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AFILS Field Strength Meter Instructions Version 1.3
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Version 1.3 AFILS Field Strength Meter Instructions
Background information
There are a number of important features on this meter that distinguish it from other
similar meters on the market:
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The indicating scale is referenced to a magnetic field strength of 0.56A/m, as
recommended in BS7594. The long term average field strength required from an
AFILS in the UK is 0.1A/m rms. However, speech has a peak to mean ratio (or crest
factor) of 12 to 15dB so to maintain good speech intelligibility, a correctly set up
AFILS must deliver a peak rms field strength 12 to 15dB (4 to 5.6 times) higher
than 0.1A/m. Note that it is not sufficient to simply set up a loop system to
deliver a field strength of 0.1A/m using a constant amplitude source such as a
tone or pink noise because these sources do not give a representative speech-
like power spectrum with a realistic peak to mean ratio. If the loop amplifier uses
compression, then it may be possible to produce a constant field of 0.1A/m but
that does not mean that the amplifier can provide sufficient current to deal with
the speech peaks. Conversely, a loop amplifier that can provide a constant field
of 0.56A/m may be overspecified and potentially unnecessarily costly for the
application.
Safety, Care and Maintenance
Keep the unit away from heat sources such as radiators, heaters or other appliances
that produce heat.
Do not expose the unit to direct sunlight.
Do not use the unit near water.
Avoid objects or liquids getting into or onto the unit.
Clean only with a damp cloth. Do not use abrasive cleaners.
Do not remove screws from or open the unit casing. This will invalidate the calibration
and any warranty.
Damage to hearing can occur with consistent exposure to high volume when using
headphones. Always check the volume level before using headphones.
Calibration
This unit is calibrated at manufacture to meet BS7594. If measurement accuracy is
critical, the instrument should be re-calibrated annually.
Have the calibration re-checked if the instrument is dropped or otherwise damaged.
A three point calibration at 0dB, -24 & -27dB is standard at the time of manufacture. A
full 20 point calibration service (part no.40IFSCAL) is available.
Batteries
Press and slide the battery cover to remove.
Orientate the battery as shown in the battery compartment moulding. No harm will
occur if the battery is reversed but the instrument will not work.
Rechargeable batteries are not recommended for infrequently used equipment due
to self discharge, but they can be used in this equipment. Operating life will be
shorter from a rechargeable battery than from an alkaline battery.
Replace the battery as soon as convenient after the red range indicator(s) start to
flash.
Switch off when not in use to prolong the working life of the battery.
Remove battery if not required for an extended period of time and remove an
exhausted battery as soon as possible to prevent leakage.
Dispose of the old battery according to local regulations.
LO range when lit*
HI range when lit*
Limit of acceptable
background noise
FLAT ‒ AFILS
frequency
response only
NORM ‒ AFILS for
hearing aids
* The range indicator in
use flashes when the
battery is low.
HI range for AFILS
signal strength
LO range for
background
magnetic noise
0dB = 0.56 A/m
Headphone socket
Volume control
