Order of the Bath

An induction loop is a system that allows hearing aid users to receive a signal directly from microphones placed close to the users rather than rely on transmitted sound. It is of considerable benefit to them because it separates out just the sound required and removes other distracting noises and reverberation.

The building regulations (1992) require that new buildings open to the public and ones that are substantially reconstructed are fitted with systems for ticket booths and auditoriums over 100 sq m. Systems must meet BS8300:2002, BS7594:1993 and EN60118-4.

The Disability Discrimination Acts (1995 & 1999) may extend this to existing premises if it is otherwise unreasonably difficult for disabled people to use the facilities. An installed loop must be properly maintained and staff must know how to use it. I have been told that some of these regulations are retrospective and older systems may no longer conform but, apart from this maintenance requirement, I can’t find any evidence for this.

The specification for an induction loop system requires a current drive amplifier (a normal PA amplifier is voltage driven) with a single turn loop and account should be taken of losses caused by building metalwork such as girders and reinforced concrete. The input to the system can be dedicated microphones or from an existing sound system.

Ampetronic, a leading manufacturer in the field, say that the type of wire is not too important so long as it appropriately sized to give a total resistance in the range required by the amplifier. The normal arrangement is for it to run around the listening area. They also say that it should not be at head height, though I haven’t discovered why, but floor level (or ceiling if it is not too high) is more normal.

The recommended field strength is 0.1 A/m for an average level speech signal with peaks at four times that value which allow for some dynamics. Larger variations are flattened out using a compressor built into the amplifier. For a single turn loop, and bypassing all the complex equations and caveats, the peak current can be simplified to approximately 4a/9 where “a” is the length of the shortest side in metres. From this you can calculate the wire gauge and the amplifier power required.

Once installed, the easiest way to measure the field is with a pink noise generator amplified to normal listening levels (though you may want to turn off the loudspeaker amplifier) and measure the field strength with a meter in different places. With a listening device you can also test the tone over the coverage area. The frequency response is required to be flat (+/- 3dB) between 100Hz and 5kHz with an upper response limited to 16kHz. High frequency losses are caused by building metalwork and compensatory EQ (and additional current) will be needed to correct for this. You may need professional advice for particularly large, oddly shaped or difficult buildings.

This entry was posted on 29 Jan 2007 at 16:02 by Rick and is filed under ChurchTech, Technical. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.