Physiology and Biophysics of Auditory and Vestibular function Dr. Charles CapadayProfessor & DirectorBrain and Movement LaboratoryCRULRG, F-65002601 de la Canardiere, Québec City (QC), G1J 2G3, Canadahttp://www.BrainAndMovementLab.org/
The sensitivity of the ear is remarkable. • Some facts: At 2-3 kHz the threshold of hearing intensity is about 10-16 W/cm2. This corresponds to a pressure variation of 2.9 x 10-4 dyn/cm2, the background atmospheric pressure is 1.013 x106 dyn/cm2 (101.3 kPa). Random pressure variations due to thermal motion of air molecules is 0.5 x 10-4 dyn/cm2 ! The sensitivity of the ear is thus close to the theoretical limit, being just above the thermal noise level. At the threshold power level (10-16 W/cm2) the displacement of the air molecules is less than the size of the molecules themselves! From which it is readily apparent <smile> that only 0.1% of the sound energy enters the water. Water is a very good sound barrier.
The sensitivity of the ear is partly due to its mechanical construction which amplifies sound pressure The area of the eardrum is 30 times larger than that of the oval window. So by Archimide’s principle… The ossicles act as a lever system with a mechanical advantage of about 2. The ear canal has a resonant frequency circa 3 kHz, amplifying the pressure by a factor of about 2. Thus, 2 x 30 x 2 =120. However,…
What we will explain… Sensitivity curve of human ear Sound Localization
Why primary afferent fibre firing rate is nonlinearly related to stimulus intensity (but not always)
Neural Mechanisms of sound localization • The medial superior olive transforms a time code into a place code.
A measured behaviour and its underlying neural systems See, T. Carew. Behavioural Neurobiology. Sinauer Associates, 2000.