IEC 60118-4 field strength: 400 mA/m and the ±3 dB variation explained

Loop design and commissioning both come back to a few numbers from IEC 60118-4: 400 mA/m, ±3 dB, and 100 Hz to 5 kHz. Here is what they mean, how to read dB re 400 mA/m, and why a design still has to be measured on site. We describe the standard in our own words; for the exact wording, buy a copy from the IEC or your national standards body.

Key points

• 400 mA/m is the reference. It is the field a loop should give at the listening height on speech peaks. A meter reading in dB shows it as 0 dB.
• ±3 dB is the allowed spread. Across the listening area the field should stay within 3 dB either side of the reference: a 6 dB window.
• Treble counts. From 100 Hz to 5 kHz the field should stay within ±3 dB of its level at 1 kHz.
• Height matters. The field is judged at head height, usually 1.2 m seated and 1.7 m standing.
• Designs predict, measurements confirm. Installed performance is confirmed by commissioning with a field-strength meter.

What IEC 60118-4 covers

IEC 60118-4 is the international standard for audio-frequency induction loop systems, the loops that reach hearing aids and cochlear implant processors with a telecoil. It sets performance targets for the field an installed system produces and describes how to measure it. Drivers and other components have their own standard, IEC 62489-1.

The targets cover field strength, evenness across the listening area, frequency response and background magnetic noise. This guide covers the first three; Commissioning a hearing loop covers background noise, which is measured with the loop off.

As of October 2026, the current edition is the third edition (2014) with its Amendment 1 (2017), which the IEC also sells as one consolidated document.

400 mA/m: the reference field strength

The reference is 400 milliamperes per metre (mA/m) at the listening height, in the vertical direction, because that is the part of the field a telecoil mostly picks up when the listener's head is upright.

It is set for the loud moments of speech: on a meter that averages over an eighth of a second (125 ms), speech peaks should reach about 400 mA/m. Speech makes the reading jump about, so set-up and measurement normally use a steady test signal, such as a 1 kHz tone; the standard allows several kinds.

The first edition, from 1981, gave a long-term average of 100 mA/m instead, 12 dB lower. That is roughly the same loudness measured a different way, not a quieter loop. Some installers read the lower figure as a peak level, though, which left loops far too quiet.

The level keeps the loop well above background magnetic noise in most buildings without overloading the hearing aid. Going far above it does not help listeners; it can sound harsh and carries the field further into neighboring rooms. Field strength follows loop current: double the current and the field doubles.

dB re 400 mA/m, and why 0 dB is the target

Meters and design software usually show field strength in decibels relative to 400 mA/m. The reference reads 0 dB, weaker fields read negative and stronger fields positive. Every 6 dB doubles or halves the field.

To convert, use dB = 20 × log10(field ÷ 400), or field = 400 × 10^(dB ÷ 20). The same rule applies to current: 1 dB more field takes about 12% more current, 3 dB about 41% more, and 6 dB twice as much. The drive voltage rises in the same proportion, so keep that in mind when you check driver headroom.

Even coverage: the ±3 dB band

Across the listening area, at the listening height, the field should stay within 3 dB either side of the reference, so a listener who changes seats does not need to touch the volume control.

• It is a window, not a minimum. A loop reading +5 dB near the wire and −1 dB in the middle is too uneven, even though no seat is quiet.
• The listening area is agreed, not assumed. The standard has a name for the space where its targets apply: the useful magnetic field volume, meaning the floor area and the heights where listeners will be. The strip right next to the conductor, where the field changes sharply, is usually left out of it. Agree the listening area with the client and write it down, so the design and the measurements cover the same places.

In practice, installers set the level close to 0 dB and aim for every listening position to read between −3 dB and +3 dB re 400 mA/m.

In a wide room a single perimeter loop sags in the middle; a phased array is the usual answer. Height matters too: a floor loop reads differently at 1.2 m than at 1.7 m. See Listening height for seated and standing listeners.

Worked example: when turning it up works

Example only, synthetic room. A perimeter loop in Community Hall is measured at 1.2 m with the same test signal at every point.

• Case A. Highest +1.2 dB, lowest −3.8 dB: a 5.0 dB spread, inside the 6 dB window. Raising the output 1 dB (about 12% more current) gives −2.8 dB to +2.2 dB. Every position is now inside the band.
• Case B. Highest +2.0 dB, lowest −4.5 dB: a 6.5 dB spread. No level setting fits: raising the output 1.5 dB lifts the low spot to −3.0 dB but pushes the high one to +3.5 dB. The fix is a layout change, or a fresh look at whether the low spot is really in the listening area.

Check the spread first, then the level. If the spread is wider than 6 dB, the level control cannot rescue it.

Frequency response from 100 Hz to 5 kHz

A loop can read 0 dB at 1 kHz and still sound dull. Consonants such as s, t and f live in the higher frequencies, so from 100 Hz to 5 kHz the field should stay within ±3 dB of its 1 kHz level at the same spot. That is judged against the 1 kHz reading, not against 400 mA/m. Two things usually pull the top end down:

• Metal. Steel in floors, ceilings and walls weakens the field, and the high frequencies most. See Metal loss in hearing loops.
• Voltage. Loop impedance rises with frequency, so the same current takes more voltage at 5 kHz than at 1 kHz, often several times more on a large loop. A driver that runs short of voltage clips or limits, and the treble suffers first. See Sizing a hearing loop driver.

Worked example: a dull 5 kHz

Example only, synthetic room. At the centre of a 300-seat sanctuary over a steel deck, the meter reads −0.5 dB at 100 Hz, 0 dB at 1 kHz and −5.0 dB at 5 kHz. Against 1 kHz, 5 kHz is 2 dB outside the band. The driver's metal-loss or treble correction can lift 5 kHz by 3 dB to −2.0 dB, but that takes about 41% more current at the frequency where impedance is highest, so check voltage headroom first, and recheck 1 kHz afterwards.

Design targets and site measurements

A design predicts the field from the layout, the current, the listening height and your metal-loss allowance. Commissioning measures what the building actually does: hidden steel, a changed wire route, a different driver setting, or background noise nobody knew about. Installation guidance from the Hearing Loss Association of America makes a similar point: a model is a useful place to start, but no loop should go in on the model alone, not least because a model cannot detect background magnetic noise. So plan with margin:

• Keep the modeled listening area comfortably inside the band, not on its edges, so hidden steel or a small route change on site does not push seats outside it.
• Size the driver to reach 0 dB with metal loss included and voltage to spare at 5 kHz.
• Measure with a test signal the standard allows, such as a steady 1 kHz tone or a noise signal, set up the way the driver and meter makers describe. Tones, noise and speech read differently, and many drivers adjust their gain automatically.
• Record what you measured, where, at what height and with which signal. That record is what the venue keeps.

Hearing Loop Designer helps on the design side. It shows the modeled field in dB re 400 mA/m at the listening height you choose, marks the ±3 dB band on every view, and reports how much of the listening area falls inside it. It also works out drive voltage at 1 kHz, 1.6 kHz and 5 kHz, so you can spot treble problems before you choose a driver. Installed performance is confirmed by commissioning. See how the software uses the IEC 60118-4 targets, and the step-by-step design method for where each check fits.

Common questions

Is 400 mA/m a minimum?

No. It is a reference with a tolerance either side. A much stronger field does not help listeners and carries further beyond the room. Aim for 0 dB, within ±3 dB.

Does every seat have to fall inside ±3 dB?

Every position in the agreed listening area should. Note any areas left out, such as the strip along the wire, in the commissioning record.

Why do my readings change with the test signal?

Meters respond differently to tones, noise and speech, and many drivers adjust their gain automatically. Follow the equipment makers' procedure and record the signal you used.

Can software tell me a room will reach the targets?

It can tell you whether the design does. Hidden metal, the final wire route, driver settings and background noise show up on site, which is why installed performance is confirmed by commissioning.