An induction loop is a wire round the room and an amplifier that feeds the film's dialogue into hearing aids by magnetic field. Ours is measured to a standard with a field strength meter. In screen 2 the meter reads a signal with the amplifier switched off and unplugged, and two customers have complained about a man talking over the film.
I am the technical manager for a small cinema chain, and the hearing loops are mine because nobody else wants them.
How a loop is tested.
There is a standard for it. You put a field strength meter at seat height at several points in the room, play a reference signal through the loop, and the meter has to read within a band. Too weak and the aids hear nothing, too strong and the sound distorts. You test annually and after any work on the floor, and the certificate goes on the wall by the box office because the law says it does.
With the amplifier off, the meter reads nothing, because there is no current in the wire. That is not a subtle test. It is the same as a light being off.
Screen 2's loop reads a signal with the amplifier off.
Not full strength. About a third of the reference level, fluctuating, with the pattern of speech.
I found it doing the annual test in 2022. I had the amplifier switched off to take a background reading, which is the first step, and the meter was reading, and I assumed I had left it on, and I had not.
I unplugged the amplifier from the wall. The meter read.
I disconnected the loop wire from the amplifier's terminals, so the wire round the room was connected to nothing at all at either end.
The meter read.
What a loop wire connected to nothing can carry.
Nothing. A loop is a wire. A signal in it needs a source driving current round it, and with both ends free there is no circuit and no source.
Except that a loop of wire is also an aerial, and it will pick up any magnetic field in the room and carry that. That is the real explanation for stray loop signals and I chased it properly: lighting dimmers, the projector, the sound amplifiers, a neighbour's loop through the wall, a lift motor.
All of it switched off, one thing at a time, then all at once, with the building on emergency lighting only.
The loop reads a third of reference, in the pattern of speech.
What the customers say.
Two complaints in two years, both from hearing aid users, both about screen 2, both the same thing: a man's voice, talking over the film, not in the film.
One of them was kind enough to talk to me about it. She said it was one voice, a man, speaking quietly and steadily the whole way through, not in English, and she thought it was another film bleeding through until she realised it was on the loop and not in the room.
I asked her what language. She said she did not know, and that it was not one she had heard, and she has travelled.
What has been done.
The loop wire was lifted and replaced in 2023. The floor was up, the old wire came out, new wire went in on a new route, new amplifier, new certificate.
It read a third of reference with the new amplifier off before the floor was down.
I have recorded it. The meter has a line out and I put a recorder on it for a whole screening with the amplifier disconnected, and I have an hour and forty minutes of a voice at a level just above the noise, and it is speech, and the sound engineer I sent it to, without saying where it was from, said it was a man reading aloud, in a language he could not place, and asked me where I had got such a clean recording of it.
What is done about it.
Screen 2's loop is certified and it works: with the amplifier on, the film's dialogue is loud enough that the other voice sits under it, and most hearing aid users never notice.
The two who did were given tickets for another screen and told there was a fault on the loop, which is the truest sentence available.
The certificate on the wall is correct. The standard measures the loop with the amplifier on and it passes.
What I do.
I test the loops and I file the certificates.
The one change is that I do the screen 2 test with the house lights up and the doors open.
The test is normally done in the dark with the doors shut, because that is the condition the customers sit in, and the meter does not care about light.
In 2023, with the new wire in and the amplifier disconnected and the meter reading, I switched the lights off to take the reading in the dark as the standard suggests, and the meter went to full reference. The moment the lights went. Not a third: the level the loop reads when the amplifier is on and the film is playing, in a dark room with a wire connected to nothing, and the recorder was running, and the voice on that minute of tape is not quiet and is not reading.