Hearing loops and Assisted hearing
By: Barry G7KCO.
This was originally presented to the Vange Amateur Radio Society on the 17th August 2006.
Methods and equipment for assisted hearing in the home and small hall environment.
For assisting hearing in these environments there are a number of methods that are readily available. Methods include Infrared transmitter / receiver systems, these are widely used in theatres, with receivers available from the box office usually for a small rental. Licence free radio systems (about 500mHz), these are widely available in both headphone and loudspeaker versions. And Induction loop systems, old technology these days but still a reliable technique, especially for users of hearing aids, the majority of which include a loop receiver setting, 'T switch. The main advantages of using the hearing aid, as the receiver for a deaf person is that hearing aids normally have their audio response adjusted to optimise the users residual hearing. This means that there is no need to adjust the signal source to the user or group of users individual needs.
Where simple installation, and a good reproduction quality, including stereophonic reproduction are required then the use of a radio-based system is probably the best bet. I use a Sennheiser radio system driven from the headphone socket of the home stereo which can direct any required signal from the stereo, to the headphones or to remote speakers in other parts of the house as required. It is unfortunate however that self building a radio-based system is not viable as the relatively low cost of the commercial systems, combined with the difficulty in obtaining the required specialist IC's in one off quantities means that purchase is really the only route.
For home use however the use of the induction loop as a home build option can be quite cheap and simple.
The receiver
Commercial receiver units can be bought from under £15.00, and built for about half that sum,
probably less if the scrap box is raided.
For a basic unit all that is needed is a single transistor pre-amp driving an 1C headphone amplifier.
A suitable pickup would be an inductance of about lOOuH. For those wishing to optimise things a 50 Hz
filter and 300-3000 Hz pass band with peak sensitivity at about 12-1500Hz will probably get
about the best from the system.
The transmitter.
The transmitter should ideally be optimised for current output as what we are trying to achieve
is a circulating current in the load (loop) as the 'primary' of a transformer with the pickup
as the secondary. Purpose built units will have input compression to optimise signal to noise
ratio the ability to drive very low impedance loads and some kind of output meter,
normally calibrated in amps. The critical value for an induction loop system is the current
in amps per loop-meter (mA/m).
If the current density is too high, then there is a likelihood of breakthrough to other systems,
and if too low then the signal to noise ratio will be too low to give the required clarity.
In practice, for home use any of the popular amplifier chips, especially those using the bridge
load principle are suitable, but the use of some sort of series load, a 1 or 2ohm high current
resistor for example, may prove necessary to compensate for the uncertain resistance of the loop.
The loop
The loop needs to be of one or more turns of wire around the area within which the sound pickup
is required. The coverage area could be a room, or as small as a single person, a loop worn
around the neck is a common method for personal listening devices, and another technique is for
a loop to be placed under the cushions of a favourite chair or sofa. For a room sized loop a
turn or two, of low current two or three core wire (bell wire) gives some flexibility as once
the wire is installed then the resistance can be measured and the wires can be connected in
either series or parallel to optimise the resistance. With a neck or chair loop multiple turns
are probably the best option. Ideally the wire of the loop should be at the expected height of
the pick up coil, and all in one plane. Provided that the vertical component of the loop is
less than about 20% of the total length, the vertical excursions required to go over doorframes
and windows does not seem to limit the performance greatly. In the Benfleet Methodist Church,
the loop runs in a dado at about 1 meter above floor height. At Thundersley Church two runs of
'bell wire' were laid in plastic conduit on the floor screed prior to the concrete floor being
poured, it finished up about 30cm down and the turns were subsequently wired in series to give
about 6ohms resistance. In home installations around skirting boards or picture rails have both
proved to be successful.
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