3 Field Strength (set-up)
PURPOSE – set the field strength to approximately the right operational
level
• On the FSM: Select ‘Field strength’ mode.
• Turn on loop system and set output current to zero (or low) to avoid
sudden unexpected signal affecting hearing aid users in the vicinity. Run
the combination noise test signal (Test Signal track 1) into the loop
system. Ensure any equalisation or filtering on the audio source such as
‘Bass boost’ is disabled.
• Increase the input gain to achieve compression (6dB / 12dB LED on the
loop driver front panel for Ampetronic amplifiers).
• Select a typical listening position, and increase drive current until FSM
reads approximately 0dB during the bursts of 1kHz sine wave. The
reading is the maximum LED illuminated over 60 seconds. Ensure the
pick up coil is orientated the same as the hearing aid telecoil – usually
vertical.
• Take and note a series of readings at each defined measurement point
(A-F), marking the positions and measured levels on a copy of the
certificate (‘initial’ values).
• Adjust drive current such that in the useful magnetic field volume:
oMaximum reading is no greater than +3dB
o0dB is achieved somewhere
oMinimum reading is no less than -3dB
• Record the ‘final’ levels on the certificate at each measurement position.
• Alternative method:
oThis test can also be performed using the ‘Pink noise’ signal (Test
Signal track 2). With this signal the peak readings must be
between -9dB and -3dB, with at least one point in the volume
reading -6dB. Note: This method is more likely to be affected by
any subsequent adjustments of the ‘Tone’ or ‘MLC’ control, and
different amplifiers AGC / compressor characteristics.
• Not advised:
oArtificial / simulated signals such as ITU speech are not
recommended as test signals for setting up systems - it is better to
use the real thing, and then only as a final check.
8
