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Hearing Loops and induction coils

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Justice C. Bigler:
I'm being asked to research a bunch of information on augmenting our hearing impaired system with a "hearing loop" or magnetic induction coil system so that people with certain types of hearing aides can use this system to listen to our shows versus the IR or RF assisted listening systems that are more common.

http://www.hearingloop.org/

Does anybody have any kind of experience with something like this?

From what I can tell it looks like a physical wire that surrounds the audience area and transmits a magnetic signal to hearing aides that are equipped with something called a "tele coils". What are the possibilities that something like this would interfere with our microphones, speakers, wireless mics, com systems, other computers and electronic devices that we use to run our shows?

On a side note, where do I need to go to find out what our legal responsibilities are regarding what kind of assisted listening systems we are required to have? We have the more traditional Sennheiser infrared system, but always get lots of complaints regarding it not working, or sounding bad. I'm looking into an RF based system that will eliminate the line of sight issues.

Steve Bunting:
As far as interference goes, I've always had problems in venues with a magnetic loop installed inducing whistling in guitar pickups. There was no solution we could find other than turning it off, which the venue were often more than happy to do. Other audio equipment was never an issue.

Brad Weber:
Assistive listening requirements are part of ADA.  ADA defines when ALS has to be provided and how many people must be served, but does not define how to do it except to some degree for where wired systems are allowed.

Induction loop, RF and IR are all acceptable approaches for wireless systems and each has pros and cons in different applications, for example courtrooms and some corporate boardrooms and theatres use IR to limit the signal from being picked up outside the room.  An induction loop system has to be properly designed for the space and audience area and may not be the most economical or practical choice for large, existing spaces.  It has the advantage of not requiring receivers for those who have hearing aids with T-coils, however anyone else wanting to use it does require a receiver.  Interference is most often a problem with things like guitar pickups, keeping the loop away from the stage usually drops the level enouhg that this is not a factor but if there is minimal distance between performers and the audience it can be a problem.  RF and IR do require everyone to have a receiver but also have the option of personal induction loop antennas that can be connected to the receiver for those with T-Coil hearing aids, in fact supporting this is part of the ADA requirements.  RF and IR also have the possible advantage of being multi-channel if you want to use the system for both assistive listening and interpretation.

A good general summary of the different options can be found here, http://www.access-board.gov/adaag/about/bulletins/als-c.htm.  And the requirements for the US are in the two documents referenced here,http://www.ada.gov/2010ADAstandards_index.htm, you have to dig to find all the relevant information but you might start with Sections 219, 221 and 706 in the first document.

Brian Ehlers:
Ampetronic has some very good white papers on the design and operation of inductive loop systems.  You'll see in their simulation plots just how far and strong the "spill" of the magnetic field is outside the loop;  the field is not contained entirely within the loop.  There are various loop constructions which can really reduce the spillage:  picture a line-array where every other element is out of phase.  Obviously, the design and installation of such a complicated loop system costs a lot more.

Be a little careful with the hearingloop.org website.  While I know they have good hearts and good intentions, they present a VERY one-sided picture.  You can't deny the laws of physics.  If you create a magnetic field, it will be picked up by ALL loops within that field.  Whether or not it causes a problem is a matter of the size and number of turns of the receiving loop and the gain applied to the signal it receives, which is then sent back out through the building's loop, etc.  (Just like acoustic feedback with mics.)  Think about how simple physical separation between the signal and ground wire inside a guitar amp creates a loop -- and the gain then applied by the amp -- and you'll start to get nervous.

Search the old ProSound Web board for inductive or induction and you'll find many horror stories about interference.  Actually, you'll find far fewer stories than I would expect;  but many of them come from Europe, which makes sense, since loops are much more prevalent over there.

I, too, would love to hear more stories (good or horror) from people with loop experience, as I am being pressured to install one in my church -- and I"m scared silly it'll cause interference and be a waste of money.

AnttiR:
You can find a suitable area for induction loop a bit further away from stage. The reception level and quality varies depending the distance from the loop-wire. In perfect world, on the way in there's a sign which informs the audience if there's a loop used and also there would be a rough seating map with marked area for the best reception. At least that's how it is or is supposed to be in our country. Normally all churches here have the loop and quite many of the them don't have the whole seating area surrounded by it, just a certain area.

Keep it away from stage, as most of the problems (magnetic pickups and light dimmers) are on stage or closeby it.

AnttiR

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