Physics 371 - Acoustics for Musicians

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Physics 371 - Acoustics for Musicians
selected slides, March 5, 2002
Loudness at different frequencies Critical
band Masking The ear Neurological response of the
ear
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Critical Bands of Sound Perception
for pure tone of some frequency, nerve cells of
the basilar membrane are excited over a small
region. approx 30 regions each covering about 1/3
octave Adding more sound in a different band
causes much more increase in subjective loudness
than adding sound in the same band.
demo
Subjective loudness of complex tones is measured
in SONES (see Fig 4, page 99 Backus to find SONES
from Phones. Difficult and subjective - when are
tones judged twice as loud?
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1/4 octave band
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Critical Band and Masking
when will one tone mask another? (a) for small
loudness (mf or below) another tone is
masked only if it's frequency is within the same
critical band. Calculations if the
softer tone adds less than 0.5 dB to the
total intensity level it will not be heard.
(b) for loud sound such as ff tones of
LOWER pitch are masked very little tones of
HIGHER pitch are masked (can't be heard)
importance for orchestration!
see Fig. 5, p. 102
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Human basilar membrane response for different
frequencies
(may explain some aspects of masking)
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Neural Response of Ear the Neural Transmitters

• sound is transmitted to the brain not as a
• wave shape but as individual pulses
  ("discharge"
• of nerve cell).
• Pulses have same height independent of stimulus,
  
• average pulse rate depends on loudness level.
• time distribution of pulses encodes wave shape

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Rose et al. J. Neurophysiology
time interval between nerve impulses for
different frequencies.
rate 30 spikes/s at 70 dB
rate 120 spikes/s at 70 dB
217 Hz
600 Hz
192 Hz
460 Hz
167 Hz
365 Hz
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30dB
70dB
60dB
20dB