Sound

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Sound
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A- The displacement component of sound B-
Compression and Rarefraction C- model closed
auditory receptor D- Pressure as a function of
time (sound wave).
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• Displacement of molecules forms a sine wave of
  high and low pressure
• Period (seconds) 1/frequency (Hz)
• Period is inversely related to frequency- freq
  1/period
• For any given freq, the greater the fluctuations
  of pressure, the greater the amplitude (dB) and
  the louder the sound.

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• Decibel (dB) is a relative measurement- how much
  greater the sound pressure is than for a
  reference sound (average threshold of human
  hearing, 0 dB)
• Sound pressure level (SPL) measured in decibels
  (dB) is a log scale.
• SPL 10 times greater than human threshold is 20
  dB because 20 log 20 log 10 20.
• A sound 100 times greater than threshold would
  be 40 dB
• Thus the 10 to the 10 times range of human
  hearing can be represented in a roughly 200 dB
  range using a log scale.

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Sound Propagation
Displacement component- rigid chains
Pressure component- compressible chains
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• Wavelength (m)
• speed of sound (m/s) / freq.
• 334 / Hz

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• Recap
• Distance wavelength 330/ Hz
• Time period 1/Hz
• speed of sound about 1 ft/msec
• Frequency 1/period
• 1000 Hz 1 kHz
• Example Period of 1000 Hz 1 sec/1000 Hz 1
  msec
• Wavelength of 1000 Hz 330/1000 .33 m 33 cm
  1 ft

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Freq. Wavelength Period 100 3.3 m 10
msec 1000 33 cm 1 msec 10,000 3.3 cm 100
usec 100,000 3.3 mm 10 usec
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• Near field 1 wavelength air displacement
• Far field gt 1 wavelength compression/rarefrac
  tion
• Example Red-winged blackbird- can hear mate
  100 m away- 2.5 kHz mating call has a wavelength
  of .13 m- therefore bird must have pressure
  sensitive (not displacement) hearing.
• Pressure component becomes reduced as it travels
  for two reasons

1) Spherical Spreading- SPL drops by 6 dB every
time the distance from the sound source is
doubled. 2) Excess Attenuation- absorption of
sound by the air and absorption or reflection by
solid objects in the sound path. Why then, are
do higher frequencies exhibit greater Excess
Attenuation?
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• Harmonics
• Natural sounds are complex, consisting of
  several frequencies, all propagating
  simultaneously.
• Consider a piano string-

• Animal vocalizations are produced by forcing air
  over a membrane- causing the membrane to vibrate-
  thus creating harmonics

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• Sound localization
• Horizontal cue- Interaural Time Differences
  (ITDs) carrier or envelope, onset-offset or
  on-going
• Horizontal cue- Interaural Intensity Differences
  (IIDs)
• Vertical cue- Spectral cues, monaural or binaural

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• Interaural phase differences- using time
  differences in the carrier frequency- the phase
  angle of the sound wave- IPDs

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0
180
same pressure
Decreasing head diameter causes such small
interaural distances that ITDs can not be
resolved in by binaural processing in the central
auditory system- solution? IIDs
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Phase ambiguity- Interaural distance must be less
than 1/2 a wavelength for useful time comparison
between the ears at that frequency. Ideally want
1/4 the wavelength
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• Using this information
• (Useable interaural time difference) 700 us
  1/2 period
• 1 period 1400 us 1.4 ms
• freq 1/period 1/1400 714 Hz
• Wavelength 330/714 Hz .46 m 46 cm
• 46/2 23 cm 9.2 interaural distance
• If a sound is coming from right, a 714 Hz tone
  will be 180 out of phase between the two ears-
  therefore highest frequency that possesses
  useable ITDs will be slightly less than half the
  wavelength.
• So, Hz must be less than 714.

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• Duplex theory of sound localization
• 1) IIDs are poor at low frequency (due to reduced
  sound shadows)
• 2) ITDs are poor at high frequency (due to phase
  ambiguity)
• Thus, species use ITDs at low freq. and IIDs at
  high freq.
• Example- human localization w/ tones

Errors
IIDs
ITDs
100
1000
10000
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Brainstem section Superior olivary complex (SOC)-

LSO (IIDs)
MSO (ITDs)
Left ear
Right ear
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Duplex theory recap ITDs 1) Auditory nerve must
phase lock 2) Low Hz hearing 3) highest Hz used-
wavelength lt 1/2 interaural distance 4) enough
neuron sensitivity 5) MSO IIDs 1) need good
intensity differences 2) High Hz hearing 3)
enough neuron sensitivity 4) LSO
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• Exception
• Pallid bat
• Uses low frequency to localize prey
• Therefore, poor IIDs
• But also very small head- poor ITDs
• So, what to do?
• ITDs using the envelope (onset, offset)

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Barn Owl Sound Localization
IIDs
ITDs
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Vertical localization (IIDs) - asymmetrical
ears - facial ruff