Type of hearing aids:   

There are four common types of hearing aids, which differ based on size, and location on the patient.  In general, the smaller the hearing aid, the less electronic flexebility, the smaller and fewer the user controls, the fewer the possible acoustical modifications, and smaller the battery.  However, smaller aids do offer a more natural sound because the acoustic properties of the pinna and in some cases the canal is preserved.  In addition, smaller aids have higher cosmetic appeal.

Body level hearing aids(BLHA): Is the largest of the hearing aids.  This device is housed in a case, which is accommodated in one’s shirt pocket or a specially stitched pouch, and contains a microphone, an amplifier, and a battery.  This is connected to a receiver through a cord.  The receiver is fitted snugly into one’s ear canal using either custom built or standard ear molds.  The advantages of BLHA is that it is cheap, rugged and easily serviced.  The maintenance is also cheap since it uses pen torch batteries that is easily available.  It is powerful and can therefore be prescribed for persons with profound hearing impairment.  A single device with a ‘V’ cord can be used for a binaural fitting.  Hence these type of aids are prescribed for the economically backward and in case of children.  The disadvantages are that it is cosmetically unappealing and is not preferred by the more sophisticated persons.

Behind the ear (BTE) hearing aids:  This device has a module that fits behind the ear, which contains a microphone, an amplifier, a battery, and a receiver.  The microphone connects connects to an earmold in the external auditory meatus via a piece of clear tubing that travels over the top of the post auricular crease.  This device is more cosmetically appealing and is the most popular.  These devices are rugged and easily serviced.

In-the-ear(ITE) hearing aids: This device sits in the concha or the the shallow depression at the entrance of the External auditory canal(EAC) and extends into the EAC.  Most ITE aids are custom devices, with the components built into a shell made from an impression of the user’s ear.  They are more expensive than the BTE aids but having the same cosmetic value as that of BTE. 

In-the-canal(ITC) hearing aids: It is similar to the ITE device, except the components are small enough to fit entirely within the EAC. ITC aids are custom molded and have high cosmetic appeal.  They also have acoustic advantages such as reduced feedback when using the telephone.  The microphone location within the ear reduces wind noise interference and enhances high frequency sounds in the 2-4-kHz region.

As technology advances, hearing aids can be packaged into increasingly smaller housings.  Devices that fit entirely within the bony canal (the deeper portion of the EAC) have been developed, called completely-in-the-canal (CIC).  These aids have high cosmetic appeal because they are virtually undetectable. They also have several acoustic advantages.  These include reduced occlusion effect, reduced gain requirements, and preservation of the natural acoustic properties of the pinna and external ear.  Because of occlusion of the lateral portion of the EAC, amplification of low frequency vibrations in the patient’s voice produces an echoing, hollow sound. This is called occlusion effect and occurs with traditional hearing aids.  Because the volume of air between the hearing  aid and the ear drum  is reduced CIC devices have reduced gain requirements.  This allows thehearing aid to operate with lowerr acoustic power, resulting in reduced distortion and improved sound quality.  The CIC wearer has better understanding of speech in noisy conditions and localization of sounds is enhanced.

More specialized types of hearing aids also exist.  The CROS (contralateral routing of signal) devices uses a microphone on the worse hearing side to transmit sounds to a receiver at the better hearing ear.  Transmission occurs via a wire that runs around the back of the neck, or in a wireless mode by radio frequency.  In this way, the wearer can use the good ear to hear sounds from the impaired ear.  The CROS unit may benefit those with no usable hearing  in one ear and minimal hearing loss or normal hearing in the other ear.  For example, a taxi driver  with an impaired left ear may benefit from the CROS unit, since the left ear is closer to the passengers.

Until recently all contemporary hearing aids were utilizing analog circuitry.  The disadvantage with these types of analog devices is that the electroacoustic characteristics of the aids could not be modified satisfactorily to suit individual patient requirements.  This resulted in a lot of patient dissatisfaction and poor usage of these devices.  The revolutionary breakthrough in hearing aid technology occurred when programmable or digital circuitry was used in hearing devices.

Programmable hearing aids have analog type amplifiers and filters, which are controlled, by an external digital source.  These hearing aids contain a memory module such as a CMOS (complimentary metal oxide semiconductor), RAM (random access memory), or EEPROM (electrically erasable programmable read only memory).  An external microprocessor (computer) accesses the memory locations within the chip to modify the hearing aid’s electroacoustical performances.  The memory module provides more precise control of the acoustic characteristics.

In entirely digital hearing aids, the input signal is digitized, then processed with digital signal processing circuitry.  These hearing aids are fitted using software packages, and can be programmed to make changes in electroacoustic performance based on the input signal.  Digital and programmable hearing aids offer the potential for better fitting to the individual hearing loss.  Adjustments to suit individual requirements are to be made by the dispenser using a programming unit provided by the manufacturer.

Experts say that millions of urban Indians between the ages of 45 and 60 have some degree of hearing impairment.  Not only does this hearing loss comes with aging but today’s urban youth are suffering such impairment much earlier than their parents did thanks to noisier surroundings.  Too many people do not realize that hearing loss has serious consequences. A recent survey by the National Council of Aging, U.S.A., of hearing impaired individuals found those without hearing aids were more likely to be depressed, anxious and isolated from society.  Today’s fast paced lifestyle demands a keen perception of hearing. Thanks to the booming hearing aid technology the user has a wide variety of hearing aid models to suit most patient requirements.  The key to a good hearing aid is testing by a professional to uncover the type of hearing loss, and then getting an appropriate hearing aid fitted properly. If it’s not fit properly, the best hearing aid will not function.  It is therefore not advisable to get your relatives to get you a hearing aid from abroad or to mail order a hearing aid responding to an advertisement appearing either on the TV or a magazine.