Chapter 5
Good vibes 
 A vibration is a motion that repeats back-andforth about an equilibrium position.
 Wave - A disturbance transmitted progressively
from one place to the next with no actual transport
of matter.
 https://www.youtube.com/watch?v=Eab_beh07H
U
Waves and Vibrations
 What is the relationship between the two??
 The source of all waves is something that
vibrates.
 The back-and-forth vibratory motion of a swinging
pendulum is called simple harmonic motion.
 A sine curve is a pictorial representation of a
wave.
Parts of a Wave
• The high points on a wave are called crests.
• The low points on a wave are called troughs.
• The term amplitude refers to the distance from the
midpoint to the crest (or trough) of the wave.
• The amplitude is the maximum displacement from
equilibrium.
Parts of a Wave
• The wavelength is the distance between successive
identical parts of the wave.
• The wavelengths of waves at the beach are measured
in meters, the ripples in a pond in centimeters, of light in
billionths of a meter.
Frequency
 Frequency - the number of vibrations an object
makes in a unit of time (usually one second).
 A complete back-and-forth vibration is one cycle.
 If a vibration occurs in one second, the frequency
is one cycle per second.
 The frequency of the vibrating source and the
frequency of the wave it produces are the same.
Frequency
 The unit of frequency is called the hertz (Hz).
 A frequency of one cycle per second is 1 hertz. Higher
frequencies are measured in:
 kilohertz (kHz—thousands of hertz)
 megahertz (MHz—millions of hertz)
 gigahertz (GHz—billions of hertz)
Frequency
 Electrons in the antenna of an AM radio station at 960
kHz vibrate 960,000 times each second, producing
960-kHz radio waves.
Frequency
 Period - the time needed to complete one cycle
 Suppose a pendulum makes two vibrations in one
second. Its frequency is 2 Hz and the period is ½
second.
 Frequency and period are inverses of each other:
Example
 think!
 What is the frequency in vibrations per second of a
100-Hz wave? What is the period?
 Answer:
 A 100-Hz wave vibrates 100 times/s.
 The period is 0.01 s.
 think!
 The Sears Tower in Chicago sways back and forth at a
frequency of about 0.1 Hz. What is its period of
vibration?
 Answer: The period is
Wave Motion
 A wave is a disturbance that moves through matter
without displacing any matter.
 The disturbance moves, not parts of the string itself.
Wave motion
 Example: Water waves
 Remember the disturbance moves not the individual
droplets.
Wave Speed
 You can calculate the speed of a wave by multiplying
the wavelength by the frequency.
 If the wavelength is 1 meter, and one wavelength per
second passes the pole, then the speed of the wave is 1
m/s.
Wave Speed
 v = f
 where v is wave speed,  is wavelength, and f is
wave frequency.
Wave Speed of Sound
 In air, the product of wavelength and frequency is the
same for every frequency of sound.
 That’s why you don’t hear the high notes in a chord
before you hear the low notes. The sounds all reach
you at the same time.
 Long wavelengths have low frequencies, and short
wavelengths have high frequencies.
Speed of Sound
Types of waves
 Whenever the motion of the medium is at right angles
to the direction in which a wave travels, the wave is a
transverse wave.
Types of waves
 When the particles oscillate parallel to or along the
direction of the wave, the wave is a longitudinal
wave.
 Sound travels longitudinally
Interference
 Waves traveling through the same medium will
combine with each other
 Interference patterns occur when waves from
different sources arrive at the same point—at the
same time.
Doppler Effect
 The apparent change in frequency due to the motion
of the source (or receiver) is called the Doppler
effect.
 As a wave source approaches, an observer encounters
waves with a higher frequency.
 As the wave source moves away, an observer
encounters a lower frequency.
Doppler Effect
Bow wave
 A bow wave occurs when a wave source moves
faster than the waves it produces.
Bow wave
Bow wave
 The crests overlap at the edges, and the pattern made
by these overlapping crests is a V shape, called a bow
wave.
 The same thing happens when an aircraft travels at the
speed of sound.
 The overlapping wave crests disrupt the flow of air over
the wings, so that it is harder to control the plane
when it is flying close to the speed of sound
Shockwave
 A shock wave occurs when an object moves faster
than the speed of sound.
 A shock wave is a three-dimensional wave that consists
of overlapping spheres that form a cone.
Shockwave
 The sound of a shockwave is
called a sonic boom.
 The shock wave has not yet
encountered listener C, but
is now encountering listener
B, and has already passed
listener A.
Shockwave examples
 A supersonic bullet passing overhead produces a crack,




which is a small sonic boom.
When a lion tamer cracks a circus whip, the cracking
sound is actually a sonic boom produced by the tip of
the whip.
Snap a towel and the end can exceed the speed of
sound and produce a mini sonic boom.
http://www.magellanjets.com/blog/bid/117488/Supers
onic-Flight-Faster-Than-Speed-of-Sound
https://www.youtube.com/watch?v=BUREX8aFbMs
Review of Waves
 https://www.youtube.com/watch?v=0a5WyAjL1MM
 Types of waves:
 Transverse
 Longitudinal
 Sound travels as Longitudinal waves
Sound
 Sound is a form of energy that spreads out through
space.
 All sounds originate in the vibrations of material
objects.
I hear voices…
 The vocal chords in a person’s throat vibrate,
causing adjacent air molecules to vibrate.
 A series of ripples in the form of a longitudinal
wave travels through the air.
 Vibrations in the eardrum send rhythmic electrical
impulses into your brain and you hear the voice.
Sound
 The frequency of the sound waves
produced equals the frequency of the
vibrating source.
 Pitch - Our subjective impression
about the frequency of sound.
 A high-pitched sound like that from a
piccolo has a high vibration frequency.
 A low-pitched sound like that from a
foghorn has a low vibration frequency.
Sound Frequency
 A young person can normally hear pitches with
frequencies from about 20 to 20,000 hertz.
 As we grow older, our hearing range shrinks,
especially at the high-frequency end.
 https://www.youtube.com/watch?v=qNf9nzvnd1k
Sound Frequency
 Sound waves below 20 hertz are called infrasonic.
 Sound waves above 20,000 hertz are called
ultrasonic.
 Dogs can hear frequencies of 40,000 Hz or more.
Bats can hear sounds at over 100,000 Hz.
Power of Sound
 The Power of Sound:
 https://www.youtube.com/watch?v=Ude8pPjawKI
 https://www.youtube.com/watch?v=uPVQMZ4ikvM
Sound wave motion
 A compression travels along the spring similar to
the way a sound wave travels in air.
Sound waves
 Sound can travel through gases, liquids, and solids
 The speed of sound differs in different materials.
 In general, sound is transmitted faster in liquids
than in gases, and still faster in solids.
Sound in a vaccuum
 Sound cannot travel in a vacuum.
 The transmission of sound requires a medium.
There may be vibrations, but if there is nothing to
compress and expand, there can be no sound.
Loudness
 Sound intensity is objective and is measured by
instruments. Loudness, on the other hand, is a
physiological sensation sensed in the brain.
 The intensity of a sound is proportional to the
square of the amplitude of a sound wave.
 The unit of intensity for sound is the decibel (dB),
after Alexander Graham Bell, inventor of the
telephone.
Sound Intensity
Sound Intensity
 Starting with zero at the threshold normal
hearing, an increase of each 10 dB means that
sound intensity increases by a factor of 10.
 A sound of 10 dB is 10 times as intense as sound of
0 dB.
 20 dB is not twice but 10 times as intense as 10 dB,
or 100 times as intense as the threshold of hearing.
 A 60-dB sound is 100 times as intense as a 40-dB
sound.
Sound Intensity
𝑃𝑜𝑤𝑒𝑟
 Intensity =
𝐴𝑟𝑒𝑎
 Threshold of hearing = 10-12 w/m2 = I0
 Threshold of pain = 1 w/m2
 To convert to decibals
  = 10 log
𝐼
𝐼𝑜
Loudest Stadium
 The loudest crowd roar at a sports stadium is 142.2
dbA and was achieved by fans of the Kansas City
Chiefs (USA), at Arrowhead Stadium in Kansas
City, Missouri, USA, on 29 September 2014
Hearing Loss/Damage
 Hearing damage begins at 85 decibels. The damage
depends on the length of exposure and on frequency
characteristics.
 A single burst of sound can produce vibrations intense
enough to tear apart the organ of Corti, the receptor
organ in the inner ear.
 Less intense, but severe, noise can interfere with
cellular processes in the organ and cause its eventual
breakdown.
 Unfortunately, the cells of the organ do not regenerate.
 http://www.chsl.org/soundchart.php
Natural Frequency
 When any object composed of an elastic material is
disturbed, it vibrates at its own special set of
frequencies, which together form its special sound.
 natural frequency - the frequency at which an
object vibrates when it is disturbed
 Depends on: elasticity and shape
Forced sound
 A forced vibration occurs when
an object is made to vibrate by
another vibrating object that is
nearby.
 Sounding boards are an important
part of all stringed musical
instruments because they are
forced into vibration and produce
the sound.
Resonance
 If the frequency of a forced vibration
matches an object’s natural frequency,
resonance dramatically increases the
amplitude.
 An object resonates when there is a
force to pull it back to its starting
position and enough energy to keep it
vibrating.
 You pump a swing in rhythm with the
swing’s natural frequency.
Beats
 When two tones of slightly different frequency are
sounded together, a regular fluctuation in the
loudness of the combined sounds is heard.
 This periodic variation in the loudness of sound is
called beats.
Beats
 The vibrations of the forks will be momentarily in
step, then out of step, then in again, and so on.
 Beat frequency:
 When one fork vibrates 264 times per second, and
the other fork vibrates 262 times per second, they
are in step twice each second.
 A beat frequency of 2 hertz is heard.
Real world example
 This is how band members tune their instruments:
 If they hear a beat then the instruments are out of
tune.
 What is the beat frequency when a 262-Hz and a
266-Hz tuning fork are sounded together? A 262-
Hz and a 272-Hz?
Assessment Questions
1.
The sound waves that humans cannot hear are those with frequencies
a. from 20 to 20,000 Hz.
b. below 20 Hz.
c. above 20,000 Hz.
d. both B and C
Assessment Questions
1.
The sound waves that humans cannot hear are those with frequencies
a. from 20 to 20,000 Hz.
b. below 20 Hz.
c. above 20,000 Hz.
d. both B and C
Answer: D
Assessment Questions
2.
Sound travels in air by a series of
a. compressions.
b. rarefactions.
c. both compressions and rarefactions.
d. pitches.
Assessment Questions
2.
Sound travels in air by a series of
a. compressions.
b. rarefactions.
c. both compressions and rarefactions.
d. pitches.
Answer: C
Assessment Questions
3.
Sound travels faster in
a. a vacuum compared to liquids.
b. gases compared to liquids.
c. gases compared to solids.
d. solids compared to gases.
Assessment Questions
3.
Sound travels faster in
a. a vacuum compared to liquids.
b. gases compared to liquids.
c. gases compared to solids.
d. solids compared to gases.
Answer: D
Assessment Questions
4.
The speed of sound varies with
a. amplitude.
b. frequency.
c. temperature.
d. pitch.
Assessment Questions
4.
The speed of sound varies with
a. amplitude.
b. frequency.
c. temperature.
d. pitch.
Answer: C
Assessment Questions
5.
The loudness of a sound is most closely related to its
a. frequency.
b. period.
c. wavelength.
d. intensity.
Assessment Questions
5.
The loudness of a sound is most closely related to its
a. frequency.
b. period.
c. wavelength.
d. intensity.
Answer: D
Assessment Questions
6.
When you tap various objects they produce characteristic sounds
that are related to
a. wavelength.
b. amplitude.
c. period.
d. natural frequency.
Assessment Questions
6.
When you tap various objects they produce characteristic sounds
that are related to
a. wavelength.
b. amplitude.
c. period.
d. natural frequency.
Answer: D
Assessment Questions
7.
When the surface of a guitar is made to vibrate we say it undergoes
a. forced vibration.
b. resonance.
c. refraction.
d. amplitude reduction.
Assessment Questions
7.
When the surface of a guitar is made to vibrate we say it undergoes
a. forced vibration.
b. resonance.
c. refraction.
d. amplitude reduction.
Answer: A
Assessment Questions
8.
When an object is set into vibration by a wave having a frequency
that matches the natural frequency of the object, what occurs is
a. forced vibration.
b. resonance.
c. refraction.
d. amplitude reduction.
Assessment Questions
8.
When an object is set into vibration by a wave having a frequency
that matches the natural frequency of the object, what occurs is
a. forced vibration.
b. resonance.
c. refraction.
d. amplitude reduction.
Answer: B
Assessment Questions
9.
Noise-canceling devices such as jackhammer earphones make
use of sound
a. frequency.
b. interference.
c. resonance.
d. amplification.
Assessment Questions
9.
Noise-canceling devices such as jackhammer earphones make
use of sound
a. frequency.
b. interference.
c. resonance.
d. amplification.
Answer: B
Assessment Questions
10. The phenomenon of beats is the result of sound
a. destruction.
b. interference.
c. resonance.
d. amplification.
Assessment Questions
10. The phenomenon of beats is the result of sound
a. destruction.
b. interference.
c. resonance.
d. amplification.
Answer: B