Michael J. Harrison

         Department of Physics and Astronomy, and
                Department of Epidemiology

         304 Physics & Astronomy Building
         (517) 355-9278,   harrison@pa.msu.edu

The Role of Thermally Excited Eardrum Pressure
Fluctuations in Establishing Primate Auditory Thresholds :
Michael J. Harrison.

The effects of thermal pressure fluctuations and particle Brownian motion in 
determining minimum audible sound signals in human subjects have been 
previously discussed [1]. Recent work has studied how variations in external
ear structure alters its role in bringing incident sound signals to the tympanic 
membrane in humans [2]. I have calculated the equilibrium mean square pressure 
fluctuation on the tympanic membrane of a model mammalian auditory apparatus 
due to single particle Brownian motion and also collective resonant modes 
thermally excited in the external auditory meatus. The result includes dependence 
on the geometric dimensions of the meatus, as well as on the temperature of
the air that transmits the sound to the tympanic membrane. For adult humans of 
typical size and acoustic band-width capacity, and also other primates, the clinical 
threshold for hearing has a mean square pressure that is remarkably close to the 
noise level given by calculations of statistical thermodynamics. This phenomenon 
provides striking evidence for the effectiveness of natural selection in leading to 
physically optimal auditory sensitivity, as measured by evolved signal to noise 
ratios in primate auditory thresholds. Such auditory sensitivity  would provide a 
comparative advantage that helps to avoid predators and to detect other unseen
quiet hazards, as well as potential food sources.

[1]  L.J. Sivian and S.D. White, J. Acoust. Soc. Am. IV, 4, 288(1933);
       H.L. deVries, J. Acoust. Soc. Am. 24,5, 527(1952)

[2]  B.B. Ballachanda, J. Am. Acad. Audiol. 8, 411(1997)