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A Brief Journey Through The Ear
    by Gordon Whitehead, B.S., MA., Aud(C), Audiologist

    This paper has been recommended by Digital Recordings as suggested reading for their AUDIO-CD product.



The human ear is a rather wondrous instrument. It is composed of tens of thousands of component parts, can work quite flawlessly from well before we are born to more than a century of age, and is capable of performing extremely sophisticated auditory tasks. And, it works 24 hours a day! In addition to helping us hear, the ear also contains our primary organ of balance; this information, however, will concentrate on the auditory function.

The normal healthy human ear can hear sounds with a frequency as low as 20Hz (20 vibrations of the eardrum per second), all the way up to a frequency as high as 20,000Hz (20,000 vibrations of the eardrum per second)!

The ear is able to hear the very slightest whisper of sound, 0 dB SPL (0 decibels Sound Pressure Level) or less, and yet, is able to tolerate very high intensity sounds in excess of 115 dB SPL (115 dB exposes the eardrum to millions of times more pressure than 0 dB SPL) for short periods of time.

Two normally functioning ears allow us to make extremely accurate decisions about the exact location of a sound source.

The normal ear is able to discriminate a complex sound such as speech against a background of noise.

The normal ear is able to differentiate the component parts of complex sounds whose characteristics have extremely sophisticated inter-relationships, such as a musical chord.

The ear is capable of analyzing tiny changes in the frequency of sound, the intensity of sound, the duration of sound, the direction of sound, and in the interplay of harmonies.

When all aspects of the human ear are considered, the ear is far more complex and sophisticated in design and function than any auditory apparatus that current technology has thus far allowed us to develop.

How does it work?

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The Outer Ear

The outer ear consists of the pinna (the part we can see), and the ear canal down to the eardrum. This part of the ear has several functions relating to hearing. The pinna acts like a funnel; it collects sounds and directs them down our ear canal. The pinna, to a small extent, also rejects sounds coming from behind us, and assists in determining the location of a source of sound. The pinna and the ear canal, in addition to functioning as a conduit, also amplify some frequencies of sound so that the outer ear performs a filtering action. It tends to enhance sounds in the frequency range important for the discrimination of speech. The outer ear is a mechanical system, or a "conductive" system.

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Middle Ear

The middle ear consists of the structures enclosed in the space directly behind the eardrum. There are three bones (ossicles) in the middle ear: the malleus (hammer) is attached directly to the eardrum; the incus (anvil) is attached to the malleus; and the stapes (stirrup) is attached to the incus and to the oval window (which is a membrane not dissimilar from the eardrum). The purpose of this chain of bones, or ossicular chain, is to transmit the movements the eardrum makes as sound strikes it, to the oval window. It is more sophisticated than just a chain linking two structures, however. The ossicular chain acts like a lever; it takes relatively large movements of the eardrum, and translates them into very small movements at the oval window. The ossicular chain also takes relatively weak movements of the eardrum, and translates them into more powerful movements at the oval window. The ossicular chain is made up of the smallest bones in the human body.

The middle ear is also the site of the two smallest muscles in the human body, the stapedius muscle and the tensor tympani muscle. The muscles attach to the bones of the ossicular chain, and to the wall of the middle ear space. These muscles contract and hold the ossicular chain relatively rigid any time a noise of high intensity occurs; the primary purpose of the muscles would appear to be to protect the more delicate inner ear from very loud noises gemerated within our head when we sneeze or cough or speak. It is also very possible that these muscles may react to help tune our ear, to make it more sensitive to a specific sound that we are attempting to hear or attend to. These muscles are very weak, however, and tire easily, so they are only able to provide short periods of protection to the auditory system.

The middle ear contains the Eustachian tube. This is the tube that goes from behind the eardrum down into the back of the nose and throat. This tube has two purposes: one is to allow air to enter the normally closed middle ear space, so that the air pressure on both sides of the eardrum is identical. This tube opens to allow air pressure equalization about every third time we yawn, or chew, or swallow. The other purpose is to allow any fluid that accumulates in the middle ear (which is lined with mucus membrane) to drain down the throat.

The middle ear system, in addition to functioning as another link in the chain, enhances some frequencies of sound through the changing resonant frequency as the muscles "tune" the ossicular chain, and thus perform a filtering action. The middle ear is also a mechanical system, or a "conductive" system.

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The Inner Ear (Cochlea)

The inner ear, or cochlear, is a snail-shaped organ. One end of this organ is the balance mechanism, and the other is for hearing. The inner ear is the beginning of the ear's electrical system, or the auditory nerve system. The inner ear works similarly to a microphone. A microphone very simply takes the physical vibration of sound into one end, and turns that vibration into an electrical signal that passes out the wire at the bottom of the microphone. The inner ear does much the same thing. The inner ear contains about 30,000 or so tiny hair cells. Each time the vibration of sound causes one or more of these hair cells to move, a signal is sent to the brain telling it that a sound has occurred. Each hair cell responds to a slightly different frequency. The inner ear contains both mechanical and electrical components, but is essentially the beginning of the nerve pathway, so it is generally considered part of the "sensorineural" components rather than the "conductive" system.

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The Eighth Nerve

The eighth nerve, or auditory nerve, is quite a complex and sophisticated structure. For the purposes of our discussion, however, we can consider it as the "wire" that connects the output of the inner ear to the brain.

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The Brain

The brain is very much a part of our ear, or auditory sytem. The brain is the organ that takes all of the auditory information gathered and processed by the outer ear, the middle ear, and the eighth nerve, and interprets it in such as way that we are able to glean useful information relating to all aspects of sound, whether it be a simple enviromnental sound, or complex sounds such as music or speech.

The brain also has some other less important tasks!

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Closing Remarks

The human ear is quite a complicated system, but, the basic concepts relating to how it works are not complicated. The above material is a simple, elementary review of the constituent parts of the human ear, and very basically how it works. The reader who has specific, or more technical questions, or wants to delve more deeply into the function of the ear, may contact us for referral to more detailed information.

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