Anatomy of the Outer Ear

The Outer Ear

• Collects and directs sound waves towards the tympanic membrane
• Consists of the pinna, auditory canal, and the tympanic membrane

Pinna

• Collects sound and directs it towards the ear
• Acts as a funnel to amplify sound
• Adds directional information to the sound

Auditory Canal

• A tube running from the outer ear to the middle ear
• Approximately 2.5 cm in length and 0.7 cm in diameter
• Functions include:
  • Protecting the eardrum from shocks
  • Preventing harmful items from entering the ear canal
  • Amplifying sound by acting as a resonator
  • Producing standing waves

Standing Waves

• A closed cylindrical air column produces resonant standing waves at a fundamental frequency and odd harmonics
• The length of the column (L) and wavelength (λ) are related as: L = n(λ/4), where n = 1, 3, 5, 7,...
• The velocity of sound in air is approximately 330 m/s
• The fundamental frequency of the standing wave in the ear canal is given by: F1 = v/(4L) Hz

Standing Waves in the Ear Canal

• Produced by the reflection of sound waves at the eardrum
• Frequencies of the standing waves are given by: fn = n(v/(4L)) Hz, where n = 1, 3, 5, 7,...
• The ear is most sensitive to frequencies in the range of 2000-10000 Hz, with best sensitivity in the region of 2000-4000 Hz

Tympanic (Eardrum) Membrane

• A cone-shaped piece of skin, 10mm in width, separating the outer ear from the middle ear
• Very sensitive, responding to even the slightest pressure variation
• Main function is to transfer sound from the air into the ossicles of the middle ear

Reflection and Transmission at the Tympanic Membrane

• The acoustical signal travels along the ear canal and hits the eardrum, causing partial reflection and transmission of the signal
• To optimize hearing sensitivity, reflection should be minimized and transmission maximized
• Reflected intensity (Iref) is 99% of the incoming intensity (Iin)
• Transmitted intensity (Itrans) is 1% of the incoming intensity (Iin)