Power supply: 9V at 18-26mA
Display: –21dB to +6dB in 3dB steps
Meter response: “S” (slow) response of 1s
Weighting: A-weighting or wide (see Fig.4)
When installing a hearing aid loop, it is important to set the magnetic field strength to the correct level. This ensures that a hearing aid with a Telecoil (or T-coil) will deliver the best signal-to-noise ratio without signal overload.
The same applies if you are using a hearing loop receiver such as the one described in the September 2010 issue of SILICON CHIP (or a commercial equivalent).
Additionally, when setting up a hearing aid loop, it is important to verify that any background magnetic noise is at an acceptable level. Both background noise and signal strength from the hearing aid loop can be measured with this Hearing Loop Tester.
Of course, if you are setting up a small hearing loop in your home, you can usually get away without using a level meter. In that case, it’s usually just a matter of setting the level to give good results from the hearing aid without any overload occurring.
However, for a system that will be used by more than one person or the general public, it is important for the level to be correct. That way, the loop will be suitable for all who use it.
As shown in the photos, the SILICON CHIP Hearing Loop Tester is housed in a small hand-held plastic case that includes a battery compartment. A power switch and an indicator LED are located on the top panel, while the front panel carries 10 bargraph LEDs arranged in a vertical column on the lefthand side.
In operation, this bargraph displays signal levels ranging from -21dB to +6dB, with each LED representing a 3dB step. However, to conserve battery life, the display is normally set to dot mode which means that only one display LED is lit at any time. The current consumption is 18mA when no bargraph LEDs are lit and 26mA when one bargraph LED is lit. This is quite satisfactory for an instrument that is normally only used for short durations.
Alternatively, you can install a link under the PC board to convert to a conventional bargraph display. This is not recommended though, due to the increased current drain.
An important feature is that the unit can be accurately calibrated to indicate 0dB at a field strength of 100mA/m. This specification is based on the Australian Standard AS60118.4-2007 – “Hearing Aids: Magnetic Field Strength In Audio-Frequency Induction Loops For Hearing Aid Purposes”.
Once calibrated, the meter can then be used to set the field strength level in a hearing loop to the correct level. It can also be used to measure the environmental background noise, to determine whether this is low enough for a hearing loop to be successful.
In operation, the unit is simply held at right-angles to the plane of the hearing loop for both signal level and noise measurements.
Refer now to Fig.3 for the circuit details. It’s based on four low-cost ICs, an inductor (L1), 11 LEDs and a handful of minor parts.
Inductor L1 is used to detect the magnetic field from the hearing loop. This inductor is actually an Xenon flash-tube trigger transformer (Jaycar MM-2520) which has a high inductance, suitable for loop monitoring.
Fig.1: the basic arrangement for a hearing loop. The loop creates a varying magnetic field in response to the driving signal and this is picked up by suitably-equipped hearing aids and receivers.