Waves and Sound

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Waves and Sound
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1 The Nature of Waves

1. A wave is a traveling disturbance.
2. A wave carries energy from place to place.

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1 The Nature of Waves
Longitudinal Wave
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1 The Nature of Waves
Transverse Wave
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1 The Nature of Waves
Water waves are partially transverse and
partially longitudinal.
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2 Periodic Waves
Periodic waves consist of cycles or patterns that
are produced over and over again by the
source. In the figures, every segment of the
slinky vibrates in simple harmonic motion,
provided the end of the slinky is moved in simple
harmonic motion.
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2 Periodic Waves
In the drawing, one cycle is shaded in color.
The amplitude A is the maximum excursion of a
particle of the medium from the particles
undisturbed position. The wavelength is the
horizontal length of one cycle of the wave. The
period is the time required for one complete
cycle. The frequency is related to the period
and has units of Hz, or s-1.
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.2 Periodic Waves
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.2 Periodic Waves
Example 1 The Wavelengths of Radio Waves AM and
FM radio waves are transverse waves consisting of
electric and magnetic field disturbances
traveling at a speed of 3.00x108m/s. A
station broadcasts AM radio waves whose frequency
is 1230x103Hz and an FM radio wave whose
frequency is 91.9x106Hz. Find the distance
between adjacent crests in each wave.
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16.2 Periodic Waves
AM
FM
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3 Wave Speed Versus Particle Speed on a String
Conceptual Example 3 Wave Speed Versus Particle
Speed Is the speed of a transverse wave on a
string the same as the speed at which a particle
on the string moves?
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4 The Nature of Sound Waves
LONGITUDINAL SOUND WAVES
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4 The Nature of Sound Waves
The distance between adjacent condensations is
equal to the wavelength of the sound wave.
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4 The Nature of Sound Waves
Individual air molecules are not carried along
with the wave.
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4 The Nature of Sound Waves
THE FREQUENCY OF A SOUND WAVE
The frequency is the number of cycles per
second. A sound with a single frequency is
called a pure tone. The brain interprets the
frequency in terms of the subjective quality
called pitch.
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Check your Hearing
https//www.youtube.com/watch?vqNf9nzvnd1k   http
s//www.youtube.com/watch?vVxcbppCX6Rk
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4 The Nature of Sound Waves
THE PRESSURE AMPLITUDE OF A SOUND WAVE
Loudness is an attribute of a sound that depends
primarily on the pressure amplitude of the wave.
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4 The Speed of Sound
Sound travels through gases, liquids, and solids
at considerably different speeds.
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4 The Speed of Sound
Conceptual Example 5 Lightning, Thunder, and a
Rule of Thumb There is a rule of thumb for
estimating how far away a thunderstorm is. After
you see a flash of lighting, count off the
seconds until the thunder is heard. Divide the
number of seconds by five. The result gives
the approximate distance (in miles) to the
thunderstorm. Why does this rule work?
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4 The Speed of Sound
LIQUIDS
SOLID BARS
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1. What is the wave speed if the period of a wave
is 4 seconds and the wavelength is 1.8 m?
2 A fisherman noticed that a float makes 30
oscillations in 15 seconds. The distance between
to consecutive crests is 2 m. What is the wave
speed?
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3 What is the wavelength of a wave traveling
with a speed of 6 m/s and a period of 3s?
4. What is the period of a wave traveling with a
speed of 20 m/s and the wavelength is 4.0 m?
5. What is the wave speed if the period is 4.0
seconds and the wavelength is 1.8 m?
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5 The Doppler Effect
The Doppler effect is the change in frequency or
pitch of the sound detected by an observer
because the sound source and the observer
have different velocities with respect to the
medium of sound propagation.
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demo
http//www.animations.physics.unsw.edu.au/jw/doppl
er.htmmedium visual effect https//www.youtub
e.com/watch?vh4OnBYrbCjY Lewin
demo https//www.youtube.com/watch?vwfcG0IRuffA

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5 The Doppler Effect
A. MOVING SOURCE
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5 The Doppler Effect
source moving toward a stationary observer
source moving away from a stationary observer
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5 The Doppler Effect
Example 10 The Sound of a Passing Train A
high-speed train is traveling at a speed of 44.7
m/s when the engineer sounds the 415-Hz warning
horn. The speed of sound is 343 m/s. What are
the frequency and wavelength of the sound, as
perceived by a person standing at the crossing,
when the train is (a) approaching and (b)
leaving the crossing?
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5 The Doppler Effect
approaching
leaving
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5 The Doppler Effect
B. MOVING OBSERVER
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5 The Doppler Effect
Observer moving towards stationary source
Observer moving away from stationary source
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16.9 The Doppler Effect
GENERAL CASE
Numerator plus sign applies when observer moves
towards the source
Denominator minus sign applies when source
moves towards the observer
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6. Standing wave

• Wave reflection at boundaries
• Principle of superposition, interference
• Standing waves on a string
• Normal modes

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Reflection of a wave pulse at a boundary
time
Pulse incident from right is reflected from the
boundary at left HOW the pulse is reflected
depends on the boundary conditions For fixed
end, reflected pulse is inverted For free (in
transverse direction) end, reflected pulse is
same way up.
Check using phet simulation
Frictionless sliding ring
Fixed end
Free end
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Reflection of a wave pulse at a boundary
Behaviour at interface can be modelled as sum of
two pulses moving in opposite directions at the
interface
Transverse force always 0 at interface
Transverse displacement always 0 at interface
free end
fixed end
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Standing Waves

• A standing wave is produced when a wave that is
  traveling is reflected back upon itself. There
  are two main parts to a standing wave
• Antinodes Areas of MAXIMUM AMPLITUDE
• Nodes Areas of ZERO AMPLITUDE.

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Comparison between standing wave and travelling
wave
Travelling wave particles undergo SHM all
particles have same amplitude all particles have
same frequency, adjacent particles have
different phase
Standing wave particles undergo SHM adjacent
particles have different amplitude all
particles have same frequency all particles on
same side of a node have same phase. Particles on
opposite sides of node are in antiphase
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Some very basic physics of stringed
instruments.
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The fundamental frequency determines the pitch of
the note. the higher harmonics determine the
colour or timbre of the note. (ie why
different instruments sound different)
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Fundamental wavelength 2L From v f?, f1
v/2L So, for a string of fixed length, the
pitch is determined by the wave velocity on the
string..
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Example Calculation The string length on
standard violin is 325mm. What tension is
required to tune a steel A string (diameter
0.5mm) to correct pitch (f440Hz)? Density of
steel 8g cm
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Sound Waves

• The production of sound involves setting up a
  wave in air. To set up a CONTINUOUS sound you
  will need to set a standing wave pattern.
• Three LARGE CLASSES of instruments
• Stringed - standing wave is set up in a tightly
  stretched string
• Percussion - standing wave is produced by the
  vibration of solid objects
• Wind - standing wave is set up in a column of air
  that is either OPEN or CLOSED
• Factors that influence the speed of sound are
  density of solids or liquid, and TEMPERATURE

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Closed Pipes

• Have an antinode at one end and a node at the
  other. Each sound you hear will occur when an
  antinode appears at the top of the pipe. What is
  the SMALLEST length of pipe you can have to hear
  a sound?

You get your first sound or encounter your first
antinode when the length of the actual pipe is
equal to a quarter of a wavelength.
This FIRST SOUND is called the FUNDAMENTAL
FREQUENCY or the FIRST HARMONIC.
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Closed Pipes - Harmonics

• Harmonics are MULTIPLES of the fundamental
  frequency.

In a closed pipe, you have a NODE at the 2nd
harmonic position, therefore NO SOUND is produced
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Closed Pipes - Harmonics

• In a closed pipe you have an ANTINODE at the 3rd
  harmonic position, therefore SOUND is produced.
• CONCLUSION Sounds in CLOSED pipes are produced
  ONLY at ODD HARMONICS!

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Open Pipes

• OPEN PIPES- have an antinode on BOTH ends of the
  tube. What is the SMALLEST length of pipe you can
  have to hear a sound?

You will get your FIRST sound when the length of
the pipe equals one-half of a wavelength.
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Open Pipes - Harmonics

• Since harmonics are MULTIPLES of the fundamental,
  the second harmonic of an open pipe will be ONE
  WAVELENGTH.

The picture above is the SECOND harmonic or the
FIRST OVERTONE.
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Open pipes - Harmonics

• Another half of a wavelength would ALSO produce
  an antinode on BOTH ends. In fact, no matter how
  many halves you add you will always have an
  antinode on the ends

The picture above is the THIRD harmonic or the
SECOND OVERTONE. CONCLUSION Sounds in OPEN
pipes are produced at ALL HARMONICS!
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Example

• The speed of sound waves in air is found to be
  340 m/s. Determine the fundamental frequency (1st
  harmonic) of an open-end air column which has a
  length of 67.5 cm.

251.85 HZ
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Example

• The windpipe of a typical whooping crane is about
  1.525-m long. What is the lowest resonant
  frequency of this pipe assuming it is a pipe
  closed at one end? Assume a temperature of 37C.

353.2 m/s
57.90 Hz
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Resonance demo
https//www.youtube.com/watch?v1K5p9DfsXGo Dest
ructive https//www.youtube.com/watch?vj-zczJXSx
nw Sound wave energy on water Sound,Bass,Water,
Sound makes water come alive with
cymatics Wave with Bill Nye https//www.youtube.
com/watch?vYsKC_EtUHcA Bill Nye The Science Guy
Waves Full Episode
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16.10 Applications of Sound in Medicine
By scanning ultrasonic waves across the body and
detecting the echoes from various locations, it
is possible to obtain an image.
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16.10 Applications of Sound in Medicine
Ultrasonic sound waves cause the tip of the probe
to vibrate at 23 kHz and shatter sections of the
tumor that it touches.
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16.10 Applications of Sound in Medicine
When the sound is reflected from the red blood
cells, its frequency is changed in a kind of
Doppler effect because the cells are moving.
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16.11 The Sensitivity of the Human Ear