The Human Ear and Sound

Jackie Kaleta

Sound is the creation of particles moving. Sound waves move in a longitudinal wave, or in a parallel direction to the energy that is being transported. Due to the back and forth movement of longitudinal motion, there are moments of high air pressure (compression) and moments of low air pressure (rarefaction). The distance between each moment of compression and each moment of rarefaction is called a wavelength. These waves can also be referred to as pressure waves because of the deference between the moments of high and low pressure.

What is sound?

The shell of the human ear, the pinna or auricle, is the portion of the ear in which people can see and is considered part of the outer ear. The main purpose of this cartilage is to collect sound. It acts as a funnel: collecting the sound and amplifying it down to be directed into the ear canal. During the collection process, some sounds are enhanced and the pinna deciphers the direction in which the sound was created. The pinna works differently for high and low frequencies. In the case of low frequencies, sound is bounced directing into the ear canal. For high frequencies though, sounds are reflected in at a slight delay known as phase cancellation and causing other frequencies to drop. This creates what is known as the “pinna notch”.

The Pinna

The Ear Canal

The ear canal, also known as the external acoustic (or auditory) meatus, is only about 2 centimeters in length. It is considered part of the outer ear. The main purpose of the ear canal is to amplify sound waves to be received by the middle ear. It is a passage between the pinna and the eardrum and helps to protect the ear from infection. The portion closest to the pinna contains nerve endings and is covered with sensitive skin. Further into the ear canal are small hairs that help to filter debris. With the help of sweat glands (which produce earwax) and electrostatically charged wax fibers called cerumen strands, the inner ear is well protected.

The EardrumThe ear drum, or the tympanic membrane, is the key to hearing. It is considered either a part of the outer ear or middle ear. The “drum” vibrates when it is “hit” by sound waves and turns the sound energy into mechanical energy. Once these sound waves have been changed into mechanical energy they are transferred to the next bones of the middle ear. There are three layers that make up the eardrum. The layers consist of: a layer of skin, a layer of fibrous and elastic material, and a layer that has a mucus producing lining. The ear drum is held into place by the annulus (a fibrocartilaginous ring).

The HammerThe malleus, which means hammer in Latin (hence the “nickname”), is the first tiny bone that makes up the middle ear. There are three of these tiny bones are called ossicles and are the three smallest bones in the human body (the term may be applied to any small bones throughout the body). The hammer is attached to the inner surface of the eardrum by the manubrium (“handle”) and transmits the vibrations of the eardrum to the incus by the head. The manubrium has both a lateral and an anterior process. The space between the manubrium and the head is called the neck.

The incus is the second of the tiny bones within the ear. It is named so for it’s shape, which looks like an anvil (in Latin, incus means anvil). The incus is the ossicle that connects the malleus to the stapes. Like the malleus, it transmits vibrations through the middle ear. It receives vibrations from the hammer and transfers them to the stirrup. The incus is divided into three parts: a body and two processes. The head of the incus is in contact with the head of the malleus. There are also three processes that make up the incus: a lenticular process and both a short and long process.

The Anvil

The Stirrup

The final of the ossicles of the middle ear is the stapes. It is known as the stirrup due to it’s shape and is also the smallest bone in the human body. The stapes is the connection between the middle ear and the inner ear by connecting the incus and “oval window” together. It is comprised of four parts. The footplate connects and covers the oval window. The head is in contact with the lenticular process of the incus. Then there is an anterior crus and a posterior crus on either side of the bone that connect the head to the footplate.

The Cochlea

The cochlea itself is the auditory portion of the inner ear. The name comes from the Latin or Greek words for “snail” or “spiral shell”. The cochlea is a very complex portion of the ear, containing many very important parts in the process of hearing. For the most part, the cochlea is a hollow, spiraled chamber of bone. It is filled with a watery liquid which moves with the vibrations received by the middle ear. When the fluid moves small hairs are triggered to convert the motion into an electric single.

The actual process of receiving and sending the vibrations is complex. The stapes transmits the vibrations to the fenestra ovalis or oval window which is on the outside of the cochlea. The oval window vibrates the fluid inside the cochlea, known as perilymph. In turn this causes movement of the hairs within the organ of Corti. The thousands of hair cells within the organ of Corti vibrate and send electronic signals. The frequency of the sounds being received effects which location of hair cells are moved. The higher the frequency, the closer to the entrance of the cochlea the hairs will move, due to stiffness in the basilar membrane.Very strong frequencies can actually cause the hair cells to die. This is the a common cause of hearing loss. It can also be noted that there is a difference between hair used for hearing between different species. While humans have inner and outer hair cells, birds have tall and short hair cells.

The Cochlea (Continued)

The Semicircular Canals The semicirular canals are, like the cochlea, filled with fluid called endolymph. They are attached to the cochlea and help in maintaining balance. There are three canals: the horizontal semicircular canal (or the lateral semicircular canal), the superior semicircular canal (or the anterior semicircular canal), and the posterior semicircular canal. Each one detects a different movement of the head. The horizontal canal detects the horizontal movement of the head and the superior and posterior detect when the head moves vertically. As the skull moves, the endolymph is moved and rushes past little hairs known as cilia which send signals to the brain.

Other Parts of the Ear

The tympanic cavity is the small cavity for which the bones of the middle ear are housed. (Picture to the left)The Eustachian tube (or auditory or pharyngotympanic tube) links the pharynx to the middle ear. It prevents pressure buildup in the middle ear. This pressure is released by letting a small amount of air through while yawning, swallowing, and chewing gum, among other ways. This tube also drains mucus that has built up in the middle ear. (Picture above)

Otitis Media is caused by Eustachian tube dysfunction. Due to the tube not transferring air as it should, pressure builds up and “sucks” fluid from the lining of the middle ear. There are four kinds of otitis: serous otitis, secretory otitis, acute otitis media, and chronic otitis media.Cholesteamtoma is when skin cells grow in the wrong place within the middle ear and mastoid. The ossicles are often destroyed when the skin mass expands.Otosclerosis is an ear disease where a soft bone starts growing around the footplate of the stapes. This causes the bone not to move like it should, creating conductive hearing loss which is often repairable. If this soft bone replaces other parts, sensorineural hearing loss can occur which cannot be repaired surgically.

Infections and Diseases of the Ear

Meniere’s disease is caused by an imbalance of fluid in the inner ear, normally in excess. This causes sudden hearing loss, vertigo, ringing, and/or pressure. If left untreated, it may progress to total hearing loss.Acoustic neuroma is a benign tumor between the inner ear and the brain that can cause vertigo, hearing loss, and loss of function of the facial nerve. Ears can also become infected by fungus. Otitis externa or an ear infection is caused by many factors: becoming wet due to swimming, sweating, or humidity; insertion of foreign objects; chronic dermatological disease; or other trauma to the ear canal. There are many symptoms including inflammation, pain, tinnitus (ringing of the ear), and even hearing loss that can come from an ear infection. There are also many home remedies that can cure an infection, but if it persists a doctor should be consulted.

Infections and Diseases of the Ear (Continued)

http://library.thinkquest.org/05aug/00386/hearing/ear/index.htm - for all slides

http://www.buzzle.com/articles/different-parts-of-the-human-ear.html - for all slides

http://www.physicsclassroom.com/class/sound/u11l1c.cfm - for “what is sound?”

http://audilab.bmed.mcgill.ca/~daren/3Dear/mid1.html - for “The Hammer”, “The Anvil”, and “The Stirrup”

http://www.hearnet.com/features/feature_articlepumpup.shtml - for “The Cochlea”

http://www.ncbi.nlm.nih.gov/books/NBK10863/ - for “The Semicircular Canals”

http://www.whonamedit.com/synd.cfm/1463.html - for “Other Parts of the Ear”

http://www.affoto.com/eardisease.html - for “Infections and Diseases of the Ear”

Pictures from:

http://www.umm.edu/otolaryngology/ear_infections.htm - Otitis media

http://www.marshfieldclinic.org/patients/?page=ent_ear_otosclerosis – Otosclerosis

http://www.marshfieldclinic.org/patients/?page=ent_ear_cholesteatoma – Cholesteatoma

http://www.mayoclinic.org/acoustic-neuroma/enlargeimage5277.html - Acoustic neuroma

http://www.wilsonear.com/education/balance/menieresdisease.html - Meniere’s disease

Works Cited

This is the finished picture. The entire thing is done in colored pencil. It is approximately 20 inches by 20 inches. The picture was based off of two pictures: http://www.arthursclipart.org/medical/senseorgans/anatomy%20of%20the%20ear.gif And http://s581.photobucket.com/albums/ss260/icancook30907/?action=view&current=sound-waves.jpg&newest=1#!oZZ11QQcurrentZZhttp%3A%2F%2Fs581.photobucket.com%2Falbums%2Fss260%2Ficancook30907%2F%3Faction%3Dview%26current%3Dsound-waves.jpg%26newest%3D1