Amplification Systems For Individuals with Hearing

Impairement: Individual & Group

Presented by :, Pargat Singhpargatsingh0007@gmail.com

Lecturer in spl. education

We know that there are different types and degrees of hearing loss. Not all the hearing losses can be treated with medicines and/ or with surgery. When the hearing loss is irreversible like in cases of sensori-neural or mixed loss or in some chronic cases with conductive hearing loss then these individuals can have the option of use of amplification devices. Amplification devices basically function by amplifying the sound or increasing the loudness of the sound and reaching it to the ear of the user. An amplification device is an electronic device that helps a person with hearing impairment to hear better. There are different types of amplification devices and each has its advantages and limitations.

Definition of Individual and Group hearing aids:

Hearing aid is an amplifier whose function is to increase the intensity of sound and deliver it to the ear with as little distortion as possible.

A hearing aid can be defined as an electronic device that amplifies sounds to a degree and in a manner that will enable a person with hearing impairment to utilize his remaining hearing in an effective manner. Amplification devices are now being used for several centuries and have undergone a lot of changes as development in technology has occurred. Individual hearing aids have gone from being bulky, cumbersome table models to tiny, delicate instruments that can be placed deep inside the ear canal and are invisible.

Amplification devices can be classified into

Individual amplification system

Group amplification systems

Parts of the hearing aids

•Microphone•Amplifer•Receiver•Volume Control•Battery

How does a hearing aid function?

•Microphone captures the acoustic signal from the air changes in to electrical energy.

•The signal is amplified (increased in intesity) by the amplifier

•This modified signal is converted back into acoustic signal by receiver and delivered to the ear.

•The power source of the hearing aid is a battery

Types of hearing aids:

Individual hearing aids are classified on the basis of several parameters. These include:

A.] The place where they are worn on the user:

Based on the place where the hearing aids are worn on the user’s body, they can be classified into body level aids and ear level aids. As the names suggest, the body level hearing aids are placed on the body at the level of the chest, while the ear level hearing aids are worn on the ear.

1. Body worn hearing aids:

These are also known as pocket hearing aids. A pocket hearing aid can be worn in the pocket or with the help of a harness and is bulky in size. This type of hearing aid consists of a rectangular box like case, a cord and an external receiver attached to the cord. The receiver is attached to an ear mould. These hearing aids can be attached to an external air or bone conduction receiver. Body-worn hearing aids are very commonly used in our country.

Body worn hearing aid with single cord

Advantages:

•Can provide high gain and has less chances of acoustic feedback due to more distance between microphone and receiver.•Larger controls and larger batteries allow old patients to handle it easily.•These are more sturdy and durable.•Body level hearing aids are less expensive to buy and maintain.•They also have lesser problems of feedback squeal if the ear moulds are not good.

Disadvantages:

•Body worn hearing instruments are less appealing and cumbersome because of their large size and cords.•The microphone placement is not very ideal as it is chest mounted. •There is a difficulty in eliminating the effects of cloth rubbing noise.•The cords are difficult to manage and are prone to damage easily.

2. Ear level hearing aids: As the name suggests, ear level hearing aids are worn on the ear. These can be of different types such as post-aural (behind-the-ear aids) and intra-aural (in-the-ear aids, in-the-canal aids, and completely-in-the-canal) aids.

a] The behind-the-ear hearing aids :

The behind-the-ear hearing aids are designed to fit behind the pinna. The microphone, receiver and amplifier are mounted in the characteristic banana shaped curved case that fits neatly behind the pinna and rests against the mastoid prominence. A thin plastic tube or hook connects the case of the aid to the ear mould.

They can incorporate a wide range of performance characteristics and circuitry such as programmable and digital. The behind-the-ear hearing aids are more expensive to buy and maintain than the body level hearing aids. They are small and thus easy to wear in children. They offer more flexibility with respect to adjustments in the output signal. As the microphone is at the level of the ear, these aids provide all advantages of binaural hearing when worn binaurally. Localization of sounds is easy compared to two pocket model hearing aids mainly due to position of the microphone. The figure shows the parts of a BTE aid.

Advantages:

•Cosmetically these are more popular due to the smaller size.

•Microphone placement in the hearing aids is at the ear level and hence resembles natural hearing to a great extent.• Disadvantages:

•BTE hearing aids are costlier than the pocket hearing aids.

•As compared to body worn aids, these aids are delicate and prone to early damage. Servicing and repairing can be quite costly. Sweat can be the main cause of problem in these instruments.

b] Intra-aural (In the ear) hearing aids:

The intra-aural hearing aids are of three main types - the in-the-ear, in-the-canal and completely-in-the-canal aids. These are very small hearing aids that are placed inside the ear pinna, inside the external auditory canal and deep inside the external auditory canal respectively. These in-the-ear type instruments are generally considered to be acoustically superior to the other types as the microphone is within the ear and the receiver is near the tympanic membrane. All the parts of these hearing aids are housed in a hard plastic casing called as the shell, which is made by taking the ear impression of the user.

These instruments are also called as custom instruments as they are housed within a custom made mould specifically for the user. These instruments are generally suitable for individuals with mild to severe hearing impairment. The following figures show the parts of an in-the-ear hearing aid and different types of in-the-ear aids.

Advantages:•Natural enhancement provided by pinna is utilized due to position of the microphone at the ear level.

•Cosmetically these are more appealing than the other hearing aids

Disadvantages:•Due to smaller and fewer user controls, fewer acoustical modifications are possible.

•These are not useful for average losses (500, 1k, 2k) beyond 70 dB, due to feedback problems

•They are expensive and not recommended for children because the entire hearing aid will need to be re-cased if the size and shape of the ear changes. In BTEs and pocket model aids, only ear moulds need to be changed.

•These aids are less durable and expensive to purchase and maintain.

B.] Mode of Operation:

Based on the mode of operation or presentation of the signal, hearing aids can be classified as air conduction aids and bone conduction aids. This refers to whether the amplified signal is delivered to the ear via the air conduction route or the bone conduction route.

1.] Air-Conduction hearing aids:

These are designed to convert the amplified electrical energy back to acoustical energy by an air conduction receiver and direct it through the ear canal via the ear mould.

These are designed to convert the amplified electrical energy into mechanical vibrations. The mechanical signal is then delivered to the ear through a bone vibrator that is placed behind the ear on the mastoid bone, bypassing the outer and middle ear to a great extent. It is mainly used in cases of congenital abnormalities of outer ear or in cases of chronic ear discharge. Bone conduction fitting is possible with body worn hearing aids and not with ear level hearing aids. In this case, the external air conduction receiver of a body level hearing aid is replaced by the bone conduction vibrator. The cord is attached to a headband and a vibrator. The following figure shows a body worn hearing aid attached to a bone conduction vibrator.  

Bone conduction hearing aid

C.] By the type of circuitry:

Hearing aids can also be classified based on the manner in which the signal processing is carried out by the circuit of the hearing aid.

1.] Analog hearing aids:

Analog hearing aids have a microphone that picks up continuously varying sounds in the environment and converts the sound into electrical signal and amplifies the electrical signal which is transduced via the receiver back to acoustic sounds to fit the needs of the hearing aid user.

2.] Programmable hearing aids:

These aids have a circuitry that functions in the same fashion as the analog circuitry; however the parameters of the signal are controlled digitally. These aids are more comfortable and compatible with the person’s hearing loss as they can be programmed as per the needs of the client.

3.] Digital hearing aids:

Fully digital hearing aids have the most advanced and most expensive type of circuitry called as Digital Signal Processing (DSP). These aids are nothing but wearable computers, in which the analog signal is converted into a digital signal. All functions of this aid are programmed by software. This can be fitted for all degrees of hearing losses.

D.] By the amount of maximum gain:

Gain refers to the amount of amplification provided by the hearing aid. Some hearing aids provide less amount of gain while some amplify sounds to a greater extent. There are international and national standards that classify hearing aids based on the amount of gain provided by the aid. Generally, the classification is as follows:

•Strong class – full on gain 65 dB

•Moderate class - full on gain 55 dB

•Mild class - full on gain 45 dB

E.] Number of ears amplified:

Based on the number of ears provided the hearing aid and the manner in which this amplification is provided hearing aids can be classified in three different ways.

1.] Monaural Fitting: When a hearing aid is used only in one ear the fitting is called as monaural fitting. This aid thus has one microphone, one amplifier, and one receiver.

2.] Binaural fitting: When amplification is provided to the two ears by way of two different hearing aids, it is referred to as a binaural fitting. This hearing aid thus has two different microphones, two amplifiers and two receivers. Binaural fitting helps a person to locate the sound and improves speech intelligibility in noisy environment. 3.] Pseudo binaural fitting: One body worn hearing aid fitted with V or Y cord gives amplification simultaneously to both the ears then this type of fitting is called pseudobinaural fitting. This type of fitting is still preferred in Indian scenario as it is economical compared to binaural fitting.

Tactile Aids

Tactile aids help those individuals with hearing impairment who get limited benefit from hearing aids. In this these devices, the signal are transmitted through the tactile mode to help those individuals who do not benefit from the auditory mode.

Tactile aids are of two types :

1. Vibro-tactile2. Electro-tactile

•A vibro tactile aid convert sounds (acoustical energy) to vibrations. •Electro-tactile devices convert acoustical signals in to mild electrical signals that cause a tingling sensation on the skin. •Both types of tactile devices have of a microphone that picks-up sounds and a processor that codes the signals picked up. •Vibro-tactile devices have one or more vibrators which transmit the coded signal to the skin through vibrations. •Electro-tactile devices have electrodes that transmit the signal to the skin through electrodes.• The vibrators/electrodes can be placed on various places on the body such as the wrist, fingers, arm, chest, abdomen, waist or thighs.

Stages in hearing aid selection for children:

1. First step is to quantify the child’s residual hearing and to determine candidacy for hearing aid fitting.

2. Before hearing aid trial, child should have custom earmoulds. Clinician can make parents aware about the various options available with respect to types of earmould, allow them to choose the appropriate one for their child.

3. During pre-selection of hearing aids, issues like type and style of aid, ears to be fitted and specific electroacoustic characteristics to be chosen are considered before further assessment.

Cleaning, Care and Maintains of Hearing Aid

1. Hearing Aid should be cleaned every day2. To clean hearing aids, use dry cloth/tissue or brush.3. Store hearing aids overnight in a clean, dry and safe place.4. Keep the hearing aid in the box.5. If possible place the hearing aid with the silicon bag to

remove moisture from the hearing aid.6. Try to avoid hearing aid from getting wet in rain, or during

washing face.7. Avoid keeping near electronic & hot places or magnetic

fields. E.g. On electronic equipments like Fridge, TV music system, Computers etc.

8. Try to avoid breakage or falling of hearing aid.

Classroom Strategies

General Strategies

If the student lip-reads :oHave students sit closer to the teacher.oSpeak slowly, naturally, & clearly.oDo not exaggerate your lip movements or shout.

Group amplification devices:

Imagine a typical classroom situation, where it is not possible for the teacher to be very close to each and every child at the same time or the classroom environment could have lot of reverberation. In such cases, in spite of using a hearing aid, the student with hearing impairment may face lot of difficulties in listening. Group amplification devices could be of great help to the listener in this situation as it enhances signal to noise ratio Group amplification systems generally consist of a mic, an amplifier and several receivers for delivering signals to individuals with hearing impairment. There are different types of group amplification devices commercially available such as hardwire system, induction loop system, FM system, and infrared system.

A.] Hard wire system:

These systems couple the sound source via a microphone, amplifier, and external receiver through the use of hard wire. The microphone is worn by the teacher in the neck loop. The system is fixed on the desk of the students in the classroom. Each child’s desk has a control box with volume control for both ears separately. It functions exactly like a hearing aid; the signal is picked up by the microphone worn by the teacher. It is amplified by the amplifier and delivered to the receiver or headphones worn by students which convert it back to acoustic stimuli. Student can adjust the intensity of the signal with volume control. The child cannot wear his individual hearing aid while using this system. 

Hard wire system

Advantages:

•It improves the signal to noise ratio as the microphone placement is nearer to the source.•It is cheaper compared to the other classroom amplification devices.•Student can adjust the gain of the signal as per his/her requirement.

Disadvantages:

•In the older version a microphone was worn only by teachers so conversation among students was difficult.•Head phones are bulky so students find it uncomfortable to wear it for a longer time. •Students’ mobility is severely restricted due to connected wires.

B.] Induction loop system:

This is one of the oldest and comparatively cheaper forms of assistive listening technology still popular in educational set ups. Main parts of this system are microphone, amplifier, a loop of wire and receiver. The input signal is picked up by a microphone and transmitted through the loop wire. This wire generates magnetic field, the strength of which is proportional to the input signal. This energy is picked by the individual hearing aids with an activated T coil. The received signal is further amplified by the amplifier of the hearing aid and then converted into sound signal.

Induction loop system

Advantages:

•This system can cover a large area, providing amplification for large groups of individuals with hearing impairment.•It can have an individual use too, with an induction loop worn around the neck by an individual listener.•Irrespective of the distance the user gets similar kind of input constantly.•Use of induction loop improves signal to noise ratio. •The child is not physically restrained and can move freely.•The system is used with the child’s own hearing aids, so the electro-acoustic characteristics selected for a particular child are maintained in the signal received in the input that the child receives.•The child can use the hearing aid on the MT setting, whereby the child can receive the teacher’s voice through the tele-coil and the other students’ voice through the microphone. This promotes interaction between the students. •It does not involve high cost for purchase as well as maintenance.

Disadvantages:

•Spill over: If there is more than one room looped in the same premises, magnetic field generated in one room could be picked up by a tele-coil in an adjacent room.

•Fluorescent lights, transformers, electric power wires in the area may cause interference in the induction system, may sound like a hum or distortion in the output.

•There could be dead spots in the room which means areas in the room where electromagnetic field does not reach.

•To get the optimum benefit of the induction loop, the tele-coil of the hearing aid of the user has to be efficient.

C.] Frequency modulated system:

This is a wireless system. FM amplification systems usually consist of a mic, a transmitter, and a receiver. In this sound is picked up by a microphone and transmitted in the form of radio frequency modulated carrier waves to a personal receiver worn by the listener with hearing impairment. The personal FM receiver worn by the listener must be set to the same channel and radiofrequency as the microphone and transmitter worn by the person speaking. The frequency of the FM signals has to be within a bandwidth specified by the National norms.

Frequency modulated system

Advantages:

•These can be used outdoors as well as indoors. Distance as much as 30 TO 200 sq feet can be easily covered by FM systems.•Clarity of signal is very good.•These systems being cordless do not restrict mobility of the speaker or the receiver.

Disadvantages:

•It can pick up the interference from signals passed along the same FM carrier waves.•FM systems are very expensive so though they are very effective hardly any schools for children with hearing impairment have installed it till date.•A personal FM system is coupled with a person’s own hearing aids. Sounds are channeled from hearing aid through the FM system making this system dependent on the integrity of hearing instrument function. Coupling options for personal FM amplification system include neck loop, silhouette and DAI. Unlike individual FM system sound field FM system allows everyone within the area such as a classroom setting to benefit from amplification.

D.] Infrared system:

These are wireless type of assistive technologies used to transmit signals by invisible light waves to the listener. The sound that is picked up by the microphone is converted into infrared light waves with the help of diodes which are spread across the room. The receiver worn by the user converts this signal back to the sound. The receiver also acts like an amplifier and can be adjusted by the listener as per his need. Any number of receivers can be used with the emitter (Crandell et al 1995).

Infrared System

Advantages:

•Infrared systems can cover large areas or also can be used as personal amplification systems.

•It can be used in theaters, auditoriums.

Disadvantages:

•A factor that can inhibit the performance of an infra red system is the amount of interference caused by lighting within the room as well as natural sunlight

•This can be used only in enclosed rooms.