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Sound

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Interference affects the loudness of sounds. Destructive sound ... Maximum amplitude of the composite wave occurs when both waves are in phase, ... – PowerPoint PPT presentation

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Title: Sound


1
Sound
2
  • Imagine an entire room is filled with ping
    pong balls, and in the middle of the room is a
    big paddle. Shake the paddle and observe what
    happens
  • As we keep shaking the paddle back and forth
  • what we are doing is creating a
    longitudinal wave.
  • Molecules of air behave like tiny Ping-Pong
    balls.
  • The Origin of Sound
  • All sounds are produced by the vibrations of
    material objects. In a piano, violin or guitar, a
    sound wave is produced by vibrating strings.
  • The source of all sound waves is vibration.
  • The original vibration stimulates the
    vibration of something larger or more massive.
    Then a disturbance is sent through a surrounding
    medium, usually air.
  • The frequency of the vibrating source equals
    the frequency of the sound waves produced.
  • The subjective impresion of the frequency of a
    sound is called pitch.
  • (infrasonic) 20 Hz ? Audible frequencies ?
    20,000 Hz (ultrasonic)

3
  • Sound in Air
  • Produce a sound and each particle
    moves back and forth along the direction of
    motion of the expanding wave.
  • This process is better understood with the
    following example
  • Open the door and this pulse of compressed
  • air is called a compression.
  • Quickly close the door and this process is
  • called rarefaction.
  • AgainIt is not the medium that travels across
    the
  • room, but a pulse that travels.

4
  • Media that Transmit Sound
  • Most sounds you hear are transmitted
    through the air. But sound also travels in solids
    and liquids Examples, floor, metal fence, etc.
    A sound is transmited louder and faster by the
    metal than by the air.
  • Solids and Liquids are generally good conductors
    of sound much better than air.
  • Sound cannot travel in a vacuum.
  • The transmission of sound requires
  • a medium !!!
  • Speed of Sound
  • Sound is much slower than light. The speed of
    sound in dry air at 0C is about
  • 330 m/s or 1200 km/hr or 750 miles/hr
  • Question
  • How far away is a storm if you note a 3-second
    delay between a lightning flash and the sound of
    thunder?

5
  • Loudness
  • The intensity of sound is
    proportional
  • to the amplitude of a sound wave. It can be
  • measured by instruments such as the
  • oscilloscope, shown in the figure.
  • Loudness, on the other hand, is a physiological
  • sensation sensed in the brain. It differs for
    different
  • people. But it is related to sound intensity.
  • The unit of intensity of sound is the decibel
    (dB) where zero (0) dB is at the threshold of
    hearing for a normal ear.
  • Natural Frequency
  • When any object composed of an elastic
  • material is disturbed, it vibrates at its own
    special
  • set of frequencies, which together form its
    special
  • sound.
  • A natural frequency is one in which minimum
    energy

6
  • Resonance
  • When the frequency of a forced
    vibration of an object matches the objects
    natural frequency, a dramatic increase in
    amplitude occurs. This phenomenon is called
    resonance.
  • A common experience illustrating resonance
    occurs
  • on a swing. When pumping a swing, you pump in
    rythm
  • with the natural frequency of the swing.
  • More important than the force with which you
    pump, is the
  • timing.
  • Resonance is not restricted only to sound waves,
    it also occurs with mechanical vibrations. Troops
    must break step when crossing bridges.
  • The Tacoma Narrows Bridge in 1940 is another
    example of destruction due to mechanical
    resonance.

7
  • Interference
  • Sound waves, like any waves, can be
    made to interfere. Interference occurs for both,
    transverse and longitudinal waves. Interference
    affects the loudness of sounds.

8
  • Beats.
  • An interesting and special case of
    interference occurs when two tones of slightly
    different frequency are sounded together.
  • A fluctuation in the loudeness of the combined
    sounds is heard loud, faint, loud, faint, loud,
    and so on.
  • Maximum amplitude of the composite wave occurs
    when both waves are in phase, and minumum
    amplitude occurs when both waves are completely
    out of phase.

9
  • Concept Summary
  • Sound waves are produced by the vibration of
    material objects.
  • A disturbance in the form of a longitudinal wave
    travels away from the vibrating source.
  • High pitched sounds are produced by sources
    vibrating at high frequency, while low pitched
    sounds are produced by low-frequency sources.
  • Sound waves consist of traveling pulses of high
    pressure zones, or compressions, alternating with
    pulses of low-pressure zones, or rarefactions.
  • Sound can travel through gases, liquids and
    solids, but not through vacuum.
  • Sound travels fastest through very elastic
    materials, such as steel.
  • Every object vibrates at its own set of natural
    frequencies.
  • When an object is forced to vibrate at one of its
    own natural frequencies, resonance occurs and the
    sound becomes much louder.
  • Like any waves, two sound waves can exhibit
    interference and make sound louder or softer.
  • Rapid changes in loudeness, known as beats, occur
    when two tones very close in frequency are heard
    at the same time.
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