Induction loop crackling

[CassieT]

Hi,

 

I had an odd experience in a venue with a temporary induction loop a few days ago and am hoping someone can explain it. So maybe I can sort it out if it happens again.

 

The room we were in didn't have a built-in loop so the organisation's technician set up a temporary loop. Cable taped down round a big block of seats. Microphone on the desk at the front where the speaker would stand. Microphone and loop were both plugged into a box, also on the table.

 

10 minutes into the first talk, loop volume suddenly decreased, varied and sound was very crackly. Often crackles in time with the words, a bit like distortion when volume has been turned up way too high, but more crackly and less buzzy than that. We got the technician down in the tea break who found someone had put a bag on the cable, or moved a chair onto it, and caused a loose connection. Somehow, the message didn't go out to everyone to make sure they kept clear of the cable and the same thing happened partway through the next talk. I don't know why, but we didn't seem to get the technician down again and I swapped to using my personal fm system, giving my radio mike to each speaker.

 

The receiver for my fm system has a neck-loop that is an induction loop. So I have to have my hearing aids on the T setting to use it. So I needed to turn off the temporary loop so I didn't keep hearing the crackly speech it was transmitting. The box at the front, that had loop and mike plugged into it had an on/off switch. So I turned it off, assuming that would stop the loop doing anything. The LEDs on it went off, so it really did seem to be off.

 

But the crackly transmissions continued. It made it more difficult to hear via my fm system over the loop crackles.

 

How was that happening when it was turned off? Or do temporary loops have another on/off or power supply somewhere else?

 

It was great of the organisers to arrange to have a loop but I'd have been better off if we didn't have it and I just had my fm system. But having a loop mean other hearing aid users had the option to use it. Apart from making sure there's a general announcement to keep well clear of the cable, what can I look for in future if I have similar problems? How do I stop a loop transmitting?

 

Thanks.

[Stuart91]

I wouldn't have expected loop cable to be any less robust than, say, a normal electrical flex. Certainly I wouldn't expect it to disintegrate when someone lays a bag on top of it.

 

It's hard to work out what's going on with the box without actually seeing it - is it possible that the on/off switch you saw is a mute for the mic, and the loop amp is elsewhere?

 

The other possibility is that there's another source of electromagnetic interference in the room. Hearing aids on the T setting would get interference if they were near old CRT monitors for instance, but I'm not sure what could be around that would give you the sort of problems you describe.

[bruce]

What sort of venue is it? Are there other meeting rooms adjacent, which also have loops? Could you be getting interference from these?

 

 

When installing loops in these sort of rooms, it’s normal to install fairly complex loops to minimise spill to the neighbouring room. But what’s often forgotten about is spill upwards and downwards - a typical “cancellation loop” will not reduce that.

 

I had one instance where we were getting lots of interference and external noise on a loop. But we couldn’t trace it to a fault with the installation. We eventually realised that the problem still occurred when switched off, and we traced it to the empty room upstairs - the pickup from that room was almost as strong as the installed one. And since the room was empty, the compressor/AGC was ramping the gain up, and noises from the street were quite clearly audible.

[smalljoshua]

Agreed on looking at external sources. Using a quite dense (less than 1m between runs) low-spill design can get vertical down a bit. As can good absorption in the floor (metal system grid floor etc). I've been to service loops in a number of places where this wasn't done and ot was as Bruce described.

 

A lot of things produce electrical noise though. I've had HVAC, grey water reclamation pumps and all sorts of motor based bits of kit putting out horrible EM noise. Pretty much anything with VFD control on it.

 

Josh

[Simon Lewis]

Indeed - it's quite possible that something else was causing the crackling....

 

Can your aid accept a direct input?

[CassieT]

Thanks for the replies.

 

I am sure it was the temporary loop in the room that was causing the problem:

To begin with, there was no crackling.

It suddenly began partway through the first talk and continued until the technician found the loose connection and corrected it.

The crackles were in time with spoken words: it was clear the microphone or something in the loop setup was "hearing" what was being said in our room.

It wasn't a monitor, or unshielded mains etc. They give a constant hum when aids are on T setting.

 

The box had the microphone and both ends of the loop cable plugged into it. There was a button with on/off symbol. When I switched that, all the LEDs went out. So I'd assumed it was turning the loop off. It didn't have a "Mute" symbol. But if it was something specific for the microphone, maybe it wouldn't have.

 

Yes, direct audio input would have worked well. Unfortunately I don't have "shoes" for these hearing aids as we kept having to adjust the tuning (long story) and didn't know if I'd need different aids. So I didn't want to splash out on buying shoes only to find I needed new ones 3 months later. As it turns out, audiologist has referred me to senior and probably will be getting new aids. I think this time I may fork out regardless, just so I always have the option.

 

I think I'll see if I can get in touch directly with the technician to let them know what happened so they can try to sort it out for next time. It was the first time they'd been responsible for a temporary loop, so tough situation for them.

 

Thanks for your help. If anyone has more suggestions about what happened, I'd be glad to hear them. Thanks.

[richardash1981]

I'm still puzzled why the loop cable seemed to be in such a fragile state. It should be at least 1mm2 for most loops (much thicker in many cases), so will be comparable to mains cable. It might not like a chair being stood on it, but I've seen a lot of abuse to fixed loop cables and never had such a problem. I wonder if the cable for the temporary loop wasn't up to the job, or was unsuitable for the loop amplifier (which could easily cause the amplifier to clip, giving very crackly distorted sound). This might just be trying to drive too long a length of cable for the amplifier rating (the larger the loop, the more current is needed to produce enough signal, and the more voltage is needed to drive the current, so loop amplifiers have to get bigger with the square of the size of the space (roughly) - so quickly!).

 

Was there any other sound system the room which could have been picking up the people speaking? I wonder if what you heard was in fact the power supply current of the sound system, which would be in time with the words but very distorted (full wave rectified for a start). This might happen due to defective power wiring (either fixed or extension leads) to the sound system.

 

Last thought: might the 'portable' loop box have been meant to be battery powered, but with a fairly-defective battery? As a result the technician had left it connected to it's mains charger so that the charger kept it powered. When you switched the charger off, the lights on the charger went out, but the loop amp kept flailing on it's mostly-flat battery to produce signal, which could leave the distorted mess you describe. In this case if the charger is a trickle type, the problem may have been that the loop worked OK when it was only spoken into intermittently during testing (the charger kept up), but as soon as someone spoke continuously (in the sessions) the battery was flattened. Hence it broke in both sessions but seemed to work in the break? This would not be too surprising if the kit has been in a cupboard without being charged correctly and the re-chargeable battery has rotted (they don't last that long anyway).

 

What you technicians really need is a Field Strength Meter - works like a T coil hearing aid, but with a headphone socket for normal hearing users so they can hear what you are getting, and a display of signal strength so that the loop can be set to the correct signal levels. They aren't very common or that cheap unfortunately.