IEC 60118-4: how the software uses the targets

The standard in brief

IEC 60118-4 is the international standard for audio-frequency induction loop systems: the loops that carry sound to hearing aids and cochlear implant processors with a telecoil. In our own words, it covers four things that matter at the design stage:

• Field strength. A reference magnetic field strength of 400 mA/m at the listening height.
• Even coverage. Across the listening area, the field should stay within ±3 dB of that reference.
• Frequency response. From 100 Hz to 5 kHz, the field should stay within ±3 dB of its level at 1 kHz, so speech keeps its consonants.
• Background noise. Limits on the magnetic noise already present in the listening area, which a listener would hear as hum.

It also describes how an installed system is measured with a field-strength meter. The standard is published by the IEC. We describe it in our own words and never quote it; for the exact wording, buy it from the IEC webstore or your national standards body.

How the software uses the targets

• Field strength in dB re 400 mA/m. The model shows the field across the room at the listening height you choose, with 0 dB as the target. Seated (1.2 m) and standing (1.7 m) are the usual heights; you can set your own.
• The ±3 dB band. Every view marks the band, and the software reports how much of the listening area falls inside it, with the lowest and highest values.
• The listening area. Figures leave out a 0.6 m (2 ft) strip just inside the wire, where the field changes sharply.
• Frequency response. Resistance, inductance, impedance and drive voltage are worked out at 1 kHz, 1.6 kHz and 5 kHz for the conductor and length you choose, so you can see whether the high frequencies hold up.
• Metal loss. The loss you allow for is applied to field strength and frequency response before you choose a driver.
• Driver headroom. The current and voltage the design needs are compared with the ratings of the driver you choose.

When a design falls short, the software says where. A perimeter loop that sags in the middle of a wide room is flagged, for example, with the suggestion to consider a phased array. You adjust the layout, the conductor, the driver or the metal-loss allowance until the plan reaches the targets.

Commissioning confirms the installed loop

A model is only as good as what it knows about the building. Hidden steel, background magnetic noise, a changed wire route or a different driver setting all change the result. That is why installed performance is confirmed by commissioning: on site, with a field-strength meter, at the listening height, across the listening area and at more than one frequency, with the results written down for the venue.

The software helps on both sides of the job: a site survey sheet before the design, and a commissioning record for the measurements afterwards. Commissioning a hearing loop explains the steps.