Passage Sound Waves Propagate Through Many Conducting Structures in the Ear

Passage
Sound waves propagate through many conducting structures in the ear before they are transduced into neuronal signals.  When sound waves reach the fluid-filled cochlea, they are detected by hair cells lining the basilar membrane.  The cochlear spiral can be modeled as a resonator system because each section of the basilar membrane is sensitive to a specific frequency (Figure 1) .
Figure 1  Amplitude pattern of the basilar membrane for different frequenciesPresbycusis (age-related hearing loss) results from a combination of factors that lessens an individual's mechanical and/or neurological sensitivity to sound.  Pure tone audiometry (PTA) is used to determine an individual's ability to detect different sound frequencies and can be used to evaluate presbycusis.  A "pure" tone is characterized by a single sinusoidal waveform.  An audiogram plots the relative intensity required of a given frequency to be detected by the individual (Figure 2) .
Figure 2  Audiogram of an elderly patient exhibiting presbycusis
-Audiometry can also be used to evaluate a subject's ability to localize a moving sound source.  When the sound source is moving toward the subject, which of the following increases?

A) The observed wave velocity
B) The observed period
C) The observed frequency
D) The observed wavelength

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