Ask the Tech Expert: How to stop our old speakers picking up the signal from our hearing loop?
Hearing loop problem
Neil Coode from Cranleigh in the UK asked us for help with a T-Loop problem. He says:
“We have a rather nice hearing loop installed in our church. We also use one or two (depending on band) older wedge speakers for foldback. Unfortunately the wedges pick up signal from the hearing loop which causes problems with distortion and feedback. How do we solve this without spending loads of cash on individual foldback monitors?”
We asked Pat Smith from SFL Group to respond:
It is difficult to fault find an issue like this at a distance without knowing significantly more about their set-up, but we can give some hints.
It is extremely unlikely that the loop is interfering directly with the foldback monitors so buying new monitors is unlikely to help with the issue. It is more likely that the loop is interfering with one or more of the sources which they happen to be feeding into the monitors. The prime culprits are usually electric or bass guitars. The pick-ups in these instruments work on the same principles of electromagnetic induction as hearing loops, so these will pick up signal being transmitted via the loop. If these are feeding back into the monitors you will hear content from the loop in the monitors via this route.
There are a couple of solutions to this. Ideally we would fit the guitars with humbucker pick-ups which are designed to reject this form of interference and should effectively eliminate the issue. If this is not possible then we need to think about the signal chain and what we can do to minimise the associated problems. In the first instance we should think about what sources are feeding the hearing aid loop, advice from the hearing aid manufacturers is not to send musical content via the loop (it just doesn’t work, the loop system is not designed to accommodate that kind of dynamic and frequency range) so we should really only be sending it the spoken vocal mics. Generally there is not a lot of overlap in use between these mics and sung worship so we can minimise loop interference issues by keeping these mics muted when not in use, and similarly by muting the monitors when not in use.
If it’s not guitar pick-ups then identifying the issue is harder. If the amplifiers for the wedges are very close to the loop (this could potentially be the case if they are powered foldback wedges and the loop runs across the front of the stage area) then it is possible this could be where the interference is occurring, though I would suggest that they would need to be in extremely close proximity (i.e. amplifier sat directly on the loop) for this to occur.
Of course we’re also assuming that it is the loop that is the issue here. I’ve certainly been in scenarios before where even quite experienced engineers have incorrectly inferred that the hearing loop is the cause of an issue. Generally when fault finding we have to be very cautious about jumping to conclusions that potentially overstep the evidence. Without knowing more I can’t say whether the Neil’s assertion that the hearing loop is the issue is sound or not.
Our Church Sound Tech Course (on DVD, download and via our subscription site) is designed as a very comprehensive guide to church sound – you could watch it as a group over a series of training evenings. Watch a sample section on the “hierarchy of importance” for free here.
For those of you near London or the UK South East, Pat is running a day of Sound Tech training for Musicademy on 19 March.
If you have any other PA and Sound Tech questions, please post them below so we can reply in a future article.