Magnetic screening of the pit

[boswell]

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,

[Simon Lewis]

try reversing the loop connections , this can sometimes have a significant effect

 

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

[boswell]

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?

[revbobuk]

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.

[boswell]

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!

[paulears]

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.

[boswell]

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

[paulears]

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!

[Brian]

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?

[Simon Lewis]

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

[boswell]

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

[Simon Lewis]

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 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