The Nature of Sound
Sound pressure easier to measure
dB SPL = 20log10P(measured)/P(reference)
P(ref) = 2x10-5 Pascals
For Intensity
dB = 10log10I(measured)/I(ref - 10-12W/m2)
So 3dB increase in intensity is doubling of sound energy.
The Nature of Sound
• Previous parameters describe pure-tones
• Real sound is mixture of pure tones
• Breakdown into component pure tones requires Fourier Analysis
• Performed by cochlea!
Klinke 22-1
The Nature of Sound
Sound pressure easier to measure
dB SPL = 20log10P(measured)/P(reference)
P(ref) = 2x10-5 Pascals
For Intensity
dB = 10log10I(measured)/I(ref - 10-12W/m2)
So 3dB increase in intensity is doubling of sound energy.
The Nature of Sound
• Previous parameters describe pure-tones
• Real sound is mixture of pure tones
• Breakdown into component pure tones requires Fourier Analysis
• Performed by cochlea!
Klinke 22-2
Anatomy of the Ear Sounds waves in air
Air Eustachian tube Middle ear is sealed air filled cavity
Ear drum compliance
Gives ability to gather sound
Selective valve that permits air to enter middle ear
Connects back of nose to middle ear
Equalises pressure outside and inside ear
Permits normal hearing
When pressure imbalance hearing decreases – reduced compliance
Equalising pressure causes ‘pop’
Anatomically divided into 3 parts
– External: lateral to tympanic membrane
– Middle
– Inner: cochlea, utricle, saccule and semicircular
tympanic membrane oval window Amplification mechanisms: intensity. Attenuation mechanisms 1) 2) Bones of middle ear Tensor tympani and stapedium
muscles are activated by V and
VII motor nuclei to attenuate low
frequency intensities.
Klinke 22-3
Klinke 22-7
Sound Transmission Vestibular duct Cochlear duct Tympanic duct 1. Sound waves in the air strike the tympanic membrane 2. Sound wave energy is transferred to bone of the middle ear which vibrate 3. The vibrations are transmitted via the oval window to the fluid within the vestibular duct and create a fluid wave within the cochlea 4. The fluid waves push on the flexible membrane of the cochlear duct 5. Sound waves are transmitted to the tympanic duct and dissipated back into air by the movement of the round window. 6. Deformation of the cochlear duct causes the tectorial membrane to move and activate the stereocilia of the hair cells. 6
The Cochlea The cochlea is the organ where sound waves are converted first into fluid waves, then into chemical signals and finally into action potentials The movement of the tectorial membrane by sound waves moves the cilia on the hair cells and effects neurotransmitter release by the hair cells Or T
Klinke 22-4
Klinke 22-5
Klinke 22-6
Klinke 22-8
Outer hair cells Cylindrical shaped Have stereocilia at top Nucleus at the bottom When stereocilia are bent by sound waves an electromotive force occurs Cell changes shape – lengthens then shortens Pushes against the tectorial membrane Amplifies vibrations in the basilar membrane allowing us to hear very quiet sounds
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