6
Measuring Frequency Response
Detailed below are two processes that can be used to con-
firm the system properly reproduces all of the required frequencies
equally in the bandwidth of 100 to 5KHz. A test signal generator
such as the Contacta Loop-TSG will be required to perform these
tests and both methods will yield very accurate results.
Method I - Sine wave signal
Send at a minimum the following sine wave signals through
the hearing loop system without adjusting any of the audio or loop
level controls: 100Hz, 250 Hz, 500 Hz, 1000 Hz, 2500 Hz, and 5000
Hz. Using the Flat Spectrum mode record the RMS level generated
by each of these. To meet the IEC specification the levels should not
vary by any more than ±3dB.
Method II - Pink noise signal
Send a pink noise signal into the hearing loop system and
select the Third Octave mode on the meter. It will initially start at a
1000Hz center frequency.
Pressing button “B” will cycle through the frequencies of
100Hz, 200Hz, 500Hz, 1000Hz, 2500Hz and 5000Hz.
IEC 60118-4 Notes and Requirements
The standard as revised notes that within the frequencies of
100Hz to 5000Hz the hearing loop system should equally reproduce
all signals. At a minimum the systems is to be tested at 100Hz, 1KHz
and 5KHz.
5
Program RMS/Peak Levels
The second mode is used to set up the hearing loop signal
level using an A-Weighted filter. It is very difficult to set the field
strength to an average of –12dBA as it will constantly vary based on
the program used to set up the system. We will use the PEAK read-
ing to confirm that both our design and equipment meet the IEC
specification. Pressing button “B” resets the PEAK reading.
How to test then adjust the loop system level:
Once the hearing loop system has been installed, play some
bandwidth limited (100Hz to 5KHz) pink noise or 1KHz sine (use the
driver manufacturers recommendation) though the loop system.
Walk throughout the audience area holding your meter vertically.
Note the readings and confirm that the A-RMS level does not vary by
more than ± 3dBA. This lets you know that the perceived signal level
in the hearing aids will be the same, no matter where the person is
sitting.
Next: using program audio similar to what is normally used in
the facility, adjust the audio program level to a level just above
“normal”. Now turn on the hearing loop system and adjust the drive
level until a –3dBA to 0dBA PEAK level is obtained in the center of
the loop. Reset the PEAK level and confirm this level averages 0
dBA across the audience listening plane.
IEC 60118-4 Notes and Requirements
The revised standard states that the signal level across the
loop area should not vary by more than ±3dB. Once that is confirmed
the signal level based on the facility’s normal program should peak
at 0 dB as referenced to a 400mA/m. This will confirm there is ade-
quate loop current to produce both the peak and average (Avg. is
-12dB or 100ma/m) signal levels that work well with T-coil equipped
hearing aids.
