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Air molecules ripple through the air in sound waves like water waves rippling across a pond. Sound Waves: Pitch Pitch: the ... relative to our solar system. – PowerPoint PPT presentation

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Title: Sound

Sound http//
CH 7 SOUND Sounds are a form of energy
produced by rapidly vibrating objects. Sound
needs a material medium for its transmission.
Sound cannot travel through a vacuum. The
vibrating object causes compressions and
rarefactions in the medium. A receiver senses the
sound by sensing the compressions and
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Sound is made when something vibrates.
  • The vibration disturbs the air around it.
  • This makes changes in air pressure.
  • These changes in air pressure move through the
    air as sound waves.

  • The sound waves cause pressure changes against
    our ear drum sending nerve impulses to our brain.

This is similar to throwing a rock into a pond.
  • Air molecules ripple through the air in sound
    waves like water waves rippling across a pond.

Sound Waves Pitch
  • Pitch the frequency of a sound wave.
  • The human ear is not as sensitive to pitch as it
    is to loudness.
  • Most people cannot hear frequencies below 20 Hz
    or above 16,000 Hz.
  • Most audible sounds lie in the range of 1000 to
    5000 Hz.
  • Exposure to loud noise damages our ability to
    hear higher pitched sounds.

  • When the frequency of a sound doubles we say that
    the pitch goes up an octave.
  • We can hear a range of pitches of about ten
  • Many animals can make sounds and hear frequencies
    that are beyond what we can hear.

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  • Sounds with frequencies less than 20 Hz are
    infrasonic while sounds above 20 000 Hz are

p. 238-241 in text p. 241 1,2,3 extra 4 p. 242
1,4,5 extra 2,3,6,7
  • To create vibrations energy is used.
  • The greater amount of energy used the louder the
  • The strength of the changes in air pressure made
    by the vibrating object determines loudness.

The loudness of sound depends on the amplitude of
the sound wave. The human ear is extremely
sensitive to the pressure differences caused by
sound waves.
Air pressure and elevation do not significantly
affect the speed of sound in air. p. 243-246
p. 243 1-4, p.246 3-5 extra p.246 1,2,6-8
Notice any patterns?
THE INTENSITY OF SOUND p. 247-248 p. 248 1-4,
p. 249 2-4 Sound intensity is the power of sound
per unit area (W/m2). Sounds can be emitted with
an extremely large variance in intensity and
likewise humans can sense extremely soft sounds
as well as loud sounds. The quietest whisper is
about 10-12 W/m2 while a sound with an intensity
of 104 W/m2 will instantly perforate an eardrum.
The decibel scale is utilized for sound intensity
and gives an easy scale to judge relative
intensities. The least intense sound we can hear
is given the intensity of 0 dB. For every 10 dB
increase in intensity the sound increases its
true intensity by 10X. The scale is logarithmic
so if the intensity increases by 30 dB then the
true intensity has increased by a factor of
1000X. The intensity of sound we hear depends on
the power of the source and the distance between
us and the source.
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As the sound spreads out from its source, the
concentration of power becomes less.
  • As the distance from the source increases the
    amount of power is spread over a greater area.
  • The amount of power per square meter is called
    the intensity of the sound.

Humans do not perceive sound intensity linearly.
  • For us to perceive a sound as twice as loud its
    intensity must be ten times greater.

The scale begins (0 dB) on the softest sound that
a person can hear. This is called the threshold
of hearing.
  • The scale ends at the volume that causes pain
    (120 dB) and is therefore called the threshold of

Sound Waves Loudness
  • Decibels dB, logarithmic scale of sound level.
  • 10 dB Barely audible.
  • 30 dB Quiet breathing.
  • 50 dB Normal classroom conversations.
  • 70 dB Noisy traffic.
  • 100 dB Fire engine horn.
  • 110 dB Concert/Airplane.
  • 110-120 dB Human Pain Threshold.

read The Human Ear p. 249-253 -will not test on
parts of the human ear
  • read The Reflection of Sound Waves p. 254-257
  • -understand echoes and echo problems (remember
    to double distance)
  • -p. 257 1-4, p. 258 1-3
  • -know echolocation, who uses it
  • -know ultrasound applications

  • Not responsible for diffraction and refraction of
  • p. 258-260.
  • Not responsible for section on interference of
  • p. 260-263.
  • Responsible for beat frequency calculations
    p.264-266, p. 266 1,2 p. 266 1-7

The Doppler Effect (p. 267-272) The apparent
changing frequency of sound in relation to an
objects motion is called the Doppler effect,
named after Christian Doppler (1803-53). If a
sound emitter is moving towards a listener (or
vice versa) then the listener hears a higher
frequency than is actually emitted. If a sound
emitter is moving away from a listener (or vice
versa) then the listener hears a lower frequency
than is actually emitted. The Doppler effect
(Doppler shift) has been used to estimate the
speed of distant stars and galaxies (using light
waves) relative to our solar system. The Doppler
shift is also used in police radar for speeding.
You are not responsible for the equation which
quantifies the Doppler effect.
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The Mach Number is the ratio of an objects
velocity to the speed of sound. When flying at
Mach 1, an object is flying as fast as the sound
it gives off. When the object emits another sound
the crest will alongside the original crest so
these crests pile up, producing an area of very
dense air. This intense compression of air is
called the sound barrier. Extra thrust is needed
to break through this barrier. Objects must be
designed to cut through this dense air leading to
sleek and pointy shapes. At hypersonic speeds,
the crests are left behind the object which
constructively interfere with other crests to
create a double cone. This intense acoustic
pressure is called the sonic boom.
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"A "sonic boom" is heard when a plane exceeds
the speed of sound. "
Did you know .....   The crack of a whip is the
sound of the tip breaking the speed of sound a
small sonic boom