Hearing Anatomy - External and Middle Ear

What are the main structures of the external ear?

• Pinna (Auricle)

• External Auditory Meatus (Ear Canal)

What is the Pinna and its Physiology?

• Aka the auricle, it is the external part of the ear, made from cartilage

• It collects the sound waves and funnels them to the inner ear to help produce sound

• Enhances 1500-8000 Hz frequencies

What is the Ear Canal and its Physiology?

• ~2.5 cm long

• Outer 1/3 is cartilage + hair/cerumen

• Inner 2/3 is an opening in the temporal bone of your skull

• Slopes up and then down — keeps things out of the ear

• Resonates mid-to-high frequencies (~10-20 dB gain)

• Conducts sound to tympanic membrane

What is the Function of the External Ear?

• Collects and amplifies sound

• Protects inner structures

• Resonance → boosts speech-relevant frequencies

What is resonance?

Refers to the reinforcement of certain frequencies; sounds can be made stronger within a particular space

What is the Transfer Function of the External Ear?

• A process that demonstrates the volume gain that the external ear provides

• Transfer function = relationship between the input and the output of some system

• Broad resonance between 1500 and 8000 Hz

• This gain helps humans understand speech more

How can we tell where sounds are coming from? 

• The differences in the gain between ears helps us to determine where sounds are coming from (there usually is a distinction in the amplitude)

What are the main structures of the middle ear?

• Tympanic membrane

• Ossicles (three smallest bones in the body)

• Middle ear muscles

• Eustachian tube

What are the Three Layers of the Tympanic Membrane? 

1. Outer (epithelial) → Connected to skin in ear canal

2. Middle (fibrous) → Help maintain structure — can be more difficult to heal if damaged

3. Inner (mucous) → Connected to middle ear

• Sound hits tympanic membrane/eardrum, causing it to vibrate; first step of processing sound

What are the Middle Ear Ossicles?

1. Malleus (attaches to eardrum)

2. Incus

3. Stapes (attached to the oval window, gateway to the cochlea)

Eustachian Tube & its Physiology?

• An opening that connects the middle ear with the nasal cavity

• Physiology: Adjusts air pressure in the middle ear

• Drainage of mucus from the nose

Eustachian Tube and Ear Infections

• When you have an infection, the lining of the nose becomes irritated and inflamed, which can narrow the Eustachian tube opening

• This creates a negative pressure that pulls the eardrum inward. This is what makes your ears feel uncomfortable and blocked

• Fluids can also start to accumulate in your middle ear, which creates a breeding ground for bacteria

Why are Middle Ear Infections More Common in Children?

• Children’s eustachian tube is narrower, shorter, and more horizontal, which makes it hard to drain 

• Larger adenoids can press on the tube, making it difficult to open

• Children are more prone to frequent respiratory viruses

What is a Conductive Apparatus?

• The system that transmits sound from the external to middle ear

• Damage to these structures causes a “conductive” hearing loss

What is the Importance of the Middle Ear?

The middle ear helps overcome impedance mismatch. The middle ear helps boost level of energy reaching inner ear

What is Impedance Mismatch?

• Going through two different environments each contain two different resistances to the flow of energy.

• This is because sound travels differently through different media

• External and middle ear are filled with air

• Inner ear is filled with fluid

What are the three parts of overcoming impedance mismatch?

1. Area ratio

2. Lever ratio

3. Buckling action of the tympanic membrane

What is area ratio?

• The middle ear provides a boost to the level of sound pressure reaching the inner ear by virtue of the difference in the tympanic membrane area as compared to the stapes footplate

• Area ratio (smaller area = more pressure)

• The sound is being funneled into a smaller, more concentrated space

What is lever ratio? 

• The arrangement of ossicles is such that they resemble a level with a long and short arm

• Differences in lengths of malleus and incus produces a boost in pressure because they are arranged like a level system

• (Think that sound from the eardrum goes to the malleus, which is larger and “tips” sound into the smaller incus like a seesaw)

What is the Buckling Action of Tympanic Membrane?

• Some parts of the membrane move a greater distance due to the curved shape of the membrane. The manubrium of the malleus tends to be more fixed in place

• Therefore, parts of the membrane move with less force/large displacement. The malleus can then be moved a small distance with more force

What are the Middle Ear Muscles?

• Tensor Tympani

• Stapedius

What does the Tensor Tympani Do? 

• Contracts to pull the manubrium of the malleus medially, decreasing vibrations, which happens during chewing/swallowing

• Tenses eardrum

What Does the Stapedius Do?

• Stabilizes your stapes bone

• Can contract to pull the stapes away from the cochlear

• Contracts in loud sound → acoustic reflex

What does the acoustic reflex do?

• Most effective at reducing low frequency sounds (primarily anything below 1000 Hz

• Can reduce intensity by up to 15 or 20 dB

• Latency of reflex is approximately 40-150ms

Implications of the Acoustic Reflex?

• Inner ear protection from excessive stimulation (sustained loud sounds)

• Improve our understanding of speech in the presence of low-frequency competing sounds

• Only kicks in when the sounds are uncomfortably loud

External and Middle Ear Physiology Conclusions

1. Passive properties of the external and middle ear modify the spectrum of sounds entering the ear (sounds get 10-20dB higher when processed through the external ear canal)

2. The middle ear functions to overcome the impedance mismatch between air and inner ear fluid

3. The auditory system regulates middle ear stiffness to modify the sound delivered to the inner ear (through the middle ear muscles)

What happens when our outer and middle ears don’t work?

Sounds would not be amplified enough to overcome the impedance mismatch so we perceive them as softer and muffled