Morning,

I have recently installed a new induction loop and have noticed that the mic signal is bleeding though all the other speakers in the theatre, I have moved the loop amp away from sound room, re routed the mic lead and made sure that the loop cable is well away from the speaker lines, but still it bleeds though.

Any ideas?

You need to make sure that no part of the multicore, mic cable, mixing desk, returns lines, or microphnes are inside the loop field, otherwise they will become excited be the electro-magnetic field, and cause thebleed you hear.

It may also be, if everything has been put in correctly, to much power going into the loop, causing it to radiate outwards a huge distance.

Have you had a listen with a loop tester? Or just checked all the lights come on on the front?

When you say 'all other speakers' are you talking about the show relay speakers? If so, do you feed them from a dynamic mic?

Medina, Rusty,


The loop radiates inside and outside the cable perimeter! Therefore, simply placing kit outside the loop will not necessarily stop this problem.



Possible, but for it to work satisfactorily, the loop needs to achieve the required average and peak mA/m stated in IEC 60118-4.


This can be a useful diagnostic tool, but I suspect that the loop signal is getting into the audio system via cable or components. The following may be a useful procedure:

Try switching the loop on, and feed it with a signal (CD / test tone etc.).
Switch off and disconnect the audio system. Check that nothing can be heard in the loudspeakers on their own, then plug in the loudspeaker cables one by one. Then proceed with individual amplifiers, switching them on with no input. Then connect the amps to your mixer, and see if that is picking up the signal. Finally, connect the multicore (with nothing plugged in) and then connect each individual input. This should at least help to pinpoint where the loop signal is being picked up.

Typical problems include dynamic microphones with poor magnetic shielding, older pieces of audio equipment (pre EMC regs), and single coil guitar pickups.
Also, there is a variation of the earth loop problem where plugging the loop amp and the audio system into different mains outlets can cause the loop signal to be heard on the audio output.
Lastly, from the point where the ends of the loop meet, the two wires should be tightly twisted to cancel out the radiated signal between the perimeter loop and the loop amplifier. The actual signal strength should be checked using a suitable loop strength meter, and the frequency response checked for compliance using a calibrated receiver and suitable analysis device (e.g. NTI Minilyser, Smaart etc.).
The position of the loop cable is quite important, and the installation instructions shoudl have given you good advice on placement, height, aspect ratio and required loop current.

A listening test is recommended (so hopefully, you have a loop receiver?) as this helps to differentiate between wanted signal and background noise (although that should have been identified in the pre installation noise survey). Personally, I would also try and conduct a speech intelligibility test using something like a NTI Talkbox and a STI-PA meter, so that the input to the system can be checked.

If you find that a key piece of equipment (mixer etc.) is picking up the signal, it may be possible to reduce the breakthrough, but many theatre installations avoid such potential problems by installing infra red system instead.

Do let us know how you get on...


Simon Lewis

show relay seems to be the only speakers not effected! its bleeding though F.O.H, rear fill and the entire cinema set up
the loop cable runs around the entire auditorium and because of the design of the theatre around the rear of the stage. I have kept the loop cable as far away from tie lines as possible but speakers are still within the loop field. the mic for the loop in hanging from the gantry in the middle of the theatre could this be the problem?

Medina,

Unless that mic is also connected to your audio system, I suspect it isn't the problem. However, although it is common practice, suspending a mic above the stage is often a very poor way of driving a loop. It does not improve the signal to noise ratio - the very problem that the hearing impaired listener is suffering from!

Try the troubleshooting guide above, and see if you can determine where the signal is getting in.


Simon

This smells like unbalanced audio feed to power amps...

I second the vote for something being unbalanced. Good quality, well screened, balanced lines should be fairly immune to the loop. Most venues are gonna have cabling in the vicinity of the loop by definition.

We often have problems with some guitarists getting the loop in their amps. But thats because they are using miles of unbalanced audio cable in their rigs and its acting like a giant arial

Electrostatic screening provide little protection against magnetic fields ;-)

The best way is to use twisted cable pairs, and then use distance to separate the receiving cable from the source. Generally, ferromagnetic material will be needed to make screening effective.

Screening, no. But the CMR of a properly balanced line should still be effective on interference from a magnetic field, at least as far as cabling goes. Dynamic mics, on the other hand...

Bob