Download Sound (Chap. 20)

Chapter 20
Sound
24-May-17
Physics 1 (Garcia) SJSU
Origin of Sound
Sound is a wave that is
produced by the
vibrations of material
objects.
Drumhead
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Guitar string
Physics 1 (Garcia) SJSU
Tuning
fork
Nature of Sound in Air
Sound in air is a longitudinal wave created
by compressions and rarefactions.
Demo: Sound is not Wind
With sound, air molecules oscillate in place.
With wind, air moves from place to place.
Smoke rings are not
sound because the
air moves from
place to place.
24-May-17
Physics 1 (Garcia) SJSU
Demo: Light & Sound
Sound waves can only travel
through a material, such as air,
water, etc.
Light and radio waves can travel
through vacuum.
See the cell phone
ringing inside vacuum
chamber but don’t hear
any sound.
24-May-17
Physics 1 (Garcia) SJSU
Check Yourself
Do light waves have energy?
What do we call the type of heat transfer
that occurs when light transfers energy?
Do sound waves have also have energy?
24-May-17
Physics 1 (Garcia) SJSU
Media That Transmit Sound
Sound travels better
through elastic liquids
and solids, such as
water and rocks, than
through air.
This is due to the close
proximity of the atoms
as they vibrate.
24-May-17
Physics 1 (Garcia) SJSU
Hear richer, louder
sound transmitted
by string
What Your Voice Sounds Like
Your voice sounds different
to you when you hear it
from a recording.
This is because when you
are speaking aloud, most
sound waves reach your
ear traveling through the
solid flesh and bone of
your skull.
24-May-17
Leave yourself
a voice-mail
Physics 1 (Garcia) SJSU
Human Ear
Pressure variations of sound waves push the eardrum, whose vibrations
are transmitted by the ossicles (ear bones) to the cochlea (hearing canal)
Cochlea
Vibrations transmitted by the ear bones create oscillations
in the fluid with the cochlea (snail in Latin), which is a
spiral-wrapped tube.
These oscillations within the cochlea cause the basilar
membrane to ripple, like a waving flag.
Organ of Corti
The organ of Corti forms a ribbon of sensory epithelium
that runs lengthwise down the entire cochlea.
The hair cells of the organ of Corti selectively transform the
oscillations of the basilar membrane into nerve signals.
Loudness & Amplitude
Loudness depends on amplitude of pressure
and density variations in sound waves.
24-May-17
Physics 1 (Garcia) SJSU
Decibels
Loudness of sound depends on
the amplitude of pressure
variations in the sound waves.
Loudness is measured in decibels
(dB), which is a logarithmic scale
(since our perception of loudness
varies logarithmically).
From the threshold of hearing (0 dB) to the
threshold of pain (120 dB) the pressure
increase is a million times higher.
At the threshold of pain (120 db) the pressure
variation is only about 10 Pascals, which is
one ten thousandths atmospheric pressure.
Demo: Make Some Noise
Let’s experience the loudness of sound like
by clapping at various decibel levels.
Sound
Meter
Start clapping softly and slowly increase or decrease
loudness,
the 1sound
meter.
(Garcia) SJSU
24-May-17 as I direct you usingPhysics
Hearing by Age & Sex
Absolute thresholds of hearing by age in males and females
Male, Age 60
Male, Age 50
Female, Age 60
Male, Age 40
Male, Age 30
Male, Age 20
Hearing acuity decreases with age, especially in the high frequencies.
In general, women have greater acoustic sensitivity than men.
Hearing Loss
The hair cells that line the cochlea are a delicate and
vulnerable part of the ear. Repeated or sustained
exposure to loud noise destroys the neurons of the
Organ of Corti.
Once destroyed, the hair cells are not replaced, and the
sound frequencies interpreted by them are no longer
heard.
Hair cells that respond to high
frequency sound are very
vulnerable to destruction, and
loss of these neurons typically
produces difficulty understanding
human voices.
Much of this type of permanent
hearing loss is avoidable by
reducing exposure, such as to
loud music.
What?
Speed of Sound in Air
Speed of sound in air
is about 340 m/s.
Sound travels about
one kilometer in
three seconds,
about one mile in
five seconds.
Light is a million times
faster than sound.
24-May-17
Physics 1 (Garcia) SJSU
Demo: Helium Voice
Sound speed in helium is
higher than speed in air.
Wavelength of sound
unchanged (size of vocal
cords is unchanged).
Frequency of voice is higher
since
(Wave speed)
(Frequency) =
(Wavelength)
24-May-17
Physics 1 (Garcia) SJSU
Breath Helium…
He
Talk like me!
Reflection of Sound
Sound reflects strongly from rigid surfaces.
Softer surfaces absorb sound.
Quiet after a fresh
snowfall because the
soft, irregular surface
of the snow absorbs
sound instead of
reflecting it.
24-May-17
Physics 1 (Garcia) SJSU
Check Yourself
When crowded, which restaurant will be quieter?
24-May-17
Physics 1 (Garcia) SJSU
Singing in the Shower
Multiple reflections from
the hard walls create
reverberation.
Hear your voice from
several sources,
slightly shifted in time.
Reverberation extends
each note and smears
(smoothens) the pitch.
24-May-17
Physics 1 (Garcia) SJSU
Your voice sounds better
when singing in the shower
Refraction of Sound
Sound speed can vary by material or conditions.
This causes the sound to bend in direction, in the
same way that light bends when it passes
through a glass lens.
Fig. 20.8
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Physics 1 (Garcia) SJSU
Ultrasound
Ultrasound is high
frequency (Megahertz),
short wavelength
(0.1 mm) sound.
Reflections and refractions
of ultrasound by flesh
and bone allow “seeing”
inside the human body.
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Physics 1 (Garcia) SJSU
Forced Vibrations
Vibrating guitar strings force the vibration of the
guitar’s body, producing most of the sound.
553 Hz
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731 Hz
Circular rings indicate where the
is vibrating up and down
Physics 1 surface
(Garcia) SJSU
Demo: Tuning Fork & Sound Box
Tuning fork by itself is not
very loud.
Sound is much louder if it
is held against a sound
box, such as the body
of a guitar or any similar
rigid surface.
The tuning fork forces the
surface into oscillation
at the same frequency.
24-May-17
Physics 1 (Garcia) SJSU
Natural Frequency
Metal wrench and
wooden bat sound
very different when
dropped to the
floor.
Different materials
and shapes vibrate
at their own natural
frequencies.
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Physics 1 (Garcia) SJSU
Demo: Singing Rod
Stoking an aluminum
rod with rosin-covered
fingers induces loud
vibrations at the rod’s
natural frequency.
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Physics 1 (Garcia) SJSU
Resonance
Resonance occurs when forced vibrations match an
object’s natural frequency.
Oscillations grow in amplitude due to synchronized
transfer of energy into the vibrating object.
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Physics 1 (Garcia) SJSU
Acoustic Resonance
Sound at an object’s natural frequency
can produce resonant vibrations.
If the amplitude of the sound is
sufficiently large, resonant
vibrations can shatter a wine glass.
As shown by Myth Busters, this
may even be achieved by
exceptionally powerful singers (and
by average singers using electronic
amplifiers).
Tacoma Narrows Bridge
In 1940, the first Tacoma Narrows bridge
was destroyed by resonance.
First Bridge
Second Bridge
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Physics 1 (Garcia) SJSU
Movie: Tacoma Narrows Bridge
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Physics 1 (Garcia) SJSU