Outer Ear

3 parts of the outer ear

• Pinna (auricle)

• Tympanic Membrane (ear drum)

• External Auditory Meatus (ear canal)

Functions of the outer ear

1. Funnel sound into your ear

2. Some sound localization

Pinna (auricle) functions

1. Protects ear canal (external auditory meatus)

2. Small amplification of sounds in the range of its resonant frequency

3. Helps differentiate sounds that come from behind vs in front

4. Helps with localization of high frequency sounds

Pinna (auricle) anatomy

• Helix and anti-helix

• Concha

• Tragus and anti-tragus

• Lobule

Ear canal (external auditory meatus) functions

1. Produces cerumen (ear wax)

• lubricates ear

• Helps remove debris from ear canal

• Antibacterial and anti fungal properties

2. Amplifies sounds in the range of its resonant frequency (2-4kHz)

3. Protects the tympanic membrane (ear drum) and middle ear

Ear drum (tympanic membrane)

• Cone shaped structure that completely closes off one end of the ear canal

• Connects to the bones of the middle ear

• Can be ruptured by things like cotton swabs or major pressure changes

Ear drum (tympanic membrane) function

Cone shape funnels the acoustic energy of the sound to its center for MECHANICAL TRANSMISSION to the middle ear

Sound localization

The process of determining where a sound is coming from in space in 3 dimensions

Binaural hearing

Having 2 ears

Sound shadows are most prominent at high frequencies

Since diffraction (sound waves wrapping around objects) occurs primarily for low-medium frequencies, sound shadows are most prominent for high frequencies

Two binaural acoustic cues for horizontalsound localization

1) Interaural level difference (ILD)

2) Interaural time differences (ITD)

Interaural level difference (ILD)

• Most prominent at high frequencies due to greater sound shadow

• Also greater head baffle - head acts as barrier and reflects/amplifies sound on the same side

• Can be seen in head related transfer functions (HRTF)

Interaural time differences (ITD)

• Most prominent at low frequencies

HRTFs (head related transfer functions)

The amplitude response curves of the head/torso/pinna

• Often measured with acoustic dummies with microphones inside their ears

A monaural acoustic cue for sound localization: Pinna filtering

• The pinna is asymmetrical, so it filters sound from high vs. low locations differently

• This helps us judge the elevation of sounds

Several pathologies can affect outer ear function

• Cerumen Impaction

• Otitis Externa

• Aural Atresia and Microtia

Cerumen Impaction

• Ear wax can build up and block the ear canal, preventing sound from getting through

• Can create hearing loss, but treatable

Otitis Externa

• Infection of the ear canal (bacterial, fungal); also called swimmers’ ear

• Accompanied by itching, swelling, pain, excretions; infection and symptoms are treated with ear drops

• Can create hearing loss if severe and chronic

Aural Atresia and Microtia

• Abnormal or absent pinna and/or ear canal can create a ~60-dB hearing loss

• Middle ear transform is minimal due to lack of impedance matching – more on this next time!