College of Medicine Dept. of Medical physics Physics of ear and hearing /CH 13 2017-2018 ***************************************************************** o Introduction : The ear is the organ that detects sound. It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system. Hearing is a complex process, everything moves happen a voice, and the sound consists of vibrations of air molecules that travels in waves, and then enter these sound waves into the ear, where the turn of the movement of wavelengths to nerve signals are sent to the administrator for a hearing in the brain, which in turn translate these nerve signals to what we hear from votes. Another function of ear in addition to hearing is keeping a balance heard, they contain in the inner ear to the members of a private responds to head movements shall give the brain information on any change in the status of the head. Who shall brain to dispatch letters to various muscles that keep the head and body balanced, as in the case of stand, or sit, or walk, or any other movement. The auditory system of the body is structured into a ; Mechanical system to catch and to amplify acoustical information (ear) Sensory (electrical) system which converts mechanical pulses into electrical signals which are passed on by the auditory nerves to the brain. Auditory system to decode and analyze the electrical nerve signal in the auditory cortex (brain)
The ear is the body's main receiver system for acoustic wave information. The main objective of the ear is to receive, the acoustic waves, to amplify the intensity, to analyze the frequency and intensity structure of the wave and to reject random background noise. The ear is divided into three parts : 1) The outer ear 2.5 cm long ear canal terminated by the eardrum. 2) Middle ear cavity section containing by three small bones ( ossicles with connecting tube to the mouth cavity (Eustachain tube) 3) Inner ear spiral-shaped, fluid-filled tube system (chochlea )with internal organ of corti. Hair cells in the organ of corti convert vibrations of sound waves hitting the eardrum into coded nerve that inform the brain of these sound waves.
1. The outer ear : Consists of two parts, namely: 1- Pinna. 2- External auditory canal. 1- Pinna: Is the curved part and the tip of the ear outside the head, which is free from the bones, and consists mainly of a strong fabric and elastic called: cartilage, which is covered with a thin layer of skin. 2- External auditory canal: Is the slot that you see given direct to the ear, which is a road leading to the eardrum, a thin membrane taut strongly and has a radius of about 10 mm which separates the outer ear and the middle, and also called the tympanic membrane. The eardrum or tympanic membrane is about mm thick (paper thin).and has an area about 65 mm2. It couples the vibration in the air to the small bones in the middle ear. The main purpose of the membrane is to absorb and transmit the pressure variations caused by acoustical waves in the outer ear canal. It is possible for sound pressures above 160 db to rupture the eardrum. A ruptured eardrum normally heals just as other living tissue dose. It contains the skin in the outer third of them on the hair, and sweat glands, and other glands secrete (ear wax) is a liquid secreted by glands greasy wax
job and ear protection from bacteria and fungi. Which helps protect the drum captures dirt that may accumulate around. It takes two-thirds of the temporal bone tunnel external auditory, and this bone is more bone stiffness in the body, which also surrounds the middle and inner ear, which thus protects the precision components for those parts of the ear. 2. The middle ear : Dominant feature of the middle ear are three small bones, the ossicles, malleus(hammer), incus (anvil), and stapes(stirrup). Purpose of the bones are to serve as mechanical impedance matching and amplifying system for the transmission of the eardrum vibrations towards the inner ear. These bones are full adult size before birth. The fetus can hear while it is still in the womb. The malleus is attached directly to the eardrum membrane to absorb the vibration, the incus couples the malleus with stapes which in turn is attached to the oval window membrane which separates the middle ear cavity from the inner ear. The ossicles amplify the pressure of the sound waves at the entrance to the inner ear. The lever action of the ossicles is such that the motion of the plate of the stapes at the oval window of the inner ear is about 0.7 that of the malleus at the eardrum. The middle ear contain air, and it is important for the air pressures on both sides of the thin eardrum to be essentially the same, the Eustachian tube serves to equalize the pressures.
3. The inner ear : The inner ear, hidden deep within the hard bone of the skull, is man's bestprotected sense organ. The inner ear consists of a small spiral-shaped, fluid-filled structure called the cochlea. The ossicles of the middle ear communicate with the cochlea via a flexible membrane (the oval window) ; the stapes its pressure variations of incoming sound waves across this membrane to the cochlea. The cochlea communicates with the brain via the auditory nerve a bundle of about 8000 conductors that inform the brain via coded electrical pulses which parts of the cochlea are being stimulated on both the frequency and the intensity of the sounds that we hear. It is divided into three small fluid-filled chambers that run its full length the oval window is on the end of the Vestibular chamber,the middle chamber is the cochlea duct, and the third chamber is that Tympanic chamber, the vestibular and tympanic chamber are interconnected at the tip of the spiral. A sound wave entering at the oval window produces a wave-like ripple in the basilar membrane of the cochlear duct. This duct contains the sensors that convert the sound into nerve signals. The motions of this membrane are about 10 times smaller in amplitude than the motion of the eardrum. Stimulation of nerves in the cochlear duct near the oval window indicates high-frequency sounds. Low-frequency sounds cause large motions in the basilar membrane and stimulation of nerves in the cochlear duct near the tip of the spiral. The transducers that convert the mechanical vibrations into electrical signals are located in the bases of the fine hair cells in the ogan of croti.
o Sensitivity of the ears: The ear is not uniformly sensitivity over the entire hearing range. Its best sensitivity is in the region of 2 to 5 KHz. this sensitivity changes with age. The highest frequency you can hear will decrease as you get older, and the level of sound will need to be greater for you to detect them. o Hearing test: The tests are normally done in a specially constructed soundproof testing room. Each ear is tested separately ; test sounds can be sent to either ear through a comfortable headset. The subject is asked to give a sign when he hears the test sound. Selected frequency from 250 to 8000 Hz are used. At each frequency the operator raises and lower the volume unit a consistent hearing threshold is obtained. The hearing threshold are then plotted on a chart and can be compared to normal hearing threshold fig. (13-8a). the normal hearing threshold at each frequency is taken is taken to be 0 db. The chart may show a general loss in one or both ears. Usually a hearing loss is not uniform over all frequencies. Fig (13-8b) shows the hearing threshold of a person with imperfect hearing. Notice the sharp hearing loss in both ears at about 4KHz. In this case, the loss was due to nerve damage of that frequency portion of the cochlea. o Deafness and hearing aids The frequency range most important for understanding conversational speech is from about 300 to 3000 Hz. A person who is deaf above 4000 Hz but who has normal hearing in the speech frequencies is not considered deaf or even hard of hearing. Hearing handicap are classified according to the average hearing threshold at 500,1000 and 2000 Hz in the better ear. A person with a hearing threshold 30 db above normal would probably not have hearing problems. People with hearing threshold of 90 db are considered deaf or stone deaf. The average sound level of speech is about 60 db. But this average become more or less According quiet place.
There are tow common causes of reduced hearing : a) Conduction hearing loss: in which the sound vibrations do not reach the inner ear. it may be temporary due to a plug of wax blocking the eardrum or fluid in the middle ear. It may be due to a solidification of small bones in the middle ear. This condition can be corrected by an operation in which the stapes, which pushes on the oval window. it is replaced with a piece of plastic. If a conduction hearing loss is not curable, a hearing aid can be used to transmit the sound through the bones of the skull to the inner ear. b) Nerve hearing loss: in which the sound reaches the inner ear but no nerve signals are sent to brain. It affect only a narrow band of frequency or it may affect all frequency. The hearing threshold that requires a person to use a hearing aid is quite variable according to: 1- Reflection :the sound reflects about 6-8 db of additional sound into ear canal. 2- The earliest artificial hearing aid was the ear trumpet. The size and shape of the ear trumpet affect its efficiency. 3- Electronic hearing aids: it is like a small public address system. It consist of a microphone to detect sound, an amplifier to increase its energy, and loudspeaker to deliver the increased energy to the ear.