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Auditory System: Loudness, Complex Sounds, ETC
How
do we perceive the large # of separate sounds? How do we localize sounds?
How do we Identify individual sound sources?
SENSITIVITY
AND LOUDNESS
Factoid: - if we were 10-15 dB more sensitive, we could hear the hiss
of colliding air molecules
*We Are Not Sensitive to all Freqs Equally*
Audibility curve:
relationship between sensitivity -vs- frequency; a plot of the minimum
SPL needed to detect a tone as
a function of freq (dB -vs- Hz)
<<< FIGURE: The Audibility Curve >>>
(Similar to spectral
sensitivity curve, intensity plotted as function of wavelength)
Low sensitivity at Low freq / High sensitivity at High freq
- e.g. 20 Hz tone must
be 80dB to be audible, 200 Hz about 20dB
Sensitivity to freq for human speech 400-3000 Hz
The area above the U-shaped function up to the Threshold of Feeling (tones
become painful) is the Auditory response area
Loudness, Intensity,
and Frequency
How will holding freq. constant and increasing intensity affect loudness?
Stevens used magnitude estimation to measure this relationship
Sone: the unit of loundess; set at 1,000Hz @ 40dB
Stevens found a relationship of 0.6 between loudness and decibels
*Loudness is a function
of SPL AND Freq*
- if we say a tone is 60dB it can be unhearable (at 30Hz) OR easily heard
(at 5K Hz)
Equal loudness curves: plot of the number of decibels by freq that
create the same perception of loudness
Note that the higher ELC is flattened, so at 80dB all tones from 30-5K
Hz have about = loudness
Stereo 'Loudness' button - at high volumes (ex. 80dB SPL) all freq heard
about equally well; at low volume (10dB SPL) the very low (400Hz and below)
and very high freq (10K Hz and above) are inaudible; Loudness button boosts
low & high freq ends
PHYSIOLOGICAL ASPECTS
OF LOUDNESS
Freqs in middle range of hearing have about a 120dB intensity range
PROBLEM: Most nerve fibers increase firing over a range of 40-50dB, above
these intensities they
saturate (can't fire any faster)
How can nerves signal loudness over large range of intensities?
2 SOLUTIONS
- some fibers don't fire until middle of intensity range & increase firing at high intensities
- place theory and area of maximum activity on the basilar membrane
Low intensity = most
neural activity centered on one place on membrane; Higher intensities
= neurons begin to saturate and neurons at other places begin to fire
more rapidly
*Broader firing patterns may signal loudness*
PERCEIVING PITCH
Pitch and Frequency
Relationship between perception of Pitch -vs- Freq
Mel: the standard of pitch; 1,000 mels = 1,000Hz @ 40dB SPL
There is a good match btwn perception and physiological vibration of membrane
Periodicity
Pitch (Effect of the Missing Fundamental)
Musical tones have a fundamental freq and harmonics
The fund. freq determines pitch
Removing the 400Hz fundamental does not result in an 800Hz pitch
Changes the timbre slightly but no effect on pitch; the fact that the
pitch does not
change is called Periodicity pitch or Effect
of the missing fundamental
If tones with different Fourier spectra can have same pitch, this poses
a problem for place theory of
pitch (should be diff places on basilar membrane)
Periodicity pitch occurs even if the harmonics are presented to different
ears
- e.g., 1600Hz to one ear/1700Hz
to other = pitch of 100Hz
This indicates that this effect happens where auditory signals from both
ears are first combined
- e.g.,
could be at the superior olivary nucleus)
Central pitch processor: a central mechanism that analyzes the
pattern of harmonics and selects the most likely fund freq
*Auditory nerve firing contains place information regarding freq, but
pitch is perceived after analysis by central mechanisms*
Same pitch can be signaled by: large amount of neural firing to a tone's
fund freq OR by the pattern of neural firing to a number of diff freqs