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Assistive Listening Devices
Hearing Aids
Hearing aids and assistive listening devices have been very beneficial to children and adults with hearing losses. Additionally, there are many types of hearing aids and amplification devices available to the hearing impaired. There are four standard components of a hearing aid. The components consist of a microphone, amplifier, receiver and a battery. The microphone detects the signal and turns this acoustic signal to an electrical signal in order to be processed by the hearing aid. The amplifier modifies and amplifies the signal depending on the controls set by the Audiologist or hearing aid dispenser. The receiver converts the electrical signal back to an acoustical signal to be interpreted or delivered to the external auditory canal. The batteries of the hearing aid come in different sizes depending on the type of hearing aid. The batteries give off strong signals and they are short-lived. Hearing aids also have some optional features that can enhance the hearing aid. There is an on/off switch, a volume control switch and a telecoil or t-coil on the hearing aid. The t-coil is used in instance when a person wants to use the phone or an induction loop system. The t-coil converts an electromagnetic signal to an electrical signal to then be shaped and amplified by the amplifier of the hearing aid. The t-coil makes speech more intelligible on the phone because the phone is giving off electromagnetic signals that the t-coil can pick up. Sound amplified by a hearing aid without a t-coil is more distorted and cannot pick up these electromagnetic signals very well.
There are five typical hearing aid styles in use. The styles of hearing aids are, body, eyeglass, behind-the-ear (BTE), in-the-ear (ITE), in-the-canal (ITC), and completely-in-the-canal (CIC). Body hearing aids consist of a box that holds the microphone, amplifier, receiver, and battery and is strapped to the body of the user. There is a tube attached to the receiver in the hearing aid that runs up to attach to an earmold in the person’s ear. The body aid is good for people with profound hearing losses, but is bulky, visible and it is hard to localize sound with the microphone so far from the ear. A second type of hearing aid is the eyeglass hearing aid; they have the hearing aid in the eyeglasses. The microphone, amplifier, receiver and battery are in temporal portions of the glasses. There is a tube connecting the receiver to an earmold. A third type of hearing aid is the behind-the-ear aids, which are placed behind the pinna, with all of the components of the hearing aid built into a small shell. The small shell is connected to an earmold in the ear by a small tube. This type of aid is great for children and people with arthritis because it is larger, making it easier to manipulate. The fourth type is the in-the-ear hearing aid style, which has all of the components of the hearing aid in a small shell that fits to the size and shape of the person’s external auditory canal. ITE hearing aids fit into the person’s external auditory canal. This is the most popular type of aid. A possible problem with this type of hearing aid is that it may be too small to have extra features like at t-coil or volume control switch. The fifth type of hearing aid is the in-the-canal aid. It is one of the smallest types and all of the components of the hearing aid fit into a small shell casing and fits almost all the way in the external auditory canal. ITC are good for mild to moderate, flat or gently sloping hearing losses. A disadvantage of ITC aid is that they may be too small to have extra features like a t-coil or volume control switch, plus you may only be able to obtain moderate amounts of gain because of feedback. The last type of aid to be discussed is the completely-in-the-canal hearing aid. However, it is not visible since it fits all the way in the ear canal. It has a small wire that is used to help pull the hearing aid out of the ear. This type of hearing aid is good for reducing feedback since it completely fills the canal. It is cosmetically appealing and less electronic gain is needed because the distance between the end of the hearing aid and the tympanic membrane is minimal.
Some people are not able to wear air-conduction hearing aids, so they can wear bone-conduction hearing aids. Bone-conduction hearing aids are worn if the person has a deformed or absent pinna or external auditory canal. Bone-conduction hearing aids have a bone-conduction oscillator, which replaces the receiver of the hearing aid. The bone-conduction hearing aid still has a microphone and amplifier. The oscillator is held on the mastoid process of the temporal bone by a headband, eyeglass or is permanently affixed there by a doctor. The bone-conduction hearing aid bypasses the middle ear and directly stimulates the temporal bone.
Classroom amplification
There are many types of classroom amplification devices. The goal of classroom devices is that they provide a good signal to noise ratio. A signal to noise ratio is simply the ratio that represents the signal present to background noise. A good signal to noise ratio is when the signal is louder than the background noise. Classroom amplification includes but is not limited to, hardwire system, induction loop system and a FM amplification system. The hardwire system provides a direct connection between the source (teacher) and the listener (student) by means of wire. In a hardware system there is a microphone close to the teachers voice, the electrical signal is sent to the listener through a wire, the signal is then amplified and changed to an acoustic signal. The listener picks up the signal by wearing a headset or by wearing a hearing aid. This system is used in schools, churches and auditorium. The problem with a hardwire system is that movement is limited because of the wire between the teacher and the student. This type of system is not very common anymore. Another type of system is the induction loop system. This was the first improvement made in classroom amplification. The teacher wears a wireless mike; the microphone is placed near the teacher’s voice. The microphone transmits FM frequency or an electromagnetic field to an amplifier. The amplifier is connected to a wire; the wire surrounds the listening area and creates an electromagnetic field around the listeners. The t-coil in the hearing aids pick up the electromagnetic signals and converts it to an electrical signal to be processed by the hearing aid. This system is not very expensive and is easy to maintain. A possible problem with this is that there could be signal spillover to adjacent rooms. The child also must have access to a telecoil in order for them to benefit from an induction loop. The third type of classroom amplification device is the FM amplification system. This type of device is the most versatile wireless system. With this type of system, the teacher wears a wireless mike that transmits FM frequency to a receiver worn by the child. It uses radio waves to transmit sound from the source to the user. The microphone is placed near the speaker. The signal is sent to a FM transmitter and carried on radio frequency waves to the receivers worn by the child. The receivers have the ability to decode the FM frequency waves. The receivers can be built into the child’s hearing aid or clipped on to the hearing aid. There are many advantages to this system. There are no spillover problems, it is portable and it improves the signal to noise ratio. One problem with this type of device is that sometimes there is FM interference.
Cochlear Implant
A cochlear implant is an electronic device designed to provide sound information by directly stimulating the auditory nerve fibers in the cochlea for people who have a profound hearing loss, either congenital or acquired in both ears, and can obtain no benefit from hearing aids. The cochlear implant by passes damaged hair cells and helps establish some degree of hearing by stimulating the auditory nerve directly. It does not eliminate the disease that resulted in deafness, nor does it restore the normal function, and each outcome varies.
The first research done on cochlear implants was conducted in France more than thirty years ago. However, in 1790, a French surgeon by the name of Volta conducted an experiment by sticking two metal rods in each of his ears and changed it with electricity to stimulate the auditory nerve. The results were serious blows to the head, and the experiment was not performed again. In 1961 several patients under went cochlear implantation performed by the House Ear Institution. While work proceeded through the 1960s and 1970s on improvements in basic design, cochlear implants were controversial in the auditory community. This was because many people were concerned about the long-term affect of the auditory nerve being directly stimulated by electronic impulses. By the 1980s views were changing for the better. The Food and Drug Administration approved the 3M / House Single – channel implant in 1984. The Nucleus multichannel implant was approved in 1990. Both of these devices are approved to be used by adults and children.
Cochlear implants are still controversial, expensive and have uncertain results. The U.S. Food and Drug Administration has limited the implants to people who qualify for the following: those who get no significant benefit from hearing aids, those who are at least two years in age and those with severe to profound hearing loss.
Hearing loss is caused by a number of different problems that occur either in the hearing nerve or parts of the middle or inner ear. The most common type of deafness is caused by damaged hair cells in the cochlea, the hearing part of the inner ear. In a healthy or normal ear hair cells stimulate the hearing nerve, which transmits sound signals to the brain. When the hair cells stop functioning the auditory nerve remains unstimulated, and the person cannot hear. Many things can destroy hair cells including infection, trauma, loud noise, aging, or birth defects.
All cochlear implants consists of a microphone worn behind the ear that picks up sound and sends it along a wire to a speech processor, which is worn in a small shoulder pouch, pocket, or belt. The processor boosts the sound, filters out the background noise, and turns the sounds into digital signals before sending it to a transmitter behind the ear. A magnet holds the transmitter in place through its attraction to the receiver-stimulator. This part of the device is surgically attached beneath the skin into the skull. The receiver picks up digital signals forwarded by the transmitter, and converts them into electrical impulses. This flow through the electrodes contained in a narrow, flexible tube that has been threaded into cochlea. This entire process takes seconds to be done. The sounds heard through an implant are different than normal hearing sounds, and have been described as artificial and robot like. Simply because its an electronic mechanism, the implants’ small number of electrodes cannot possibly match the complexity of a person who has fifteen-thousand functioning hair cells.
During the surgical procedure, the surgeon makes an incision behind the ear and opens the mastoid bone leading into the middle ear. Then the surgeon places the receiver-stimulator in the bone and gently threads the electrodes into the cochlea. This surgical procedure takes from an hour and a half to five hours.
Before a person receives an implant, specialists at an implant clinic conduct a careful evaluation, including extensive hearing tests to determine how well the candidate can hear. Unfortunately, it’s not possible to predict who will benefit from the implant more than someone with a damaged nerve. First, a candidate undergoes a trial with a powerful hearing aid. If the aid cannot improve their hearing well enough, then a doctor performs a physical exam and orders a hearing screening. If a candidate has a scarred cochlea, then it is a possibility that they are not a good candidate. The doctor also recommends a psychological evaluation to get a clearer understanding of the candidate’s expectations as well as there understand what the implant can and cannot do.
After the surgery, the patient stays in the hospital for a day or two. After a month, the surgical wounds will have healed and the patient returns to the implant clinic to be fitted with external parts of the device, which are the speech processor, microphone, and the transmitter. Then the speech processor is tuned and set to the levels for each electrode, from soft to loud. The patient is then trained in how to interpret the sound head through the device. The length of the training varies from days to years; it all depends on how well one can interpret the sounds heard through the device.
Some risks after the surgery are dizziness; due to the acoustical nerve which has to do with balance and facial paralysis infection at the incision sight. Great care must be taken with many children and precautions must be made during the surgery because the child’s skull bones are thinner and more fragile than adults. So care must be taken to avoid puncturing during drilling of the mastoid cavity. Scientists are not sure about the long-term effects of electrical stimulation of the nervous system.
Most profoundly deaf patients who receive implants are able to discern medium and loud sounds, including speech at comfortable listening levels. Many use sound clues from the implant together with speech reading and other facial cues. Almost all adults improve their communication skills when combining the implant with speech reading. Some adults can even understand spoken words without speech reading. More than half of adults who lose their hearing after they learned to speak can understand most speech without speech reading. However, about 30% can understand spoken sounds well enough to use a telephone. Unfortunately, some people have reported that after receiving the implant that they feel alienated from the deaf community however, at the same time they do not feel as a part of the hearing world or mainstream.