Vibrating Strings and Air Columns

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Vibrating Strings and Air Columns

• Standing Waves Harmonics

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Source of sound (Animation)

• The source of sound is a vibrating object.
• Standing waves are produced in musical
  instruments by striking, blowing, plucking,
  bowing
• Pitch - Frequency
• Amplitude - Loudness (Energy)
• Sound Quality - Wave form
• Vibrating strings (Violin, Guitar)
• Vibrating columns of air (Flute, trumpet, pipe
  organ)

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Standing Wave

• When a wave reflects off of something, it can
  interfere with its own reflection. The
  interference is alternately constructive or
  destructive as the two waves move past each
  other. This creates a standing wave.

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Node Antinode

• Certain points along the standing wave never
  move. These points are called nodes. The points
  that move the most are called antinodes.

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Fundamental and Harmonics n

• Fundamental - the lowest resonance frequency
• Harmonics n (overtones)- multiples of the
  fundamental frequency
• The closed end must be a node and the open end is
  an anti node

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Standing waves on a string
Harmonics in violin string (always nodes at the
ends)
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Closed on both ends (Piano)
 
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Piano Guitar

• Notes are just standing waves of a specific
  frequency.

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Tube Open on both ends (Flute)
Tube Open at Both Ends                          
                                            
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Tube Open Closed (Pipe organ)

• Only odd harmonics n 1, 3, 5, 7, (We need a
  node at the closed end and an antinode at the
  open end)

Tube Closed at One End                          
                                            
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Timbre (French, from tambourine)

• If notes are just waves of different
  frequencies, why do all the different instruments
  sound different when they play the same notes?
• An instrument always plays several frequencies at
  once.
• This spectrum of frequencies is called the
  instruments timbre (tone color).

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Sound Color- Timbre
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Beat frequency (Interference)

• Beats
• A "wah wah" is heard when two close frequencies
  are heard. The number of Beats equals the
  difference between the frequencies
• Example
• 230 HZ 232Hz
• Beat frequency is 2Hz

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Sound intensity level decibel dB

• Intensity of a sound was is defined as the energy
  carried by the wave per unit time across unit
  area. Intensity is measured in watts per square
  meter (W/m2). The intensity is proportional to
  the square of the amplitude.
• Sound level intensity is related to the perceived
  loudness of a sound, measured in decibel (dB). A
  logarithmic scale is measured for this quantity.

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Sound intensity in dB
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dB Calculations
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Audible range of frequencies

• Good human ears can detect sound from about 20 Hz
  to about 20,000Hz (20 kHz)
• This can be called the audible range of
  frequencies.
• As people grow older, we loose our ability to
  detect high frequencies.

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Ultrasound - Above Hearing
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Infrasound- Below human hearing - sound of
seismology

• 10 Hz to 0.001 Hz
• Earthquakes, Auroras , Tornados, Tsunamis
• Brown University seismic
• Uni Fairbanks Infrasound Page
• Tsunami Columbia Uni