Hearing loop design: a step-by-step method

A loop that works is planned before the first meter of wire goes down. This guide follows the design method in the order the work happens, from site visit to handover, with a deeper guide linked at each step. New to loops? Start with what a hearing loop is and how it works, and see how loops compare with FM, infrared and Auracast when a client asks which system to choose.

Key points

• Start on site. Drawings rarely show all the steel, and they never show background magnetic noise.
• Design at head height, across the area where people actually sit or stand.
• Use a perimeter loop for smaller rooms with little metal; consider a phased array for wide rooms, heavy steel or a neighboring room that must stay quiet.
• Allow for metal loss before choosing the driver. Steel takes the most from the treble.
• A design aims at the IEC 60118-4 targets. Commissioning confirms the installed loop.

1. Survey the site

Everything later depends on what you find here. Note:

• Use: where people sit or stand, and whether neighboring rooms have loops or hold confidential conversations.
• Construction: rebar in slabs, steel decks, raised access floors, metal ceiling grids and steel framing.
• Access: the floor finish and whether it is being replaced, ceiling and below-floor access, and the route from the rack.
• Rack, power and audio: where the driver goes and which output feeds it.
• Background magnetic noise: measure it with a field-strength meter while lighting and building equipment run. A model cannot predict it.

Use the site survey checklist, and read metal loss in hearing loops beforehand so you recognize the signs of hidden steel.

2. Define the listening area and listening height

Mark the seats, standing areas and lectern or stage positions that need coverage. Leave out space nobody uses: covering it costs current and pushes the field further than needed.

Then set the listening height, from the finished floor to a listener's head: usually 1.2 m seated and 1.7 m standing. Tiers, stages and balconies change the distance between loop and listener, and that distance sets the field strength. See listening height for seated and standing listeners.

3. Choose the layout

The layout decides how even the field is, how far it travels and how it copes with steel.

• Perimeter loop. One loop around the area. Simple, and a good fit for smaller rooms with little metal. As rooms get wider, the middle sags compared with the edges.
• Figure-8 and sectioned loops. Smaller loops side by side. The field drops to almost nothing where sections meet, so plan seats and aisles around those lines.
• Phased array. Two sets of narrow loops, each one series circuit on its own channel of a two-channel driver. The sets are offset so one channel is strong where the other is weak, and driven 90° apart so their fields never cancel. Arrays keep the field even in wide rooms, hold up better over steel and help limit overspill, at the cost of more wire and labor.
• Cancellation loop. A loop outside the area, carrying current the opposite way, that weakens the field beyond one edge. It adds load and can raise the field just outside that edge.

See phased array hearing loops and hearing loop overspill.

4. Allow for metal loss

The loop induces currents in nearby steel, and those currents oppose its field. The whole field weakens, and the effect grows with frequency, so the consonant-carrying treble suffers most. Rebar, steel decks and raised access floors are the usual causes.

Allow for it in the design rather than hoping the driver's metal-loss correction covers it on the day. Loss at 1 kHz means more current; a steeper loss at 5 kHz means more treble boost, which needs voltage. Narrow array elements are generally affected less than one wide loop. See metal loss in hearing loops.

5. Choose the conductor and the feeder

Round wire suits ceilings, saw cuts and concrete pours; flat copper tape lies flat under carpet. Conductor size and length set resistance; loop size and shape set inductance, which holds back the high frequencies. More turns raise the field per amp, but resistance rises with the turns and inductance with their square.

The feeder adds resistance and inductance but no useful field, and spread conductors radiate. Keep outgoing and return tightly paired, or use star-quad with opposite cores joined at both ends. See loop wire vs copper tape.

6. Size the driver

The driver must deliver the design current, plus the metal-loss allowance, with enough voltage to hold that current through loop and feeder as the frequency rises. Resistance barely changes across the speech range, but reactance rises in step with frequency, so impedance at 5 kHz is often two to four times the 1 kHz figure.

Worked example (illustrative figures). The Community Hall loop in 1.5 mm² wire with a 15 m two-core feeder: about 0.69 Ω and 62 µH in total. At 3.7 A, no metal loss:

Now add a metal-loss allowance of 2 dB at 1 kHz and 5 dB at 5 kHz. The 1 kHz current rises to about 4.7 A, at about 3.7 V. At that current the 5 kHz field sits 3 dB below the 1 kHz level, right at the edge of the frequency-response target. Holding it level would take about 6.6 A and roughly 14 V at 5 kHz.

Speech carries much less energy at 5 kHz than around 1 kHz, so drivers are not normally sized for full current there, and data sheets often state the rated load at 1.6 kHz. Check 1 kHz and 5 kHz as well: a driver that looks generous at 1 kHz can run short of voltage at the top once treble correction is added.

Leave headroom on current and voltage. See sizing a hearing loop driver.

7. Check field strength and frequency response against the IEC 60118-4 targets

In our own words, IEC 60118-4 targets a reference field strength of 400 mA/m at listening height, a field within ±3 dB of that across the listening area, and a frequency response within ±3 dB of the 1 kHz level from 100 Hz to 5 kHz. Field strength is shown in dB re 400 mA/m, so 0 dB is the target. See IEC 60118-4 field strength explained.

Check the whole listening area, not just the middle. Weak spots sit in the middle of wide perimeter loops, where sections meet and over heavy steel. If the plan falls short, change the layout, turns, conductor, allowance or driver, and check again.

Hearing Loop Designer brings steps 2 to 7 into one workspace: choose the layout, including phased arrays, set the metal-loss allowance, pick the wire, feeder and driver, and the software checks the design against the IEC 60118-4 field-strength and frequency-response requirements at your listening height. Installed performance is still confirmed by commissioning.

8. Draw it and list the materials

The crew installs from the drawings, so show:

• Dimensions from a marked origin, noting that written dimensions govern.
• The route, turns, winding direction and, for arrays, each section's channel. Each channel is one continuous series circuit; a crossing is not a joint.
• The feeder route, driver location, and how the conductor is fixed, protected and joined.

List the driver, the conductor's full length including turns and splice allowances, adhesive tape for copper tape and feeder cable for each channel. In a concrete pour, a separately labeled spare circuit is inexpensive insurance. The outputs page shows what a full handover set contains.

9. Install and commission

Before covering anything, disconnect the driver, test each circuit for continuity, resistance and insulation, and photograph the routes. Test again once the floor or ceiling is finished.

Commissioning measures the installed loop with a field-strength meter: background noise with the loop off; then, with the driver set up to its maker's instructions, field strength across the listening area at listening height, then frequency response. HLAA also recommends a listening check by an experienced telecoil user before the installer leaves. The measurements, as-built routes and driver settings form the record the venue keeps. See commissioning a hearing loop.

Common questions

Can I design a loop from drawings alone?

Drawings are a good start for the layout and a first driver estimate, but they rarely show the steel accurately and never show background magnetic noise. Treat a drawings-only design as provisional until the site is surveyed.

When should I choose a phased array over a perimeter loop?

When a perimeter loop would sag in the middle of a wide room, when there is heavy steel in the floor or ceiling, or when the field has to stay out of a neighboring room. For a smaller room with little metal, a perimeter loop is usually simpler.

What listening height should I design for?

1.2 m above the finished floor for seated listeners and 1.7 m for standing listeners. If a space has both, check both; tiers and balconies need their own heights.

If the design reaches the targets on screen, will the room?

Not necessarily. Hidden steel, background noise, a changed wire route or different driver settings all change the result. That is why installed performance is confirmed by commissioning.