Sound and the auditory system

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Sound and the auditory system
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Sound

• What is sound?
• The experience we have when we hear (perceptual
  definition)
• Pressure changes in the air or other medium
  (physical definition)
• Sound occurs when movement or vibrations of an
  object cause pressure changes in the air, water
  or other medium surrounding the object - sound
  wave
• Speed of sound in air 340 m/s in water 1500m/s

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Sound

• What is the perceptual experience associated with
  these (air) pressure changes?
• Sound waves can cause
• Tactile perceptions
• Hearing the main experience caused by sound
  waves
• The physical characteristics of the pressure
  changes related to hearing are amplitude and
  frequency

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Sound terms

• Computer generated
• Pure tone sound of a single frequency
• Sine wave
• Frequency - the number of times per second that
  the pressure changes repeat
• Amplitude size of the pressure changes
• Phase - relative position in time

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Amplitude and Loudness

• Pressure difference between high and low peaks of
  the sound wave
• Increase amplitude increase perceived loudness
• Large range of amplitudes from events in the
  environment (see table 11.1)
• Sound pressure to decibels (named after who??)
• dB 20 Log (p/p0) p sound pressure, p0
  standard sound pressure of 20 micropascals

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dB and loudness

• The dB scale useful as it compresses the large
  range of sound pressures into a manageable range
• Decibels is related to the psychological
  experience of loudness
• Graph showing the relationship between dB and
  loudness for a pure tone (Fig. 11.4)

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Frequency and Pitch

• Measured in Hertz (Hz) 1Hz is 1 cycle per
  second
• Period / periodic tone
• Pitch the psychological quality of a tone that
  we can describe as high or low closely
  linked to frequency (see Figure 11.5)
• tone height / pitch / tone chroma / octave - the
  piano keyboard (Figure 11.6)

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The range of hearing

• Humans 20Hz 20kHz
• Elephants hear below 20Hz dogs hear above 40
  kHz cats 50kHz and dolphins above 150kHz
• Figure 11.7 curve is for pure tones
• Audibility curve auditory response area equal
  loudness curves
• The perceived loudness of pure tones depends on
  sound pressure and frequency

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Timbre

• Distinguish among different musical instruments
• Used to distinguish between sounds with the same
  loudness and pitch e.g. a piano and violin
  playing the same note
• How is timbre related to the physical properties
  of sound?
• Complex tones more than one frequency component
  most of the sounds around us are complex tones
• These additional frequencies and their relative
  strength contribute to the timbre of the sound

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• Additive synthesis
• Fundamental frequency (first harmonic)
• Second, third fourth etc. harmonics
• Harmonic series
• Frequency spectrum - indicating frequency and
  amplitude of the harmonics
• Compare the frequency spectrum of guitar, bassoon
  and alto sax playing G3

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• Timbre also depends on the attack and decay
  pattern of the note
• Play backward and forward piano tone timbre
  difference yet the spectrum is the same for each
  time reversed waveform amplitude of waveform
  would be changed