Hearing loop overspill: how far the field travels and how to control it

A hearing loop's magnetic field does not stop at the wall. It carries into the room next door and the floors above and below. When the space next door has its own loop, or the room's audio is private, that overspill becomes a design problem, and it is far easier to solve on paper than after the cable is down.

Key points

• A loop's field always reaches past the wire, sideways and up and down. How far depends mostly on loop size and level.
• Overspill matters next to other loops, in confidential rooms and near sensitive audio equipment. Elsewhere it is usually harmless.
• IEC 60118-4 sets its targets for the listening area, not a limit for overspill, so agree with the client what is acceptable next door.
• Smaller loop elements spill less: figure-8 loops and phased arrays fade sooner, and a cancellation loop can help on one side.
• Paired feeders and a carefully set level matter too. Measure the protected spaces on site.

Why the field reaches beyond the loop

Inside a perimeter loop, the vertical part of the field from every side points the same way at listening height, so the contributions add up. Outside, the nearest side still produces a strong field and the far side, now pulling the other way, cancels part of it. So just outside the wire, the field can be nearly as strong as in the middle of the room.

Further out, the field fades. How quickly depends mainly on the loop's size: a big perimeter loop around a wide hall carries its field a long way, while a small loop fades much sooner.

The field is just as strong at a given distance below the wire as at the same distance above it, so a floor loop also reaches down into the room below. With a large perimeter loop, a seated listener directly upstairs can receive a field not far below the one in the hall. Steel in the slab absorbs some of it, unevenly (see metal loss).

The field next door also rises and falls with the level you set, and a feeder whose two conductors run apart acts as an extra piece of loop.

When overspill matters

• Neighboring loops. Two nearby loops carrying different audio, such as meeting rooms side by side or a divisible ballroom with its partitions closed. Listeners in one room hear the other in the background. If both loops always carry the same program, overspill between them is rarely a problem.
• Confidential rooms. Boardrooms, courtrooms, jury and interview rooms. Someone outside the room with a telecoil or a loop receiver may be able to listen in. If the room must be truly private, a loop may not be the right tool on its own; see hearing loop vs FM, infrared and Auracast.
• Floors above and below. Offices over a hall, classrooms over a sanctuary. A ceiling loop sends as much field up as down.
• Stages and audio equipment. Electric guitar pickups and some other stage equipment can pick up the loop's audio. A guitar is usually held upright, so its pickups respond mostly to the horizontal part of the field, not the vertical part hearing devices use. The horizontal part is strongest close to the wire, so keep loop runs away from the stage where you can.

Overspill into a lobby or corridor is usually harmless. At the site survey, agree with the client which spaces need protecting, including rooms that may be looped later.

Layouts that limit overspill

The smaller the loop elements, the sooner the field fades beyond them. Each option has trade-offs; the hearing loop design method shows where this choice fits.

Perimeter loop, set back or made smaller

The simplest layout spills the most. Pulling the wire back from the sensitive wall moves the strongest outside field away from the neighbor, but weakens the field in the nearest seats. Looping a smaller area also helps but serves fewer seats.

Figure-8 loops

A figure-8 is one loop crossed over in the middle, so current runs round the two halves in opposite directions. At a distance their fields partly cancel, so the field fades sooner than around a perimeter loop of the same size. The catch is a weak line along the crossover: put it under an aisle.

Phased arrays

A phased array covers the area with narrow loop elements on two channels. Each channel is wired as one series run, and the two are driven 90° apart so one channel fills the other's weak lines. Within a channel, neighboring elements carry current in opposite directions, so beyond the array their fields largely cancel. The field outside fades over a distance set by the element width rather than the room width, which is why low-spill arrays use narrower elements.

The trade-off: the field also fades more quickly with height, so the array must sit close to the listeners and the listening height must be right. Low-spill arrays usually go in the floor and need more cable and driver current. See phased array hearing loops.

Cancellation loops

A cancellation loop is a narrow extra loop just outside the listening area on the side you want to protect, with current running the opposite way round to the main loop. Beyond a crossover point, the field on that side drops away more steeply. Know the limits:

• Before the crossover point it can raise the field outside the room. If the protected space starts right at the wall, the benefit may come too late.
• It adds load on the driver and can make the field less even in the seats along that edge. It never adds coverage.
• Small changes in its width make a large difference, so tune it on site. If the benefit is small, leave it out.

Feeders and level

Run the feeder as a closely paired cable all the way to the loop (see loop wire vs copper tape), and set the level carefully at commissioning: a loop run hotter than it needs to be pushes its field further into the neighbors.

Worked example: Community Hall and the meeting room next door

A made-up example; dimensions are for illustration.

Community Hall is 12 m × 18 m, single story, with seated audiences at 1.2 m. Beyond its long east wall is a 6 m × 8 m meeting room with its own loop, often in use at the same time.

1. Name what you are protecting: the meeting room, at its seated listening height of 1.2 m, in the vertical orientation hearing devices use.
2. See why the simple layout falls short. A perimeter loop puts 18 m of wire along the shared wall, and the hall's 12 m width carries the field a long way.
3. Compare the alternatives in the table below.
4. Model each option at 1.2 m in both rooms and pick the one that keeps the hall on target and leaves the least field next door. In Hearing Loop Designer you can try each layout on the same room: the cross-section shows the field beyond the loop edge at your listening height, and the cancellation-loop tool compares the field across a protected area with and without the extra loop. It is a planning model; installed performance is confirmed by commissioning.
5. Plan the site check so the results can go in the commissioning record.

Checking overspill on site

Real overspill depends on metal, wire positions and feeder routing, so check the protected spaces once the cable is in, ideally before it is covered.

1. Measure the background magnetic noise in each protected space with every loop off.
2. Drive the loop with a steady 1 kHz tone at a conservative level, within the driver maker's guidance, with the neighboring loop off. Take a reading in the loop's own listening area at the same level, so each reading next door can be compared with it.
3. Measure where the neighbors listen, at their listening height, reading the vertical field for hearing devices or the horizontal field near stage equipment. Include the floors above and below.
4. Find the source. If the stray field follows a corridor or wall, check the feeder route for split conductors.
5. Tune any cancellation loop before fixing it. Tape it down with about 0.5 m (1.5 ft) of slack at each end of its outer edge. One person reads the meter in the protected space while two move the outer edge evenly toward and away from the main loop; secure it at the best position.
6. Listen with both systems running different audio, using a loop receiver in the protected space.
7. Record where, at what height, in which orientation and at what frequency you measured, and how each reading compares with the listening area, in the record handed to the venue (see commissioning a hearing loop).

Common questions

Does IEC 60118-4 set a limit for overspill?

No. Its field-strength and frequency-response targets describe the listening area. It gives some guidance on spill between neighboring positions for small counter systems, but it sets no overspill limit for a room loop. How much field is acceptable next door depends on the space, so agree it with the client and record what you measured.

Can two rooms side by side both have hearing loops?

Yes. Choose layouts whose field fades quickly beyond the shared wall, pair the feeders, set levels carefully, and check each room while the other runs different audio.

Does a phased array stop overspill completely?

No layout does. A well-planned array makes the field fade much sooner beyond its edge than a perimeter loop would, but some field always reaches past it.

Will steel in the building block overspill?

Not reliably. Steel absorbs part of the field unevenly, and weakens the listening-area field too. Treat any help as a bonus.