The Ear Canal
In this column, I'll cover a variety of topics related to hearing loss and hearing help. In an attempt to balance the technical and practical, I'll alternate between hearing aids and hearing science.
Not just a hole in the head ---
This month, we'll look at one of the least understood and underestimated parts of the auditory system:
The ear canal.
The external ear canal is a tube of skin approximately 2.4 cm long.
The outer 1/3 is supported by cartilage, and the inner 2/3 is supported by
bone. The two main functions of the ear canal are to protect the eardrum and
to amplify sound.
The protective nature of the ear canal is related to its shape and the
process of cerumen (wax) production. The ear canal has two curves along
its length. These curves make it difficult for foreign objects to make
their way to the eardrum. The curve of ear canal , along with your hearing
loss, will also determine if you are appropriate for canal or Completely In
the Canal (CIC) hearing aids.
Near the second curve, or bend, the ear canal narrows slightly at a
place called the isthmus. The isthmus is very sensitive and stimulation by
foreign objects like cotton swabs may cause a reflexive cough. Along
the ear canal are small hairs that point outward. At the base of each hair
is a sebaceous, or wax, gland. Whenever a hair is pushed, the sebaceous
gland is triggered, and more wax is produced. This is the reason that the use of
cotton swabs inside the ear canal actually causes more problems than it
solves. Left alone, the ear canal is self-cleaning.
The amplification of sound by the ear canal is accomplished via two
mechanisms. The fact that the opening of the ear canal is much smaller
than the pinna increases the pressure of the sound. This funnel-like action
collects and increases the sound pressure of the incoming sound. The
second part of the amplification of the ear canal is related to an acoustical
phenomenon called resonance. Any tube that is closed on one end will
have a resonant frequency related to its length and the volume of air in the
tube.
The resonant frequency of the average adult ear canal is about 3000 Hz.
Smaller ear canals, like in children, have higher resonant frequencies,
usually around 4000 Hz. If you've have surgery on your ear, like a
mastoidectomy, your ear canal may be larger than average, and your ear
canal resonance would be lower.
When we place a hearing aid into the ear canal, we interfere with this
natural resonance. To overcome this "insertion loss" a little more gain needs to be
provided by the hearing aid. Some audiologists and dispensers can measure these
resonance characteristics of your ear using a probe microphone or "real
ear" system.
So, clearly the ear canal is an important part of the hearing and
hearing aid system. Take good care of them. Your mother's advice was the best:
Never put anything smaller than your elbow in your ear!
Next month we'll look at the available types of hearing aid circuits.
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I'd like the following credit:
AudioLogic is re-posted courtesy of the Greater Boston Area SHHH
Chapter.
Brad Ingrao is the audiologist with Tobias & Battite, Inc. Hearing
Healthcare Centers of Boston
www.tobias-battite.com
©www.hear-ri.org
