DRAFT TEXT - STILL BEING WRITTEN
Before we start. Being of perfect hearing, I cannot justify getting a hearing aid. If there is a manufacturer of hearing aids that would like to contribute one for my experimental use, then I would be most grateful if you would contact me.
On to the article....
Installing a hearing aid loop using either an old PA amp (see An Economical Hearing Aid Loop) or a purpose built loop amplifier (which is nothing more than a "sliding bias class-A amplifier" with current - as opposed to voltage - feedback) is not the trickiest of things to do, but ensuring it is generating the correct amount of magnetic field is. It is all very well adjusting a loop dedicated for use by only one person (e.g. a TV loop for a relative who's hard of hearing) but ensuring a loop is performing properly in a public place (Church, cinema, auditorium, etc.) takes a little more than tweaking volume controls!
Before we get on to testing the loop, time for a few simple explanations of what goes on with loops.
The 'T' setting on a hearing aid simply switches the input of the hearing aid from the in-built microphone to a small induction coil also housed within the hearing aid.
The primary use of this function is with telephone handsets (hence 'T') where the induction coil picks up the small magnetic field emitted from a telephone earpiece. This induction coil is sensitive to any varying magnetic field within the audio frequency spectrum - limited, in the case of hearing aids, to about 50..5000Hz.
All that is therefore required to 'transmit' an audio signal direct to the ear of a hearing aid user is to create a magnetic field of sufficient intensity - and this is done with the aid of a loop whose primary purpose is to set up a magnetic field, rather than relying on the secondary effect of a magnetic field from a speaker or earpiece etc.
The intensity required for satisfactory coupling is laid down in the standards EIA 118-4 and BS 6083-4 and is accepted as 100mA/m. Like all specifications it lacks definitions (e.g. what is understood as 100mA/m) and methods of testing. In more simple terms; This refers to the field set up in a loop of wire 1 metre in diameter with 100mA (RMS) flowing through it.
Contrary to popular belief....
Hearing aid loops do not use supersonic carrier frequencies to deliver the audio to the hearing aid, and therefore do not require specialised circuits to create modulated carrier frequencies.
Hearing aid loops do not use radio frequencies to deliver the audio to the hearing aid, and therefore do not employ any form of radio transmitting equipment whatsoever.
In simple terms; This, therefore, excludes e.g. 32kHz and 40kHz carrier systems as well as e.g. 27MHz, 404MHz, 432MHz, and 2.4GHz 'wireless' systems.
Background interference is, based on the above, not caused by any specialised radio systems (TETRA, cellphone masts, etc.) and/or 802.11b/g 'wireless networking' systems. It is amazing just how many poor souls have been led up garden paths through the ignorance of others!
Background interference, should it exist, is limited to being purely magnetic in nature, and, as the variations must occur within the audio frequency spectrum, is limited to conductors capable of carrying such a varying current which in turn sets up the varying magnetic field.
Electrical cables (of all sizes!) are the most common carrier and/or creator of magnetic interference. However, water pipes are just as conductive and could be the second means to conduct currents from badly installed or faulty equipment.
When tackling the setting up of loops we are faced with two issues, the first being knowing what is the accepted level of the magnetic field and then a method by which we can test and measure the loop.
After much scrabbling about, a decent technical specification and relevant data was obtained and describes not only the specification, but also the methods of ensuring the loop is set up correctly. This is available here. Having a reference point now to work from we can create a reliable method to test deaf aid loops.
There are professional units available to measure magnetic field strengths at about £300 (they are also available for hire) but this is most certainly beyond the means of the well meaning individual trying to help a person or small community set up a loop correctly. With only a little bit of electronic knowledge and some ingenuity a reasonably reliable method can be created. The only part believed to be a problem was the pickup. Whatever is used must resemble, as closely as possible, the loop that will be found in a hearing aid. Once the signal is available, amplifying it will be easy.
The k.i.s.s. principle (keep it simple, stupid) is one adopted when personally approaching all problems and designs. Initial instincts led along the lines of a telephone suction style pickup (used for recording telephone conversations) fed into a miniature tape recorder set to record, with the output fed to an earphone. This would very closely resemble a hearing aid circuit. The only problem with this is the level is unknown and the combination may be more sensitive than a hearing aid proper.
Two methods of setting up a loop are available to us. The first is to take the loop size and ensure the accepted level of current is generated within the loop. This level is clearly defined within the document mentioned earlier (how to measure the current is not but we cover that later). The second method is to actually measure the magnetic field with some reliable means and we'll investigate a reasonably reliable way of doing that in the second half of this document.
The first, less desirable method of testing the loop is to take the area of the loop and ensure the current flowing in the loop is correct. The instinctive thought is to use a low value resistor and measure the voltage across it. This may work but only when the current in the loop is high with a somewhat high loop resistance e.g. 5 or more amps into a loop of 4 to 8 ohms. If the loop resistance is low or the current flowing in it is minimal then the loop to resistor ratio will be such that is likely to affect the loop currents when taken back out of circuit.
If we are going to use the current into the loop as a guide then the real answer is to use a current transformer to measure the current, this is created fairly easily. Any small audio transformer is suitable as long as a single winding can be introduced between the outer of the former (windings) and the core. The winding does not have to be round, a flat copper strip is extremely suitable.
The next stage is to determine what load resistor to use on the output of the CT. Here it would be advisable to choose something that would relate to the measurements made, e.g. 1V for 100mA and 1A (switchable). This is best done with a signal generator into an audio amp feeding the current transformer via a suitable resistor (4 to 10 ohms but not lower than the output impedance of the amp) and then adjust the output of the amp until the volts reaches the value of the resistor (e.g. 4.7 volts for a 4.7 ohm resistor). This equals 1A into the transformer.
More to follow..... (stay tuned!).
© 25.04.01 / 07.10.04