Hearing - Ear Anatomy Bones that transmit sound to the...

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Unformatted text preview: Ear Anatomy Bones that transmit sound to the cochlea Liquid filled Processes sound and transmits to brain Outer and middle ear amplify sound pressure 1 The Cochlea 2 The Cochlea (cont'd) (Stirrup) 3 The sound waves resonate at different points in the basilar membrane (tonotopic relationship) The location of maximum resonance depends on the frequency of the sound High frequencies: close to the oval window (entrance of the cochlea) Low frequencies: closer to the apex 5 kHz 500 Hz 100 Hz 20 Hz 1 kHz 15 kHz 20 kHz 4 The organ of Corti contains the auditory sensor cells (hair cells) 15,000 to 20,000 auditory nerve receptors Each receptor has its own hair cell Nerve fibers from haircell connect to the brainstem, then to the primary auditory nuclei, up to the thalamus, then to the primary auditory cortex Auditory nerve has 30,000 nerve fibers 5 How do We Measure Sound? Pressure = Force/Surface Force = Surface Pressure Force is measured in Newtons [N] Surface is measured in m2 Pressure is measured in Pascals [P] or N/m2 1 Kg 9.8 N 1 Kg 9.8 N 0.2 m2 At each point, the pressure is 9.8 N/0.2 m2 = 49 N/m2 0.05 m2 At each point, the pressure is 9.8 N/0.05 m2 = 196 N/m2 Force refers to a body Pressure refers to a point in space 6 What is Sound Sound is the variation of pressure in the air High pressure Low pressure 7 A simple sound (tone) is a periodic variation of pressure (at a given point) in time A = 0.8 N/m2 T=0.01 s T = period = 0.01s F = frequency=1/T= 100 Hz (Hertz=cycles/second) A = amplitude = 0.8 N/m2 8 Sound Pressure Level The Sound Pressure Level measures sound in dB SPL = 20 log10 A/A0 (A = amplitude) Reminder: if 10y = x, then y = log10 x Example: 103 = 1000 then log10 1000 = 3 What is log10 10x? x = 10y then 10x = 10y10 = 10y+1 Then log10 10x = y+1 = log10 x + 1 If we increase the amplitude of a sound by 10, its SPL becomes 20 dB higher If we increase the amplitude of a sound by 100, its SPL becomes 40 dB higher A0 is the amplitude of a barely perceptible tone at 1 KHz SPL is a better measure of perceived loudness than amplitude is 9 Threshold of injurious sound 10 Equal Loudness Contours (Fletcher-Munson Diagram) Sound Pressure Level (dB) 11 Degrees of Hearing Loss Hearing Threshold (dB) 0-25 26-40 41-55 55-70 71-90 >90 Degree of Loss Normal Mild Moderate Moderately Severe Severe Profound 12 Loud but not Clear? We may hear the conversation but not understand it. Speech discrimination is measured by the percentage of monosyllabic words recognized in a list Played back well above the hearing threshold in quiet environment 90-100% 75-90% 60-76% < 60% Normal Slight disability Moderate disability Severe difficulty 13 ...
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