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## Sound

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### 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 ... – PowerPoint PPT presentation

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Title: Sound

1
Sound
• Mechanical Vibrations in Gas, Liquid or Solid

2
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.

3
Anatomy of a Sound wave
• Sound waves consist of alternating compressions
rarefactions in a longitudinal wave

Courtesy University of Wisconsin
4
Sound Waves Passes By Us
Courtesy University of Sydney
5
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

Sound-Harmonic Applets
6
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 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

7
Speed of Sound
• About 331 m/s at 00 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)

8
Experiencing Speed of Sound
• Name two experiences you have had caused by the
relatively slow speed of sound
• Echoes
• Delay between seeing lightning and hearing thunder

9
Thunder Delay
• 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

10
Sounds in Liquids and Solids
• 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?

11
Wave Properties Review
• 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

12
Analogy
• Complete Loudness is to intensity as pitch is
to ________________

frequency
13
Intensity in decibels
• Energy in a sound wave in a certain area
• dB 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
14
dB(SPL) Source (with distance)
194 Theoretical limit for a sound wave at 1 atmosphere environmental pressure
180 Rocket engine at 30 mKrakatoa 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)
15
Questions
• Use dB 10 log10 (I/I0) to answer
• (1) If the sound of a siren is twenty times
more intense than that of a person speaking, how
many decibels more is this?

Answer 13 dB more since log 20 1.3
16
Questions
• Use dB 10 log10 (I/I0) to answer
• (2) If the sound of a rock concert is fifty
times more intense than the sound of street
traffic, how many decibels more is this?

17
Forced Vibration, Natural Frequency and Resonance
• 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

18
Examples of Resonance
• How do you need to push the person on the swing
to get the maximum amplitude?
• At natural frequency!

19
The Tacoma Narrows Bridge
• What do you think happened next?
• What was the cause?
• Wind induced vibrations

Resonance Applets from New Zealand
20
• 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.

21
Wave Interference
• Complete constructive
• Complete Destructive
• Applets
• http//www. mysite.verizon.net/vzeoacw1/wave_inter
ference.html

22
Describe Each of These
Answers from top down complete destructive,
partial destructive, constructive
23
Anti-noise Technology
• 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

24
Speakers Out of Phase
• 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
25
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 frequency
• Beats simulation applet

26
Unequal Comb Spacing Produces Moire Pattern like
Beats
27
Beats Questions
• 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

28
Another Example of Beats
• How could you use the phenomenon of beats to get
these engines to run at the same speed?
(synchronize them)

29
Doppler Effect in Sound
• Source or observer moving towards pitch
increases
• Source or observer moving away pitch decreases
• Airplane Flyby

Wavelength Increases Frequency Decreases
Wavelength Decreases Frequency Increases
Doppler from New Zealand
30

Opposing Waves
Con/Destructive
Standing Waves
Beats