HEARING AIDS
SELDA TÜRKOĞLU
Hearing aid technology and its application to people who are hearing impaired are barely beyond the embryonic stage of development. However, within the past decade, significant gains have been made in the sophistication of electroacoustic devices and their subsequent applicability to a variety of auditory deficits. (Ref3)
HEARING IMPAIRMENT
It has been estimated that about more than 10% of people around the world suffer from hearing impairment. In adults hearing loss affects their ability to obtain, perform, and keep a job. Throughout life, hearing loss causes people to be isolated and stigmatized. They suffer from emotional, communicative and social dysfunctions. (Ref10)
HISTORY
Hearing impairment has been recognized as a handicap for many centuries. Amplification devices to ameliorate its effects date back at least several centuries, with their effectiveness increasing rapidly when electrical and electronic technologies become available.
Hearing aid technology today maybe divided roughly into four main periods. The first is the acoustic era, during which devices such as horns, trumpets, and speaking tubes were utilized to amplify the sound. The second is the carbon hearing aid era, during which telephone technology was adapted to hearing aid construction. The third is the vacuum tube era, which made possible greater amplification, wider response range, and reduced internal noise. The fourth era is the one we are still in, the transistor era.
1- The Acoustic Era
Perhaps the first acoustic hearing was simply the hand cupped behind the ear. There is considerable benefit from this universally available aid, which collects and reflects sound toward the pinna. Animal horns were used to aid hearing as early as thirteenth century. References to man-made trumpets to aid hearing appeared in the seventeenth century. The idea of devices with a large opening tapering down to a small exit hole at the ear canal became well established in that century.
Acoustic devices are still being patented and used today. The gain obtained from these devices is nearly 25-30 dB. They have had the longest history by far of any form of hearing aid. Although not very acceptable cosmetically, the larger acoustic devices actually provided a great deal of gain and were of real benefit when no other means of amplification was known.
2- The Carbon Era
The telephone, invented by A.G. Bell in 1876, used a magnetic microphone, which did not amplify sound. The invention of carbon transmitters by Blake and Hughes in 1878 provided the amplification capability that made telephone technology adaptable to hearing aids. The first practical commercially available wearable carbon hearing aid in the United States was made by Miller Reese Hutchinson in 1902. His invention became the basis for ‘Acousticon’ hearing aids. Berger (1984) gave excellent historical information of telephone techniques to hearing aids.
Construction
The simple carbon hearing aid consisted of 3 elements: a sensitive carbon microphone, a magnetic receiver, and a battery, plus necessary connection cords. (Ref3) The restriction of amplification was developed by the use of carbon amplificatory in 1925. The basic principle is; while transferring the sound from microphone to receiver, the vibration of another diaphragm that is related with the existing magnetic medium takes place. These vibrations are transferred to receiver by the carbon cells, which are filled with carbon granules. (Ref2)
Vibration of the diaphragm produces relatively large variations in the microphone resistance and the battery current passing through the magnetic receiver winding is correspondingly modulated.
Carbon granules are similar to those used in telephone transmitters. These were held in a shallow cylindrical pocket between the diaphragm and a back plate. A washer of felt or similar material held the granules in place. The surface area of the diaphragm (if metal) and the back electrode were often gold plated.
Magnetic receivers, either bipolar or monopolar, were used with carbon aids. Early receivers were of the large ‘watch-case’ size; miniaturized receivers followed in the 1920s.
During its approximately four-decade duration, the carbon era, in spite of its many limitations, contributed much to the well being of the hearing impaired. During this era, many techniques and concepts were developed that enhanced later hearing aid technologies.
3- The Vacuum Tube Era
Several researches were performed in order to increase the limited frequency range and the gain of the carbon hearing aids. The vacuum tube era is the result of these researches. Lee DeForest invented the triode vacuum tube in 1907. It was quickly adapted to radio and telephone applications. The first hearing aid using a vacuum tube was ‘Vactuphone’ invented by Hanson (1920). It used a carbon microphone and a peanut tube. In 1924 a large multitube hearing aid was developed by E.A. Myers. It became available as the ‘Radioear’ hearing aid. It employed a moving coil microphone, which was much superior to a carbon microphone because of its amplitude linearity and freedom from internal noise. A rubber diaphragm was used in an effort to obtain a smoother response. (Ref3)
The high gain of the vacuum tube amplifier allowed the use of much less sensitive microphones. The technology of choice for this was the piezoelectric crystal with its ability to produce electrical signals when mechanically stressed. The lower sensitivity of devices using the technique was accompanied by the potential of great linearity and a better impedance match to the vacuum circuit. (Ref5)
The Rochelle salts crystal microphone appeared in nonwearable vacuum tube hearing aids as early as 1936. Crystal receivers have the advantage of lightweight and wider frequency response than most magnetic receivers. However, both crystal microphones and receivers were affected by humidity and temperatures exceeding 110°F. Wearable vacuum tube hearing aids did not appeared in U.S. until 1937 and was preceded by such hearing aids in England, where initial production of small battery –operated vacuum tubes took place. Hyvac prepared the first Automatic Gain Control (AGC) aids, which can be used in vacuum tube hearing aids in 1938.
Vacuum tube hearing aids had higher gain, wider frequency range, and lower distortion than carbon hearing aids. The vacuum tube era served the needs of the hearing impaired extremely well. Many technological advances were made. It should also be mentioned that it was during this era that the birth of audology took place.
3- The Transistor Era
The current transistor era started late 1952 and has produced dramatic improvements in hearing aid technology. (Ref3) Hearing aids have become more effective and smaller in size as compared to the older models. The need of a small amount of energy (1.4-1.6 V) to perform its duty by transistors and other technological developments have enabled the hearing aids to smaller and smaller sizes that can be carried easily. (Ref2)
The first transistor was a point-contact type, possibly never used in hearing aids. Late in 1952, a CK718 PNP alloy junction germanium transistor for hearing aid use was introduced. The change from vacuum tubes to transistors did not significantly change acoustical performance, but did vastly reduce the cost of operation and the size of the battery supply; only a single low voltage battery was needed. Several improvements and size reductions were made in germanium transistors. Planar silicon transistors replaced the germanium types about 1960. Besides providing high reliability and low noise, these also greatly reduced the number of components needed. The higher input impedance of the silicon transistors proved valuable when ceramic and electrect microphones became available.
During 1953, miniaturized balanced armature magnetic microphones for hearing aids appeared. The production of small, rectangular shaped balanced- armatures were started to be produced.
With increased transistor and component availability, a wide variety of transistorized body aids were produced. Size dropped as smaller microphones, batteries, and other components became available. The hearing aid industry and the hearing impaired are heavily indebted to the fore-sight and competence of component suppliers, who made continued size reduction and improved acoustical performance possible.
By 1955, the size of body aids had come down significantly. That year is important, because it marked the beginning of ear level hearing aid development. (Ref3)
HEARING AIDS
Partially surgical operations or hearing aids are usually effective for solving hearing impairments.
Hearing impairment is defined as the inefficiency in taking and processing the sonar stimuli. The reason for the hearing loss is usually caused by the physiological changes in the hearing system. This results with two important hearing impairment groups:
-There is a loss of auditory sensitivity in taking sonar stimuli in the hearing system. They are detected in a lower frequency than their actual value. This is called ‘attenuation factor’ and is seen in the conductive type hearing impairments. (Ref1) The problems arise from ear infection, earwax or a disease process blocks the normal flow of sound to the inner ear. (Ref11)
-The characters of sonar stimuli are changed in this second group. Sonar stimuli are sensed in a different character. This is called ‘distortion factor’ and usually is seen in sensorineural hearing impairment. When sound is heard sufficiently loud, its quality in other terms may be poorer than for normal hearing listener. This is called qualitative effect. Distortion makes the hearing of background noise difficult. (Ref1) most common cause of sensorineural hearing loss in older persons is presbycusis, which is the deterioration of the hearing system associated with normal aging and almost always can be helped with hearing aids. Presbyacusis occurs in both ears and effects over half of people over 60 years old, making it the second most common disability in older people. The most common hearing loss associated with ageing results from degeneration of a part of the inner ear, which contains microscopic blood vessels. This type of hearing loss does not disproportionately affect the individual’s ability to hear and understand the speech. However, other changes within the inner ear associated with ageing, including the loss of the ear’s tiny hair cells, can have a more serious affect on the ability to understand the speech. Vital components of speech sounds, usually the higher pitched consonants, which give intelligibility to speech, are missing or distorted and, for this reason, many people first experience difficulty in understanding women and children- the lower pitched male voices often being easier for them to hear and comprehend. As hearing deteriorates the ability to understand speech becomes more severely affected. (Ref7) Some of the many other causes of sensorineural hearing loss are exposure to loud noise, heredity, head injuries or taking certain drugs. (Ref11)
Correction of attenuation factor is easy. It is corrected by the amplification of the noise. However, correction of distortion factor is difficult.
Hearing aids are mostly not needed in conductive type impairments. Some special cases of conductive type may require the use of these aids. However, nearly 90% users of hearing aids are sensorineural. They do not fully benefited from hearing aids because of the distortion factor. (Ref1)
Hearing aids have always considered as a sign of physical disability and people have had a tendency of not wearing it. For this reason, hearing aids should meet some cosmetic concerns. (Ref1) In this circumstance, the smaller the better is probably true. The smallest transducers possible, regardless of performance, may be needed. If completely in the channel is attempted, some protection from cerumen occlusion would probably be best. (Ref5)
Technology is developing everyday for meeting these demands. Hearing aids are categorized into two main groups, which are:
- Conventional hearing aids
- Implantable Devices
1- Bone anchorage hearing aids
2- Middle ear implants
3- Cochlear implants
Conventional Hearing Aids
These are categorized into two main groups: 1) Acoustic devices, 2) Electronic devices
Acoustic devices are not used today. The idea here is, any space makes the sound stronger in a certain frequency. The trumpets and horns are in this group.
Electronic Devices
Hearing aids are in this category. Assistive Listening Devices may also be considered in this group. They have been developing since 1930. A hearing aid mainly consisting of these parts: Microphone, amplifier and filter, receiver, volume control, battery, and T switch which is available in only some devices. (Ref1) The mechanism is explained in the following figure.
1- Microphone
A microphone is a device that converts the acoustical energy of a sound field to electrical energy. The hearing aid microphones in current use are electret devices that have rather good linear behavior over a frequency range of 50 Hz to 600 Hz, and the technology is available to extend this range of 20 Hz to 20,000 Hz and beyond (Ref5). Some microphones can collect the sounds from all directions. These are called omnidirectional, which may be useful in some situations for example when listening to music or when crossing the road. Others can collect the sounds only from one direction. These are named as directional. (Ref1) A directional microphone is a microphone with an electrical output level that varies with both the amplitude and the direction of the sound source relative to the microphone. The logical application is the differentiation between wanted and unwanted sounds by proper orientation of the microphone with respect to the sound sources (Ref5). The hearing impaired people working in a noisy environment can get benefit from the unidirectional microphones. The unidirectional ones are especially preferred by sensorineoral hearing impaired people. Unidirectional microphones are more sensitive to talking while less sensitive to noisy environment. (Ref1) By employing two microphones, directional instruments can make sounds originating behind your head softer than in front of you. This offers tremendous help for listening in noisy environments. Some products have directional microphones, available behind the ear aids (BTE), full in the ear aids (ITE), and a few half-shell instruments. The newest directional microphones in some digital aids can even track the location of the loudest sounds on the side of you, as well as behind you, and decrease the volume wherever they are. Industry research supports the claim that users who have directional microphones are 50% more satisfied with their hearing aids’ performance in noise. Directional microphones are now available in all digital aid sizes, except the tiny completely in the channel size. (Ref8) The implacement of the microphone is an important factor when considering its performance. The performance is higher when it is emplaced in the entrance of outer ear, which is the normal entrance direction of the voice. It should be remembered that humidity affects the performance of the microphone. Diaphragm loses its vibration ability and the voice becomes stifling. Some microphones may become unusable by humidity penetration. (Ref1)
2- Amplifier and Filter
A hearing aid amplifier consists primarily of small transistors that are built into an integrated circuit. The primary function of transistors in hearing aids is to increase the power of an electrical signal, i.e. to provide amplification. Most hearing aids require hundreds of transistors to function, a potential problem were it not for the fact that transistors are incredibly small in size, and likely to become smaller in future. (Ref1) Today’s amplifiers are used in two separate groups, linear amplifier and compression amplifier.
In linear amplifiers, all sounds from the microphone are amplified in the same strength no matter what their intensities are. (Ref1) That is, for a given amount of gain, a linear hearing aid will produce a 10 dB change in output for 10 dB change in input until the onset of saturation. Linear amplification represents the most common type of hearing aid fitting in use today. (Ref5). Simple linear amplification would be ineffective in providing the controlled acoustic behavior needed to accommodate the auditory deficit and environmental listening situations. Linear amplification is not the solution to the auditory requirements (Ref3) However, in compression types the sound is amplified to a certain level and after reaching this level no matter how high the frequency of the sound coming to the microphone, amplification stays same. (Ref1) Compression amplifiers are designed to prevent some of the problems associated with linear amplification. Compression amplifiers reduce gain when either the input to device or output from the device exceeds a predetermined level. This process tends to result in comfortable amplification for the hearing aid wearer.
Being the most important part of the hearing aid, amplificatory is a very complex part with the circuits in it. The circuit can be named as signal processing for the better understanding of this part. Three different signal processing is used in hearing aids today;
- Analog Circuits
- Digitally Programmable Circuits
- Digital Circuits (Ref5)
1- Analog Circuits
Traditional hearing aids are of this type. The term ‘analog’ means they use conventional electronics. In these aids microphone picks up the sound, which is amplified using transistors, and is then reproduced by a miniature receiver. There may be a choice of settings, for example for noisy situations, and these may be selected automatically.
Many analog aids is incorporate some from Automatic Gain Control (AGC) or compression system which can give a lot of amplification for weak sounds while preventing loud sounds from becoming uncomfortably loud. Sometimes, the AGC operates independently in two or more frequency ranges, called bands or channels. This can be advantageous for a person whose amount of hearing loss varies with frequency. (Ref7)
Microphone
Receiver
Mechanism in an Analog Hearing Aid copied from (Ref2)
2- Digitally Programmable Hearing Aids
These are analog aids that use a digital memory to select between various settings. This makes it much easier for the audiologist or dispenser to provide the user with a range of suitable settings for different listening environments. In some models, the user can select settings using a remote control. Programmable aids have a number of advantages. They can be more precisely tailored to the individual’s needs, settings can be changed and then restored and, than in conventional analog aids, more appropriate settings may be found for different situations.
Microphone
Digitally Programmable Hearing Device copied from (Ref2)
3- Digital Hearing Aids
Digital hearing aids incorporate a miniature computer to process sounds. In truly digital aids, the sound is converted from analog to digital form. In this form, numbers represent the electrical signals to which it is possible to apply mathematical techniques. The digital signals are then converted back to analog form and sent to the receiver (in some aids the digital signal is sent directly to a special receiver). This makes it possible to process the sound in ways that would be difficult or impossible with analog aids.
Digital hearing aids may include several features that are not found in analog aids, such as systems for reducing:
- Feedback (the annoying whistling sound that occurs when the sound generated by the aid leaks back to the aid microphone)
- The loudness of steady background noises, improving listening comfort.
In addition, they may contain more sophisticated automatic gain control systems than analog aids, and they may have directional microphones with enhanced directionality. Digital aids are also generally highly adjustable to suit the individual.
One potential problem with digital hearing aids is that they introduce a slight delay in the sound, typically between 1 and 10 milliseconds. Usually this small delay is not noticeable. However, if a single digital aid is fitted, the time difference of the sound at the two ears may disturb the ability to localize sounds. Therefore, it is recommended that digital aids are fitted to both ears, not just one ear (Ref5).
M R R
Digital Hearing Aids copied from (Ref2)
3- Receiver
A receiver is a device that converts electrical energy into acoustic energy of the sound field. The receiver is usually a balanced armature magnetic transducer. These transducers contain a plethora of electrical, mechanical, and acoustical reactive elements with coupled resonances. Magnetic receivers superseded the piezoelectric materials which were poor transducers used in the past. In general the larger the receiver, the larger the output signal it can supply. However, because of cosmetic reasons, most users prefer as small a hearing aid as possible. A receiver produces sound by using a varying electrical signal to modulate the total force on the metal armature exposed to the magnetic field supplied by one or more magnets. The receiver outlet is usually open to the ear canal. The cerumen, secreted from the ear canal is possible to work its way further into the receiver. This can affect the frequency response of the hearing aid. Cleaning of ear mold and the tubing must be performed regularly (Ref5).
Volume Control
This switch arranges the amplification ability of the hearing aid.
Batteries
Button type batteries are used in today’s hearing aids. They are in four different sizes. Their power increases with increasing sizes. They are made of zinc oxides. They have a life of 2 weeks in a normal usage. Their voltages are 1.4 V. They become unusable when their voltage drops 1.2 V.
Telecoil or T switch
A telecoil, or telephone pickup coil, is a device that converts the energy in an electromagnetic wave into electrical energy. This is available only in some hearing aid devices. Electromagnetic waves are found in telephones. For this reason, the hearing aid wearer cannot use telephone in the absence of T switch. This switch enables the hearing aid wearers to talk on the phone. It is also needed for asistive listening devices. (Ref1)
Assistive Listening Devices
Various instruments, generally called assistive listening devices, are available for specialized use in particular listening conditions or more generally in very difficult listening situations. In general, these devices improve signal to noise ratio by moving the microphone of the amplifying system closer to the person speaking. For example, personalized FM, radio transmitting/receiving systems utilize a speaker-worn microphone and transmitter broadcasting the signal to the hearing impaired listener who wears the receiver/amplifier/earphone system. Since the speaker talks directly into the system’s microphone in a noisy room, the signal to noise ratio is greatly improved. There are many others and are usually inexpensive. (Ref3)
Earmold
It is a device that is placed in the outer ear and is used for sending the signals from receiver to ear membrane. It is essential for body, eyeglass, and behind the ear aid types. It is made of silicone or acrylic. It consists of 2 main parts,
1) The larger part providing the retaining of the device in its place is placed to concha.
2) A tube for sending the signal from receiver to ear membrane.
The tubes that are made of silicone are soft and easily shaped. They provide a good plugging in the ear channel. However, the tubes that are made from acrylic are harder. Acrylic tubes are used in severe hearing impairments and silicone tubes are used in mild to moderate impairments.
I.CONVENTIONAL HEARING AID TYPES
They are collected in 7 main groups;
1) Body Aids
2) Eye glass aids
3) Behind The Ear (BTE)
4) In The Ear (ITE)
5) In The Canal (ITC)
6) Completely In The Canal (CIC)
7) CROS and biCROS
They have many advantages and disadvantages.
1) Body Aids
These types are the oldest version of the hearing aids. The microphone and the receiver are separate from each other in this version. They are connected by a cable. Microphone is put to the lumbar region or to the chest region. The receiver is in the entrance of the ear.
Advantages
b) It provides a high gain.
c) For this reason it can be employed in all types of hearing impairments.
d) The distortion is prevented since the receiver and the microphone are far from each other.
e) The volume control and T switch is the main advantage of them.
Disadvantages
f) It does not meet esthetical concerns of the hearing aid wearers.
g) The friction of the cloth on the body causes some problems in the microphone since it is connected on the body.
h) The cable that connects microphone to the receiver deteriorates quickly.
2) Eyeglass Type
Microphone is placed around the frame of the eyeglass. It contacts with the receiver, which is placed in front of the ear on the stem of the frame. These devices were introduced in 1960s for cosmetical reasons. Some firms used them with behind the ear devices. They placed the hearing aid on the stem, which is on the back of the ear. This solved the problem of large frame and enabled users to wear both eyeglass and the hearing aid at the same time.
Advantages
- It has the similar advantages to the behind the ear hearing aids.
- It enables users to wear both eyeglass and the hearing aid at the same time.
Disadvantages
- When the eyeglass is taken off, the device cannot work.
- It has some esthetical problems due to its larger sized frame.
3) Behind The Ear (BTE)
They have attained their popularity in between 1970 and 1980. Microphone is placed behind the ear, by an earmold. Receiver is placed on the concha (outer ear canal entrance). According to the USA statistics, the 20% of the hearing aids used in 1990 was this type. The center of the ear mold is open that enables refreshing of the concha. The humidity problem is also partially solved by this opening.
Advantages
- It provides a high amplification.( Lesser than body aids)
- It is esthetically better than body aids..
- The friction problem does not exist.
- It is powerful and is not failed easily.
- It can be used by children.
Disadvantages
- An earmold is needed.
- There is an assistive system for retaining in its place in children.
- Since microphone is in the entrance of the ear, its performance decreases by the movement of the head.
3) In The Ear (ITE)
This type is nearly 80% of the hearing aids that are in use in the USA. They show similar performance to the BTE aids. They are fitted in concha and microphone is at ear canal level.
Advantages
- It is esthetically more preferable than BTE .
- Placement is easy.
- Since microphone is exactly at ear canal level, its performance is higher.
- It can be used in all types of hearing impairments.
Disadvantages
- It is visible.
- Some dexterity is needed for inserton, removal and adjustment..
- Humidity is an important problem.
- Since the batteries are small, they have to be changed regularly.
- It does not take the advantage of pinna and concha.
5) In The Canal (ITC)
They are placed in the concha. Only the face sticks out the concha. Microphone is opening at the ear canal. It is proper for mild to moderate hearing impairments.
Advantages
It is usually preferred by younger age group for esthetical appearance.
- Its performance is good.
- It takes advantages of most of the auricle.
Disadvantages
- Distortion problem arises since microphone and receiver is very near to each other.
- The gain is inefficient.
- It cannot be used in heavy hearing impairments.
- Small battery has to be changed regularly. (Ref1)
6) Completely In The Canal (CIC)
They are available in digital and programmable (computerized) hearing aids, and are favored by patients with cosmetic concerns. These tiny aids are almost invisible in most ears. The fully digital CIC hearing aids are normally best suited for mild to moderate hearing losses and patients with good manual dexterity. (Ref11)
Advantages
- It is hidden in the canal so invisible in the ear.
- It takes the full affect of auricle.
- Receiver is very close to tympanic membrane so requires less amplification.
Disadvantages
- Patients need very good dexterity to place and remove.
- It can be easily lost because of its small size.
- Some circuitry is not available in this size.
7) CROS and biCROS
If one of the ears is normal, while the other is completely deaf and cannot get benefit from hearing aids ‘Contralateral Routing of Signal’ -CROS can be used in these people. The microphone is placed in the bad ear side and the receiver is put in the normal ear side. This enables sending of the signals that come to the bad ear side to the normal ear. The shadow effect is prevented by this method. Another application of this method is biCROS application. The method is same except that while one of the ears is unable to hear, the other can hear with the help of a hearing aid. There are two microphones and a receiver in this type. One of the microphones is placed in front of the bad ear. The other is placed in front of the better ear that is wearing hearing aid. It provides amplification to better ear. (Ref1)
II. IMPLANTABLE HEARING AIDS (IHD)
IHD use the same technology as the conventional devices. However, there is no receiver in IHD. There are many requirements from these devices;
- The percutaneous connection must not cause any irritation.
- It must be tissue compatible.
- It should have a long life.
- It should be small and esthetically acceptable.
- If the application is not effective, the removal should be easy and after the removal, it should not affect the usage of conventional devices.
There are also many advantages from these devices;
- The patient feels more comfortable.
- The damping of the outer ear canal is prevented because it is open now.
- Since the outer ear canal is open, distortion problem is prevented partially.
- When the outer ear canal is filled with a conventional device, patient hears his voice higher. This is also solved by IHD. (Ref1)
1) Bone Anchored Hearing Devices
A fairly recent development, bone anchored hearing aid is helpful to patients who are unable to use a conventional hearing aid. This may be for a variety of reasons, such as if the patient’s ears are absent or malformed, or if they have a disease that regularly causes the ears to discharge, clogging up their hearing aid. It is estimated that some 30,000 people fit into this category.
These types of aids works for transmitting sound through the bones of the skull to the nerves of the inner ear.
The aid is attached to a small titanium screw, which is drilled into the mastoid bone behind the ear. The operation is quite simple and is carried out under local anesthetic. The screw is then left for 3 months to allow the bone to grow back, locking the screw firmly into place, after which a plastic socket is attached. The bone anchored hearing aid fits onto this, and can be easily removed for sleeping or bathing. (Ref7)
2) Middle Ear Implants
A middle ear implant can be thought of as a hearing aid, where either the receiver, or the entire unit, is surgically inserted into the middle ear. The potential advantages of MEIs are three-fold; first, if the ossicles can be driven directly (also referred to as "direct drive") there may indeed be improved sound quality, with essentially no acoustic feedback. Second, a completely implantable MEI may one day be available, with no external components at all. Indeed two manufacturers have designed completely implantable middle ear devices.
Thirdly, depending on the MEI, if there is no device in the ear canal, there is no insertion loss with a net boost in high-frequency sound transmission. Resulting in a higher fidelity, more useful sound product. (Ref6)
In general, middle ear implants are attached to the middle ear bones to enhance their vibration. This amplifies nerve impulses to the brain, increasing hearing ability. But all devices currently available also use an external component -- a small sound processor, much like a microphone, that usually is worn behind the ear.
The external device enhances the sound because it's "capable of being programmed" so small changes in hearing or listening can be adjusted. (Ref9)
MEIs come in two general categories; electromagnetic and piezo-electric. While an electromagnetic approach in the 1930s was made, the first clinically wearable middle ear implants were of the piezo-electric type.
Piezo-Electric:
A piezo-electric crystal has some interesting properties. When such a crystal is bent, it generates an electric charge, and when an electric charge is applied to the crystal, it bends. This bending of the crystal generated a small electric current-- enough to drive the system. Such a crystal can be used in the middle ear. When attached to the middle ear bones, a small electric current from the microphone would cause it to bend and flex. This in turn causes the middle ear bones to vibrate and thereby transduce sound to the inner ear. Since 1984, this type of middle ear implant has been used successfully.
Some advantages of the piezo-electric approach are that the components of the implant are physically small and the design is quite simple. In contrast, the electromagnetic approach uses bulkier components, and depending on its implementation, can be quite complex. Interestingly, the magnet should weigh less than 50 mg. If the magnet’s mass is above this, there will be "loading" of the ossicular chain with the introduction of an associated high-frequency conductive component.
Electromagnetic:
In contrast to the piezo-electric method of transducing sound, there are several programs around the world (mostly in the United States) using the electromagnetic approach. With this method, the microphone sends sound energy to a coil of wire that creates a magnetic field. The magnetic field communicates (through induction) with an implanted magnet -- much like a cochlear implant. Instead of a crystal being implanted, a magnet is connected to the ossicular chain, which vibrates in synchrony with the magnetic field. The mechanical vibration of the ossicular chain is transferred to hydraulic, and then bioelectric signals via the inner ear.
There are technical issues regarding where to place the magnet, as well as the nature and orientation of the magnetic coil. Nonetheless, many of the research protocols and findings are similar. Essentially, the magnet has to be medial enough on the ossicular chain to derive benefit from the high-frequency rotational characteristics of the ossicular chain; yet lateral enough to be near the transducing magnet to optimize gain and output. A compromise location is near the incudo-stapedial joint.
One major advantage of the electromagnetic approach is that this technolgy can generate significantly more gain and output than can piezo-electric technology. Piezo-electric hearing aids will only be useful for someone with up to a moderate or moderately-severe hearing loss in the mid and high frequencies (maximum output of 110 dB SPL), the electromagnetic approach can be useful for those with severe hearing loss. In both technologies, there is greater gain and output in the mid-and-high frequency region, than for lower frequency sounds.
These devices are In general, middle ear implants are attached to the middle ear bones to enhance their vibration. This amplifies nerve impulses to the brain, increasing hearing ability. But all devices currently available also use an external component -- a small sound processor, much like a microphone, that usually is worn behind the ear.
3) Cochlear Implants
A cochlear implant is a surgically implantable device that provides hearing sensation to individuals with severe-to-profound hearing loss who do not benefit from hearing aids. Typically, people with hearing losses in the severe-to-profound range have absent or malfunctioning sensory cells in the cochlea. In a normal ear, sound energy is converted to mechanical energy by the middle ear, which is then converted to mechanical fluid motion in the cochlea. Within the cochlea, the sensory cells-the inner and outer hair cells-are sensitive transducers that convert that mechanical fluid motion into electrical impulses in the auditory nerve.
Cochlear implants are designed to substitute for the function of the middle ear, cochlear mechanical motion, and sensory cells, transforming sound energy into electrical energy that will initiate impulses in the auditory nerve.
Although it has been known since the late 1700s that electrical stimulation can produce hearing sensations, it was not until the 1950s that the potential for true speech understanding was demonstrated. In 1957, two French surgeons placed an electrode on the auditory nerve of a deaf man during an operation for facial nerve repair. When current was passed through electrode, the patient was able to discriminate some sounds and understand a few simple words.
Based on that observation, several research groups around the world began exploring the feasibility of implantable electrical stimulators that could be used on a long-term basis by hearing-impaired individuals. Through their efforts, cochlear implants have evolved from single-channel devices, introduced in the late 1970s, into technologically advanced microprocessor systems that deliver a higher level and greater range of hearing benefits than ever would have been predicted or expected. Currently, four cochlear implant systems are available or under investigation in the United States.
Components of a Cochlear Implant
All cochlear implant systems consist of both internal and external components. The external components, which are worn on the head, over or next to the ear, include (1) a microphone, which converts sound into an electrical signal, (2) a speech processor, which manipulates and converts the signal into a special code (i.e., speech processing strategy), and (3) a transmitter, which sends the coded electrical signal to the internal components. The surgically implanted components include (1) a receiver, which decodes the signal from the speech processor, and (2) an electrode array, which stimulates the cochlea with electrical current. The systems are powered by batteries located in the speech processor.
Although all cochlear implant systems have basic features in common, they differ in how those features are implemented. Important differences exist in the (1) implanted electronics packaging, (2) electrode design, (3) stimulation waveform and temporal pattern of stimulation (4) speech coding strategy, and (5) telemetry.
The efficacy of cochlear implants for the remediation of severe-to-profound hearing impairment is now well accepted. Research and development of cochlear implant technology will continue to focus on modifying electrode/neural interfaces to reduce power consumption and to limit undesirable channel interaction. Speech processing strategies that use a variety of stimulation waveforms and patterns will allow implants to be better tailored to the needs of each individual user. New miniaturization processes will result in smaller behind-the-ear processors and, eventually, completely implantable systems. In addition, technological advances should result in improved implant reliability and enhanced speech perception benefit. As the perceptual benefits improve, the candidacy criteria will likely expand to include other individuals with hearing impairment who do not benefit satisfactorily from traditional acoustic amplification. (Ref6)
REFERENCES
- AKYILDIZ, N., ‘Kulak Hastalıkları ve Mikrocerrahisi’,Ankara, 2002
- CELIK,Onur ‚’ Kulak Burun Bogaz Bas ve Boyun Cerrahisi’, Istanbul, 2002
- SANDLIN, R.E., ‘Handbook of Hearing Aid Amplification Vol I’’, Singular Publishing Group, London, 1995
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- VALENTE, M. ‘Hearing Aids: Standards, Options and Limitations’, Thieme Medical Publishers, New York, 1996
- http://www.audiologyonline.com/audiology/newroot/Articles/arc_disp.asp?id=333&catid=2274
- http://www.defeatingdeafness.org
- http://www.ear-responsible.com/digital.html
- http://www.cnn.com/2002/HEALTH/conditions/08/21/middle.ear.implants/
- http://www.who.int/pbd/pdh/pdh_home.htm
- http://audionicsnyc.com/AdultCause.cfm
