Digital Hearing Aids
Sample (i.e. analyse ) incoming speech sounds as well as the ambient noise and then electronically processes the incoming sound which consists of speech and ambient noise and then finally deliver the amplified sound to the user’s (i.e. patient’s) ear such that the user hears the desired sound (i.e.,speech also called ‘signal’) most comfortably and clearly.
The analogue amplifier works with transistors, which amplify the sound and capacitors and resistors, which determine the filter and compressor characteristics. Even a slight distortion of the electrical current running through the analogue amplifier (which is quite frequent due to minor changes in the electronics of the capacitors and transistors) is heard as distortion of the amplified sound. Noise created by the electronics of the analogue hearing aid (caused mainly by the random movements of the electrons in the resistors) is heard as noise and it directly affects the incoming sound signal. This is the limitation of analogue circuits. In digital signal processing (technically known as DSP), the sound parameters are encoded into a sequence of numbers. Digital processing is done by calculations on the sequence of numbers that are passing through the processor. A 10 times amplification is achieved through multiplication of each number in the sequence by a factor of 10 before re-conversion. Hence distortion is much much lesser in digital hearing aids compared to analogue hearing aids
Filtering to cut-off unwanted sounds is achieved through other calculations. In the conversion, a binary number system is applied, creating a stream of bits. This stream runs through the processor, which makes calculations and modifies it accordingly. The net result of these manipulations emerges after re-conversion in a Digital-to-Analogue converter and presents an analogue signal to the receiver. The receiver finally transforms the signal into sound. This is a very brief idea of how the analogue and digital hearing aids work. If you are not interested in all these technicalities, do not break your head trying to understand it; you will only get more confused. Just know that digital hearing aids are much better than the conventional analogue hearing aids as they offer clearer and distortion-free sound output and the electronics is less prone to defects. Moreover the amplified sound can be customised according to the patient’s specific hearing loss and according to the acoustic ambience (of the patient’s environment) much more in digital hearing aids as compared to analogue hearing aids. Hence now-a days analogue hearing aids are being phased out and today practically nobody buys analogue hearing aids. If you want to know more about this you may read up the book CONTEMPORARY HEARING AIDS AND RELATED LISTENING DEVICES authored by Dr. Anirban Biswas and available from Bhalani Medical Book House, Mumbai e-mail firstname.lastname@example.org)
Digital hearing aids give us the ultimate in hearing aid technology that is available today featuring:
1. Programmability i.e. setting amplification parameters according to your individual requirements. An example will clarify this. If you have a deafness for high frequency sounds only, you will be able to hear that people are talking to you but you will not be able to make out what they are saying. This happens because the consonant sounds that give us the intelligibility of speech are mostly high frequency sounds (from 2000 to 8000Hz) ; so if you have selective high frequency deafness, you will only be able to hear the loud vowel sounds and miss the faint consonant sounds. If you use a conventional hearing aid that cannot be programmed in accordance to this hearing loss, the low frequency (vowel) sounds and the background noise (which mainly consists of low frequency sounds) will get over-amplified (i.e. will become very loud) and mask the faint consonant sounds and thereby further reduce your speech intelligibility. You will hear the sounds very loudly but not understand what is being spoken.
In multichannel digital hearing aids, by proper programming, only those high frequency sounds where you have the deafness can be selectively amplified without amplifying the low frequency sounds. This increases your understanding of speech i.e. enhances speech intelligibility. Digital hearing aid technology provides us the opportunity for finer control over a wide range of acoustic parameters and hence has more potential for accurate and very precise fitting to the individual hearing loss.
2. Digital quality sound processing with very clear distortion-free sound.
3. Monitoring the background noise and amplifying the speech sounds accordingly. The sophisticated digital hearing aids have sensors that sample i.e. analyse the background noise thousands of time every second and try to amplify the speech sounds in such a way that the background noise is much less disturbing and speech more clearer to the user. The high end sophisticated digital hearing aids automatically sense the acoustic environment and change the hearing aid’s sound output spontaneously and instantly according to the changes in the acoustic environment. They are smarter than the Multi-program hearing aids as the user does not have to operate a toggle switch when he goes from one acoustic environment to another.
4. Fully automatic control – for comfortable, natural loudness.
5. Sophisticated signal processing for speech enhancement and reduction of background noise .
6. Digital Processor making several thousand calculations per second for producing the exact quality and intensity of the sound output which is best suited for that particular user at the particular listening environment in which the user is placed. The nature and extent of your hearing loss as obtained from the audiological tests, your hearing preferences and the nature of the ambient i.e. environmental noise are all taken into account by the microprocessor chip of your hearing aid to calculate the sound output which will be best suited for you. If the digital hearing aid is programed properly (which of course is mandatory for the success of the aid) it will produce one type of amplified sound if the user is in a traffic crossing and another type of sound output when the user is placed in a concert hall or in a quiet bedroom. If two different users having two different types of hearing losses are placed in the same listening environment, the same digital hearing aids will produce different types of sound output for the two users, if of course the aid is programed differently according to the hearing loss of the two different users. This does not happen with the non-digital conventional hearing aids.
7. Some of the digital hearing aids are equipped with fine-scale noise canceller which continually monitors the speech to noise ratio (SNR) in multiple bands where noise is interfering with speech. It then intelligently adjusts gain in each band separately according to the intensity of the ambient noise and produces a sound output where background noise is very minimum or even absent.
And all these advanced functions can be carried out in a very small piece of hardware that can even be hidden completely in your ear canal. A microchip more efficient than the processor of a Pentium computer does numerous calculations very very fast and ultimately produces a sound which is not only very close to the original sound but also most suited for your type of hearing loss. Modern digital hearing aids are a marvel of modern science.
A digital hearing aid is the right choice provided you can afford it. The next choice should be a very well- programmed analogue hearing aid followed by the conventional non-programmable analogue hearing aids. However these are also being phased out now and possibly in the very near future only the completely digital hearing aids will be available. The term “programmable” means that the amplification characteristics can be adjusted according to the hearing loss of the person for different types of sound. If this adjustment can be done through the computer, the hearing aid is called “digitally programmable”. In 100% digital hearing aids, the amplification can be programmed through a computer and the amplification of sound is done by a digital amplifier, whereas in the digitally programmable analogue hearing aids, the amplification can be programmed by the computer but the sound is amplified by an analogue amplifier.
However, if you have a profound i.e. near total deafness with low speech discrimination scores, the sophisticated high-priced digital hearing aids may not be the correct choice for you as you will not be able to utilise the advantages of the digital hearing aids that you are paying for. A high powered (technically termed as strong-class) conventional hearing aid will be adequate to provide the limited benefit that you will derive from the hearing aid. An honest and ethical hearing aid dispenser will be the right person to guide you on this.
FAQ on Hearing Aids:-
1. Will my hearing worsen if I use the hearing aid(s)?
No, not at all. If your hearing aid has been properly selected by a responsible and knowledgeable person, taking into account all details of your audiogram (results of the hearing test done before dispensing of the aid), there will be no harm whatsoever to your hearing.
2. Will using the hearing aid stop any further deterioration of my hearing?
No, If you are having a progressive deafness, the hearing aid cannot stop or check the deterioration of your hearing. The hearing aid is just an amplifier and it can never alter or influence the disease process. The disease usually follows its own course and cannot be either stopped or increased by the hearing aid. If due to the progress of disease, the hearing deteriorates, the amplification parameters of the hearing aid can be adjusted (at least partially) to suit your requirements of increased amplification. This facility is better with programmable hearing aids.
3. Will the tinnitus in my ear stop if I use a hearing aid?
No, The hearing aid cannot cure tinnitus, However if you use a hearing aid, the ambient noise (i.e. the background noise) gets amplified and this partially masks the tinnitus but do not have any misconceptions that your tinnitus (if any) will be cured by the hearing aid.
4. Will I become dependent on the hearing aid if I use one and not be able to hear at all without the aid later on?
The hearing aid will definitely improve your hearing and it is desirable that you use the hearing aid always. However when you want to hear without the hearing aid, your hearing will be exactly as before; using the hearing aid will not worsen your natural hearing at all.
5. Will a costlier hearing aid give me better hearing?
Not always. You will sometimes find that your are doing quite well even with comparatively cheap hearing aids. However as already discussed, the digital hearing aids are expected to give you clearer sound. The benefit you derive from the hearing aid depends primarily on the how perfectly the selection and programming of the hearing aid has been done, not on the cost only. Even if you buy the very costly multichannel digital hearing aids (that cost 60-70 thousand rupees), your hearing will not be satisfactory if the hearing aid has not been programmed properly. The insight, dedication and experience of the person dispensing the aid matters a lot. Do not go by cost alone and do not have any misconceptions that a costlier hearing aid will suit you better.
6. I have giddiness also along with the deafness, will my giddiness be corrected if I use a hearing aid?
No, The hearing aid has nothing to do with your balance system.
7. I have discharge in my ears. How will I use the hearing aid?
You may use either a spectacle bone conduction hearing aid or treat the ear discharge by medical/ surgical methods and then use a hearing aid. It is always advisable not to use a hearing aid (other than the spectacle bone conduction hearing aids) while the ear is discharging. As long as there is no active discharge from the ears, any type of hearing aid may be used.
8. A multiband 100% digital hearing aid costs nearly ten times the price of a non-programmable analogue hearing aid. Will I hear ten times better with such a sophisticated aid as compared to the conventional ones?
No, you are not expected to hear 10 times better just because the aid costs 10 times more. Your hearing may improve by (say) 20-25% at the most. But for many people, especially lawyers, musicians, stenographers, doctors, actors etc this 20-25% improvement of hearing makes all the difference. If your hearing is very important for you, and if of course you afford the money, it is always better to have the high-end sophisticated hearing aids. While selecting the hearing aid, the dispenser takes into account the hearing loss, the life-style of the patient and also the affordability of the patient.
9. Where should I buy the hearing aid from?
The hearing aid has to be DISPENSED and not merely sold. The reason why most hearing aid users are dissatisfied with their hearing aids is that their hearing aids had not been selected and programmed properly. For proper selection of the hearing aid, a lot of audiological and medical tests need to be done by the audiologists and ENT specialists. These include clinical examination of the ears, audiometry test including air-cond, bone cond audiometry, speech audiometry etc. While dispensing the hearing aid, unaided and aided trials (i.e. repeated audiometry tests in a free field setup with different types of hearing aids), speech discrimination scores, programming of the hearing aid through the computer etc etc needs to be done for perfect selection of the hearing aid. This is a team work and is a very time consuming process (usually 1½ to 2 hours –sometimes even more). It requires the services of a very dedicated and committed programmer who is determined to give you the best. There is no short-cut to this. Be aware of shopkeepers whose only consideration while selecting the hearing aid is your purse. Though 5 or 10% concession is often offered by some shops or the so-called “hearing-aid camps”, to lure prospective buyers, a very perfect selection is usually not possible in such places and the 90% to 95% of the MRP that you pay may be totally wasted if your selection of the aid is done hurriedly or by incompetent persons with no medical ethics. Look for a person with integrity to dispense the hearing aid to you. A lot depends upon the knowledge, commitment and honesty of the person dispensing the hearing aid. Buying the hearing aid is like making a long term investment. It is the beginning of a relationship with the dispenser and this relationship will continue for years to come. Buy the hearing aid only from a decent person/set-up who appears reliable to you, does less of sales-talks and really cares for your hearing. Going back repeatedly to a repulsive character or a shady dealer is like making a bad investment and then running after the company to get back your dues. Choose the hearing aid dispenser properly. This is quite a difficult job as hearing aids are not uncommonly sold by “not the right people”. If you have been successful in selecting the right person to dispense the aid, the dispenser will select the right hearing aid for you. Selecting the proper dispenser is probably more difficult than selecting the hearing aid.
10. Is it better to use a digital hearing aid?
Yes, the digital hearing aids offer a lot of advantages which are discussed in the previous pages. The digital aids are smaller in size and a lot of electronic jugglery can be packed in a much smaller space. Hence even in very small CICs and BTEs a lot of special features and higher amplification can be obtained in hearing aids using digital technology. Even if the hearing profile changes drastically later on, there is much more scope of adjusting the hearing aid according to the changed hearing loss. Understanding of speech is much better with digital aids as many of the digital aids contain separate speech processors for different frequency bands by virtue of which the faint consonant sounds (which provide the intelligibility of speech) can be enhanced over the loud vowels. Feedback in hearing aids that is a annoying whistling sound can be selectively filtered off in digital aids. Feed -back manager is a special feature in most digital hearing aids. Most of the digital hearing aids can sense the environmental (i.e the ambient) noise and automatically adjust the volume so that you do not have to fiddle with the volume control every now and then. Some of these aids have special features like selective programmes for different hearing situations like heaaring a concert, hearing in traffic, hearing in closed spaces etc.
11. What are the common problems one encounters while using the hearing aid?
Some minor problems are not unexpected since you are now hearing sounds artificially amplified. These usually pass off once you get used to the hearing aid. The common problems are :-
(a) It may appear to you that your ear is blocked and you are hearing inside a barrel,
(b) sudden loud sounds may be very disturbing and annoying in the initial stages
(c) the background noise in the room i.e. the ambient room noise may appear disturbing and
(d) the speech sounds especially the vowel sounds may appear loud in contrast to the faint consonant sounds
(e) your own voice may appear a little different.
Most of these problems can (at least partially) be corrected by your dispenser by making a vent in the ear mould, re-programming the hearing and other methods but to get the best out of your hearing aid and to override these teething troubles, you must also be sufficiently motivated and try to adapt to this new form of hearing. Moreover, with time as you get acclimatised to the hearing aid, these minor problems disappear.
12. Will I hear normally as soon as I start using the hearing aid ?
No, it will take sometime. If you are sufficiently motivated you will achieve this faster. In the initial stages, the sound usually appears a little different on using the hearing aid. This happens because your brain has come to accept your impaired hearing as normal and anything else as abnormal. Moreover it becomes a little difficult for the brain to identify new amplified sounds through the hearing aid which you had not been hearing for some years. It is believed (but not fully proved) that the brain forgets how to process sounds that it had not heard for sometime. So when the hearing aid re-introduces these sounds to the brain , the brain being unable to recognize it, perceives it as noise. In about 6 weeks time , the brain re-learns these sounds and can then interpret these long-forgotten sounds provided you give some effort in getting used to this new form of hearing. So the first few weeks are a little frustrating to the new hearing aid user. You must be able to bear with this.
A write-up that I had presented in a forum of ENT surgeons on requirement of hearing aids in a patient with bilateral mild to moderate SENSORINEURAL hearing loss limited ONLY to the high frequencies:-
May I have the audacity to offer my views on the necessity of hearing aids in the type of hearing loss being discussed i.e., a bilateral mild or mild to moderate hearing loss limited to the high frequencies only. I use the term ‘audacity’ as it contradicts the opinion of most if not all the learned clinicians who have voiced their opinion on this vexing and controversial issue including one of my mentors and idols Prof S K Kackar. This is a forum where everybody has a freedom to offer his views and not be ridiculed and also learn from others and it is only on this premise that I am penning my experience and the little that I have learnt on it over the years. Please point out if I am wrong in my logic and in my understanding of this issue. I have been asked about this in many forums and I need to have a very clear understanding on this so that I can give the right advice to my patients and also to many juniors who approach me with these problems. Hence I need to be corrected if my understanding is incorrect.
1. For proper hearing i.e., for proper understanding of speech which requires perfect central auditory processing that is vital for our understanding of speech and hearing in challenging acoustic environments ( like a noisy classroom or in the AGMs of our AOI especially the most recent one), we need to hear NORMALLY across the entire speech frequency spectrum and that too EQUALLY in both ears. Hearing can NEVER be normal even if there is a partial hearing loss however mild in one or both ears
2. It has been proved beyond doubt that central auditory processing is best when there is equal input from both ears in all the frequencies. Even if in one particular octave there is a difference in the hearing threshold level between the two ears the central auditory processing of sound cues with that particular frequency will be comparatively poor. If central auditory processing is poor, the subject will have poor speech understanding in challenging acoustic environments.
3. Though a mild or mild to moderate hearing loss limited to the higher frequencies only with normal hearing in the low and middle frequencies is usually passed off as NEAR NORMAL HEARING and clinicians often ask such patients to refrain from using hearing aids, yet even a mild deafness in the high frequencies is far from normal and the subject with such hearing deficit is bound to have problems in understanding speech when he is in difficult hearing situations and in challenging acoustic environments even though he may not want to accept or realise it.
4. A person with a mild high frequency hearing loss only in the high frequencies of even about 40 dB has a significant problem in understanding speech as he misses the high frequency consonant sounds not only because he has a mild hearing loss in the high frequencies but also because the high frequency consonant sounds are spoken softly. In normal human speech, high freq consonant sounds are pronounced about 30dB softer (i.e., lower in intensity) than the low frequency vowel sounds. Hence the handicap of a person having a 40dB high freq hearing loss is not just a hearing loss of 40dB but much more (say 40+25dB as the high freq consonant sounds which provide intelligibility to the speech are pronounced at much lesser intensity). So the handicap of a person with a 40dB high frequency hearing loss is actually much more than 40dB.
5. When a person has a high freq hearing loss even if it is mild, there is usually a concomitant cochlear outer hair cell damage which in turn jeopardizes cochlear processing of sounds of those particular frequencies. Cochlear processing involves selective amplification, frequency resolution, temporal resolution, temporal summation etc etc which are very important requirements for clear hearing and speech understanding. The function of the cochlea is not mere amplification of the sound but much much more than that. The cochlea processes and cleans the sound so that a very clean image of the sound in the form of action potentials reaches the central auditory system for more efficient processing in the brain. When there is even a minor localized damage of the cochlea, this function is lost for those frequencies. Selective amplification which is a specific cochlear function means higher gain (amplification) for soft sounds (say input sounds of 30dB or lower) but lower gain (very slight or no amplification) of loud sounds (say input sounds of 80dB). Without this, recruitment will take place and hearing will become uncomfortable. Similarly, frequency resolution and temporal resolution will also be improper for those frequencies leading to poor speech understanding. These problems will be there but the subject may not realize it adequately but is definitely handicapped especially in difficult hearing situations. By doing an audiometry test we are measuring only the hearing threshold for those frequencies, we are not testing these specialized cochlear functions and so the handicap appears lesser to us on paper but the actual handicap is much more than what is revealed by the audiometric thresholds. A sophisticated digital hearing aid if programmed perfectly can mimic many of the cochlear functions and provide near natural and comfortable hearing in these types of so called MILD hearing loss.
6. Another important aspect is there regarding central auditory processing. The brain is tonotopic as regards processing of the different frequencies. If the brain is deprived of certain frequencies even partially, the functionality of those areas is reduced or is replaced by other functions. Stimulating these areas later on by amplification is not expected to yield the desired results and central processing will be poor if the amplification is provided late. Hence amplification should be provided as soon as the hearing loss is detected. And what is most important in such cases where there is just a minor cochlear damage limited to just a few frequencies, is that the hearing handicap may not be very perceptible and so the patient is not so much bothered and so ignores it, even though he is bound to have a problem but dysfunction of central auditory processing induced by the hearing impairment will have permanent and more serious handicapping effects with deleterious results that cannot be corrected later on even by providing amplification.
7. A study about which I had read somewhere (I do not subscribe to audiological journals but do happen to get a free copy from here and there sometime which I take care to read but do not retain due to lack of space at home and also because I did not have to pay for it) had carried out speech discrimination scores (SDS) in two groups of patients having a similar mild high frequency hearing loss, five years after the hearing loss had been audiometrically detected. The patients who had been fitted with a hearing aid immediately on detection of the hearing loss had better SDS after 5 years than the patients who had not been fitted the hearing aids even though the hearing thresholds had not deteriorated in five years. Other studies have suggested that perfectly programed suitable digital hearing aids gives better SDS scores when fitted early proving that central auditory processing deteriorates if the deafness, however mild, is not corrected early.
8. Lastly, from personal experience I can tell you that in quite a number of such high profile patients having just a mild or mild to moderate hearing loss limited to the high frequencies only whom I had advised to use high end digital hearing aids perfectly programmed and customized according to their requirements, each and every one of them has immensely benefited and have complimented me saying that they now have a much better quality of life and better quality of hearing. This is all the more true for those in the legal profession esp. judges and lawyers (I have a fair number of them as my clients /patients) as they work in acoustically challenged noisy environments do not afford to miss a single word. Just about 10 days back a VVIP judge had come to me with just a heaviness and blockage in his ears and occasionally missing some words of some old lawyers. A PTA showed a mild to moderate SN deafness above 4000Hz (40 to 50dB)in both ears with perfectly hearing (below 30dB) in the low frequencies, tympanometry was normal. He is extremely happy and has complimented me times without number. I myself was so very happy seeing him so satisfied that I was wondering why had it not been done earlier. Whenever I have met him or talked to him on phone ; he always compliments me for fitting him with the hearing aid and always mentions that it has given him a better quality of life both professionally as well as socially. These high profile persons who can afford the special high quality (and pricey) hearing aids that have facilities for selective high frequency amplification and inbuilt special features that can mimic cochlear function, should definitely be fitted a proper hearing aid(s) but it MUST be done by a very very competent person in perfect hearing aid fitting setups with real ear measurement devices and other requirements which is unfortunately not the case in most set-ups or so called clinics that dispense (rather sell) hearing aids.
I understand that I have made this very very long and few will have the patience to go through it but if anybody reads it and sends me a feedback I will be grateful.
Dr ANIRBAN BISWAS
Vertigo and Deafness Clinic
BJ-252, Salt Lake, Kolkata – 91