Sound
Mechanical Vibrations in Gas, Liquid or Solid
California State Standards
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.
Anatomy of a Sound wave
Sound waves consist of alternating compressions and rarefactions in a longitudinal wave
Courtesy University of Wisconsin
Sound Waves Passes By Us
Courtesy University of Sydney
What Causes Sound
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 and Pitch
Frequency is the technical term of the number of vibrations per second in a sound wave going past us
Pitch is how we hear frequencyRange of human hearing is about 20 to
20,000 HzAs we age, we can no longer hear the
highest pitch soundsListen to Different Frequencies
Speed of Sound
About 331 m/s at 00 CIncreases by 0.6 m/s for every degree1200 km/hr1100 feet per secondDepends on properties of medium such as
Density Elasticity(ease of changing shape)
Experiencing Speed of Sound
Name two experiences you have had caused by the relatively slow speed of sound
EchosDelay between seeing lightning and
hearing thunderStarting gun delay between seeing smoke
and hearing
Thunder Delay
You hear thunder five seconds after you see lightning. How far away from you did the lightning strike?
D = s x tD = 1100 feet/sec x 5 s =5500 ft or about one mile Metric: 340 m/s x 5 s = 1700 m = 1.7 km
Sounds in Liquids and Solids
Sound travels much faster in liquids than in air
About 1500 m/s in waterMuch faster still in solidsAbout 4500 m/s in steelCan you think of a scene in a movie based
on the high speed of sound in solids?
Wave Properties Review
What are the characteristic properties of all waves?
Frequency, wavelength, speed, amplitudeIntensity is proportional to the square of
amplitudeThe sensation we experience due to the
intensity of a sound wave is called loudness
Analogy
Complete: Loudness is to intensity as pitch is to ________________frequency
Intensity in decibels
Energy in a sound wave in a certain areadB = 10 log10 (I/I0) dB is “decibels”
Logarithms Review
Log10 (10) = 1
Log10 (100) = 2
Log10 (1000) = 3
Log10 (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
dB(SPL) Source (with distance)
194 Theoretical limit for a sound wave at 1 atmosphere environmental pressure
180 Rocket engine at 30 m;Krakatoa explosion at 100 miles in air(160 km)[1]
150 Jet engine at 30 m
140 Rifle being fired at 1 m
130 Threshold of pain; train horn at 10 m
120 Rock concert; jet aircraft taking off at 100 m
110 Accelerating motorcycle at 5 m; chainsaw at 1 m
100 Jackhammer at 2 m; inside disco
90 Loud factory, heavy truck at 1 m
80 Vacuum cleaner at 1 m, curbside of busy street
70 Busy traffic at 5 m
60 Office or restaurant inside
50 Quiet restaurant inside
40 Residential area at night
30 Theatre, no talking
10 Human breathing at 3 m
0 Threshold of human hearing (with healthy ears)
Questions
Use dB = 10 log10 (I/I0) 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
Questions
Use dB = 10 log10 (I/I0) 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
Forced Vibration, Natural Frequency and Resonance
All objects have frequencies they vibrate at naturally – natural frequencies
Any object can be forced to vibrateWhen object is forced to vibrate at its
natural frequency, the result is called resonance
Examples of Resonance
How do you need to push the person on the swing to get the maximum amplitude?
At natural frequency!
The Tacoma Narrows Bridge
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 self-excitation or "negative damping" . . . This paper emphasizes the fact that. physically as well as mathematically, forced resonance and self- excitation are fundamentally different phenomena.
Wave Interference
Complete constructive
Complete Destructive
Applets http://www.
mysite.verizon.net/vzeoacw1/wave_interference.html
Describe Each of These
Answers from top down: complete destructive, partial destructive, constructive
Anti-noise Technology
Example of destructive interferenceUse microphone, amplifier and speaker to
produce opposite soundThis combines with the original sound by
destructive interference to produce…No sound!Sometimes called active noise
cancellationBuy headphones
Speakers Out of Phase
Face speakers toward each otherChange one wire so they are connected
wrong (out of phase)What do you predict will happen
Two speakers sound less loud than only one
Beats
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 frequencyBeats simulation applet
Unequal Comb Spacing Produces Moire Pattern like Beats
Beats Questions
Tuning forks of 254 and 256 Hz are played. What is the beat frequency?
2 HzTuning forks of 514 and 518 Hz are
played. What is the beat frequency?4 Hz
Another Example of Beats
How could you use the phenomenon of beats to get these engines to run at the same speed? (synchronize them)
Doppler Effect in Sound
Source or observer moving towards – pitch increases
Source or observer moving away – pitch decreases
Fun applets http://www.ndt-ed.org/EducationResources/HighSchool/Sound/
dopplereffect.htm http://library.thinkquest.org/19537/java/Doppler.html http://www.lon-capa.org/~mmp/applist/doppler/d.htm
Wavelength decreases
Wavelength increases