|
WHAT
IS AN INDUCTION LOOP SYSTEM? HOW DOES IT WORK? In
its simplest form an induction loop consists of a length of wire which is
run around the required room or area. This loop of wire is connected to
the output of an amplifier and a microphone connected to its input, Any
sound picked up or 'heard' by this microphone is converted by the system
into a varying magnetic field within the area enclosed by the loop of
wire. This
magnetic field can be detected by a hearing aid and is re-converted into
sound. Because this magnetic field cannot be detected by those with normal
hearing, it is important to understand that A LOOP SYSTEM WILL BE OF
BENEFIT ONLY TO HEARING AID USERS. It
is estimated that there are over two million hearing aids in the U.K.
which are capable of receiving induction loop transmissions. In fact,
since 1974 most National Health hearing aids have been fitted with a two
position switch which is marked 'T' and 'M'. The 'M' position is for the
aids own internal microphone and is the normal position of use. The 'T'
position was originally designed to pick‑up the magnetic field
generated by a telephone, but this is also the position used for loop
reception. THE
HEARING AID WILL ONLY RECEIVE SIGNALS FROM A LOOP SYSTEM IF IT IS SWITCHED
TO THE 'T' POSITION. Some modern aids now have a third switch position
marked 'T + M' where both microphone and loop pickup coil can be used
together.
In
most public buildings, theatres, halls and Churches, the listener (the
hearing impaired person) is some considerable distance from the performer,
or person who is speaking. Room acoustics, (reverberation or echoes)
environmental, audience, or congregation noises and other disturbing
sounds all serve to mask sounds the hearing aid wearer wants to hear. In
a loop system the 'pickup microphone' is placed close to the required
sound source and thus the sound picked up by the hearing aid from the loop
is free of all these unwanted sounds and is much clearer. (Effectively the
hearing aid user is now as close to the sound he wants to hear as the loop
system 'pickup microphone' is.) A
further advantage is that when switched to the 'T' position a hearing
aid's own internal microphone is disconnected thus eliminating the
possibility of acoustic feedback (that high pitched whistle which
sometimes occurs when a hearing aid is turned up too far, and which can be
so distressing to those with normal hearing). The system requires that a wire be run around the required room or area. The loop of wire can be fixed at, or beneath floor level, or sometimes even above ceilings if these are not very high. The usual procedure is to run the wire around the skirting board on an existing building, and in a duct or conduit beneath the floor when constructing new buildings. The
actual installation of this cable is often the most difficult part,
particularly in Church buildings. A comprehensive survey of the proposed
site, including listening tests with inductive pickup test equipment, is
essential before any system costs can be evaluated. Generally speaking the
larger the area to be enclosed by the loop, the greater the cost. We do
not, however, advocate the use of loops which cover only part of the
public area in any room. The segregation of hearing impaired persons to
specific areas is, in our view, entirely unacceptable. A hearing aid user
should be able to sit in any part of the room and still receive induction
loop transmissions. EXISTING MICROPHONES
AND SOUND SYSTEMS Where
a sound reinforcement system is already installed, the addition of a loop
system is made much simpler. The loop amplifier is connected to the
existing 'sound' amplifier and utilizes the existing microphones. If
properly installed by a competent engineer, the loop system should have no
effect on the operation of the existing sound system, When a loop system
is to be installed where there is no sound reinforcement system, then it
is often necessary to provide several microphones in order to give the
required clarity of signal for the loop transmission, Remember the
'pickup' microphone should be as close as possible to the required sound
source (or sources). At
this point it may well be worth considering the installation of a sound
reinforcement system as an alternative to the loop system, as the cost can
often be comparable when several microphones are required. A properly
installed sound reinforcement system can be of benefit to everyone rather
than just hearing aid users as is the case with a loop system. Quite often
those with some hearing deficiency are unwilling to wear a hearing aid and
sometimes they are unaware of their deficiency. It is estimated that more
than 50% of people over the age of 70 have a significant hearing loss.
Even those with normal hearing can have difficulty understanding the
spoken word in situations of high reverberation such as those often
encountered in churches. The ideal situation is, of course, to install
both a sound reinforcement and a loop system and although cost
considerations may preclude this, remember that the two systems will not
be the sum of each individual system, as the microphones, microphone
wiring, and amplifier mixer section are common to both. CHOOSING EQUIPMENT
AND INSTALLERS A
BRITISH STANDARD SPECIFICATION exists for induction loop systems and both
the R.N.I.D. and S.C.I.F. organisations are concerned that these standards
should be adhered to. Any proposed installer should be fully conversant
with these recommendations and be experienced in loop installation work.
Once the loop is installed it should be calibrated and adjusted using a
Field Strength Meter and an Induction Loop Monitor Set. The whole loop
area should then be tested to ensure satisfactory results. In order to
avoid unauthorised adjustments, possible malfunction, or inter
interference to other equipment, the loop amplifier should have no user
operable controls. When considering the installation of a loop system, the
prospective purchaser should ensure that he is fully aware of the work
involved, and the route to be taken by the cable. This cable should be as
inconspicuous as possible, the colour being chosen to best blend with the
surroundings, and any cable clips should be similarly coloured, The loop
cable should be securely fixed. Loose, hanging, or taped down wiring is
not acceptable.
Any damage to the loop cable could lead to failure of the system, and
this may well go unnoticed until a complaint is received from a user.
Whilst an ordinary 'sound' amplifier can be used in some cases to drive a
loop system, its use is not recommended and it will invariably give
inferior results. A properly designed DEDICATED LOOP AMPLIFIER should
always be specified. This will normally have a 'constant current output
stage' and incorporate limiting and automatic gain circuits. These will
ensure that even though the sound level being picked up by the loop
system's microphone may at times vary considerably, the volume received by
the hearing aid user remains relatively constant. This avoids the user
having to frequently re-adjust his hearing aid, and prevents possible
discomfort when loud sounds occur. We
hope that these details provide useful information for those considering
loop systems. |
