Chapter 20 Sound

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Chapter 20 Sound
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Origin of Sound

• Sound is a wave that is produced by the
  vibrations of material objects.

Drumhead
Guitar string
Tuning fork
3
Nature of Sound in Air

• Sound in air is a longitudinal wave created by
  compressions and rarefactions.

4
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.
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Demo Light Sound

• Sound waves can only travel through a material,
  such as air, water, etc.
• Light and radio waves can travel through vacuum.

Radio Wave
See the cell phone ringing inside vacuum chamber
but dont hear any sound.
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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?

7
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.

Hear richer, louder sound transmitted by string
8
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.

Leave yourself a voice-mail
9
Human Ear
Pressure variations of sound waves push the
eardrum, whose vibrations are transmitted by the
ossicles (ear bones) to the cochlea (hearing
canal)
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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.

11
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.

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Loudness Amplitude

• Loudness depends on amplitude of pressure and
  density variations in sound waves.

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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.
14
Demo Make Some Noise

• Lets experience the loudness of sound like by
  clapping at various decibel levels.

Sound Meter
Start clapping softly and slowly increase or
decrease loudness, as I direct you using the
sound meter.
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Hearing by Age Sex
Hearing acuity decreases with age, especially in
the high frequencies. In general, women have
greater acoustic sensitivity than men.
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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?
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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.

18
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

Breath Helium
He
Talk like me!
(Wave speed)
(Frequency)
(Wavelength)
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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.
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Check Yourself

• When crowded, which restaurant will be quieter?

21
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.

Your voice sounds better when singing in the
shower
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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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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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Forced Vibrations

• Vibrating guitar strings force the vibration of
  the guitars body, producing most of the sound.

731 Hz
553 Hz
Circular rings indicate where the surface is
vibrating up and down
25
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.

26
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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Demo Singing Rod

• Stoking an aluminum rod with rosin-covered
  fingers induces loud vibrations at the rods
  natural frequency.

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Resonance

• Resonance occurs when forced vibrations match an
  objects natural frequency.
• Oscillations grow in amplitude due to
  synchronized transfer of energy into the
  vibrating object.

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Acoustic Resonance

• Sound at an objects 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).
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Tacoma Narrows Bridge

• In 1940, the first Tacoma Narrows bridge was
  destroyed by resonance.

First Bridge
Second Bridge
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Movie Tacoma Narrows Bridge