Dr.George Thomas Kovai Hearing,Speech and Balance Centre
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Kovai Hearing Speech & Balance Centre,
7A, Artsan towers,
Ramanathapuram, ( South India )
Coimbatore - 641045
Telphone: 0422 - 4390909
Mobile:(o) 9994390909
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The reason no one wants to be seen wearing hearing aids is probably that they don’t work well, or at least they didn’t. When the problems with such hearing aids are solved (Digital aids – high Fidel –ity), a new / old problem arises are: Background noises are often blamed on the hearing aid. The real problem is that the user has lost ABONSO (automatic brain – operated noise suppressor option), and this problem persists until the user relearns how to recognize and localize background noises (at which time the brain automatically performs as a highly effective noise suppressor option).

Ongoing attempts to replace the brain with a tiny circuit that will somehow reject noises, we don’t want to hear are unlikely to result in useful devices.
 
Some of us must have taken the Hippocratic Oath; a vast majority of us might not even know who Hippocrates was (Hippocrates of cos 460 to 375 B.C. the Father of Medicine), a few of us will be aware of the first and foremost rule of the oath ‘’Never do Harm’’. This is entirely true when it comes to selecting and fitting candidates with hearing aids. Another important issue to consider, which is debated in the literature is the possibility of ‘’overuse, underuse, or misuse’’ of hearing aid amplification. There is a natural tendency when new technology becomes commercially available for each of this misapplication to occur.

Despite some misconception, it has never been the case in the provision of amplification that any hearing aid is better than none. There is great potential to do harm with an aid that over amplifies or under amplifies sounds for a child. In the former scenario, the child’s residual hearing may be damaged. In the latter, wearing aids with too little gain may quickly convince children that hearing aids are of little use to them.

Finally a word of caution, the awareness of certain conditions like Auditory neuropathy etc. is of paramount importance since in advent fitting of hearing aids in the special group of patients will lead to total destruction of their hearing capabilities leading to permanent handicap and disability. Diagnoses of this condition in the hearing impaired is imperative, with the help of specialized testing to assess the auditory pathway status. This condition should be ruled out before amplification procedures are implemented; otherwise you will have in your hands a case of iatrogenic induced profound hearing loss. Recapitulating the earlier mentioned point ‘’NEVER DO HARM ‘’.

The prevalence of hearing loss is increasing in the population at large. There is growing concern about meeting medical, audio logical, emotional, financial, phychological and social needs of these clients. Also a lack of understanding of hearing impairment and it is true all over the world. As hearing loss is an invisible handicap. People have been finding their own solutions to combat this problem and ‘’hand behind the ear ‘’ may be considered as first of its kind of solution. Hearing aid development has matured since then.

Decreasing size has been a constant trend during the history in the development of the hearing aid. This history can be divided into five eras : acoustic, carbon, vacuum, transistor and digital. The last of these eras, which we are just entering, promises to hold advances at least as significant as the eras that preceded it.
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All hearing aids operate upon similar principles. A simple block diagram is shown below.

A microphone converts mechanical acoustic energy into electric current. The amplifier in the hearing aid increases the amplitude in the electrical current, which corresponds to an increase in the intensity of the sound energy. The receiver or ear phone converts the amplified electrical signal into acoustic energy and delivers it to the ear via an ear mould.

Hearing aids may be worn on the body, whereby the microphone and amplified are worn on the chest and a separate receiver is worn at ear level. Ear-level aids can behind-the-ear (BTE), in-the-ear (ITE) or in-the-canal (ITC).
 
The main purpose of a hearing aid is to amplify speech so that it is audible and comfortable to listen to. Audibility is critical- speech that cannot be heard cannot be understood – but it is not everything. Audibility is not perfectly correlated with intelligibility. There is little point amplifying sound to the point of discomfort because it will inevitably lead to aid rejection or a reduced volume setting. It has to be noted that a hearing impairment increases the contribution made by a given amount of audibility decreases. Increasing high frequencies may even result in reduced speech recognition. What appears to be critical is the balance between low-frequency and high-frequency amplification and the contribution of the overall loudness and comfort associated with aided listening.
 
Fitting hearing aids involves a careful process of selecting and adapting a particular hearing aid to an individual. A first step in understanding this process involves defining hearing aid performance; that is establishing how much amplification a hearing aid provides. The characteristics of hearing aid are described using the terms gain, frequency response, saturation sound pressure level (SSPL) and distortion.

A brief description of each of these electro acoustic hearing aid characteristics:
 
GAIN: it is the difference in decibles between the input signal and the output signal and can be calculated at individual frequencies.

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FREQUENCY RESPOSE : Hearing aid frequency response determined by measuring hearing aid gain across a wide range of frequencies. All sound systems are limited in the range of frequencies that they can amplify and typically hearing aids amplify across the range of 200 to 6000 Hz. (speech range).

SATURATION SOUND PRESSURE LEVEL: The saturation sound pressure level, also know as the Maximum power Output and the SSPL 90, is the greatest sound pressure that can be produced by a hearing aid. It is measured with the aid on full volume using an input signal of 90 dB SPL.

DISTORTION: Distortion occurred in a hearing aid when the sound entering the hearing aid. The addition of distortion to the amplified sound may reduce speech intelligibility for the hearing –impaired listener. Harmonic distortion occurs when new frequencies are generated in the amplifier of the fundamental frequency input signal. Harmonic distortion increases with increase in volume setting making it undesirable to fit an aid that is to be worn near maximum.

MEASURING HEARING AID PERFORMANCE: The electro acoustic characteristics of an individual hearing aid can be measured by attaching the hearing aid to a 2cc coupler to simulate some of the properties of an average adult ear. They are critical for quality control to ensure that individual hearing aids match up to manufacture’s specification. It also provides a valuable starting point for hearing aid selection for a hearing –impaired individual.
The fallacies associated with the above mentioned method have been overcome with the recent development of equipment to measure hearing aid performance via a microphone placed near the tympanic membrane and represents a resolution in hearing aid fitting –REAL –EARPROBE MICROPHONE MEASUREMENTS. Using this equipment it is possible to measure thereal-ear gain, output, SSPL and harmonic distortion of a hearing aid while it is worn by a hearing impaired child or adult. The procedure is queck, objective, accuaate and reasonable non-invasive. Hearing aids behave quite differently in the real ear than in the 2cc coupler. Real-ear probe microphone measurement offers a very reliable tool for verifying hearing aid performance in the real ear but real - ear measure how the hearing aid amplifies sound in an individual ear, thus reflecting the acoustic characteristics of that ear.

Digital - The New World : Truly digital hearing aids may offer the clinicican and the hearing impaired listener yet another level of flexibility and sound quality. These are a relatively recent development. Digital aids may offer children additional benefits in terms of acoustic feedback control, frequency transposition and beam forming microphone arrays. However, these aids will come at a cost. It is critical that well designed research is undertaken to evaluate the benefits of digital aid technology for children, particularly those with serve to profound hearing impairment.

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It is interesting to note that 96% of user satisfaction with hearing instruments can be explained by four criteria, 1) Improvement in hearing and value, 2) Reliability, 3) Sound quality, 4) Ability to hear in multiple environments.
Those who believe sise is the ultimate criterion, need to see that there is a further more important factor in fitting of hearing aid that is “hearing performance”. This break through in hearing performance has been achived with digntal technology

Didital hearing aids promise many advantages over completely analog (old) hearing aids. Potential features include 1) programmability 2) greater precision in adjusting electro acoustic parameters 3) Self monitoring capabilities 4) control of acoustic feedback 5) Noise reduction 6) Automatic control of signal levels 7)Self adaptive adjustment to changing acoustic environments. These features provide more flexibility to professionals in evaluation and fitting of hearing aids. These instruments allow true in situ circuit selection of various performance characteristics, such as, gain, saturation output, low frequency adjestements etc. Leading to more individualized fitting, especially for medically fluctuating losses, skislope hearing loss and for reduced dynamic range cases.

It has been well documented that persons with conchlear hearing impairnments generally have mpre difficulty in discriminating in noisy background, than normal hearing persons. This difference appears to be due to a greater susceptibility to masking than for individuals with normal hearing and also due to abnormal widening of the auditory filters of pathological ears. This altered physiology in the impaired leads to difficulty in processing information in the presence of background noise. Hearing aids in wide spread use today generally have a major drawback, that is, significant amounts of undesirable environmental noises are amplified together with the derived stimuli.

Digital signal processing has solved this difficulty of signal to noise processing by way of suppression of noise. Digital hearing instruments have also taken into account reduced dynamic range which is by and large a characteristic feature of cochlear hearing loss. Digital hearing aids function in a comfort zone of every individualwhich means no over amplification, no distortion and most important of all less fatigue to the users. After all what is a dream of a hearing impaired person? “ Clarity of speech in noise, no built – in noise from the instrument and no stress during listening”. The only answer certainly is “Digital Way to Hear”.

The high technology era has just begun. It carries the powerful potential of delivering the performance hearig impaired users crave for. It is upto competent qualified professionals who have an in-depth knowledge of the subject to put this technology to work and gain patient satisfaction.
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