1 Review of Physical Terms Sound –By definition, all sounds require three components Source - must vibrate with sufficient force to affect the surrounding

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Review of Physical Terms

• Sound– By definition, all sounds require three components

• Source - must vibrate with sufficient force to affect the surrounding environment

• Medium - required to propagate energy from one location to another

• Receiver - a perceiver or device that can have its physical state altered by the energy produced by a vibrating source

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• Source - Requirements– Must vibrate– To vibrate, it must have mass and stiffness

• Vibration is the interplay between inertial force, associated with mass, and restoring force, which is associated with the stiffness of the source

• The rate of vibration is the FREQUENCY• The amount of time for one cycle to be completed is the

PERIOD• The size (power) of the vibration is its AMPLITUDE• The amount of time vibration lasts is its DURATION

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• Medium - Requirements– Mass and stiffness– May be comprised of air, fluid, solid

• The type of medium dictates the speed of sound• More rarefied the medium, the slower the

propagation– Inverse square law – mathematical

representation of the loss of acoustic energy as sound travels

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• Receiver - Requirements– Must have its physical state changed by the

presence of the sound– Change in physical state excites neural

mechanism – for us, it’s a sensory cell and, ultimately, a series of neurons

– Auditory mechanism, therefore, features structures that vibrate in synchrony with the source and medium, and transduce the vibration into another form of energy (bioelectric)

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Review of Physical Terms• Vibration - moment by moment account

– Initial force creates displacement– Interplay between inertia and stiffness keep object in motion

• The restoring force (proportional to stiffness) contributes to the damping the object will experience and applies force in the direction of the resting position

• The inertia contributes to the continuation of the vibration, driving the object in whatever direction it happens to be moving at that moment

• High damping, short duration• Low damping, long duration

– PHASE is the term for the object’s location at any moment (out of 360 degrees)

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• Filtering - types of filters and their depiction on an amplitude spectrum– low pass – passes energy below a certain

frequency– high pass – passes energy above a certain

frequency– band pass – passes a range of frequencies– band reject – eliminates a range of

frequencies

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• Filtering – analogy to your stereo– The bass control shifts the pass band of the filter to

progressively lower frequencies– The treble control shifts the pass band upward in

frequency– The selectivity, or tuning of the filter indicates how wide

the pass band is• For example, a radio or TV is tuned to a very narrow

bandwidth at any given moment• In this way, only one station at a time is “passed”

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• Basic Associations between physical components of sound waves– F=1/T– F=1/λ– Displacement 1/F– Duration 1/Damping– Damping is proportional to stiffness, inversely

proportional to mass (Tacoma Narrows Bridge)– Fres stiffness/mass

• Therefore, the greater the stiffness, the higher the resonance frequency; in general, the higher the mass, the lower the Fres

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RESONANCE

Resonance - occurs when a force is applied to a body at the frequency at which the body would vibrate if set in motion, and then left to itself (Stevens and Davis, 1938)– Equation: F(res) s/m– Consider effect of increasing stiffness – will raise the

resonance frequency– Consider effect of increasing mass – will lower the

resonance frequency– Recognize the effect of damping is minimal at the

resonance frequency– Therefore, the object will vibrate at the greatest duration

and amplitude at its resonance frequency

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RESONANCE

Resonance – May also occur when an object is driven at a rate equal to the rate at which it would vibrate if left alone: in this way, the energy of the vibration is reinforced– Tuning forks of different frequencies versus forks

‘tuned’ to the same frequency– Child on a swing– Pushing a car out of ditch– The Tacoma Narrows Bridge (1940)

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RESONANCE

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RESONANCE

http://www.youtube.com/watch?v=3mclp9QmCGs

http://www.youtube.com/watch?v=17tqXgvCN0E&feature=related

http://www.youtube.com/watch?v=3mclp9QmCGs



Audition: Anatomy

• Outer ear• Middle ear• Inner ear• Peripheral Nerves• Central Pathway• Primary Auditory Cortex

Outer ear: Primary function is to focus acoustic energy onto the eardrum and increase high-frequency components.

Middle ear: Primary function is to amplify mechanical vibration produced by the soundwave’s pressure.

Inner ear: Primary function is to convert mechanical vibration into electrical impulses. Also houses balance organ.

Divisions of the Auditory System:

“Outside” “Inside”Conductive Sensorineural

Sensori-neural system

A & P of the Auditory System

• Two pathways that sound can take but both must route sound through the cochlea– Air conduction pathway includes the outer and

middle ear, as well as the cochlea and neural structures (conductive and sensorineural systems)

– Bone conduction pathway includes the cochlea and neural structures (sensorineural system)

• The cochlea is where vibratory energy is transformed into neural energy

• Sound is filtered/processed throughout the auditory system at all stages


• The Outer ear - the visible portion of the ear, consists of:– Pinna – External auditory meatus – Lateral surface of the TM– Comprised of cartilage (lateral 1/3) and

bone (medial 2/3)

Helix: the prominent ridge which begins superior to the opening of the EAM

Antihelix: a ridge which runs parallel to the helix

The lobule (ear lobe): bottom of the pinna (no cartilage)

Concha: a deep depression which forms the mouth of the EAM

Tragus: a backward folding ridge located on the anterior wall of the ear canal

External Auditory Canal

Architecture:• An average horizontal diameter of 6 mm• An average vertical diameter of 9 mm• Approximately 23-25 mm in length• The skin of the cartilaginous portion contains glands that

produce cerumen (earwax)• Earwax keeps the epithelium in the meatus moist

(prevents chapping)• Shape of the canal, the hairs at the entrance, the

secretion of cerumen helps keep foreign objects out of the canal and aids in self-cleansing it

• The ossification is typically completed by the end of the third year

• Bony portion fixed in diameter• Cartilaginous changes with jaw movement (note that HA

fittings occupy cartilaginous portion)


Congenital microtia (small, malformed auricle / pinna) and external canal atresia

Congenital microtia without canal atresia.

Preauricular appendage (supernumerary auricle)

Bifid earlobe

TYMPANIC MEMBRANE

• Doorway between outer ear and middle ear• Takes up the vibration of air particles in the EAC and

passes it to the middle ear system• Primary function is to overcome the loss of energy as

vibration moves from the air to the fluid of the inner ear

TYMPANIC MEMBRANE

TYMPANIC MEMBRANE

Tympanic membrane (eardrum)

The medial limit of the EAM is formed by the TMTM sits at an angle of about 55 to the ear canal

Tympanic membrane (eardrum)The TM is thin (0.1 mm), very compliant but resistant to breaking

Cone-shaped (like a miniature loudspeaker), the TM is displaced inward near its center by about 2 mm; the peak of this broad cone is called umbo

Very small mass (15 mg), compliance and cone shape make the TM suitable for sound absorption

The pars tensa formed of several layers of tissue

The pars flacccida contains very few fibers

Tympanic membrane (eardrum)

Umbo coincides with the tip of the manubrium of the malleus

Malleus extends upward, appearing at “1 o’clock” in a right ear, “11 o’ clock” in a left ear

Healthy TM is translucent

Development of the Outer Ear

At birth the floor of the external auditory canal has no bony portionExternal Auditory Canal (EAC) is cartilaginous at birth, ossifies through the first seven yearsIn the infant the EAC is short and straight, whereas in the adult the canal is longer and curvesTM is present at a nearly horizontal angle at birth

Development of the Outer Ear

Conductive system maturation

The pinnaContinues growing until the child is 5-9 yrs

External Auditory MeatusOssification continues for the first few yearsChanges primarily the flexibility of the canal, as newborn canals are very flexible

Maturation of the Peripheral Auditory System

The TMAngled obliquely at birthBecause the superior side is shorter, pulling upward on the pinna lengthens the canal, and straightens the TM so that it may be viewedAs canal ossifies, TM straightensAlong with changes to the ossicles (specifically the stapes) conductive system transmission changes

Anatomy

• An adult middle ear cavity is an almost oval, air-filled space of roughly 2 cm3

and is enclosed within the temporal bone.– Lined with mucous

membrane with cilia (small hair-like projections) that periodically moves in a wiping action to cleanse the middle ear via the eustachian tube

It contains a chain of movable bones (ie., ossicles), which connects the lateral wall to its medial wall.

MIDDLE EAR STRUCTURES



Walls of the Middle ear

– Note - Lateral wall (the tympanic membrane) has been removed to see the interior of the middle ear.

• RED: Internal carotid • BLUE: Internal jugular vein • PURPLE: Tensor tympani • YELLOW: Facial nerve (VII) with nerve to stapedius & chorda tympani • DARK GREEN: Eustachian tube, oval and round windows • LIGHT GREEN: Cochlea • Black: Mastoid air cells


• The Labyrinthic or Medial Wall is vertical in direction, and constitutes the oval window and round window, the promontory, and the prominence of the facial canal.


• The promontory is a rounded hollow prominence, formed by the projection outward of the first turn of the cochlea; it is placed between the round and oval windows.

• The prominence of the facial canal indicates the position of the bony canal in which the facial nerve (CN VII) is contained.

The Middle Ear Ossicles• The tympanic cavity contains a chain of

three movable ossicles (2 - 6mm in length), the malleus, incus, and stapes (latin for mallet, anvil, and stirrup).

• The malleus is attached to the tympanic membrane,

• The stapes - to the circumference of the oval window

• The incus being placed between and connected to both by delicate ligaments.

The Malleus

• The Malleus consists of a head, neck, and three processes, viz., the manubrium, the anterior and lateral processes.

• 3D ossicles

http://audilab.bmed.mcgill.ca/~daren/3Dear/3d_ear_homepage.html

The Middle Ear Ossicles

• The Head has a cavity or facet for the incus to articulate.

• The manubrium is connected by its lateral margin with the tympanic membrane.

• Near the upper end of the manubrium is a slight projection into which the tendon of the Tensor tympani muscle is inserted.

The Incus

• The Incus consists of a body and two crura


• The body of the Incus articulates with the Malleus.

• The long crus ends in a rounded projection, the lenticular process, which is tipped with cartilage, and articulates with the head of the stapes.

The Stapes

• The Stapes consists of a head, neck, two crura, and a base.

Stapes

• The stapes is the smallest bone in the human body being roughly the size of a grain of rice.


• The head presents a depression, which is covered by cartilage, and articulates with the lenticular process of the incus.

• the base is fixed to the margin of the oval window by a ring of ligamentous fibers.

Animation

http://www.neurophys.wisc.edu/~ychen/auditory/animation/animationmain.html

Muscles of the Tympanic Cavity

• Two muscles - Tensor Tympani & Stapedius

• The Tensor tympani is the larger muscle (about 25 mm in length).

• It originates from the anterior wall near the cartilaginous part of the eustachian tube.

• Inserts into the manubrium of the malleus.


• Tensor Tympani• Innervated by the Trigeminal (CN V)

nerve.• Action - The Tensor tympani draws the

tympanic membrane medial-ward, and thus increases its tension

• Non-auditory stimulation


• The stapedius is the smallest muscle in the human body (length - 7mm).

• It originates from the mastoid or posterior wall.

• It inserts into the neck of the stapes.• Innervated by the facial (CN VII) nerve.


• Stapedius Muscle• Action - pushes the stapes into the oval

window and thereby tenses the oval window and tympanic membrane

• Modifies the mode of stapedial vibration, greatly reducing energy delivered


• Serve a protective function for the inner ear by increasing middle ear impedance– Elicited by acoustic stimuli of 70-90 dB above

threshold– Reflex contraction stiffens the ossicular chain,

reducing sound transmission by 5-10 dB, primarily at frequencies below 1.2 kHz.

• The acoustic stapedial reflex is an important clinical tool and serves as a reliable objective test.

Ligaments of the Ossicles

• The ossicles are connected with the walls of the tympanic cavity by 5 ligaments: three for the malleus, and one each for the incus and stapes

• Connective tissue that stiffens with age

Middle Ear - Muscles & ligaments

Middle Ear Muscles

Tensor Tympani muscle

Stapedius muscle

Parts of the Middle Ear

Eustachian Tube• The Eustachian or auditory

tube is the channel through which the tympanic cavity communicates with the nasal part of the pharynx (nasopharynx).

• Its length is about 36 mm, and it is oriented downward, forward, and medial-ward, forming an angle of about 45 degrees with the sagittal plane and from 30 to 40 degrees with the horizontal plane.

Eustachian Tube• It is formed partly of

bone(1/3rd), partly of cartilage and fibrous tissue (2/3rd).

• The osseous portion is about 12 mm. in length. It begins in the carotid or anterior wall of the tympanic cavity.

• The cartilaginous portion about 24 mm. in length, is formed of a triangular plate of elastic fibrocartilage.

Tensor Veli PalatiniLevator Palatini

Eustachian Tube

• The diameter of the tube is not uniform throughout, being greatest at the pharyngeal or oral orifice (opening), least at the junction of the bony and cartilaginous portions, and again increased toward the tympanic cavity.

• The narrowest part of the tube is termed the isthmus.

Eustachian Tube

• In adults, the tube is normally kept closed by the spring mechanism of cartilage.

• It is opened by the action of three sets of muscles at the nasopharynx orifice.– One of which is the tensor veli palatini

muscle.– Contracts and opens the Eustachian

tube during yawning, sneezing, or swallowing, or when excessive pressure is applied from the nose.


A & P of the Conductive System

• The Outer ear - function– Not a mere passive conduit of sound– Along with the EAC, it high-pass filters

sound - acts like a HF amplifier (10-15 dB of gain at frequencies above 1500 Hz)• Helps with the localization of sound• Helps with speech perception

– Cerumen is produced by sebaceous glands in the cartilaginous portion - protective

OUTER EAR TRANSFER (Filter) FUNCTION

At the opening to the ear

At the eardrum

MIDDLE EAR FUNCTION

• Eustachian Tube - responsible for aerating the middle ear space– The only pathway for air to enter the middle ear– Middle ear must remain aerated so that the TM can vibrate

most efficiently– The most common precursor to middle ear infection is E.

Tube dysfuction or obstruction• More common in pedes than adults

Middle Ear Impedance Mismatch

• When sound waves travel from gaseous to a liquid medium, energy is lost (99.9% would be reflected back).– auditory meatus air cochlear fluids – (low impedance) (high impedance)

• This lost of energy equates to an intensity loss of about 30 dB.

• The middle ear serves to overcome this loss

Middle Ear Transformer Function

• Area ratio – The tympanic membrane’s area (~50 mm2) is

larger than the oval window’s area (~3.5mm2).– This results in a 14-fold (or about 25 dB)

increase in acoustical pressure at the footplate relative to the pressure at the eardrum.


• Lever action-– The manubrium of the malleus is about 1.3

times as long as the long process of the incus.

– This yields a lever advantage of about 2 dB increase in acoustical pressure at the oval window


• Thus it can be seen that the middle compensates for ~27 dB of the 30 dB energy loss due to the impedence mismatch.

• Recent data indicate that the pressure gain contributed by the middle ear is greatest at the lower frequencies. – The middle ear pressure gain is relatively flat at

frequencies below 1 kHz, averaging 23 dB and peaking at 26-27 dB at 0.9 kHz.

– At frequencies higher than 1 kHz, the gain decreases by 8-9 dB per octave, contributing little amplification above 7 kHz.

MIDDLE EAR FUNCTION

• Passage of acoustic energy through the middle ear– The TM vibration is taken up by the ossicular chain

• Malleus, Incus, and Stapes pivot against each other• Their movement is facilitated by the coupling of ligaments to

the walls of the middle ear space, as opposed to the articulation of bone into bone

• The ossicular chain selectively stimulates the oval window• They transform airborne energy into mechanical energy

MIDDLE EAR FREQUENCY RESPONSE

Input to the middle ear

Resonance Peaks

MIDDLE EAR FUNCTION

• The Acoustic Reflex– Changes the stiffness of the middle ear system, thereby

reducing the efficiency of sound transmission– Protects the cochlea from intense sound by reducing the

amount of stapes footplate vibration– Carried out by the stapes muscle (primarily) and also by

the tensor tympani• Stapes is considered the primary effector of the reflex

because the TT can by stimulated by non-acoustic stimuli• Tightens the stapes footplate in the oval window

MIDDLE EAR FUNCTION

• Eustachian Tube Dysfunction– The E. Tube normally opens passively during swallowing,

yawning, chewing– If occluded or not functioning properly, it doesn’t open

often enough to keep the tympanum’s air pressure equal to that of the atmosphere• At any other pressure, sound may be blocked from

passing through the conductive system– More importantly, fluid may build up in the space and

result in a middle ear infection

MIDDLE EAR FUNCTION

• Summary of Middle Ear Function:– Mechanical amplifier that helps overcome loss of energy to

the cochlea• Surface area advantage, lever action, buckling

action, and selective oval window stimulation– Protects the cochlea from intense sounds through the

contraction of the Acoustic Reflex– Must remain aerated to facilitate TM vibration and health

of the ME tissues

Eustachian Tube

• In infants the Eustachian tube is shorter and wider, and in a more horizontal plane.

• The nasopharyngeal orifice remains open to till approximately 6 months of age.

• This has clinical implications and, eventually, educational.

Documents

1 Review of Physical Terms Sound –By definition, all sounds require three components Source - must vibrate with sufficient force to affect the surrounding