modulation FM - Frequency Modulation
535 kHz to 1605 kHz
FM - Frequency Modulation
88 MHz to 108 MHz
Modulation - an impression of the sound wave on a higher frequency radio waves

78. Modulating Radio Waves
© 2000 Microsoft Clip Gallery
Modulation - variation of amplitude or frequency when waves are broadcast
AM – amplitude modulation
Carries audio for T.V. Broadcasts
Longer wavelength so can bend around hills
FM – frequency modulation
Carries video for T.V. Broadcasts

79. Humans use sonar to locate or map objects
An instrument that uses reflected sound waves to find underwater objects
For example,
Humans use sonar to locate or map objects
Animals use sonar or echo location to find their prey; these sounds have such a high pitch or frequency that the human ear cannot hear

80. Bibliography http://science.pppst.com/sound.html
schools.birdvilleschools.net/ /lib/.../Chapter_12.ppt
kisdwebs.katyisd.org/.../Waves%20and%20sound%20powerpoint.ppt