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Tutorial 5- Fixing the Mismatch.docx

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Paul Faure

October 9 , 2013 Psych 3A03: Audition Tutorial 5: Fixing the Mismatch Middle Ear Impedances - The tympanic membrane, the middle ear ossicles and inner ear fluids all contribute to the opposition of vibratory motion from airborne stimulus - All of the above contribute to acoustic impedance of auditory system Our Old Firiend – Impedance (Z) - Impedance (Z) is the total opposition to motion - Units of impedance: Ohms - The impedance of a system ahs 2 components: 1. Resistance (R) (energy dissipating component) 2. Reactance (X) (energy storing component) - The acoustical impedance of a system is the complex sum of the resistance (R) and the reactance (X) components of a system √ - Phase relationships between resistance and reactance  X meads R by 90 o  R leads X cy 90 o  X leads X by 180 (i.e. when X = X , mass reactance and compliance m c m c reactance cancel each other) Acoustic Resistance (R) - Resistance is a force that opposes or resits motion - Resistance opposes the motion of an applied force resulting in the transfer of KE to thermal energy (i.e. dissipation of energy as heat) - Units of resistance - Resistance forces are frequency independent - Thus, a given magnitude of resistance provides opposition to an input force regardless of the frequency Middle Ear Resistance (R) - Two main resistive components contribute to impedance of middle ear: 1. Most important resistive component is the resistance of fluid of inner ear to motion of the stapedial footplate at cochlear oval window 2. Friction of middle ear bones also contribute - Resistance is frequency independent - Resistance of the ligaments and muscles - Broad contact to narrow contact Acoustic Reactance (X) - Reactance forces also opposes motion but in a different way from resistance. Some energy applied to an elastic system is stored in the system as PE - Reactance is the component of the system that is responsible for energy storage - Reactance forces are frequency dependent - Reactance varies with mass and compliance of system - Thus, acoustic reactance has 2 components: 1. Mass reactance (X ) m  Xm= 2πfm 2. Compliant reactance (X ) c  Xc= 1/2πfc, where c = object compliance (the inverse of stiffness) Middle Ear Reactance - Two types of reactance components contribute to the impedance of middle ear: 1. Mass reactance of middle ear bones 2. Complia
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