SOUND

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SOUND a range of compression wave frequencies to
which the human ear is sensitive
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Remember a sound wave is an example of a
longitudinal wave.

• This wave is vibrating directly parallel to the
  waves motion.

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Range of Some Common Sounds
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Intensity Range for Some Common Sounds
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Sounds are produced by vibrating matter.
1. reeds
3. membranes
click
4. air columns
2. strings
Sound is a mechanical wave (longitudinal). It
will not travel through a vacuum.
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Speed of Sound

• Speed of sound in a material DOES NOT depend upon
  materials density, but its ELASTICITY!
• Sound wave speed depends on the medium
• Sound waves travel 3 to 4 times faster
  in water then in air.
• Sound waves travel even faster in
  solids than air or in water.
• Sound waves have speeds 15 to 20 times faster
  in rock or metal
• than in air.
• Solids Liquids Gases
• How it varies a) increase with humidity
• b) increase with
  temperature
• c) increase with
  density

SLOWEST
FASTEST
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The velocity of sound in air depends on the air
temperature. The speed of sound in dry air is
331.5 m/s at 0 ºC.
This speed increases with temperature about 0.6
m/s for every 1 ºC increase in temperature.
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Sound generally travels fastest in solids and
slowest in gases, but there are some exceptions.
Medium Velocity (m/s) Medium Velocity (m/s)
Air 330 Carbon dioxide 260
Helium 930 Hydrogen 1270
Oxygen 320 Water 1460 Sea water
1520 Mercury 1450 Glass 5500
Granite 5950 Lead 1230
Pine wood 3320 Copper 3800
Aluminium 5100
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Forced Vibrations

• the setting up of vibrations in an object by a
  vibrating force.
• Examples of Forced Vibration
• A tuning fork touching a wood surface
• Sounding boards for stringed instruments
• Matching tuning fork boxes

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Natural Frequency

• the frequency at which an elastic object
  naturally tends to vibrate.

• At this frequency, a minimum energy is required
  to produce a forced vibration.

• The natural frequency of a body depends on its
• elasticity, size, shape.
• Mass on a Spring
• Ringing Small and Large Bells
• Xylophone
• Rubbing a Wine Glass
• Dropping Aluminum Rods

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The human ear relates amplitude
to loudness and frequency to pitch.
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Listen to various sound frequencies here and
mixtures of sound waves here. Click here to
make your own sound waves. You should hear that
frequency relates to pitch and amplitude
relates to loudness (for a given frequency).
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Fact
All objects have a natural frequency of vibration.
Resonance - the inducing of vibrations of a
natural rate by a vibrating source having the
same frequency
sympathetic vibrations
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Resonance

• is the result of forced vibrations in a body
  when the applied frequency matches the natural
  frequency of the body.
• The resulting vibration has a high amplitude and
  can destroy the body that is vibrating.
• eg.
• Mass on a spring at resonance
• swinging your legs in a swing
• breaking a wine glass using sound
• a singing rod caused by forced vibration
• a tuning fork exciting a guitar string
• a truck driving on a rough road
• In 1940, the Tacoma Narrows Bridge was destroyed
  by wind-generated resonance.

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Famous Bridge Collapses Evidences of
Resonance? Tacoma Narrows link Others link
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A resonant air column is simply a standing
longitudinal wave system, much like standing
waves on a string.
closed-pipe resonator tube in which one end is
open and the other end is closed
open-pipe resonator tube in which both ends are
open
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A closed pipe resonates when the length of the
air column is approximately an odd number of
quarter wavelengths long.
l (1,3,5,7,)/4 l

• With a slight correction for tube diameter,
• we find that the resonant wavelength of a
• closed pipe is given by the formula
• 4 (l 0.4d),
• where ? is the wavelength of sound,
• l is the length of the closed pipe,
• and d is the diameter of the pipe.

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An open pipe resonates when the length of the air
column is approximately an even number of
quarter wavelengths long.
l (2,4,6,8,)/4 l

• With a slight correction for tube diameter,
• we find that the resonant wavelength of an
• open pipe is given by the formula
• 2 (l 0.8d),
• where ? is the wavelength of sound,
• l is the length of the open pipe,
• and d is the diameter of the pipe.

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Click here to see a simulation of standing waves
in a resonant tube (closed and open). Learn more
about resonance here.
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Sound Interference

• Overlapping compressions of a sound wave will
  result in
• constructive interference.
• and a louder sound.
• Overlapping a compression and a rarefaction
  results in...
• destructive interference.
• and a softer sound.
• Compressions and rarefactions traveling from the
  tuning fork through tube.

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Wave Interference

• Constructive when two waves are in-phase
• result increase of twice the amplitude of the
  individual waves.
• Destructive when two waves are in opposite
  phase
• result in decrease of amplitude and cancel each
  other out.
• eg. Anti-noise technology noise-canceling ear
  phones for pilots.

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Sound waves refract.
Click here to view a simulation of the refraction
of sound waves.
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The interference of sound waves can cause
beats Click here and here to run
computer simulations of interfering sound
waves that result in discernable beats. View
interference beats here and here. What are
evidences of reflection and the diffraction of
sound?
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Beats
Figure 26.16 The interference of two sound
sources of slightly different frequencies
produce beats.

• Beats - the periodic variation in loudness of two
  sounds played together
• The beat frequency is equal to the difference in
  the frequency of the two sounds.

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Comparison of Waves to Sound Characteristics

• Characteristics of Waves
• Longitude and Width
• Compression
• Ex. Compression travels along spring or clap your
  hands pulse travels (vibrates) back n fourth
  along direction of motion.
• Rarefaction- A disturbance in air (or matter) in
  which pressure is lowered.
• Ex. Slamming door closed
• A Amplitude
• ? Wavelength

• Characteristics of Sound
• Sound
• Pitch
• High / Low
• Vibrating Frequency Sound
• Timbre
• Sound Quality
• Intensity
• Energy
• Tempo
• Rhythm

Relate - ? f, ? ?, ?P ? f, ?
intensity