Ear Notes

INNER EAR: TRANSDUCTION OF SOUND

What is sound

-Sounds are signal of danger so ability to perceive them is needed for survival- evolutionary necessity-Sounds-general noises and speech- means of communication- massively aided development of human race

-Sound refers to audible pressure variation that propagates away from a source of vibration such as a loudspeaker cone or tuning folk- consists of compression and rarefraction

-Sound is characterised by frequency (number of cycles per second measured in Hertz)-humans have a range of 20Hz to 20000Hz- the frequency determines the pitch of the sound and by its peak amplitude.

-Simplest sound is pure tone-made of sinusoidal increases and decreases in pressure

-Natural sounds, such as speech, tend to have more complex specra-contain several frequency components that vary in amplitude. The spectrum of a more complex sound is related to sound quality

-Very important property of auditory system that underlies ability to understand speech is the capacity to resolve individual components of a complex sound.

-Evolution of ear is tracked through natural history-fossil records show how ossicles developed from bones of the jaw-but as these bones are separated from the jaw they are able to evolve specifically for the function of hearing

Anatomy of the ear

-Split into 3 parts: Outer, middle and inner ear

-Outer ear + middle ear: involved in conduction of signals to inner ear

-Inner ear: receptors that can detect sound

Anatomy of the outer ear

Pinna

-Visible part of outer ear-pinna/auricle – thin plate of yellow elastic cartilage-covered with skin

-Function:

-Collects sounds from a wide area

ii) Resonance

-Amplitude of different frequencies is modified in different ways depending on location of sound source- this filtering effect is referred to as colouration-spectral cues for sound location- useful in distinguishing sounds in front of and behind the head for localising sounds at different elevations.

-This is why some sounds are heard better than others

-Within the flange/helix of the pinna there is a 2dB pressure gain at 4KhZ, whereas in the concha there is a 9dB increase

-Produces an increase in pressure between 2-7 kHz

-Also allow sounds to be localised to some degree using one ear alone

Localisation of sound in vertical plane

-To impair vertical sound localisation one must place a tube into the auditory canal to bypass the pina

-Sweeping curves of outer ear are essential for assessing elevation of source of sound- bumps and ridges produce reflections of entering sound-Delays between direct path and reflected path change as sound source moves vertically

-Sound from below will produce slightly more delayed echo than if the same came from above

-Echoes are produced in the range of 3.2 to 10KhZ-high frequencies are particulary affected-but ability to vertical locate sound deteriotes with age

Eliminates segment of frequency spectrum-pinna notch-seen for high frequencies

-High frequencies-sounds reflect off the contours of the pinna- some of the sounds that enter the ear travel directly to the ear, other sounds eflec off the contours of the pinna first- so these enter the ear canal after a slight delay-this delay causes phase cancellation-eliminates frequency component whose wave period is twice the delay period

-Some frequencies are amplified-acoustic gain

-Spectra of notches and gains- processed in the brain-vertical localisation

-Vertical localisation of sound is seriously impaired if convolutions of pinna are covered

-Pinna= specific to each individual in the auditory system and is calibrated to interest spectra patterns based on specific pinna- when pinna are altered system must re-learn how to recognise new spectral patterns for localisation

-In studies in which subjects wore pinna moulds that dramatically altered spectral cues

-Immediately after inserting the moulds, subjects perceived all sounds as coming from roughly the same elevation regardless of the actual elevation

-Overtime, localisation abilities become normal supposedly due to recalibration of spectral cues

-After removal of moulds, the subjects were able to perform normally without a learning period-suggests the pattern recognition representations were maintained separately

-This gives evidence for the importance of pinnae in vertical sound localisation-hofman et al 1998

iv) Other animals- pinnae may move- allows detection of sounds from different locations more effectively

-Many mammals can move the pinna with auriculares muscles in order to focus their hearing in a certain direction- most humans unlike other animals- don’t have this ability

v) It also has a protective funciton

Ear canal

-From the pinna the sound pressure waves move into the ear canal- also known as the external acoustic meatus

-Tube leads inward from the bottom of the auricular and conducts vibration towards tympanic cavity

Function:

-Cells of canal are protected by layer of cerumen (Ear wax) –secreted by apocrine sweat glands and sebaceous glands that line canal

-Secretion protects other cells and against pathogens but too much can hinder conduction of sound to other cells

-Amplifies frequencies in range of 3kHz to 12kHz

Clinical disorders of external ear

-Preauricular cysts: developmental defects in 1^st/2^nd branchial arches- can become chronically infected

Genetic syndromes

-Konigsmark syndrome- small ears, atresia of the external auditory canal-conducting hearing loss and inherited in autosomal recessive manner

-Treacher Collins syndrome- dysplasia of the auricle, atresia of the bony part of auditory canal, hypoplasia of auditory ossicles and tympanic cavity-mixed deafness

Middle ear

-Middle ear: Air filled Cavity behind the tympanic membrane (ear drum is also part of middle ear)-contains 3 smallest bones in human body (collectively known as ossicles)- malleus, incus and stapes (hammer, anvil and stirrup respectively)- these bones connect tympanic membrane to flexible membrane (oval window) in wall of fluid...

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