Contacta Loop-FSM Gebruikershandleiding - Pagina 5

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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.

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.