Archive for the 'Instrumentation' Category
March 29, 2008
For past blogs I have always tried to find some sort of quirky, eye catching title. Once again though ear moulds remain unglamorous, unloved, unappreciated and strictly utilitarian. These days though they are quite high tech and vital to successful operation of modern air conduction aids, particularly at the high power end of the range. Individually fitted moulds are a fairly recent innovation, being developed in the mid 1900’s by adopting dental methods and materials, both for impression taking and construction. They are a far cry from the early injection moulded plastic, rubber tipped, cones that came in “small, medium & large” only. These had a poor seal, worked lose continuously, and fell out at the most inconvenient moments.
Modern moulds have to be a good fit into the ear canal and remain in position under difficult circumstances. The ear canal is very close indeed to the jaw’s “hinge” mechanism and is continuously flexed by the jaw and its muscles during speech and eating. The outer ear and canal together form a complex three dimensional shape, and making a nearly exact matching shape that can be inserted and removed relatively easily is challenging. All ears are different and insertion can be difficult, particularly with a new or especially deep mould. The action is a bit like a bayonet light bulb fitting, but with four stages instead of the bulb’s simple two stage – push then turn.
A poorly designed mould (or one that is not quite inserted correctly) can quickly cause extremely painful pressure sores. These form by the same mechanism as bed sores and are caused by continuous pressure at one localised point. They can be initiated inadvertently by many very minor changes, but are easily corrected once the cause is found.
With ‘behind the ear instruments’ amplified sound is channelled through the solid mould by a replaceable plastic tube. This passes through a duct drilled during the later stages of production. ‘In the ear’ aids are more complex, the moulds being produced as a hollow shell into which the entire instrument is assembled.
One very well known drawback of hearing aids is feedback. Also called ‘whistling’ and a host of other descriptive terms. Another very similar effect used to be the ‘howling’ common with poorly set up public address systems and microphones at local ‘hops’ etc. It is caused when amplified sound from a speaker has a relatively direct path back to the microphone. This causes some sensitive frequencies to be amplified even more, and round and round they go!
With hearing aids the microphone and speaker are, of necessity, extremely close together and feedback becomes increasingly difficult to avoid as the level of amplification is raised. Digital aids are much better at controlling this effect than the old analogues but a well fitted mould is still a major requirement for good operation. It must seal off any air path from the sound output to the ear drum, back to the microphone. Even with a good mould, feedback is still possible if anything (or anybody!) is placed very close to the aid. This is due to a different, sound reflection, mechanism. Even when part prepared, you can bet that an aid will always start to scream at the most inconvenient time when working in a very awkward position. Under a car bonnet, or wiring under the floor are obvious examples that come to mind immediately. The reader can perhaps imagine others.
The seal of the mould in the ear is so important, yet so difficult due to movement of the jaw. Moulds made from hybrid materials are now common in an attempt to combine the rigidity, required for good location, with flexibility for a good seal. In this case the bulk of the mould is rigid but with the innermost tip being a more rubbery formulation.
Moulds sealing as well as this can lead to their own problems. They don’t cause infections but they do constitute an unnatural environment in the ear which can incubate any existing problems. Remember many people, such a myself, are wearing aids due to damage caused by chronic infection effects. Condensation can also be a problem with behind the ear units, as the sound tube acts as a very efficient condenser particularly in cold weather. Droplets of condensation blocking the tube can cause sudden loss of hearing and cannot be removed without removing the aid. Also a drop of condensation running back into the ear is harmless but intensely irritating.
For these reasons ‘vented’ moulds are available for people who have a persistent condensation problem. After all I have said above about feedback and a good seal, I have no idea how this can be accommodated. I can only assume that the vent channel follows an indirect path and somehow emerges pointing away from the microphone area.
Pretty clever things really, all being considered.
March 4, 2008
I got through college and my first job using an NHS body worn OL56. It was very robust and tolerant of industrial atmospheres, though the flimsy wire used to crack quite frequently. I only managed to destroy one instantly, and that was by dropping it into a bowl of tomato soup at a formal dinner. (Not the most successful evening ever!) It was also quite effective as a stethoscope, but mainly for machinery rather than humans. It was however clumsy to use with a telephone and useless under heavy clothing. Many chemical plants are essentially open air installations, and the young development engineer’s uniform was often a works issue duffle coat.
In 1967 I married, took a step up the career ladder with a new job and decided to invest in a private “behind the ear” instrument, as the NHS did not supply these at that point. It had a lot of practical advantages but was a considerable item of expense at the time. It doesn’t just stop with the purchase either. They have a finite (3- 5 year) life and need service / repair every so often at a further cost. In addition, and importantly, these repairs have up to two weeks turn-round time. That is a major drawback compared with the “walk-in” repair or replacement service provided by most NHS audiology centres.
The above situation demonstrates very well the balance to be made in this decision. Something that satisfies a personal preference of some sort, but with an ongoing cost and perhaps a little personal inconvenience versus run of the mill at no cost and excellent service.
In 1974 the NHS introduced the BE11 behind the ear instrument. Since then I have changed many times between NHS and private depending on the state of the art available from each of them, and my requirements. I am currently using NHS in normal day to day activities but have a pair of (fairly expensive) ‘waterproof’ aids which I use for the gym and with very wet ‘corrosive’ hair after swimming.
The NHS has massive buying power and doesn’t lag behind the private sector technology very long in important functional matters. Just occasionally it even has technical advantage because of its extremely wide usage. The greater part of my career was associated with plants processing highly flammable vapours, which by law require all electrical equipment to have standard safety certification. There was no way that a private supplier could afford to provide testing and certification but the NHS unit was accepted for use down coal mines, which have similar regulations.
Another case in favour of the NHS is my current venture into bone conduction, something that I would probably never have considered through private channels.
The NHS is perhaps weak, for very good reasons, on aesthetic counts. It maintains its flexibility by using only instruments with a separate ear mould. In this case the personal fitted mould is very long lasting, and all the wearing bits are easily replaceable and interchangeable. This is in contrast to the very tiny “in the ear” instruments where the electronics are effectively built into the personal mould. A breakdown or replacement of one of these is always a “one off” job and therefore on extended turn-round. Until the advent of digital aids there were also technical drawbacks to the higher powered in the ear instruments.
As a summary then, and purely my personal opinion. The first port of call should always be the NHS. You will receive very good impartial advice both personal and technical. They will also make available very functional instruments at zero cost with an excellent maintenance service.
If you want that little bit extra as regards looks, or technical sophistication, there is always the private sector. You will pay for this in both price and down-time for repairs, but that may well be acceptable in your personal circumstances.
I have used (and am currently using) the private sector many times and have nothing major to say against it. However, the general rule of purchase “caveat emptor” probably applies to this market slightly more than normal everyday goods.
February 22, 2008
I’m not sure just how well qualified I am to blog on this subject now as I’m not really a mainstream user these days, in that I’m retired and out of the more demanding working environment. However, if readers will bear in mind that this is my personal view and experience only – here goes. It may well also have to be spread over more than one post, it’s a very involved subject when you get into it.
The two main things that people consider when thinking about a hearing aid are; What does it look like? and How effective is it (for me)? The relative importance of these two is strictly a personal matter. Regular readers will already know my own views. Actually the two are not unrelated and the other possible decision, private or NHS, tends to creep in here as well.
One point for thought before going further is the question of what exactly do we want an aid to do? Is that a silly question? After 40 odd years of significant hearing loss, if someone restored my hearing to normal tomorrow the result would probably be devastating. I certainly wouldn’t be able to tell what people were saying immediately and, at my age, I would probably never completely learn to adjust to the new range and volume of sounds. People considering an aid obviously have a hearing loss, will probably have had it for some time, and their brain will have already started to make adjustment. If this person is now provided with an aid that restores their hearing to normal, this “adjustment” has to be undone and the person has to re-learn to hear normally again. One of the difficulties I’m sure audiologists find when trying to prescribe aid settings for a patient, is to balance technical reality against what the patient has already become adjusted to.
One thing I would stress amongst this confusion is that a hearing aid never actually restores normal hearing . Whatever that is – we all have our own illusions of reality. (Quite deep that, I must have read it somewhere) They are merely a tool to help us mitigate the loss. As with all tools, skills are needed to use them effectively, and these skills involve both conscious and unconscious learning processes. If you are going to have to learn these skills, it is better to start earlier rather than later.
If we now overlook the previous two paragraphs and take the sound engineer’s view of hearing instruments. What we would want to do is to look at a patient’s audiogram and provide them with an instrument that exactly corrected the loss at each frequency. It would also have automatic adjustment to cope with very loud or quiet sounds, and a means of tuning in to concentrate on selected groups of frequencies when needed. All of this has been possible for some time of course – it’s called a recording studio!! We are back again to balancing size & looks against technical specification & power.
The basic “behind the ear” analogue instrument had a three position switch and a volume control. A little manual dexterity is required to manipulate these accurately in situ, especially with ageing fingers and, particularly, the non dominant hand. The instrument however is normally large enough to accommodate controls big enough for most users after a little practise. The tinier instruments have very little room for controls other than on/off, and in these circumstances the wearer perhaps would not want to draw attention to them by making running adjustments anyway. However, people very often do prefer to make their own adjustments, and versions are even available with radio remote control of settings for this reason, operated from a pocket or handbag if needs be.
In addition to the “user controls” the technician usually had three small adjustment screws (potentiometers) concealed in the instrument body, with which they could roughly “tune” the aid to the patients requirements.
Just about everyone now knows that the latest development in hearing instruments is “digital processing”. With this technology the amplified sound signal fed to the patient is controlled as a computer program, rather than with a “collection of knobs and switches”. The technician actually connects the instrument to a computer to program it to the patient’s prescription. The the aid can be considered to be a mini computer in its own right, but is relatively limited as to the amount of memory, power and processing capability that can be fitted into the body. The technician must decide how to allocate these resources to the best advantage, considering the individual patient’s circumstances. With such a complex system, the other consideration for the designer and/or technician is;- just how much running adjustment can the patient be allowed to make for him/herself? Restricted adjustment is probably the source of most adverse comments I have heard about “digitals”, though their superior performance in many other areas is readily acknowledged.
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