Sound

Sound

Definition of Sound
Sound is a wave created by vibrating objects and
propagated through a medium from one location to
another.

If a tree falls in a forest, and there
is no one there to hear it, does it
make a sound?
Sound is a physical
disturbance in a medium.
Based on our definition, there IS
sound in the forest, whether a
human is there to hear it or not!.
A person to hear it is not required. The
medium (air) is required!

What type of waves are sound waves?

Review: Sound is a mechanical wave
• The sound wave is transported from
one location to another by means of
particle-to-particle interaction.
• 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 position.
• 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 wave.

Check your understanding:
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 is not.
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.

When a tuning fork vibrates, it creates areas of high
pressure (compressions) and low pressure (rarefactions).
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.

Check your understanding
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.

Frequency of Sound
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 Example
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.

The frequency of sound
• We hear frequencies of sound as having different
pitch.
• 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.

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.

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
Intensity: the rate at which a wave’s energy flows
through an area
Sound intensity depends on
Amplitude
Distance from source
Measured in decibels (dB)

Loudness is sort of like
intensity, but…
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
brain.

Intensity is caused by the
Amplitude of the vibration
Example:
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 sound. The decibel scale is a little odd because
the human ear is incredibly sensitive. 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!

The Decibel Scale
On the decibel scale, the smallest
audible sound (the threshold of
hearing) is 0 dB.
A sound 10 times more powerful is
10 dB.
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:
Source
Intensity
Level
# of Times
Greater Than TOH
Threshold of Hearing 0 dB 10 0
Rustling Leaves 10 dB 10 1
Whisper 20 dB 10 2
Normal Conversation 60 dB 10 6
Busy Street Traffic 70 dB 10 7
Vacuum Cleaner 80 dB 10 8
Large Orchestra 98 dB 10 9.8
Walkman at Maximum Level 100 dB 10 10
Front Rows of Rock Concert 110 dB 10 11
Threshold of Pain 130 dB 10 13
Military Jet Takeoff 140 dB 10 14
Instant Perforation of Eardrum 160 dB 10 16