Assistive Listening Systems - Frequently Asked Questions
- Q. What does AFILS stand for?
A. Audio Frequency Induction Loop System.
- Q. I don't have a hearing aid. How can I tell if the loop system is working?
A Use a battery powered portable receiver such as the ILR3.
- Q. I have a loop fitted but how can I check it meets standards?
A Good question! Probably the best or easiest option is to call in a specialist to test it for you. Failing that, you can buy your own test equipment and do it yourself. This is probably the most cost effective option and allows you to test your system regularly. Finally, it might be possible to hire out the test equipment. A suitable tester would be similar to Ampetronics new FSM.
- Q. My building is difficult or historical and I can't alter the decor or floor - what can I do?
A. Put the loop high up in the roof where the wiring will not be seen. You will need to use more power (hence more amplifiers) to project the correct signal strength down to floor level but this is not a problem. Alternatively, if you have access under the floor, fit it underneath.
- Q. Can I fit an Induction Loop in an elevator (lift)?
A. Yes - contact Ampetronic email for details.
- Q. Can I install two different loop systems in adjacent rooms without them interfering with each other?
A. Yes - You can use what are called "low spill" designs - contact Ampetronic email for details.
- Q. There IS a loop fitted but it has 4 (or more) turns of wire around the hall and dosen't work very well - what can I do?
A. You have an old fashioned VOLTAGE loop fitted which uses a standard audio amplifier. These were OK at first but have been replaced by CURRENT loop systems which are much easier to fit and produce more consistent results. Contact Ampetronic email for more help.
- Q. Putting a short piece of wire or metal foil across the output of the amplifier short circuits it - surely?
A. A normal amplifier - yes, it would overload and either be damaged or blow a fuse. Induction Loop amplifiers are NOT normal amplifiers but are special and are designed to give a CURRENT output rather than a voltage output as a normal amplifier. Special circuits measure the current flow and continuously adjust it to give maximum output yet remain perfectly stable under all conditions.
- Q. I've run a design through your online design software but the amplifier(s) chosen are much larger than I expected - Why?
A. The software compensates for anything that will degrade the loop signal by adding more amplifier power. So for example a long thin room with a low aspect ratio (width / length) needs more power than a square room where the length and width are the same value. Added to this, the height at which you plan to install the loop cable is critical and should be 12-16% of the shortest dimension but related as well to the listening height. If you go even slightly out of this band, extra amplifier power is necessary to guarantee the loop signal meets specifications. So even in a small room, installing the loop cable at a high level may need an ILD9 to drive it.
Try altering the loop cable installation height, or, try to make the area covered by the loop cable closer to a perfect square (even if this makes the area covered by the loop larger). Take the longest dimension of the room and make the shortest side match it by extending the loop cable to include an additional area to the side - you may cover two rooms for the price of one by doing this!
- Q. I'm confused about 'Metalwork' - Will the signal pass through metal?
A. Yes the loop signal will pass through metal. Usually, you can ignore metalwork contained within the loop area as it will have no effect whatsoever. BUT!! There are two specific scenarios where metalwork will degrade loop performance:
- Where metal (of any type or shape) is formed into closed sections - a metal coat hanger for example is a closed section. Or a metal picture frame would be another closed section. Indeed, anything made of metal that is in the form of a closed circle or any other closed shape (ie a completed electrical circuit) would fall under this definition. In small quantities - not a problem, but in the case of steel reinforcing mesh fitted within cast concrete in the floor for example - major headache.
- Where metal (of any type, shape or thickness) is arranged in large sheets. So for example a room fitted out with foil backed plaster board, where the loop cable runs around the OUTSIDE of the room would be a problem.
In the two cases above, metalwork within the area covered by the loop system will cause losses to the loop signal. The losses are proportional to the amount of metalwork present - more metalwork - more loss. Loss implies additional design input and much more amplifier power to overcome the losses.
- Q. What is an MLC control used for?
A. MLC stands for Metal Loss Corrector and is to loop systems what a treble control is to a hi-fi system. It boosts high frequency signals to compensate for frequency selective losses introduced by metalwork. What it can't do though is compensate for absolute loss due to metalwork - this must be done by design. The high frequency signals are absolutely essential to ensure maximum intelligeability experienced by the user.
- Q. I can hear a loud buzz when I listen to the loop, is it broken?
A. Probably not. Stay listening in the same position and get someone to turn the loop system OFF. If the buzz goes then it IS coming through the loop system. If the buzz is still there, you are listening to some other source of interference, possibly from the mains electrical wiring system. Move your position until the buzz is loudest and look for the source of interference. Split live and neutral return circuits are a prime suspect or you may have a faulty electrical fitting. You may even have to re-route or re-wire the mains wiring to cure the problem.
If the buzz is coming through the loop amplifier, check your input connections. Disconnect the input signal - if the buzz is still present see the next question (faulty wiring). If not, the problem is either a poor interconnect circuit (always try to use balanced audio input / output wiring techniques), an earth loop (loop amp and source equipment both earthed - again use balanced connections), or a buzz from the signal origin. Check your microphone is not pointing towards noisy light fitting!
- Q. My loop amplifier is getting hot and I'm getting strange noises on my other equipment?
A. The most likely cause of this is a fault on your loop wiring where part of it is grounded (connected to earth). This will induce signals into the screen wires of any other signal cables and cause interference. Turn off the loop amplifier, disconnect the loop wires and test for a circuit to earth. Find the contact and insulate it.
© 1994-2019 DTSystems. All Rights Reserved.
Designers and Manufacturers of Electronic Equipment.