Sound Transduction 1Or, if the a tree falls in a forest and no one is around does it still reflect light?

What is Resonance? Resonance

Characteristic frequency response Density & Tuning Sharpness

Hi dense = Lo Sharpness

Size/Cavity

Breaking Glass Demo

The Outer Ear: All about resonance Pinna

Immobile cartilage side of head Flange

~3 - 4000 Hz resonance Concha

~1 – 7000 Hz resonance

Why such high frequencies? Thoughts?

Directionality Finger in folds demo Spectral Filter (e.g., Rayker et al., 2004)

Resonance Frequencies ~1000 – 7000 Hz

Notch Filter ~ 700, 3500, 7000, 14000 Hz

Outer Ear: con’t Meatus

Cartilaginous – bone Density & Resonance 3000 Hz resonator

Wax + Hair Dirt Filter

Tympanic Membrane Elastic Skin stretched across a bony ring Stiff cone (2 mm height)

High Fidelity Transfer

Middle Ear: The Saga Continues

The Impedance Problem Getting sound to the sensors

Tympanic Membrane to Oval Window (stapedial footplate) Orders of magnitude size

difference Ossicular chain

Malleus – Incus – Stapes – Stapedial Footplate High density benefits!

More middle ear goodness Air filled pressure equal to outside

Eustacian Tube regulation High intensity sound response

Multiple muscles Sound attenuators

Quick Interruption! TLA 1: Hearing Under Water (HUW)

Why is this important? Ingredients:

Sound source (Clicker?) Still water (bath, sink, pool)

Stick your ear, or a friend’s ear under water Make sound in air and under water and listen with:

Out of water ear Under water ear

Questions: Which produces the loudest sound? Is it difficult to determine directionality?

Into the Inner Ear Major subdivisions of the Bony Labyrinth

Vestibular & Auditory Auditory-side = Cochlea Cochlea

Cavity within bone, Fluid-filled caverns Curls like a snail

Life in the Cochlea Three major subdivisions

Scala Vestibuli: Largest cavity, filled with perilymph (e.g., Ricci & Fettiplace, 1998)

Positively charged Sodium Ions (Na+) Scala Media: Smallest cavity, filled

with endolymph (e.g., Ricci & Fettiplace, 1998)

More positively charged Potassium & Calcium Ions (K+, Ca++)

Where the action is!!!

Scala Tympani: Mid size cavity, filled with perilymph Connected to Scala Vestibuli

Scala Media, come get some! ‘Organ of Corti’ Organ o’ Corti contains

Basilar Membrane (base) Tectorial Membrane Inner Hair Cells Outer Hair Cells

Hair cells embedded in Bas. Membrane

Outer Hair Cells contact Tect. Membrane

Basilar Membrane Properties of the

Basilar Membrane Apex thin and stiff,

Base broad and flexible Standing Waves

Upward spread of masking

Why do higher Frequencies get masked by lower frequencies?

Why does it matter that the Basilar Membrane moves? Hair cell magic Outer Hair Cells

~ 12,000 in three rows Afferent and Efferent

connections Attached to muscle fiber

Inner Hair Cells ~ 3,000 in single row Afferent connection Passive Motion

Actual Transduction! Wave along Basilar

Membrane Causes inner hair cell

shearing Shearing opens channel

Endolymph in Scala Media attracts perilymph in Scala Tympani

Charges up Hair cell to cause neural firing

What are the outer hair cells doing? Outer Hair cells motile & embedded in

Tectorial Membrane Theory 1. Stiffen to attenuate sound along the

basilar membrane, shear to add energy to the basilar membrane

Theory 2. Stiffen to raise the Tectorial membrane away from the inner hair cells, shear to lower the Tectorial membrane and obstruct the inner hair cells

The big picture Outer/Middle ear filter and intensify sound Inner ear detects sound

Inner Hair Cell movement along the basilar membrane Converts Mechanical energy to Electrical energy

(nerve impulse)

Outer hair cells help modulate movement along the basilar membrane

Questions?