Sound

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Sound
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Properties of Sound Waves

• Sound saves are longitudinal waves
• The speed in air increases 0.6m/s for each oC
  increase.
• Vsound 330 0.6 x T m/sec
• The speed of sound is higher in liquids and
  solids than it is in gases.
• Sound has properties of all other waves
  reflection, refraction, interference,
  diffractions.

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Sound of sound in various materials at 20oC.
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Pitch and Loudness

• Pitch is essentially the frequency of the wave.
  The higher the frequency, the higher the pitch.
• Loudness depends on the amplitude of the pressure
  variation wave. The louder the sound, the bigger
  the amplitude.
• Sound level is measured in decibels (dB).
• Two notes with frequencies related by the ratio
  21 are said to differ by an octave.

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Equation for sound intensity

• The intensity level, b, of any sound is defined
  in terms of its intensity, I as follows

NOTE - Io is usually taken as the minimum
intensity level for an average person, which is
Io 1.0 x 10-12 W/m2.
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Some Sound Levels (dB)
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Sources of Sound

• The source of sound is a vibrating object.
• Almost any object can vibrate and hence be a
  source of sound.
• Standing waves are produced and the object
  vibrates at its natural resonant frequency.

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Sources of Sound Vibrating Strings.

• The most widely used instruments are the violin,
  guitar and piano.
• The pitch is normally determined by the lowest
  resonant frequency the fundamental.
• The wavelength,l, of the fundamental is equal to
  twice the length, L, of the string or l 2L.
• The frequency is f v/l v/2L where v is the
  velocity of the wave on the string.
• The stings on a guitar are all the same length
  but have different masses. This affects the
  velocity and hence the frequency of vibration.

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Sources of Sound Vibrating Columns of Air.

• Instruments such as woodwinds produce sound from
  the vibrations of standing waves in a column of
  air within a tube or pipe.
• The simplest mode of vibration is the Fundamental
  mode. Higher harmonics (or overtones) can also
  be produced.
• The frequency of each overtone is an integral
  multiple of the fundamental frequency.

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Interference of Sound Beats.

• When two waves simultaneously pass through the
  same region of space, they interfere with one
  another.
• Beats occur when two sounds of slightly different
  frequency interfere with one another.
• The beat frequency fb is
• fb f2 f1

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The Doppler Effect

• When a source of a sound is moving toward an
  observer, the pitch is higher than when the
  source is at rest and when the source is
  traveling away form the observer, the pitch is
  lower.
• This phenomenon is known as the Doppler Effect.
• This effect occurs for all types of waves.

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Equation for the Doppler Effect
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Doppler Effect - Problem I

• A factory whistle emits a sound at 900Hz. What
  frequency will be heard by an observer in a car
  traveling at 70 km/hr
• (a) away from the source.
• (b) towards the source.
• Answer (a) 849Hz (b) 951Hz

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Doppler Effect Problem 2

• Two trains emit whistles of the same frequency,
  380Hz. If one train is at rest and the other is
  traveling at 90 km/hr away from an observer at
  rest, what will the observer detect as the beat
  frequency?
• Answer 26 Hz