If a tree falls in a forest, and there
is no one there to hear it, does it
make a sound?
Based on our definition, there
IS sound in the forest, whether
a human is there to hear it or
Sound is a physical
disturbance in a medium.
A person to hear it is not required. The
medium (air) is required!
The sound wave is transported
from one location to another by
means of particle-to-particle
• If the sound wave is moving
through air, then as one air
particle is displaced from its
equilibrium position, it exerts a
push or pull on its nearest
neighbors, causing them to be
displaced from their equilibrium
Sound is a mechanical wave
Since a sound wave is a
disturbance that is transported
through a medium via the
mechanism of particle-to-particle
interaction, a sound wave is
characterized as a mechanical
A sound wave is different than a light wave in that
a sound wave is
a. produced by a vibrating object and a light wave
b. not capable of traveling through a vacuum.
c. not capable of diffracting and a light wave is.
d. capable of existing with a variety of frequencies
and a light wave has a single frequency.
Check your understanding:
--When a tuning fork vibrates, it creates areas of high
pressure (compressions) and low pressure
--As the tines of the fork vibrate back and forth, they
push on neighboring air particles.
--The forward motion of a tine pushes air molecules
horizontally to the right and the backward retraction
of the tine creates a low-pressure area allowing the
air particles to move back to the left.
Graphing a Sound Wave.
Sound as a pressure wave
The variation of pressure with distance is a useful way to
represent a sound wave graphically. But remember – sound is
actually a longitudinal wave.
A sound wave is a pressure wave; regions of high
pressure (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.
b. waves have a speed that is dependent only upon the
properties of the medium.
c. can be diffracted around obstacles.
d. vibrates longitudinally; the longitudinal movement of air
produces pressure fluctuations.
Check your understanding
--The vibrating object that creates sound could be
the vocal cords of a person, the vibrating string of a
guitar or violin, the vibrating tines of a tuning fork,
or the vibrating diaphragm of a radio speaker.
--As a sound wave moves through a medium, each
particle of the medium vibrates at the same
frequency. This makes sense since each particle
vibrates due to the motion of its nearest neighbor.
--And of course the frequency at which each particle
vibrates is the same as the frequency of the original
source of the sound wave.
Frequency of Sound
A guitar string vibrating at 500 Hz will set
the air particles in the room vibrating at the
same frequency of 500 Hz, which carries a
sound signal to the ear of a listener, which is
detected as a 500 Hz sound wave.
Frequency of Sound Example
• We hear frequencies of sound as having
• A low frequency sound has a low pitch, like
the rumble of a big truck.
• A high-frequency sound has a high pitch, like
a whistle or siren.
• In speech, women have higher fundamental
frequencies than men.
The frequency of sound
Frequency of Sound
• The human ear is capable of detecting sound
waves with a wide range of frequencies, ranging
between approximately 20 Hz to 20 000 Hz.
• Any sound with a frequency below the audible
range of hearing (i.e., less than 20 Hz) is known
as an infrasound.
• Any sound with a frequency above the audible
range of hearing (i.e., more than 20 000 Hz) is
known as an ultrasound.
Ultrasound is a medical imaging technique
that uses high frequency sound waves and
The technique is similar to the echolocation
used by bats, whales and dolphins.
How it works: Ultrasound
The ultrasound machine transmits high-
frequency (1 to 5 megahertz) sound pulses
into your body using a probe.
The sound waves travel into your body and
hit a boundary between tissues (e.g. between
fluid and soft tissue, soft tissue and bone).
Some of the sound waves get reflected back to
the probe, while some travel on further until
they reach another boundary and get
The reflected waves are picked up by the
probe and relayed to the machine.
The machine calculates the distance from
the probe to the tissue or organ
(boundaries) using the speed of sound in
tissue and the time of the each echo's
return (usually on the order of millionths
of a second).
The machine displays the distances and
intensities of the echoes on the screen,
forming a two dimensional image like the
one shown below.
What about animals?
Dogs can detect frequencies as low
as approximately 50 Hz and as
high as 45 000 Hz.
Cats can detect frequencies as low
as approximately 45 Hz and as
high as 85 000 Hz.
Frequency and music
Certain sound waves when played (and heard)
simultaneously will produce a particularly
pleasant sensation when heard. Such sound
waves form the basis of intervals in music.
For example, any two sounds whose frequencies
make a 2:1 ratio are said to be separated by an
octave and result in a particularly pleasing
sensation when heard. That is, two sound waves
sound good when played together if one sound
has twice the frequency of the other.
Intensity: the rate at which a wave’s
energy flows through an area
Sound intensity depends on
Distance from source
Measured in decibels (dB)
Loudness is sort of like
Loudness is Subjective! (This means it depends
on the person who is hearing it.)
Loudness is a personal, physical response to the
intensity of sound.
As intensity increases, so does loudness, but
loudness also depends on the listener’s ears and
Intensity is caused by the
Amplitude of the vibration
A vibrating guitar string forces surrounding air molecules to be
compressed and expanded.
The energy that is carried by the wave is imparted to the medium by
the vibrating string.
The amount of energy that is transferred to the medium is dependent
on the amplitude of vibrations of the guitar string.
If more energy is put into the plucking of the string, then the string
vibrates with a greater amplitude. The greater amplitude of
vibration of the guitar string thus imparts more energy to the
medium, causing air particles to be displaced a greater distance
from their rest position.
The Decibel Scale:
The decibel (abbreviated dB) is the
unit used to measure the intensity of a
The decibel scale is a little odd
because the human ear is incredibly
Your ears can hear everything from your
fingertip brushing lightly over your skin
to a loud jet engine .
In terms of power, the sound of the jet
engine is about 1,000,000,000,000 times
more powerful than the smallest audible
sound. That's a big difference!
On the decibel scale, the smallest
audible sound (the threshold of
hearing) is 0 dB.
A sound 10 times more powerful is
A sound 100 times more powerful
than near total silence is 20 dB
A sound 1,000 times more powerful
than near total silence is 30 dB.
Intensity (Loudness) is measured in decibels:
# of Times
Greater Than TOH
Threshold of Hearing 0 dB 100
Rustling Leaves 10 dB 101
Whisper 20 dB 102
Normal Conversation 60 dB 106
Busy Street Traffic 70 dB 107
Vacuum Cleaner 80 dB 108
Large Orchestra 98 dB 109.8
Walkman at Maximum Level 100 dB 1010
Front Rows of Rock Concert 110 dB 1011
Threshold of Pain 130 dB 1013
Military Jet Takeoff 140 dB 1014
Instant Perforation of Eardrum 160 dB 1016
Check your understanding
A mosquito's buzz is often rated with a
decibel rating of 40 dB.
Normal conversation is often rated at 60 dB.
How many times more intense is normal
conversation compared to a mosquito's
The speed of sound depends only
on the properties of the medium it’s
The Speed of Sound
In general, sound travels fastest through
This is because molecules in a solid
medium are much closer together than
those in a liquid or gas, allowing sound
waves to travel more quickly through it.
In fact, sound waves travel over 17 times
faster through steel than through air.
Speed of Sound
Sound also travels faster in liquids than in
gases because molecules are still more
In fresh water, sound waves travel 4 times
faster than in air!
Speed of Sound
The speed of sound in air is 343 meters per
second (660 miles per hour) at one
atmosphere of pressure and room
The speed of sound
When we look at the properties of a gas, we see
that only when molecules collide with each other
can the compressions and rarefactions of a sound
wave be passed along. So, it makes sense that the
speed of sound depends on how often the particles
collide. At higher temperatures, molecules collide
more often, giving the sound wave more chances to
move forward rapidly.
Speed of Sound
The frequency or frequencies at which
an object tends to vibrate with when
hit, struck, plucked, strummed or
somehow disturbed is known as the
natural frequency of the object.
Some objects tend to vibrate at a single
frequency and they are often said to produce a
pure tone. A flute tends to vibrate at a single
frequency, producing a very pure tone.
Other objects vibrate and produce more
complex waves with a set of frequencies that
have a whole number mathematical relationship
between them; these are said to produce a rich
sound. A tuba tends to vibrate at a set of
frequencies that are mathematically related by
whole number ratios; it produces a rich tone.
Other objects will vibrate at a set of
multiple frequencies that have no
simple mathematical relationship
between them. These objects are not
musical at all and the sounds that
they create could be described as
The same note from different instruments
has different qualities because the sounds
from instruments are rarely pure notes,
i.e. of one frequency.
Rather they consist of one main note
which is predominant and other smaller
notes called overtones.
The main note or fundamental note is also referred
to as the first harmonic and if it has a frequency f,
the overtone with frequency 2f is called the second
harmonic and the overtone with frequency 3f is
called the third harmonic and so on. The sum of all
the harmonics is the waveform and determines the
quality of the sound.
The same note sounds different when played on different instruments
because the sound from an instrument is usually not a single pure
The variation comes from the harmonics, multiples of the fundamental
A C note played on a piano and played on a guitar:
Harmonics and instruments
Caused by the interference of two identical waves
(or a wave and its reflected wave) travelling in
opposite directions in a medium.
Wave pattern of alternating nodes and antinodes.
Nodes - areas of no displacement of the medium
caused by destructive interference
Antinodes - areas of maximum displacement of
the medium caused by constructive interference.
Note: One wavelength
(shown in the diagram) is
equal to twice the
distance between nodes.
Harmonics and Overtones
1st harmonic = fundamental
2nd harmonic = 1st overtone
3rd harmonic = 2nd overtone
4th harmonic = 3rd overtone
and so on...
Sound Interference – Interference of
sound waves produce BEATS
Sound Interference – Interference of
sound waves produce BEATS
The Doppler effect is a phenomenon
observed whenever the source of waves is
moving with respect to an observer.
The Doppler effect is an apparent upward
shift in frequency for the observer when the
source is approaching, and an apparent
downward shift in frequency when the
source is receding.
The Doppler Effect
As a car approached with its siren blasting, the pitch of the siren sound (a
measure of the siren's frequency) was high; and then suddenly after the
car passed by, the pitch of the siren sound was low. That was the Doppler
effect - a shift in the apparent frequency for a sound wave produced by a
The source of sound always emits the same frequency.
Therefore, for the same period of time, the same number of
waves must fit between the source and the observer. If the
distance is large, then the waves can be spread apart; but if the
distance is small, the waves must be compressed into the
smaller distance. For these reasons, if the source is moving
towards the observer, the observer perceives sound waves
reaching him or her at a more frequent rate (high pitch). And if
the source is moving away from the observer, the observer
perceives sound waves reaching him or her at a less frequent
rate (low pitch). It is important to note that the effect does not
result because of an actual change in the frequency of the
The Doppler Effect
For the Doppler Effect to occur,
the source may be moving, the
listener may be moving, or both
may be moving.
Builders of auditoriums and concert halls avoid
the use of hard, smooth materials in the
construction of their halls. With a hard material
such as concrete, most of the sound wave is
reflected by the walls and little is absorbed.
Walls and ceilings of concert halls are made
softer materials such as fiberglass and acoustic
tiles. These materials have a greater ability to
absorb sound. This gives the room more
pleasing acoustic properties.
Reflection of Sound
You’re standing at the bottom of a
canyon, and wondering how far away
the other side is. To figure out the
distance across the canyon, you could
yell, and clock the time until you heard
your echo. Let's say this took exactly 3.0
seconds. Since you took physics, you
know that sound travels at about 0.2
miles per second. How far away is the
other wall of the canyon?
Reflection can also cause
What is sonar?
A system that uses reflected waves to determine
the distance & location of objects.
Uses the d=v/t formula
d = distance
V= wave speed
t = time it takes for wave to reflected off
High- frequency ultrasonic waves that
are use in a system called
Sound Navigation And Ranging
Ships send sound waves into water that
travels in a straight line until it hits an
object, the wave is then reflected back to
the ship, the time it takes to travel back
and forth is measured and used to
calculate the distance traveled.
A ship on the surface of the water sends
a SONAR signal, and it bounces off a
submarine in the water. Calculate the
distance of the submarine from the ship
if the signal is returned in 4.0 seconds.
(The speed of sound in water is 1450
Diffraction: Light vs Sound
Imagine going to a baseball game, and you discover that
your seat is directly behind a wide post. You cannot see
the game, of course, because the light waves from the
field are blocked. But you have little trouble hearing the
game, since sound waves simply diffract around the post.
Sound Wave Behavior:
The reason for the difference—that is, why sound
diffraction is more pronounced than light diffraction—is
that sound waves are much, much larger than light waves.
The amount of diffraction increases with increasing
wavelength and decreases with decreasing wavelength.
Why do sound waves
diffract more than light
Future test question:
If you were to take a guitar string and
stretch it to a given length and a given
tightness and have a friend pluck it, you
would hear a noise; but the noise would
not even be close in comparison to the
loudness produced by an actual guitar. If
the string is attached to the sound box of
the guitar, the vibrating string is capable
of forcing the sound box into vibrating at
that same natural frequency.
The sound box in turn forces air particles inside
the box into vibrational motion at the same
natural frequency as the string. The entire system
(string, guitar, and enclosed air) begins vibrating
and forces surrounding air particles into
vibrational motion. The tendency of one object to
force another adjoining object into vibrational
motion is referred to as a forced vibration. This
causes an increase in the amplitude and thus
loudness of the sound.
• Be careful though!
• Forced vibration is not the same thing as
• Resonance occurs when something starts
vibrating because of another vibrating object
that isn’t touching it!