Unit 3.

Unit 3

unit 3- HEARING AIDS Common tests – audiograms, air conduction, bone conduction, masking techniques, SISI, Hearing aids – principles, drawbacks in the conventional unit, DSP based hearing aids

EAR The function of the ear is to convert physical vibration into an encoded nervous impulse. It can be thought of as a biological microphone. Like a microphone the ear is stimulated by vibration: in the microphone the vibration is transduced into an electrical signal, in the ear into a nervous impulse which in turn is then processed by the central auditory pathways of the brain

EAR The ears are paired organs, one on each side of the head with the sense organ which is technically known as the cochlea, deeply buried within the temporal bones. Part of the ear is concerned with conducting sound to the cochlea, the cochlea is concerned with transducing vibration. The transduction is performed by delicate hair cells which, when stimulated, initiate a nervous impulse.

Most sound is transmitted by a vibration of air.
Vibration is poorly transmitted at the interface between two media which differ greatly in characteristic impedance (product of density of the medium and speed of sound within it, c), as for example air and water. The ear has evolved a complex mechanism to overcome this impedance mis-match, known as the sound conducting mechanism. The sound conducting mechanism is divided into two parts, an outer and the middle ear, an outer part which catches sound and the middle ear which is an impedance matching device

Contd… From the figure :The pinna and external auditory canal form the outer ear, which is separated from the middle ear by the tympanic membrane. The middle ear houses three ossicles, the malleus, incus and stapes and is connected to the back of the nose by the Eustachian tube. Together they form the sound conducting mechanism. The inner ear consists of the cochlea which transduces vibration to a nervous impulse and the vestibular labyrinth which houses the organ of balance.

Tympanic Membrane Thin membrane Forms boundary between outer and middle ear Vibrates in response to sound . Changes acoustical energy into mechanical energy

Structures of the Inner Ear
Cochlea - Snail-shaped organ with a series of fluid-filled tunnels; converts mechanical energy into electrical energy

Structures of the Inner Ear (for information)
Oval Window – located at the footplate of the stapes; when the footplate vibrates, the cochlear fluid is set into motion Round Window – functions as the pressure relief port for the fluid set into motion initially by the movement of the stapes in the oval window

Organ of Corti (for information)
The end organ of hearing; contains stereocilia and hair cells.

Hair Cells Frequency-specific High pitch sounds = base of cochlea
Low pitch sounds = apex of cochlea

Vestibular System Consists of three semi-circular canals
Shares fluid with the cochlea Controls balance No part in hearing process

Central Auditory System
8th Cranial Nerve or “Auditory Nerve” carries signals from cochlea to brain Fibers of the auditory nerve are present in the hair cells of the inner ear Auditory Cortex: Temporal lobe of the brain where sound is perceived and analyzed

How Sound Travels Through The Ear...
Acoustic energy, in the form of sound waves, is channeled into the ear canal by the pinna. Sound waves strike the tympanic membrane, causing it to vibrate like a drum, and changing it into mechanical energy. The malleus, which is attached to the tympanic membrane, starts the ossicles into motion. (The middle ear components mechanically amplify sound). The stapes moves in and out of the oval window of the cochlea creating a fluid motion. The fluid movement within the cochlea causes membranes in the Organ of Corti to shear against the hair cells. This creates an electrical signal which is sent via the Auditory Nerve to the brain, where sound is interpreted.

AUDIOGRAM The auditory system can be stimulated via sound energy that is sent through air to the ear drum (air conduction) or by placing a bone vibrator against the skull (bone conduction). Sound sent through air tests all parts of the auditory system—the outer ear, middle ear, inner ear and central auditory pathways

BONE CONDUCTION In contrast, sound conducted through bone bypasses the outer and middle ear. It directly sets up a traveling wave in the cochlea and stimulates the cochlea and central auditory pathways. By comparing the auditory thresholds using these two methods, we can determine the site of hearing loss.

W hat is an Audiogram A pure tone audiogram is a graph that shows the pitches (frequencies) across the top and the loudness (intensity) down the side. This graph is used to register the loudness.

NORMAL HEARING

AUDIOGRAM

Contd…

Contd… Loudness is measured in decibels (dB) and appears in the audiogram in a top down count from 0 dB to dB. The numbers in the horizontal axis indicate the pitches, i.e. the frequencies of different sounds. Frequencies are measured in Hertz (Hz).

Audiogram Low pitched sounds (e.g. middle C on the piano) have frequencies around 250Hz. A high-pitched sound can reach 8000Hz. Pitches are particularly important for speech. Most vowels are low-pitched sounds, whereas consonants such as “s”, “t”, “f” and “sh” are high-pitched sounds.

It is important to remember that 0 dB does not mean that there is no sound at all.
It is simply the softest sound that a person with normal hearing ability would be able to detect at least 50% of the time. Normal conversational speech is about 45 dB. The softest sounds a person hears at each pitch at least 50% of the time are considered their hearing threshold

Audiogram configurations

The configuration of an audiogram will tell you which sounds are best heard. A ‘sloping’ audiogram means the person can hear low pitched sounds but not high pitched sounds. Inversely a ‘rising’ configuration shows that high pitched sounds can be better heard than low pitched sounds

An audiogram is considered ‘flat’ when somebody needs the same amount of loudness to hear a sound, regardless of the pitch of the sound.

NORMAL HEARING The audiogram shows hearing thresholds within normal ranges for the left ear. The white area represents the sounds that the person would not hear (softer then their thresholds) and the tan area indicates all of the sounds that the person would be able to hear (louder then their thresholds).

Conductive Hearing Loss Audiogram
This audiogram shows a conductive hearing loss in the left ear. The white area represents the sounds that the person would not hear (softer then their thresholds) and the tan area indicates all of the sounds that the person would be able to hear (louder then their thresholds).

CONTD.. Conductive hearing losses occur when the "outer" or "middle" portions of the ear fail to work properly. Sound is “ blocked “ from being transferred to the inner ear at normal intensity. Conductive losses are often treatable with either medicine or surgery. Common causes of conductive hearing loss are fluid build up in the middle ear or wax blockage in the ear canal. Children are more likely to have a conductive hearing loss than a sensorineural hearing loss.

Sensorineural Hearing Loss
This audiogram shows a sensorineural hearing loss in the left ear. The white area represents the sounds that the person would not hear (softer then their thresholds) and the tan area indicates all of the sounds that the person would be able to hear (louder then their thresholds).

Sensorineural hearing losses occur when the "inner" ear or the actual hearing nerve itself becomes damaged. About 9 0 % of all people with hearing impairment are in this category, making it the most common type of hearing impairment.

Sensorineural hearing loss is often referred to as "nerve deafness.“ Nerve deafness is not really an accurate description because the damage most frequently occurs within the inner ear rather than the hearing nerve.

(FOR INFORMATION) Common causes of sensorineural hearing loss are aging and exposure to loud noises, but there are many other causes (viral infections, disrupted blood supply to the ear, metabolic disturbances, accident/injury, genetic predisposition, medications that are toxic to the ear, etc).

This type of hearing loss is frequently not medically or surgically treatable. It is typically permanent and irreversible. However, most people with sensorineural loss find wearing hearing aids to be of significant benefit and some people with severe loss can benefit from a cochlear implant.

Mixed Hearing Loss Audiogram
The white area represents the sounds that the person would not hear (softer then their thresholds) and the tan area indicates all of the sounds that the person would be able to hear (louder then their thresholds).

Mixed Hearing Loss Audiogram
Mixed Hearing Loss Mixed hearing losses are simply combinations of the above two types of hearing loss. They can occur when a person has a Permanent sensorineural hearing loss and then also develops a temporary conductive hearing loss

Ranges of Hearing Loss -10dB to 25dB = Normal range (Grey)
2 6 dB to 40 dB = Mild hearing loss (purple) 41 dB to 55 dB = Moderate hearing loss (red)

Ranges of Hearing Loss 5 6 dB to 7 0 dB = Moderately Severe hearing loss (green) dB to 9 0 dB = Severe hearing loss (yellow) Over 9 0 dB = Profound hearing loss. (blue)

Conductive vs. Sensorineural Hearing Loss
If a hearing loss is detected via air conduction, but not by bone conduction, it infers that the inner ear and central auditory pathways are normal and that the site of hearing loss is localized to the outer ear or middle ear.

CONTD.. The difference between air conduction threshold and bone conduction threshold on the audiogram is called an “air-bone gap”.

CONTD.. If a hearing loss is detected by both air and bone conduction methods, one can conclude that the cause of hearing loss is in the inner ear or central auditory pathways.

CONTD.. Diseases affecting the inner ear or central auditory pathway result in a sensorineural hearing loss. Additional tests can determine whether the site of lesion is the inner ear (a sensory loss) or central auditory pathway (a neural loss)

The table below summarizes possible results from audiometric testing and how tests results are analyzed. For instance, if bone conduction is normal and air conduction is normal, hearing is normal.

CONTD… If bone conduction is normal and air conduction is abnormal, there is a conductive hearing loss. Finally, in a sensorineural hearing loss, both bone and air conduction will be abnormal.

Types of hearing loss Conductive hearing loss
dysfunction of outer and middle ear Deficit of loudness only Sensori-neural hearing loss Dysfunction of inner ear or auditory nerve Permanent and untreatable Mixed hearing loss Both conductive and sensori-neural hearing loss Noise induced hearing loss

Hearing assessment Accurate otologic diagnosis depends on reliable and accurate tests of hearing. The results help in finding the point of lesion and type of treatment to be given. Patients may be screened in a course way or more sophisticated testing procedures may be followed

Test compares air and bone conduction hearing
Test compares air and bone conduction hearing.(turning fork test – Rinne’s test) Strike a 512 Hz tuning fork softly. Place the vibrating tuning fork on the base of the mastoid bone. Ask client to tell you when the sound is no longer heard Note the time interval and immediately move the tuning fork to the auditory meatus. Ask the subject to tell, when the sound is no longer heard..

Contd… Note the time interval and findings
A) Normal hearing clients will note air conduction twice as long as bone conduction B) With conductive hearing loss, bone conduction sound is heard longer than or equally as long as air conduction C) With sensorineural hearing loss, air conduction is heard longer than bone conduction in affected ear, but less than 2:1 ratio

RINNE’S TEST

Weber’ test The Weber test is a quick screening test for hearing.
It can detect unilateral (one-sided) conductive hearing loss and unilateral sensorineural hearing loss. The test is named after Ernst Heinrich Weber

WEBER TEST In the Weber test, a 512 Hz tuning fork is placed on the patient's forehead. If the sound lateralizes (is louder on one side than the other), the patient may have either conductive hearing loss or a contralateral sensorineural hearing loss.

Weber’s test In a normal patient, the sound is heard equally loud in both ears (no lateralization). However a patient with symmetrical hearing loss will have the same findings. Thus, there is diagnostic utility only in asymmetric hearing losses In the Weber test a vibrating tuning fork (either 256 or 512 Hz) is placed in the middle of the forehead equidistant from the patient's ears. The patient is asked to report in which ear the sound is heard louder.

Weber test Distinguishes between conductive and sensorineural hearing. 2) Strike a 512 Hz tuning fork softly 3) Place the vibrating fork on the middle of the client's head 4) Ask client if the sound is heard better in one ear or the same in both ears A) If hearing is normal, the sound is symmetrical with no lateralization B) Sound localizes toward the poor ear with a conductive loss C) Sound localizes toward the good ear with a sensorineural hearing loss

Weber’s test

WEBER ‘S TEST

Air conduction The process of transmitting sound waves to the cochlea by way of the outer and middle ear. In normal hearing, practically all sounds are transmitted in this way, except those of the hearer’s own voice, which are transmitted partly by bone conduction.

Bone conduction Bone conduction is the conduction of sound to the inner ear through the bones of the skull. Bone conduction is the reason why a person's voice sounds different to him/her when it is recorded and played back. Bone conduction tends to amplify the lower frequencies, and so most people hear their own voice as being of a lower pitch than it actually is.

Audiometric tests for adults
Audiometric tests normally includes pure-tone testing of threshold by both air and bone conduction, speech reception threshold (SRT), and speech discrimination score (SDS). These tests normally require the cooperation of the subject whose response to a sound is indicated by some gesture (e.g. raised hand).

CONTD.. Testing is typically carried out in a sound-attenuated room with the subject listening to carefully calibrated sounds.

Threshold sensitivity testing using airconducted pure tones
Testing is done using earphones thereby allowing each ear to be examined independently. Tones are reduced in intensity until they are no longer heard, at which point the examiner alternately raises and lowers the intensity of the sound until a just-detectable threshold is determined

CONTD… This is repeated at several frequencies within the audible range and the results plotted as an audiogram. The shape of the curve is a measure of the frequency sensitivity of both the middle ear and the inner ear. To differentiate between middle ear (conductive) and inner ear (sensorineural) components to a hearing loss it is necessary to conduct additional tests.

Threshold sensitivity testing using bone conducted pure tones
Testing is done with a vibrator placed somewhere on the skull (usually the mastoid). The testing and plotting procedures are the same as with air conduction testing. sound is transmitted directly to the cochlea via bone conduction, thereby by-passing the transmission mechanism of the middle ear.

CONTD… Thus, audiograms obtained using both bone and air conducted sounds provide information about the integrity of both the middle and inner ears. Difficulty in hearing only air conducted sounds results in a separation of the bone and air conduction audiograms - the so called "air-bone gap".

Tympanometry A tympanogram assesses the mobility or compliance of the tympanic membrane and thereby provides important information about the function of the middle ear including the tympanic membrane, ossicles, and Eustachian tube. When a tympanogram is performed, a sound is introduced into the ear canal.

Tympanometry A microphone in the ear canal measures the intensity of the sound as it reflects off of the eardrum. If the eardrum is functioning normally, more of the sound energy will be ‘absorbed’ and little will be ‘reflected’ (i.e., there will be little impedance to sound transmission and this will be reflected in the tympanogram).

Tympanometry If the eardrum is not functioning normally, more of the sound energy will be reflected and little will be absorbed i.e., there will be great impedance to sound transmission. For instance if there is a middle ear infection most of the sound is reflected back and the tympanogram is flat (low compliance).

Tympanometry If a part of the tympanic membrane is flabby or the ossicles are broken there is even greater compliance and the tympanogram will display an abnormal peak.

Tympanometry

Otoacoustic Emissions
Otoacoustic emission testing assesses the integrity and function of outer hair cells in the inner ear. When a very sensitive microphone is placed in the ear canal, sounds can be detected that are caused by traveling waves in the basilar membrane of the inner ear. Otoacoustic emissions are sounds that are produced by healthy ears in response to acoustic stimulation. The function of the cochlea is to receive the sound energy collected by the outer and middle ear and to prepare it for neural transmission.

Purpose of OAE’s The primary purpose of otoacoustic emission (OAE) tests is to determine cochlear status, specifically hair cell function. This information can be used to partially estimate hearing sensitivity within a limited range differentiate between the sensory and neural components of sensorineural hearing loss test for functional hearing loss.

Recording OAE’s OAEs are measured by presenting a series of very brief acoustic stimuli, clicks, to the ear through a probe that is inserted in the outer third of the ear canal. The probe contains a loudspeaker that generates clicks and a microphone that measures the resulting OAE’s that are produced in the cochlea and are then reflected back through the middle ear into the outer ear canal. The resulting sound that is picked up by the microphone is digitized and processed by specially designed hardware and software. The very low-level OAEs are separated by the software from both the background noise and from the contamination of the evoking clicks.

Clinical Use OAE’s only occur in a normal cochlea with normal hearing.
The creation of OAE’s by the cochlea and the re-emission of this energy as sound from the ear serve no important physiological purpose that can be determined. Their clinical significance is that they are evidence of a vital sensory process arising in the cochlea. OAE’s only occur in a normal cochlea with normal hearing. If there is damage to the outer hair cells producing mild hearing loss, then OAE’s are not evoked.

OAE A good rule of thumb is that OAE’s are present if hearing is 35 dB or better. Because OAE’s are evoked by signals that have a wide frequency response, a broad region of the cochlea responds, providing information on the frequency range from 1000 Hz to 4000 Hz.

Evoked potentials- Auditory Brainstem Response
When a brief acoustic stimulus (e.g., a click or short tone burst) is presented to the ear there is a synchronized burst of action potentials generated in the auditory nerve which spreads up the central auditory pathway Auditory brainstem response (ABR) testing is used to measure the function of the central auditory pathways. Recording electrodes taped to the skull record the electrical activity of the brain (EEG).

Contd… When such methods are employed the complex waveform recorded is called the auditory evoked potential and it includes contributions from many sites that are activated sequentially in time along the auditory pathway. Because of its very low amplitude (in the microvolt range) this wave of activity is generally buried in the EEG and can only be recovered using computerized signal-averaging techniques.

Contd… An averaged waveform has multiple peaks and valleys stretched out over a period of several hundred milliseconds after the presentation of the acoustic stimulus. The time period most commonly studied covers the first 10 msec after the stimulus is presented to the ear and represents the electrical activity evoked in neurons in the auditory nerve and brain stem

Contd This technique is very useful in studying hearing loss of central auditory origin, as may be caused by a lesion affecting the brainstem (e.g., acoustic neuroma or multiple sclerosis). It is also helpful in documenting the hearing loss in infants who cannot cooperate with a behavioral-based audiometric exam.

MASKING “You know I can't hear you when the water is running!”

Auditory masking Masking can be simultaneous or non simultaneous.
Auditory masking occurs when the perception of one sound is affected by the presence of another sound. The phenomenon of masking is often used to investigate the auditory system’s ability to separate the components of a complex sound. Masking can be simultaneous or non simultaneous.

Simultaneous masking Simultaneous masking is when a sound is made inaudible by a "masker", a noise or unwanted sound for the same duration as the original sound (Moore 2004). unmasked threshold : defined as the quietest level of the signal which can be perceived without any masking present.

masked threshold masked threshold:
is the quietest level of the signal perceived when combined with a specific masking noise. The amount of masking is the difference between the masked and not masked thresholds

Simultaneous masking …..
A sound of cat scratching the post is played If it is heard at 10 dB -> unmasked threshold Now hold the original sound at the same level Play the sound of vacuum cleaner simultaneously. If the cat scratching sound is not heard at all when the sound level of vacuum cleaner is increased to 28 dB.

Simultaneous masking …..
28 dB -> masking threshold The difference between unmasked and masked threshold gives the amount of masking

GENERAL PRINCIPLES Normal hearing aids perform several functions:
Signal amplifications Transmission Frequency shift Every electronic hearing aid has at minimum a microphone, a loudspeaker (commonly called a receiver), a battery, and

Contd… an electronic circuitry.
The electronic circuitry varies among devices, even if they are the same style. The circuitry falls into different categories based on the type of audio processing (Analog or Digital) and the type of control circuitry (Adjustable or Programmable).

HEARING AIDS

Working of a normal hearing aid
The correction of hearing loss is revealed by audiograms. The function of the prosthesis is to do selective amplification of the signal. The frequency dependant amplification is adapted. The wind protector system is added to stop the whistling effect seen when an air draft occurs. The anti Larsen system prevents the auto-oscillation of the system

Block diagram of a programmable hearing aid

Programmable prostheses
In this , the spectrum is divided into three ranges and each range has a specific processing. The parameters of each processing are passed to the prosthesis by an interface connected to a computer. Each channel is processed independently .

Contd… The signal from the microphone is given to set of filters ( high pass , low pass and band pass filters) to have specific amplification and compression , more adapted to the patient’s possibilities. Then , the acoustic wave is reconstructed before the power stage ( speaker). Similar to the normal aid , a general amplification and an automatic gain control ( AGC) are present at the input of the system.

Frequency shift When the patient’s hearing zone is on the low frequencies , its possible to transfer some information coming from the high frequencies into low frequencies. The signal is multiplied by itself and some information is transferred to the low frequencies. Sin (2πft ) . Sin (2π f’ t ) = [ cos (2π (f-f’)t ) - [ cos (2π ( f+f’)t ) ] / 2

Frequency shift

Frequency shift (frequency transposition )
From the signal extract the high frequency component by using high pass filter ( which compresses the low frequency) . The high frequency component is multiplied with the same high frequency component Sin (2πft ) . Sin (2π f’ t ) (f-f’) -> low frequency component (f +f’) -> high frequency component. The high frequency (f +f’) component is suppressed by using low pass filter .

Contd… Thus low frequency component(f – f’ ) is brought into the hearing zone of the subject. Can the brain cope with all these data ?  and 

Bone – Integrated Vibrator
The bone vibration transmits the signal to the inner ear , when the normal airway cannot be used. The acoustic wave is picked up by a microphone -> amplified and distributed into a coil. This will stimulates ( by induction) a “ screw” fixed to the patient’s skull.

Bone – integrated vibrator

System currently in market
BAHA ( Bone Anchored hearing Aid) In this system , the “ bone – integrated screw is placed outside (open to air) The magnetic induction stimulates directly so the transmission is more efficient but the risk of infection is more Audiant : In this device , the integrated screw is covered by the skin and the transmission is made by induction. Pros : the infection risk is kept low Cons: a high power loss

Middle Ear Aids

Middle Ear Aids To restore transmission American system :
a magnet is placed on one of the three middle ear bones. A coil is placed inside the outer ear canal and the transmission is done by induction. Japanese system : The vibration is now transmitted by an electric field. Construction : Two blades of different piezoelectric coefficients are joined together and is bound to stapes.

The entire setup is situated between the plates of a condenser.
Working: The signal is amplified -> to the coils , this will cause deformation of the blades and this deformation is passed to the inner ear.

Main functions of a numeric hearing aid

Numeric revolution ( digital processing)
The signal is split by a digital filter bank such that , the high and low frequency can be processed differently. Then the signal is reconstructed and the output is distributed to the patient’s ears. Advantages: Filters can be processed independently. Technology is simple ( a simple microprocessor ) Filters have sharp edges and the frequency bands are well separated. All the processing is done digitally so the distortion rate is very low.

Hearing aid Microphone Tone hook Volume control On/off switch
Battery compartment

Types of hearing aids Body worn hearing aids Behind the ear (BTE)
In the ear (ITE) Receiver in the ear (RITE) In the canal Completely in the canal hearing aids

Body worn hearing aids This was the first type of hearing aid invented by Harvey Fletcher while working at Bell Laboratories. These aids consist of a case containing the components of amplification and an ear mold connected to the case by a cord. The case is about the size of a pack of playing cards and is worn in the pocket or on a belt. Because of their large size, body worn aids are capable of large amounts of amplification and were once used for profound hearing losses. Today, body aids have largely been replaced by Behind-The-Ear (BTE) instruments

Contd…

BTE BTE aids have a small plastic case that fits behind the pinna (ear) and provides sound to the ear via air conduction of sound through a small length of tubing, or electrically with a wire and miniature speaker placed in the ear canal.

BTE The delivery of sound to the ear is usually through an ear mold that is custom made, or other pliable fixture that contours to the individuals ear. BTEs can be used for mild to profound hearing losses and are especially useful for children because of their durability and ability to connect to assistive listening devices such as classroom FM systems.

BTE Invisible some keep the ear canal more open so listeners may still utilise their residual natural hearing (most helpful for those with normal hearing in the lower frequencies). Ideal for high frequency losses, these miniature versions are generally used for mild to moderate hearing loss.

ITE These devices fit in the outer ear bowl (called the concha); they are sometimes visible when standing face to face with someone. ITE hearing aids are custom made to fit each individual's ear. They can be used in mild to some severe hearing losses.

ITE Traditionally, ITEs have not been recommended for young children because their fit could not be as easily modified as the ear mold for a BTE, and thus the aid had to be replaced frequently as the child grew. However, there are new ITEs made from a silicone type material that moderates the need for costly replacements.

Receiver in the ear (RITE) hearing aids
These devices are similar to the BTE aid. There is however one crucial difference: The speaker ('receiver') of the hearing aid is placed inside the ear canal of the user and thin electrical wires replace the acoustic tube of the BTE aid.

There are some advantages with this approach:
Firstly, the sound of the hearing aid is smoother than that of a traditional BTE hearing aid. With a traditional BTE hearing aid, the amplified signal is emitted by the speaker (receiver) which is located within the body of the hearing aid (behind the ear). The amplified signal is then directed to the ear canal through an acoustic tube, which creates a peaky frequency response.

RITE With a RITE hearing aid, the speaker (receiver) is right in the ear canal and the amplified output of the hearing aid does not need to be pushed through an acoustic tube to get there, and is therefore free of this distortion. is extremely not noticeable. one of the most cosmetically acceptable hearing device types.

In the canal (ITC), mini-in-the-canal (MIC) and completely-in-the-canal (CIC)
ITC aids are smaller, filling only the bottom half ofthe external ear. Cannot see very much of this hearing aid when the wearer is face to face with someone. MIC and CIC aids are often not visible unless you look directly into the wearer's ear. These aids are intended for mild to moderately severe losses.

Bone Anchored Hearing Aids (BAHA)
The BAHA is a auditory prosthetic which can be surgically implanted. The BAHA uses the skull as a pathway for sound to travel to the inner ear. The implant vibrates the skull and inner ear, which stimulate the nerve fibers of the inner ear, allowing hearing.

For people with conductive hearing loss, the BAHA bypasses the external auditory canal and middle ear, stimulating the functioning cochlea. For people with unilateral hearing loss, the BAHA uses the skull to conduct the sound from the deaf side to the side with the functioning cochlea.

What is a cochlear implant?
A cochlear implant is a device that provides direct electrical stimulation to the auditory nerve. In sensorineural hearing loss where there is damage to the tiny hair cells in the cochlea, sound cannot reach the auditory nerve. With a cochlear implant, the damaged hair cells are bypassed and the auditory nerve is stimulated directly. The cochlear implant does not result in "restored" or "cured" hearing. It does, however, allow for the perception of sound "sensation."

HAIR CELLS

COMPONENTS OF COCHLEAR IMPLANT

How does a cochlear implant work?
Cochlear implants have external (outside) parts and internal (surgically implanted) parts. External Parts: The external parts include a microphone, a speech processor, and a transmitter. The microphone looks like a behind-the-ear hearing aid. It picks up sounds - just like a hearing aid microphone does -- and sends them to the speech processor . The speech processor may be housed, with the microphone, behind the ear or it may be a small "box" worn in a chest pocket. The speech processor analyzes and digitizes the sound signals and sends them to a transmitter worn on the head just behind the ear. The transmitter sends the coded signals to an implanted receiver just under the skin.

CONTD… Internal parts:
The internal (implanted) parts include a receiver and electrodes . The receiver is just under the skin behind the ear. The receiver takes the coded electrical signals from the transmitter and delivers them to the array of electrodes that have been surgically inserted in the cochlea. The electrodes stimulate the fibers of the auditory nerve and sound sensations are perceived.

WORKING

GENERAL ORGANIZATION OF A COCHLEAR IMPLANT

working The air vibrations are captured , sampled and fed into a microprocessor system( Digital Speech Processor). Then the signal is reconstructed and used to modulate a high frequency carrier which goes through skin to a receiver surgically introduced. The cochlear implant takes the modulated signal and detects and reshapes the information, which is then distributed to electrodes situated inside the cochlea duct.

BLOCK DIAGRAM

BLOCK DIAGRAM SHOWING THE MAIN FUNCTION OF THE DSP OF A MULTICHANNEL COCHELAR IMPLANT

Working… The signal is captured , preprocessed , sampled and fed into buffers A fft calculates the spectrum lines The spectrum lines are grouped into sets The energy of each set is used to make an amplitude modulation of HF Carrier. The carrier is emitted from an aerial , situated the scalp skin.

Speak strategies SMSP ( spectral Maxima Sound processor) :In each spectral analysis only the six highest bands are kept and distributed to the electrodes. SPEAK : the frequency bands having their energy above a given level are kept. The selected band ranges from 4 to 10. The rhythm of delivery depends on the number of selected electrodes.

Block diagram of the main functions of the implanted part of a cochlear system
The signal is demodulated and the audio information ( the pulses ) are distributed to the electrodes.

Electrodes

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