PDA Pro-Range Instructions • Approved Doc. No. DCP0003168 Rev 1 • Page 13
For compliance with BS7594 (the code of practice for audio-frequency induction loop
systems), we recommend you also check the system using a pink noise generator and
magnetic field strength meter.
BS7594 recommends that the magnetic field strength of an AFILS over a covered area
should be 100mA RMS average peaking to 400mA per metre. The most efficient way of
ensuring this requirement is met is to measure the magnetic field strength of a steady
output from the induction loop amplifier.
Unlike music or speech (both of which provide a variable sound output from which it is
virtually impossible to obtain an accurate RMS reading), pink noise has an equal and
constant average amount of energy content per octave of bandwidth. When fed into
an AFILS system, pink noise therefore gives a constant magnetic field strength which
can be easily and accurately measured using a magnetic field strength meter.
The following items of cost-effective PDA RANGE test equipment can be used to verify
both the audio quality and the magnetic field strength of any AFILS system. ALWAYS
refer to the more detailed information supplied with each unit before starting any
tests.
P-Ngen Pink noise generator (part no. PNGN)
FoSmeter H Combined magnetic field strength meter & loop listener (part no. AHHM/H)
FoSmeter+ Combined pink noise generator & magnetic field strength meter (part no. AMPN)
Headphones for use with FosMeter H (part no. HEAD1)
Contact your PDA RANGE distributor/supplier for pricing details.
Metal Compensation Control
If a large amount of metal is present in or near the induction loop, you may find the
sound you hear through the loop listening device is ‘woolly’ or ‘dull’. This is caused by
the metal in the room absorbing the magnetic field at a rate that increases with
frequency, i.e. high notes are absorbed more than low notes resulting in
a ‘muffled’ sound. If this is a problem, try turning the amplifier’s
METAL COMPENSATION
control clockwise in small increments until a natural balance is achieved. Please note
however that due to the metal absorbing power from the amplifier, its area of
coverage will be reduced and further reduced as the
METAL COMPENSATION
control is
turned clockwise.
Note that when testing the installation with a loop listening device, you may hear a
slight ‘humming’ noise in the background. This IS NOT a fault with the induction loop
system but a common occurrence caused by mains wiring. This hum will normally NOT
be heard by hearing aid users as most modern hearing aids have filters which cancel
this noise out.
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