I know nothing about electric Guitars, but we have a problem with the electric bass guitar in the pit picking up the magnetic deaf aid loop and then going into feedback.
Can electric bass guitars use any other type of pickup that is not sensitive to magnetic loops? if so which?

Well - like any electric, they could use double coil pickups (hum-buckers) or the guitar could be fitted with individual string synthesis/sampling systems like the variax ones, or even midi pickups - but the majority of instruments have pickups for a particular sound. If the audio loop has been poorly installed, then the problem is often impossible to solve without changing instruments - and, as the player will tell you, there is nothing wrong with his guitar, it just can't tell the difference between string and loop! Many loops are run around the perimeter when they could have been profiled to avoid the pit and stage area.

I assume you have experimented with different positions in the pit?

A delay in the feed to the induction loop amplifier might fix it, but will probably upset the audience members who, like me, have only one hearing aid.

Your bassist is likely to be very reesistant to changing his pickups for this reason, especially if it's going to be at his cost!

Seconded!

On a more constructive note - from the OP's notes it sounds like the bass is being mixed into the signal sent to the loop amp. Assuming the pickups can't/won't be changed - could the sound operator remove the bass from that mix? For all affected instruments, I find that doing this AS WELL AS ensuring the musicians turn their amps off during "talk" sections is usually as failproof as any human.

Bit more info, the Bass is not fed directly into the Loop. The Loop amp has a mic hanging over the stage as its only input, if we turn the gain down on the loop amp then speech is not loud enough over the loop. The loop is a seperate system not directly connected to the sound system.
The pit were complaning that the bass was too loud in the pit (Local amp) and wanted the bass fed into the stage foldback monitors so they could turn their amp down in the pit. Reasonable request, espescially with the new sound regs!!
So the bass (along with keyboard) is being fed into the foldback monitors via the desk, being picked up by the loop mic, sent out over the loop and being picked up by the bass pickup so completing the chain and feedback.
Have tried using EQ on the bass channel on the desk but the MD & Guitar player complained that the 'sound' of the bass guitar was lost.
I don't think its the bass notes causing the trouble, as it can go into feedback without playing a note, it's more that the bass pickup is responding to the loop signal and then we are feeding it back into the loop mic input via the Foldback monitors

I think I'll tell the bass player that if he wants to feed his bass into the system then he will have to find a pickup that is not affected by the loop or put up with a high level in the pit!!

The loop layout is optimised for the the best audience coverage. If we move the loop cable away from the pit towards the audience then the first few rows will have reduced signal levels and be more sensitive to orientation of the deafaid(We tried this during the install of the loop)

Our audience average age is 60+ with quite a few 90+ so turning off the loop is not an option!.
You want to see 80 year old ladies screaming at 'The Full Monty'

If I was the bass player - I'd be tempted to tell you where to go (from his perspective). There are only two workable alternatives - borrow a different guitar that may be less problematic, or get him to play through his amp at a lower volume than he wants. He's already annoyed having his control being removed from him and passed to the sound dept. I hate this, and find it very difficult to play. The simplest possible solution is to try and get a guitar swap, but putting everything through one monitor mix would be totally and utterly unacceptable to most musicians - they need to hear themselves. If you have your instrument through one speaker, and the band in another, you can mentally separate them, but having them all through one speaker is a real messy problem.

Sound level is likely to be worse - as all these sources will need to be potent, and sitting next to a bass amp at X volume is less trouble than a mix at the same level.

Why is it that the musicians needs are rarely taken into account? Get a painter in to paint a portrait and say "I'd like a realistic painting please, but I don't want you to use both eyes, and can you not use Blue????"

If you are doing the Full Monty, the band have some LOUD songs to play - have you tried screening? I've not tried it, but how about a foil or mesh grounded panel between the guitar and the audience?

I have come across a smilar problem to this in the past, it turned out the solution was simple, we just moved the FOH & Monitor amps as far way from the Induction Loop Amp as possible.

So my suggestion although it may seem to be the bass pickup, is to experiment with the position of the bass players guitar and amp and also don't rule out the monitor amps.

The monitor & FOH amps are 17mtr away from the outside of the loop and don't pick up anything any other time. Good point about the guitar amp tho' I'll try to get it moved in the pit.

Just a quick tuppence on this one, I would check the bass players lead and make sure its properly shielded and of high quality, also check the shielding and earthing in the bass as this is usually the quickest way to make a bass microphonic, would be interested to know if the problem reduces if the player backs the bass guitars volume control back a little bit as it could be a problem with his volume control on the way out or just being a cheap component

Although the loop could be picked up by these devices, it's more likely to be picked up by a coil of wire - which is why the instrument's pickups are supect. Orientation can make a big difference, so getting him to turn 90 degrees can sometimes solve the issue. Magnetic shielding is rarely achieved using screening - you need to use something like mumetal.

Is it possible to feed the loop from the desk, instead of using an overhead mic? The mic approach rarely delivers the best intelligibility anyway ;-)

Simon

Can you DI the bass and give the player headphones, possibly with a local mic mixed into his cans for ambience?

The bass would still be coloured by the loop though, so would no doubt sound odd, but if you fed this through the pa, and into the loop it feedback again?

try reversing the loop connections , this can sometimes have a significant effect , In one of the local churches I have a changover switch fitted ready for any such problems

One other thing worth checking though not so common on bass amps is the old spring line reverb units.

Problem fixed tonight, he brought another guitar!! All problems went away but he does not know what type of pickup it is.

Thanks for all the suggestions,

Lufty,

it seems strange that this could have a marked effect... do you know why it works? Might it be tied in with something else (like temporarily stopping magnetic oscillation?).

Simon

I've been asked to screen the pit from the magnetic hearing aid loop as much as is reasonably possible.
I've gone down the route of moving the loop wire away from the pit wall as much as possible until it starts to effect the signal strength for the end user.
Next step is to fit some type of screen/barrier on the pit wall nearest the loop.
My question is what is the best type of screen, (discounting Mu Metal due to cost), I can get thin steel sheet at reasonable cost but would mesh/chicken wire be better for these audio frequencies., Would aluminium foil backed plasterboard be any good?

I'll be interested to hear what people suggest - I have problems with this too. Single coil guitar pickups are real terrors for picking up loop signals - even with carefully screened guitar body cavities.

It is possible to use different layouts of the loop cable to minimise the overspill - the single turn rectangle isn't the best shape for this. Phased arrays can give cancellation in the areas where you don't need signal. Ampetronic have a range of datasheets and useful information - take a look here for stuff.

Altering the array is not really possible, we would have to take the seats out to take carpet up!
We have a phased array at the moment, just like example 2 in your link, which gives a good even distribution.
We find the bass player does not take kindly to being told, 'Use a decent twin coil pickup or you can't have the job!'
In the last musical the feedback was so bad that we had to turn the loop off and then the hearing impaired complained.
The only way around it was to turn his local amp in the pit right down, put a DI box in and feed his signal via the FOH desk into the FOH speakers. They are far enough away from the Loop mic so not to pick up much. Then the MD complains that he can't hear the bass so can't balance the pit.
I'm going to take up lighting, it must be easier!

You've just proven that his guitar isn't the culprit - it's the amp. If you can DI it, it doesn't matter how far away the speakers are? So you could devote a spare send to him, and give him a proper monitor.

Not explained myself fully
The pit noise (music?) is picked up by the overhead loop mic positioned downstage angled upstage.
The guitar then picks up the magnetic loop and puts it out via his local amp in the pit, this is picked up by the overhead loop mic and off we go again.
Turning down the local pit amp & putting the guitar out via the FOH breaks the loop as the overhead loop mic does not hear the FOH speakers very well. Putting a local monitor speaker back into the pit would just take us back to square 1 (unless it was an 'in ear monitor' or headphones!)

All very interesting but digressing from original question

Thanks anyway Paul

Moderation: I had a sense of deja vu, and discovered the OP had started another version of the same topic? So I have merged them - we've been through this before!

Before you dive into building modifications...

Thinking outside the box for a moment. To stop feedback you need to change something in the feedback loop. The easy one is gain, once it's below unity: problem solved. Now, I've noticed over the years that radio mics seems less prone to feedback than wired mics. I've never fully investigated but have always assumed that this is down to a combination of various phase shifts going on. I also remember from a few years ago that someone (Surrey Electronics?) used to make a feedback killer that worked by introducing a fixed, 9Hz?, frequency shift to the audio signal. In those days it was done analogue and so probably sounded rubbish.

With the advent of cheap digital processing I'm wondering if anyone has tried introducing a pitch shift on the signal fed to the loop?
Or using a radio mic to feed the loop amp?

After managing to wipe a half written reply last night, I saw that I had responded to the original thread.... (good memory Paul!)

Screening loops is a thankless task, and your proposed techniques may reduce both the wanted and unwanted signal.

Ampetronic's technical department may be able to suggest some solutions, but typically it will involve altering the loop layout.

It does look like the gain of the loop system needs altering, the mic position changing, or better still to not use the mic.

If the show is 'reinforced', can not the loop be fed from the desk, and less from the overhead mic? Although apparently fine for people with good hearing (who are of course the ones who set them up!) overhead mics often fail to give the necessary direct sound, and pick up more of the reverberant field, (together with the band in this case).

Simon

Merging the posts has confused the issue, my original question was posted on 3rd July

Quote
"I've been asked to screen the pit from the magnetic hearing aid loop as much as is reasonably possible.
I've gone down the route of moving the loop wire away from the pit wall as much as possible until it starts to effect the signal strength for the end user.
Next step is to fit some type of screen/barrier on the pit wall nearest the loop.
My question is what is the best type of screen, (discounting Mu Metal due to cost), I can get thin steel sheet at reasonable cost but would mesh/chicken wire be better for these audio frequencies., Would aluminium foil backed plasterboard be any good?"
Endquote
The replies, altough interesting, do not address the question asked, but instead concentrate on suggestions for an earlier topic (which has been merged).
Many thanks to those who have taken the trouble to reply

The answer is that the manufacturers do not recommend trying to screen the loop signal, but prefer to use one of several low spill designs. You may achieve some degree of attenuation, but this may not be enough to solve your existing problem. When designing loops, typical figures for (frequency dependent) losses due to metal in the plane of the loop are:

reinforced concrete floor 3 to 6dB
metal system floor 9dB+
suspended ceiling 0 to 8dB but can be as high as 20dB

All figures are for a 2m loop width (source - Ampetronic training notes).

Whether you can achieve sufficient attenuation (bearing in mind that you will typically attenuate high frequencies first) is down to the loop layout and topology, building construction, current flowing through loop and the devices that are picking up the loop signal.

Metal often degrades the loop's performance, but it doesn't necessarily 'stop' the signal. I would again suggest talking to a manufacturer's technical department - it would be rash to suggest any type of screening, only for you to find that it does not provide the result you need.

Simon