Hearing loop site survey checklist

Key points

• Survey before you design. Drawings show what was planned; the site shows what was built.
• Pin down the listening area and listening heights first.
• Look hard for metal: slab, deck, framing, ceiling grid, stage and seating frames.
• Find out where the conductor can go and what the owner will allow.
• Settle the driver location, power, audio feed and feeder route on site.
• Measure background magnetic noise with the building running. A model can't predict it.
• Leave with a dimensioned sketch, photos, readings and a list of open questions.

Before the visit

Ask the venue for floor plans with the seating layout, structural or as-built drawings (they show the slab, rebar, steel deck and framing), and any plans for flooring, ceiling or AV work in the next few years.

Book the visit for a time when the lighting, HVAC and AV can run as they do during events. Noise readings in a quiet, empty building can mislead you.

How the room is used

• Listening area. Mark the seats, standing areas, lecterns, counters and stage positions that need coverage, and the space that doesn't.
• Listening heights. Seated listeners are usually planned at 1.2 m (about 4 ft) above the floor they sit on, standing listeners at 1.7 m (about 5 ft 7 in). Note tiers, platforms and balconies with their heights. See listening height.
• Changes. Does the seating move, does the room get divided, is a renovation planned?
• Neighbors. Note rooms beside, above and below: other loops, confidential spaces such as offices, and rooms in use at the same time. They decide how much overspill matters.
• Instruments. Ask about electric guitars, basses and other instruments with magnetic pickups. Near the loop they can pick up its signal and feed it back into the sound system.

Construction and metal

Metal in the structure weakens the field, and the high frequencies most. It is a common reason a loop that looked fine on paper comes up short. Collect clues:

• Age and structure. When was it built? Steel frame, concrete, timber or masonry?
• Floor slab. Rebar or mesh, post-tensioned tendons, or concrete on a corrugated steel deck, which is easy to recognize from the floor below.
• Raised floors and stages. Lift a panel or look underneath for steel panels and pedestals.
• Ceilings and walls. Metal grid or tiles, foil-faced insulation, metal studs, cladding.
• Local steel. Beams, columns, stairs, seating frames, bleachers. Mark them on the sketch.
• Quick tests. A magnet finds steel, but not aluminum, which still causes loss. A rebar scanner shows whether a slab has bars and roughly how far apart.

Note how far the conductor would sit from the nearest metal in each install option. Where the construction is unknown or clearly heavy, plan a test-loop measurement before committing to a design; the method is outside this checklist. For what your findings mean, read metal loss in hearing loops.

Never cut or drill a post-tensioned slab until the tendons have been located and the owner has approved the route and depth.

Flooring and surface access

• What is the finish: carpet, carpet tile, wood, vinyl, tile, terrazzo, sealed concrete or stage decking?
• Is it being replaced? New flooring or a new slab is the easiest time to install a loop.
• Can the carpet be lifted and relaid without affecting its warranty?
• Is saw-cutting allowed? Any heritage, landlord or finish restrictions?
• Could older floor tile or adhesive contain asbestos? Have it checked before anything is disturbed.
• Photograph thresholds, transition strips, expansion joints, floor boxes and seat fixings on likely routes.

The floor finish often decides between round wire and flat copper tape. See loop wire vs copper tape.

Ceiling and below-floor access

• Ceiling. Height, construction, and what is up there: ducts, sprinklers, cable trays, lights. Ask whether the space above the ceiling carries return air: local fire codes then limit which cable can run there. Note any fire barriers a route would cross.
• Below. Is there a basement or crawl space under the whole listening area? Is the floor timber or concrete, and how thick? The floor's depth adds to the distance from the loop to listeners. A steel deck between the loop and the listeners absorbs much of the field, the high frequencies most, so a loop fixed below one is rarely workable.
• Access. Ladder, lift or scaffold for each route, and the hours work can happen.

Rack, power and the audio feed

• Driver location. Which rack or closet, how much space, and is there airflow? Drivers get warm under load.
• Power. A spare outlet near the rack, ideally on the same circuit and earth as the sound system. Audio equipment powered from different circuits can form a ground loop, and any hum it adds to the feed goes out over the loop.
• Audio source. Which mixer, processor or matrix output will feed the loop, at balanced line level, with which connector? Who controls gain, EQ and locked settings?
• What the feed carries. Speech microphones, media playback and anything else listeners need. Ask how microphones are used: a handheld mic held far from the mouth sends a weak signal full of room sound, and no loop can fix that.
• Feeder route. Measure the path from the driver to where the loop starts along the route, not in a straight line. The feeder's resistance adds to the loop's, so the driver needs more voltage for the same current. Twisted pair or star-quad cable keeps the added inductance small. Each channel of a two-channel array needs its own feeder. See sizing a hearing loop driver.

Background magnetic noise

Background noise is the hum and buzz a telecoil picks up from the building's electrical systems. A loop can't cancel it and no model can predict it. HLAA's installation guidance makes the same point: a computer design is a starting point, but it can't show whether background noise is present.

1. Run the building as it runs during events: lighting at its usual dimmer settings, HVAC, lifts, displays and stage lighting.
2. Set the field-strength meter to its A-weighted background-noise reading.
3. Take readings across the listening area at listening height, with the meter reading the vertical field, the direction a telecoil picks up when the listener sits upright. Take more near panels, transformers, dimmers and equipment rooms.
4. Listen with a loop receiver too. Mains hum, dimmer buzz and the whine of motor drives sound different, which helps find the source.
5. Log each reading with its location, the time and what was running.

IEC 60118-4 sets a target for how quiet the background should be, as an A-weighted level relative to its reference field strength of 400 mA/m. Commissioning a hearing loop covers that target and how to read your readings against it. The noise matters even if the venue picks another system: telecoil users on FM or infrared listen through neckloops, which pick it up too.

What to bring back

• A dimensioned sketch: room, listening area, seating, platforms, an origin corner and the front of the room.
• The listening height for each area.
• Construction notes and photos.
• The install options the owner will accept, and who approves them.
• Background noise readings, with locations and conditions.
• Rack location, power, audio output details and the measured feeder route.
• Contacts, access hours and open questions.

Hearing Loop Designer's printable site survey sheet covers the same ground: a site evaluation checklist, a page for sketching the floor plan, feeder route and driver location, and a log for metal-loss readings. The room size, listening heights, construction and feeder length you bring back are what you enter to start the design. From there, the design method takes you to handover, and commissioning confirms the installed result.

Common questions

Can I design a loop from drawings without a site visit?

You can sketch a first layout from good drawings and photos. But drawings don't show background magnetic noise, and they are not always as built. Mark every assumption and confirm it on site before anything is ordered.

What equipment do I need for a survey?

A field-strength meter that reads A-weighted background noise, because nothing else gives you numbers. Add a loop receiver, a laser measure, a camera, a magnet and a flashlight. A rebar scanner helps on concrete floors.

Who should be at the site visit?

Someone who can open ceilings, closets and crawl spaces, the person who runs the sound system, and someone who can approve floor and ceiling work. Decisions made on the spot save a second visit.

What if the background noise is too high?

Find the source first: dimmers, transformers, electrical panels and wiring faults are the usual suspects. Building wiring problems go to a licensed electrician, and protective earth must never be disconnected to cure hum. If the noise can't be brought down, move the listening area away from it where you can, and tell the client before the design goes further.