Mechanical Vibrations in Gas,
Liquid or Solid

 Students know sound is a longitudinal wave whose speed depends on the properties of the medium in which it propagates.
 Students know how to identify the characteristic properties of waves: interference (beats) , diffraction, refraction, Doppler effect , and polarization.

 Sound waves consist of alternating compressions and rarefactions in a longitudinal wave
Courtesy University of Wisconsin

Courtesy University of Sydney

 Vibrations of matter produce sounds
 Sound could be a wave in air, or another medium
 The original vibration makes something more massive vibrate, such as a sounding board

 Frequency is the technical term of the number of vibrations per second in a sound wave going past us
 Pitch is how we hear frequency
 Range of human hearing is about 20 to
20,000 Hz
 As we age, we can no longer hear the highest pitch sounds
 Listen to Different Frequencies

 About 331 m/s at 0 0 C
 Increases by 0.6 m/s for every degree
 1200 km/hr
 1100 feet per second
 Depends on properties of medium such as
 Density
 Elasticity(ease of changing shape)

 Name two experiences you have had caused by the relatively slow speed of sound
 Echos
 Delay between seeing lightning and hearing thunder
 Starting gun delay between seeing smoke and hearing

 You hear thunder five seconds after you see lightning. How far away from you did the lightning strike?
 D = s x t
 D = 1100 feet/sec x 5 s =
 5500 ft or about one mile
 Metric: 340 m/s x 5 s = 1700 m = 1.7 km

 Sound travels much faster in liquids than in air
 About 1500 m/s in water
 Much faster still in solids
 About 4500 m/s in steel
 Can you think of a scene in a movie based on the high speed of sound in solids?

 What are the characteristic properties of all waves?
 Frequency, wavelength, speed, amplitude
 Intensity is proportional to the square of amplitude
 The sensation we experience due to the intensity of a sound wave is called loudness

 Complete: Loudness is to intensity as

 Energy in a sound wave in a certain area
 dB = 10 log
10
(I/I
0
) dB is “decibels”
Logarithms Review
Log
10
(10) = 1
Log
10
(100) = 2
Log
10
(1000) = 3
Log
10
(10,000) = ?
To find the logarithm of a number to a certain base is to find the exponent to which the base needs to be raised to obtain the original number

70
60
50
100
90
80
40
30
10
0
150
140
130
120
110 dB(SPL)
194
180
Source (with distance)
Theoretical limit for a sound wave at 1 atmosphere environmental pressure
Rocket engine at 30 m;
Krakatoa explosion at 100 miles in air(160 km) [1]
Jet engine at 30 m
Rifle being fired at 1 m
Threshold of pain ; train horn at 10 m
Rock concert ; jet aircraft taking off at 100 m
Accelerating motorcycle at 5 m; chainsaw at 1 m
Jackhammer at 2 m; inside disco
Loud factory , heavy truck at 1 m
Vacuum cleaner at 1 m, curbside of busy street
Busy traffic at 5 m
Office or restaurant inside
Quiet restaurant inside
Residential area at night
Theatre, no talking
Human breathing at 3 m
Threshold of human hearing (with healthy ears)

 Use dB = 10 log
10
(I/I
0
) to answer:
(1) If the sound of a siren is 100 times more intense than that of a person speaking, how many decibels more is this?
Answer: 20 dB more

 Use dB = 10 log
10
(I/I
0
) to answer:
(2) If the sound of a rock concert is
100,000 times more intense than the sound of street traffic, how many decibels more is this?
Answer: 50 dB more

 All objects have frequencies they vibrate at naturally – natural frequencies
 Any object can be forced to vibrate
 When object is forced to vibrate at its natural frequency, the result is called resonance

 How do you need to push the person on the swing to get the maximum amplitude?
 At natural frequency!

 What do you think happened next?
 What was the cause?
 Wind induced vibrations

 Resonance, Tacoma Narrows bridge failure, and undergraduate physics textbooks by Yusuf Billah and Bob Scanlan, (Am. J. Phys. 59 (2),
February 1991)
 " . . . in many undergraduate physics texts the (1940
Tacoma Narrows bridge) disaster is presented as an example of elementary forced resonance . . . Engineers, on the other hand, have studied the phenomenon . . . and their current understanding differs fundamentally from the viewpoint expressed in most physics texts. In the present article the engineers' viewpoint is presented . . . It is then demonstrated that the ultimate failure of the bridge was in fact related to an aerodynamically induced condition of selfexcitation or "negative damping" . . . This paper emphasizes the fact that. physically as well as mathematically, forced resonance and self- excitation are fundamentally different phenomena.

 Complete constructive
 Complete Destructive
 Applets
 http://www. mysite.verizon.net/vzeoac w1/ wave _ interference .ht
ml

Answers from top down: complete destructive, partial destructive, constructive

 Example of destructive interference
 Use microphone, amplifier and speaker to produce opposite sound
 This combines with the original sound by destructive interference to produce…
 No sound!
 Sometimes called active noise cancellation
 Buy headphones

 Face speakers toward each other
 Change one wire so they are connected wrong (out of phase)
 What do you predict will happen
Two speakers sound less loud than only one

 Another example of sound wave interference
 When two tuning forks close in frequency are played you hear a third sound, whose frequency is the difference between the two tuning fork frequencies
 Called beat frequency
 Beats simulation applet

 Tuning forks of 254 and 256 Hz are played. What is the beat frequency?
 2 Hz
 Tuning forks of 514 and 518 Hz are played. What is the beat frequency?
 4 Hz

 How could you use the phenomenon of beats to get these engines to run at the same speed? (synchronize them)

 Source or observer moving towards – pitch increases
 Source or observer moving away – pitch decreases
Wavelength
Wavelength decreases increases
 Fun applets
 http://www.ndted.org/EducationResources/HighSchool/Sound/dopplereffect.htm
 http://library.thinkquest.org/19537/java/Doppler.html
 http://www.lon-capa.org/~mmp/applist/doppler/d.htm