LING124 Sound waves

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LING124 Sound waves

• August 28, 2008

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Announcement

• Course website
• http//www.sjsu.edu/faculty/hahn.koo/teaching/ling
  124/ling124.html
• No password protection yet

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Class outline

• Sound wave
• Sine wave representation
• Amplitude, power, intensity, dB
• Frequency, period
• Complex wave
• Sum of two or more sine waves
• Quality and component waves
• Example speech sound
• Praat

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Sound wave

• Variation in air pressure
• Air particles oscillate around fixed locations
• Compression
• Air particles are packed tighter than usual
• Rarefaction
• Air particles are packed less tightly

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Sine wave

• Represents local pressure variation over time
• Equilibrium resting position
• Crest maximal compression
• Trough maximal rarefaction
• yAsin(2pft)

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Amplitude

• Degree of change in air pressure
• Amount of displacement of air particles from
  their resting positions
• Determines loudness of a sound

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RMS amplitude

• Amplitude over a given interval of time
• Average amplitude (Arithmetic mean) would be zero
• Root mean square

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Power, Intensity

• Power
• Total amount of energy contained on the surface
• Proportional to the square of amplitude
• Intensity
• Amount of energy over time
• Equal to the square of amplitude

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Measures

• The actual value of amplitude, power, intensity
  of a sound can be measured in standard units
• Amplitude Pascals (N/m2)
• Power Watts (Nm/s)
• Intensity Watts per square meter
• We are interested in how loud a sound is relative
  to another sound
• The absolute values can be too large
• Difference in loudness depends on the ratio of
  the powers

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Decibel (dB)

• Magnitude relative to a reference level
• Reference level
• Threshold of hearing
• 210-5 Pa
• 10-12 W/m2

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Some dB levels
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Frequency

• Period (T)
• Time required to complete a cycle
• Frequency (F)
• Number of cycles per second (Hz)
• Determines pitch
• F1/T, T1/F

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Frequency example

• Frequency 20Hz
• Period 0.05 sec. 50 ms.

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Complex waves

• Not all sounds have simple wave forms
• The shape of the complex wave determines the
  quality of sound

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Sum of waves
100 Hz
200 Hz
300 Hz
Sum
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A speech sound yes
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Praat

• Doing phonetics by computer
• Download
• http//fonsg3.hum.uva.nl/praat/
• Brief overview
• Create sound from
• Formula
• Tone complex