Transcript Chapter 20: Sound

Vibrations, Waves and Sound
Unit 7: Vibrations, Waves & Sound
Chapter 20: Sound
 20.1
Properties of Sound
 20.2
Sound Waves
 20.3
Sound, Perception, and Music
20.1 Investigation: Sound and Hearing
Key Question:
What is sound and how do we hear it?
Objectives:

Identify the range of frequencies humans can hear.

Describe the how perception influences the sound humans
hear.

Make and analyze a histogram of class data.
The frequency of sound
 The
pitch of a sound is
how you hear and
interpret its frequency.
 A low-frequency
sound
has a low pitch.
 A high-frequency
sound
has a high pitch.
Each person is saying “Hello”.
The frequency of sound
 Almost
all the sounds you hear contain many
frequencies at the same time.
 Humans
can generally hear frequencies between
20 Hz and 20,000 Hz.
The loudness of sound
 The
loudness of a sound is measured in
decibels (dB).
 The
decibel is a unit used to express relative
differences in the loudness of sounds.
The loudness of sound
 Most
sounds fall between 0 and 100 on the decibel
scale, making it a very convenient number to
understand and use.
The frequency of sound
 Sounds
near 2,000 Hz
seem louder than
sounds of other
frequencies, even at the
same decibel level.
 According
to this curve,
a 25 dB sound at 1,000
Hz sounds just as loud
as an 40 dB sound at
100 Hz.
Decibels and amplitude
 The
amplitude of a sound increases ten times every
20-decibels.
The speed of sound
 The
speed of sound in
normal air is 343 meters
per second (660 mi/hr).
 Sound
travels through
most liquids and solids
faster than through air.
 Sound
travels about five
times faster in water, and
about 18 times faster in
steel.
The speed of sound
 Objects
that move faster
than sound are called
supersonic.
 If
you were on the
ground watching a
supersonic plane fly
toward you, there would
be silence.
The sound would be behind the
plane, racing to catch up.
The speed of sound
jet “squishes” the sound waves so
that a cone-shaped shock wave forms where the
waves “pile up” ahead of the plane.
 A supersonic
 In
front of the shock wave there is total silence.
The speed of sound
 Passenger
jets are subsonic because they travel
at speeds from 400 to 500 mi/hr.
The Doppler effect
 When
the object is moving, the frequency will not be
the same to all listeners.
 The
shift in frequency caused by motion is called the
Doppler effect.
 You
hear the Doppler effect when you hear a police
or fire siren coming toward you, then going away
from you.
Recording sound
 Most
of the music you listen to has been recorded
in stereo.
 The
slight differences in how sound reaches your
ears lets you know where sound is coming from.
Recording sound
To record a sound you must store the pattern of
vibrations in a way that can be replayed and be
true to the original sound.
1.
A microphone transforms a sound wave into an
electrical signal with the same pattern of vibration.
Recording sound
2.
An “analog to digital converter” converts the
electrical signal to digital values between 0 and
65,536.
Recording sound
3.
One second of compact-disc-quality sound is a
list of 44,100 numbers which represents the
amplitudes converted sounds.
Recording sound
4.
To play the sound back, the string of numbers is
read by a laser and converted into electrical
signals again by a second circuit which reverses
the process of the previous circuit.
Recording sound
5.
The playback circuit converts the string of
numbers back into an electrical signal.
6.
The electrical signal is amplified to move the coil
in a speaker and reproduce the sound.
Unit 7: Vibrations, Waves & Sound
Chapter 20: Sound
 20.1
Properties of Sound
 20.2
Sound Waves
 20.3
Sound, Perception, and Music
20.2 Investigation: Properties of Sound
Waves
Key Question:
Does sound behave like other waves?
Objectives:

Listen to beats and explain how the presence of beats is
evidence that sound is a wave.

Create interference of sound waves and explain how the
interference is evidence for the wave nature of sound.
What is a sound wave?
 Sound
waves are pressure waves with
alternating high and low pressure regions.
 When
they are pushed by the vibrations, it
creates a layer of higher pressure which results in
a traveling vibration of pressure.
Pressure and molecules
 At
the same temperature and
volume, higher pressure
contains more molecules than
lower pressure.
 The
speed of sound increases
because collisions between
atoms increase.
 Therefore,
if the pressure goes
down, the speed of sound
decreases.
The wavelength of sound
 The
wavelength of sound in air is similar to the size
of everyday objects.
The wavelength of sound
 Wavelength
is also important
to sound.
 Musical
instruments use the
wavelength of a sound to
create different frequencies.
Reverberation
 The
reflected sound
and direct sound from
the musicians together
create a multiple echo
called reverberation.
 The
right amount of
reverberation makes
the sound seem livelier
and richer.
Sound wave interactions
 Like
other waves, sound waves can be reflected
by hard surfaces and refracted as they pass from
one material to another.
 Diffraction
causes sound waves to spread out
through small openings.
 Carpet
waves.
and soft materials can absorb sound
Ultrasound
 Ultrasound
is high-frequency
sound, often 100,000 Hz or
more.
 We
cannot hear ultrasound,
but it passes through the
human body easily.
The ultrasound image
above is a heart.
 Medical
ultrasound
instruments use ultrasound
waves to create images of the
human body’s interior for
diagnostic purposes.
Standing waves in pipes
 A panpipe
makes music as sound resonates in
tubes of different lengths.
 The
natural
frequency of a pipe
is proportional to its
length.
Standing waves in pipes
 Because
frequency and wavelength are
inversely related, longer pipes have
lower natural frequencies because they
resonate at longer wavelengths.
 A pipe
that must vibrate at a frequency 2
times higher than another pipe must be
1/2 as long.
If the long pipe has a frequency of 528 Hz, what is
the frequency of the short pipe?
Standing waves in pipes
 Blowing
across the open end of a tube creates a
standing wave inside the tube.
 If
we blow at just the right angle and we match
the natural frequency of the material and the
sound resonates (spreads).
Standing waves in pipes
 The
open end of a pipe is an open boundary to a
standing wave and makes an antinode.
 The
pipe resonates to a certain frequency when
its length is one-fourth the wavelength of that
frequency.
Wave speed and designing instruments
 Sounds
of different frequencies are made by
standing waves.
 The
length of a vibrating system can be chosen so
that it resonates at the frequency you want to hear.
Fourier's theorem
 Fourier’s
theorem
says any complex
wave can be made
from a sum of single
frequency waves.
Sound spectrum
 A complex
wave is really a
sum of component
frequencies.
 A complex
wave can be
made from a sum of singlefrequency waves, each with
its own frequency,
amplitude, and phase.
Unit 7: Vibrations, Waves & Sound
Chapter 20: Sound
 20.1
Properties of Sound
 20.2
Sound Waves
 20.3
Sound, Perception, and Music
20.3 Investigation: Sound as a Wave
Key Question:
How can we observe sound as a wave?
How can we use the speed of sound and
certain frequencies to build a basic
instrument based on wavelength?
Objectives:
Explain how pitch is related to frequency and wavelength of a
sound wave.
 Determine the lengths of pipe required to produce sounds at
certain frequencies at a specific speed of sound.
 Construct a basic instrument from PVC pipe and use it to play
musical notes.

Sound perception and music
 When
you hear a sound,
the nerves in your ear
respond to more than
15,000 different
frequencies at once.
 The
brain makes sense of
complex sound because
the ear separates the
sound into different
frequencies.
Sound spectrum
A
frequency spectrum is a graph showing the
different frequencies present in a sound.
 Sound
containing many frequencies has a wave
form that is jagged and complicated.
Sonograms

More information is found
in a sonogram which
combines three sound
variables:
1. frequency,
2. time, and
3. amplitude (loudness).
Sonograms
Which letter
represents a soft
sound lasting 5
seconds?
What is it’s
frequency?
How we hear sound

1.
2.
3.
The parts of the ear work together:
When the eardrum vibrates,
three small bones transmit the
vibrations to the cochlea.
The vibrations make waves
inside the cochlea, which
vibrates nerves in the spiral.
Each part of the spiral is
sensitive to a different
frequency.
Sound protection
 Listening
to loud sounds for a long
time causes the hairs on the
nerves in the cochlea to weaken
or break off resulting in permanent
damage.
Music
 The
pitch of a sound is how high or low we hear its
frequency.
 Rhythm
is a regular time pattern in a series of
sounds.
 Music
is a combination of sound and rhythm that
we find pleasant.
The musical scale
 Most
of the music you listen to is created from a
pattern of frequencies called a musical scale.
Music and notes
 Each
 The
frequency in the scale is called a note.
range between any frequency and twice that
frequency is called an octave.
Music and harmony
 Harmony
is the study of how sounds work together
to create effects desired by the composer.
 The
tense, dramatic sound track of a horror movie
is a vital part of the audience’s experience.
 Harmony
is based on the frequency relationships of
the musical scale.
Beats
 When
two frequencies of sound are not exactly
equal in value, the loudness of the total sound
seems to oscillate or beat.
Echolocation and beats
 Bats
navigate and find
food using echolocation.
 The
beat frequency is
proportional to how far
the insect is from the bat.
Consonance and dissonance
 When
we hear more than one frequency of sound
and the combination sounds pleasant, we call it
consonance.
 When
the combination sounds unsettling, we call it
dissonance.
Making sounds

The human voice is complex sound that
starts in the larynx, at the top of your
windpipe.

The sound is changed by passing over by
expandable folds (vocal cords) and
through openings in the throat and mouth.
Making sounds
 For
a guitar in standard
tuning, the heaviest string
has a natural frequency of 82
Hz and the lightest a
frequency of 330 Hz.
 Tightening
a string raises its
natural frequency and
loosening lowers it.
Harmonics and music
 The
same note sounds different when played on
different instruments.
 Suppose
you compare the note C (262 Hz)
played on a guitar and the same note played on a
piano.
 A single
C note from a grand piano might include
20 or more different harmonics.
Harmonics and instruments
 The
variation comes
from the harmonics.
 The
sound from an
instrument is not a
single pure frequency.
Hearing

Deafness is poorly understood in
general.

For instance, there is a common
misconception that deaf people
live in a world of silence.

To understand the nature of
deafness, first one has to
understand the nature of hearing.