Outer Ear Anatomy for Physicians

Restoration Hearing

Anatomy

Pinna (Auricle)

The pinna (also known as the auricle) is the structure most visualize when they think of the ear.  The pinna has several landmarks that give the ear its distinct and well-known shape.

The most common landmarks include: The Helix, the tragus, the triangular fossa, the scaphoid fossa, the cymba concha, the cavum concha, the intertragal incisure, the anti-tragus, and the ear lobe.

External Auditory Canal

The external auditory canal (also known as the external auditory meatus and more commonly as the ear canal) typically has an “s” shape to it as you travel down the ear canal.  The bends of the ear canal are cleverly termed “the first bend” and “the second bend.”

The ear canal is approximately 2.5 cm in length and .75 cm in diameter for the average adult.  The ear canal is usually wider at the isthmus (opening) and narrows until it reaches the isthmus of the ear canal where it flares out again.

The outer 1/3 of the ear canal is cartilaginous and the inner 2/3 of the ear canal is bony with an average skin thickness of .2 mm in the medial two-thirds of the ear canal.  The skin covering the bony portion of the ear canal is thinner than anywhere else in the body.

Physiology 

Pinna Effects

The pinna is believed to have a natural high-frequency resonance and provides a strong boost around 5000 Hz.  The pinna is also believed to aid in sound localization in the vertical plane and front to back as well.  Due to its shape, the pinna is effective and reducing high-frequency sounds that arrive from the back (see localization below for more) 

External Auditory Canal Effects

Due to its shape and nature as an open ended tube, the average external auditory canal provides a 10-15 dB boost in the frequency range of 3000-4000 Hz.

 

Localization

Localization is the ability to discern where sounds originate.  Localization is useful for safety purposes as it increases our environmental and situational awareness, however, it also plays an essential role in understanding speech (particularly in noisy environments).  When we are able to determine where sounds (or speech) originate we can quickly shift our focus and attention towards the sound source (usually a person) and block out extraneous noise or unwanted speech information in the environment.  This allows us to quickly tune into a conversation, process what is being spoken without having to ask for repetition, and provide an apt reply.  There are two ways our brain receives localization information.

Intensity Differences Intensity differences are differences in loudness between two sides.  Simply put, if a sound is louder in the left ear than the right ear our brain registers sound as coming from the louder side (in this example it would be the left side).  This type of input is most useful when processing high-frequency input as high-frequency input has a difficult time wrapping around the head.  The head essentially acts as a wall that blocks out high-frequency sounds.  High-frequency sounds have a harder time traveling around the head when compared to low-frequency sounds due to their short wavelengths.  Longer wavelengths have a much easier time traveling around objects, which is why low-frequency sounds typically travel further than high-frequency sounds of similar intensities.

Timing Differences Timing differences are differences in arrival times for two sounds.  If a sound originates to the left of your head it will arrive at your left ear before it arrives at your right ear.  These types of signals are useful in helping the brain discern where low-frequency sounds originate from.

Note: Since speech is such a complex signal that consists of multiple intensities and frequencies, the brain usually uses both types of localization cues at all times to localize sound.

Head and Torso Effects

As mentioned above, the head and torso can act as a wall that, depending on the sound origin, can reduce the overall volume, loudness, or gain of a sound as it reaches the ear.