Chapter 5 - Chapter 5 The Auditory System Sound and the Ear...

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Chapter 5- The Auditory System: Sound and the Ear A. The Physics of Sound - Understanding of sound began with Galileo in the 16 th c. o Galileo believed sound is a vibrational event and that alterations of the vibrational frequencies correlate with differences in pitch. o At the time, others believed sound was created by invisible particles. o The dispute was resolved with the bell jar experiment (Boyle, 1660), in which the sound produced by a bell becomes less audible if enclosed in a jar from which air is slowly pumped out, eventually becoming silent (proved necessity of a medium). The creation of sound - Sound, in the context of hearing, can be described by a set of perceptual qualities. - To a physicist, sound is a vibrational disturbance of a medium w/ certain physical qualities. - 3 essential requirements of audition: sound energy must have a source, travel through a medium, and be translated into a biological sound by some mechanism. Vibrational properties of objects: - For an object to be able to create sound, it must have 2 essential properties: o Inertia : tendency of an object to stay in motion; greater mass = greater inertia. o Elasticity : property that restores object from distortion to original state. o The interaction of inertia + elasticity makes an object vibrate when force is applied. The tuning fork as a sound source: - Tuning fork : narrow, U-shaped metal bar; produces one tone when struck; sound quality depends on length of the prongs (or tines). o Initial force causes the tines to move in the force’s direction; elasticity then produces a restoring force to return to equilibrium; inertia opposes this restoration, producing vibration. The end of vibration due to friction against air = damping. - Simple harmonic motion : back + forth repetitive movement through an equilibrium point (0) where the maximum displacement on both sides is equal and the interval of each complete vibration is the same. E.g. as produced by a pendulum or tuning fork prongs. - The displacement profile of simple harmonic motion is a sinusoidal function . Impact of a sound source on the medium: - The simple harmonic motion of a tuning fork imparts a corresponding vibrational disturbance to surrounding air particles (which are normally in constant but random motion, maintaining an average distance from each other). - Outward movement of prongs causes air molecules to momentarily become crowded = compression ; i.e. an increase in air pressure. - As the prong passes the equilibrium and vibrates in the opposite direction of the force, the air molecules momentarily thin out = rarefaction ; i.e. a decrease in air pressure - Alternating pressure changes move outward and away through air mass, because each air particle acts as a separate vibrator, which collides w/ others and passes along the vibration.
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