Physics of the Ear and Hearing PDF

Physics of the Ear and Hearing D r. E ntid h ar altae e c o l l e g e o f me dicine medical physics Topics of the Lecture Hearing system Parts of the ear. Outer ear  Middle ear Inner ear Hearing loss (Deafness). Hearing test (Audiometer) Hearing system Any hearing system c...

Physics of the Ear and Hearing D r. E ntid h ar altae e c o l l e g e o f me dicine medical physics Topics of the Lecture Hearing system Parts of the ear. Outer ear  Middle ear Inner ear Hearing loss (Deafness). Hearing test (Audiometer) Hearing system Any hearing system consists of: 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. The ear is a cleverly designed converter of very weak mechanical waves in air into electrical pulses in the auditory nerve. The auditory system of the body is structured into 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 auditory system The ear is divided into three parts : the outer, the middle and the inner ears. The outer ear It is the outer visible portion of the ear that collects and directs sound waves toward the tympanic membrane by way of a canal. It consists of pinna, auditory canal and the tympanic (eardrum) membrane as following  Pinna: The Pinna collects sound, acting as a funnel to  Protects the eardrum from shocks. amplify sound and directing sound toward the ear  Preventing harmful items from canal and adding directional information to the sound. entering the ear canal by the help of  Auditory canal; is a tube running from the outer ear hair and wax. to the middle ear. It is about 2.5cm in length and  Amplification the sound by acting as 0.7cm in diameter. Functions of the auditory canal are: a resonator. By producing standing wave. This resonance will enhance the sensitivity of ear in the higher frequency range 2000- 10000 Hz and the best sensitivity of the ear will be in the region 2000- 4000 Hz  Tympanic (eardrum) membrane; it is a cone-shaped piece of skin about 10 mm in width, separating the outer ear from the middle ear. It is very sensitive, even the slightest pressure variation will cause it to vibrate. The main function of the tympanic membrane is to transfer the sound that comes from the air into the ossicles of the middle ear Reflection and Transmission at the Tympanic Membrane The acoustical signal travels along the ear canal and hits the eardrum. This causes partial reflection and transmission of the signal. To optimize the hearing sensitivity reflection should be minimized and transmission maximized. From measuring the intensity ratios for reflected and transmitted acoustical waves at the eardrum, we can obtain the following (where, 𝑍𝑎𝑖𝑟 = 430 kg/m².s, 𝑍 𝑚𝑢𝑠𝑐𝑙𝑒 = 1.48x106 kg/m².s): 𝑰𝒓𝒆𝒇 𝑰𝒕𝒓𝒂𝒏𝒔 = 𝟎. 𝟗𝟗 , = 𝟎. 𝟎𝟎𝟏 𝑰𝒊𝒏 𝑰𝒊𝒏 From the above values, one can observe that most of the incoming wave intensity is reflected (bad impedance matching) and therefore lost for hearing process. Middle Ear It is the central part of the ear, behind the eardrum, through which sound travels. It consists of: hammer, the anvil and the stirrup. They transmit the vibrations of the tympanic membrane to the oval window of the inner ear. The functions of the bones of the middle ear are: a) act a lever system, in which amplify the pressure on the oval window by a factor of about 22, as shown in the following:  The force on the oval window (𝑓𝑜 ) is about 1.5 times the force on the eardrum (𝑓𝑚 ).  The area of the oval window (𝐴𝑜 ) is about 15 times smaller than the area of the eardrum(𝐴𝑚 ). 𝒑𝒓𝒆𝒔𝒔𝒖𝒓𝒆 𝐨𝐧 𝐭𝐡𝐞 𝐨𝐯𝐚𝐥 𝐰𝐢𝐧𝐝𝐨𝐰 𝑷𝑶 𝑓𝑜 𝑓𝑚 𝑓𝑜 𝐴𝑚 = ÷ = × = 1.5 × 15 = 22 𝑷𝒓𝒆𝒔𝒔𝒖𝒓𝒆 𝐨𝐧 𝐭𝐡𝐞 𝐞𝐚𝐫𝐝𝐫𝐮𝐦𝐦 𝑷𝒎 𝐴𝑜 𝐴𝑚 𝑓𝑚 (𝐴𝑜 )  Filter out noise generated in the body. Protect the ear from excessive vibrations by switching to a less-efficient mode of vibration at high sound levels The inner ear Serves to transform the energy of the compressional wave within the inner ear fluid into nerve impulses which can be transmitted to the brain. The hearing portion of the inner ear is the cochlea, a snail shaped structure that is connected to the stirrup (or stapes). As the stapes moves in and out, it produces fluid waves within the cochlea.  The inner ear It is divided into three small fluid-filled chambers vestibular chamber, the middle chamber tympanic chamber. Reissner membrane separates the scala vestibule from the scala media, Basilar membrane separates the scala media from the scala tympani. Along the basilar membrane there are two types of sensory cells (or hair cells) called the outer and the inner cells. These are arranged to response to the frequency (or pitch) of that they detect and they are covered by the tectorial membrane.. When sound energy is transmitted to the cochlea, the basilar membrane vibrates up and down causing the hair cells to shear on the tectorial membrane above them. These shearings causes the hair cells to undergo a chemical change the results in electrical charge inside the cells. The electrical charges make neural impulses which travel along the auditory nerve to the brain. These impulses are interpreted in the brain into sounds.  High frequency sounds produce the greatest motion of the basilar membrane near the oval window.  Low frequency sounds produce the greatest motion of the basilar membrane farthest from the oval window.  This results in different nerve cells, distributed along the organ of Corti, producing electrical pulses depending on the frequency of the sound waves. https://youtu.be/98-6WfdumZY