1. Sound is a longitudinal mechanical wave that propagates through a medium such as air or water by compressions and rarefactions which create regions of high and low pressure. 2. The document discusses several properties of sound waves including that frequency determines pitch, amplitude determines loudness, and speed depends on the properties of the medium. 3. Wave interference and phenomena like resonance, standing waves, and the Doppler effect are also covered as they relate to the nature and perception of sound waves.

1. Unit 1 - Waves
Sound Waves

2. Unit 1 - Waves
• Sound is a wave, which is created by vibrating objects
and propagated through a medium from one location to
another. A sound wave is similar in nature to a slinky
wave for a variety of reasons.
– First, there is a medium which carries the disturbance from
one location to another.
– Second, there is an original source of the wave, some vibrating
object capable of disturbing the first particle of the medium.
– Third, the sound wave is transported from one location to
another by means of the particle interaction.
• Since a sound wave is a disturbance, which is
transported through a medium via the mechanism of
particle interaction, a sound wave is characterized as a
mechanical wave.
Sound Waves

3. Unit 1 - Waves
• Sound waves are longitudinal waves
because particles of the medium through
which the sound is transported vibrate
parallel to the direction, which the sound
moves.
• There are regions of compressions and
rarefactions. The compressions are
regions of high air pressure while the
rarefactions are regions of low air
pressure.
Sound Waves

4. Unit 1 - Waves
• The diagram below depicts a sound wave
created by a tuning fork and propagated
through the air in an open tube.
Sound Waves

5. Unit 1 - Waves
• Even though a sound wave is a longitudinal
wave, it can be mathematically represented as
a transverse wave. The crests correspond to
areas of high pressure (compressions) and the
troughs correspond with the rarefactions or
areas of low pressure.
Sound Waves

6. Unit 1 - Waves
Sound Waves

7. Unit 1 - Waves
Check Your Understanding:
• A sound wave is a pressure wave; regions of high
(compressions) and low pressure (rarefactions) are
established as the result of the vibrations of the sound
source. These compressions and rarefactions result
because sound
a.is more dense than air and thus has more inertia,
causing the bunching up of sound.
b.waves have a speed which is dependent only upon the
properties of the medium.
c.is like all waves; it is able to bend into the regions of
space behind obstacles.
d.is able to reflect off fixed ends and interfere with
incident waves
e.vibrates longitudinally; the longitudinal movement of
air produces pressure fluctuations.
Sound Waves

8. Unit 1 - Waves
• Frequency for any wave can be defined as the
number of cycle of the wave that pass by a
given point every second. The sound wave is a
longitudinal wave.
• Each cycle of the sound wave includes one
compression and one rarefaction. If, for
example, a speaker vibrates back and forth at a
frequency of 500 Hz, then 500 compressions,
each followed by a rarefaction, is generated
every second. A sound with a single frequency
is called a pure tone.
Pitch & Frequency of
Sound Waves

9. Unit 1 - Waves
• A healthy young person hears all sound
frequencies from 20 Hz to 20 000 Hz (20
kHz). This audible range of frequencies is
called the audio spectrum.
• Infrasonic sound waves are those below
the range of normal human hearing.
These frequencies occur below 20 Hz.
• Ultrasonic frequencies occur above the
range of normal human hearing and
therefore are above 20 kHz.
Pitch & Frequency of
Sound Waves

10. Unit 1 - Waves
• Frequency is an objective physical property of a
sound wave and can be measured using an
electronic frequency counter. The listener’s
perception of sound is subjective.
• The brain interprets the frequency detected by
the ear as a subjective or perceived quality
called pitch.
• A pure tone with a high frequency is
interpreted as a high-pitched sound. A pure
tone with a low frequency is interpreted as a
low-pitched sound.
Pitch & Frequency of
Sound Waves

11. Unit 1 - Waves
Pitch & Frequency of
Sound Waves

12. Unit 1 - Waves
• The diagram below shows a pure-tone
sound wave travelling down a tube.
• In the areas of rarefaction, the air
pressure of the sound wave is low. In the
areas of compression, the air pressure is
high.
Sound Pressure, Amplitude
and Loudness

13. Unit 1 - Waves
• An air pressure versus distance graph has the
appearance of a transverse wave even though
sound itself is a longitudinal wave.
• The pressure amplitude shown in the graph
above is the magnitude of the maximum change
in pressure measured relative to undisturbed or
Sound Pressure, Amplitude
and Loudness

14. Unit 1 - Waves
• Loudness is a characteristic of sound that
depends primarily on the amplitude of
the wave. The larger the amplitude, the
louder the sound. The pressure amplitude
is an objective property of the sound
wave since it can be measured with an
instrument. Loudness, on the other hand,
is subjective. Each individual determines
what is loud, depending on the acuteness
of his or her hearing.
Sound Pressure, Amplitude
and Loudness

15. Unit 1 - Waves
• Sound waves carry energy with them as they
move from one place to another.
• The intensity of a sound wave is the energy
that is transported past a given area per unit of
time. When the amplitude of a sound wave
increases, the energy of the wave increases and
therefore the intensity is also larger. As the
sound wave travels away from its source, the
surface area is larger and the intensity of the
wave is less. The intensity is less because the
wave is spreading out over a larger area.
Sound Intensity and
Decibels

16. Unit 1 - Waves
• The decibel (dB) is a measurement that is used to
compare two sound intensities.
• The threshold of hearing is assigned a decibel level of
0 dB.
– A sound that is 10 (101
) times more intense than the threshold is
assigned a sound level of 10 dB. Rustling leaves would have a
level of 10 dB.
– A sound that is 100 (102
) times more intense than the threshold is
assigned a sound level of 20 dB. This is the sound of a whisper.
– A sound that is 100 000 (105
) times more intense than the
threshold has a sound level of 50 dB. This is the sound of an
average classroom with students actively working with each
other.
– Other examples of sound levels are a car without a muffler at 100
dB, a live rock concert at 120 dB, and the threshold of pain is at
about 130 dB.
Sound Intensity and
Decibels

17. Unit 1 - Waves
Sound Intensity and
Decibels

18. Unit 1 - Waves
• Wave interference is the phenomenon, which
occurs when two waves meet while traveling
along the same medium. Recall, there is two
types of interference; constructive and
destructive.
• Sound is a pressure wave, which consists of
compressions and rarefactions. The
interference of sound waves causes the
particles of the medium to behave in a
manner that reflects the net effect of the two
individual waves upon the particles.
Wave Interference &
Production of Beats

19. Unit 1 - Waves
• If a compression (high pressure) of one wave meets
up with a compression (high pressure) of a second
wave at the same location in the medium, then the
net effect is that that particular location will
experience an even greater pressure. This is a form
of constructive interference.
• If two rarefactions (two low pressure disturbances)
from two different sound waves meet up at the same
location, then the net effect is that that particular
location will experience an even lower pressure. This
is also an example of constructive interference.
Wave Interference &
Production of Beats

20. Unit 1 - Waves
• If two sound waves interfere at a given location in
such a way that the compression of one wave meets
up with the rarefaction of a second wave, destructive
interference results.
• The net effect of a compression (which pushes
particles together) and a rarefaction (which pulls
particles apart) upon the particles in a given region
of the medium is to not even cause a displacement of
the particles. The particles would remain at their
rest position as though there wasn't even a
disturbance passing through them. This is a form of
destructive interference.
Wave Interference &
Production of Beats

21. Unit 1 - Waves
• Constructive and destructive interference
occur when the waves are of the same
frequency. In this section we will study what
happens when sound waves have slightly
different frequencies. This gives rise to a
phenomenon called beats.
• Sound beats are the periodic and repeating
fluctuations heard in the intensity of a sound
when two sound waves of slightly different
frequencies interfere with one another.
Wave Interference &
Production of Beats

22. Unit 1 - Waves
• The sound changes from loud to softer, then loud again,
and so on. The diagram below illustrates the wave
interference pattern resulting from two waves with
slightly different frequencies.
Wave Interference &
Production of Beats

23. Unit 1 - Waves
Wave Interference &
Production of Beats

24. Unit 1 - Waves
• The beat frequency is the rate at which the
sound alternates from loud to soft and equals
the difference in frequency of the two sounds.
If two sound waves with frequencies of 440 Hz
and 442 Hz interfere to produce beats, a beat
frequency of 2 Hz will be heard.
• The human ear is only capable of hearing
beats with small beat frequencies (e.g., 8 Hz
or less).
Wave Interference &
Production of Beats

25. Unit 1 - Waves
Wave Interference &
Production of Beats

26. Unit 1 - Waves
• Recall, a standing wave pattern is formed
when reflected waves interfere with incident
waves to form a standing wave that appears
to be standing in place.
• The frequency at which the standing wave
exists is called the resonant frequency. Each
resonant frequency is a whole-number
multiple of the lowest resonant frequency
called the fundamental frequency.
Standing Wave Patterns

27. Unit 1 - Waves
• The word resonance comes from Latin
and means to "resound" - to sound out
together with a loud sound. Resonance is
a common cause of sound production in
musical instruments.
• This is known as resonance - when one
object vibrating at the same natural
frequency of a second object forces that
second object into vibrational motion.
Resonance

28. Unit 1 - Waves
Resonance

29. Unit 1 - Waves
• The way our ears separate various
frequencies is based on the principle
of resonance. An object that, when
struck, can vibrate with a certain
frequency, will also start vibrating in
response to a sound wave of this
frequency.
Resonance

30. Unit 1 - Waves
Speed of Sound

31. Unit 1 - Waves
• Like any wave, the speed of a sound
wave refers to how fast the disturbance
is passed from particle to particle. While
frequency refers to the number of
vibrations, which an individual particle
makes per unit of time, speed refers to
the distance, which the disturbance
travels per unit of time.
• Always be cautious to distinguish
between the two often-confused
quantities of speed (how fast...) and
frequency (how often...).
Speed of Sound

32. Unit 1 - Waves
• Since the speed of a wave is defined
as the distance, which a point on a
wave (such as a compression or a
rarefaction) travels per unit of time,
it is often expressed in units of
meters/second (abbreviated m/s).
In equation form, this is
speed = distance/time
Speed of Sound

33. Unit 1 - Waves
• The speed of any wave depends
upon the properties of the medium
through which the wave is traveling.
Typically there are two essential
types of properties which effect
wave speed - inertial properties and
elastic properties.
Speed of Sound

34. Unit 1 - Waves
• The density of a medium is an example
of an inertial property. The greater the
inertia (i.e., mass density) of individual
particles of the medium, the less
responsive they will be to the
interactions between neighbouring
particles and the slower the wave. If all
other factors are equal (and seldom is it
that simple), a sound wave will travel
faster in a less dense material than a
more dense material.
Speed of Sound

35. Unit 1 - Waves
• Elastic properties are those
properties related to the tendency of
a material to both maintain its
shape and not deform whenever a
force or stress is applied to it.
• Steel versus plasticine.
Speed of Sound

36. Unit 1 - Waves
• In general, solids have the strongest
interactions between particles, followed
by liquids and then gases. For this
reason, longitudinal sound waves travel
faster in solids than they do in liquids
than they do in gases. Even though the
inertial factor may favor gases, the
elastic factor has a greater influence on
the speed (v) of a wave, thus yielding
this general pattern:
vsolids
> vliquids
> vgases
Speed of Sound

37. Unit 1 - Waves
• The speed of a sound wave in air depends
upon the properties of the air, namely the
temperature and the pressure. The pressure
of air (like any gas) will affect the mass
density of the air (an inertial property) and
the temperature will affect the strength of
the particle interactions (an elastic
property). At normal atmospheric pressure,
the temperature dependence of the speed of
a sound wave through air is approximated
by the following equation:
v = 331 m/s + (0.6 m/s/C)*T
where T is the temperature of the air in
degrees Celsius.
Speed of Sound

38. Unit 1 - Waves
• Perhaps you recall an instance in which a
police car or emergency vehicle was
traveling towards you on the highway. As
the car approached with its siren blasting,
there seemed to be a change in the pitch of
the siren; and after the car passed by, the
pitch of the siren sound changed again.
Doppler Effect
In small groups discuss this
phenomenon, explaining what you
would here before car reaches you
and after the car passes you. Try
and explain why this may occur.

39. Unit 1 - Waves
Doppler Effect

40. Unit 1 - Waves
Doppler Effect

41. Unit 1 - Waves
• The Doppler effect is a phenomenon
observed whenever the source of waves is
moving with respect to an observer. The
Doppler effect can be described as the effect
produced by a moving source of waves in
which there is an apparent upward shift in
frequency for the observer as the source is
approaching and an apparent downward
shift in frequency when the observer and
the source is receding.
Doppler Effect

42. Unit 1 - Waves
• When a vehicle travels faster than the
speed of sound, a sonic boom can be
heard. As the vehicle overtakes its
own sound, the sound waves spread
out behind in a shockwave, or sonic
boom.
Doppler Effect