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Chapter 5

PSYC 251 Chapter Notes - Chapter 5: Cochlear Nerve, Sound Intensity, Fundamental Frequency

Course Code
PSYC 251
Stanka A Fitneva

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The computational task facing the auditory system is to extract from these complex
waveforms the discrete auditory objects that created them such as individual voice, musical
melodies, and car horns.
- Loudness is the perceptual attribute of a sound that corresponds most closely to its
physical intensity
- Two techniques to compare the loudness of different sounds: loudness matching and
loudness scaling
Loudness matching
- in this technique, the subject is required to adjust the intensity of a sound (known as
the comparison stimulus) until it sounds as loud as a standard stimulus with a fixed
- Equal loudness contour: a curve plotting the SPLs of sounds at different frequencies
that produce a loudness match with a reference sound at a fixed frequency and SPL
Tend to follow the absolute threshold curve at lower standard intensities, but
flatten out at high intensities
- The “bass boost” control in audio reproduction equipment – see page 130
Loudness scaling
- Equal loudness contours provide a means of comparing the loudness of different
sounds, but they cannot tell us how rapidly loudness of a sound increases with its
- The simplest method is to ask subject to assign numbers to sounds at different
intensities magnitude estimation
- Loudness does not increase linearly with intensity; loudness obeys a power law in
which sensory magnitude grows in proportion to stimulus intensity raised to a power
- Stevens (1961): the exponent for loudness is 0.3 each time sound intensity increase
by 10dB, loudness increases by a factor of two
Models of loudness perception
- Auditory nerve fiber responses are known to be frequency selective because of the
frequency-to-place conversion performed by the cochlea
- As the firing rate of auditory nerve fibers increases, so does encoded intensity
- The excitation pattern model proposes that the overall loudness of a given sound is
proportional to the total neural activity evoked by it in the auditory nerve
- Different auditory nerve fibers cover different ranges of intensity although
individual auditory nerve fiber have a restricted dynamic range, the range covered by
the whole population is sufficient to account for the range of loudness perception
- Central limitations in the brain prevent auditory nerve fibers from making optimal use
of the incoming neural info
- On limitation might be memory since loudness comparison requires the listener to
retain memory trace of one sound in order t compare its loudness with a second sound
- Pitch is the perceptual attribute of a sound that corresponds most closely to its
- In the case of pure tones, pitch is related to the frequency of the tone
- In the case of complex tones, pitch is related to the frequency of the fundamental
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- Pitch allows us to order sounds on a musical scale extending from low bass nite to
high treble notes
Frequency selectivity
- Ohm’s law: the auditory system constructs a separate representation for each
frequency component of a complex sound
Psychophysical studies of frequency selectivity
- The main psychophysical technique used to investigate the limits of frequency
selectivity is masking
- Systemic studies of masking typically measure a listeners ability to detect simple
sinusoidal signal in the presence of a noise mask
- Noise: refers to a stimulus containing a wide range of frequency components having
random phases but equal amplitudes subjectively, noise produces an unstructured
hissing sound, similar to a de-tuned radio
- Band-pass noise: a sound stimulus containing equal energy within a certain band of
frequencies above and below its certain frequency
Critical bandwidth: only noise frequencies within the pass band of
the filter affect the listener’s ability to hear the signal; the noise
bandwidth at which detectability flattens off can then be taken as an
estimate of the band width of the auditory filter
- Masking is most effective when the mask center frequency is very close to the signal
- As mask center frequency moves away from signal frequency, progressively higher
mask SPLs are required to maintain the signal at threshold
- As seen on the psychophysical tuning curves, low mask intensities are needed when
the mask has a similar frequency to the signal, but progressively more intense masks
are needed as mask frequency departs further from signal frequency
Physiological basis of frequency selectivity and masking
- The point of max displacement on the basilar membrane varies with stimulus
frequency, so frequency tuning of auditory nerve fibers reflects their place of
innervation on the basilar membrane
- We can therefore propose a link between frequency-tuned responses in the peripheral
auditory system, as revealed by physiological studies, and the auditory filters
revealed by psychophysical studies
- Critical-band masking: an experimental effect in which masking of a sinusoidal
signal occurs only when the center frequency of the noise falls within a certain band
of frequencies surrounding the signal
- The simplest explanation of critical-band masking in psychophysical tasks makes
three assumptions:
1. The presence of signal increases the activity in auditory filters tuned to its
2. When this response increment exceeds some minimum value, the signal is
detected by the listener
3. The mask also produces excitation in auditory filters, but this activity is
obviously unrelated to the presence or absence of a signal
- If the mask and the signal activate different filters, then the presence of the mask
should have no effect on the excitation produced by the signal, and therefore on its
- If the mask excites the same filter as the signal, the resultant activity will swamp the
activity produced by the signal, impairing detectability
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