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Audio-Frequency Induction Loop Systems Audio-frequency induction loop systems (AFILS) allow hearing impaired people to hear more clearly. Most hearing aids have a ‘T’ or ‘MT’ switch which allows them to pick up the electromagnetic field generated by an induction loop system. The hearing aid converts this signal into a sound suited to its user’s specific hearing requirements. Any person with a hearing aid positioned within or near the loop (see example schematics below) can hear the loop signal by switching their hearing aid to the correct position, allowing them to participate more effectively in general conversation, ordering goods or services, listening to public performances, etc.
This page aims to give you a broad overview of our PDA Range of audio-frequency induction loop equipment. For further details, click here to download our 32-page Guide to Audio-Frequency Induction Loop Systems. ABOUT THE PDA RANGE OF AUDIO FREQUENCY INDUCTION LOOP SYSTEMS C-TEC's PDA range of induction loop equipment is one of the most comprehensive in the UK. It comprises a huge range of amplifiers, microphones, connector plates and test equipment covering virtually every conceivable AFILS application. AMPLIFIERS Designed to meet or exceed the requirements of BS7594 and EN60118-4 when correctly installed, all amplifiers in the PDA range offer excellent intelligibility, true current mode amplification, phantom power (for electret microphones) and full compatibility with our ‘outreach plate’ audio input extension system (except the PL1, VL1 and DL50). Many are also available in kit format for ease of specification, purchase and installation. A brief overview of C-TEC's induction loop amplifiers can be found below. More detailed information can be found in our Guide to AFILS or via the individual PDA RANGE datasheets and instruction manuals located in our downloads section).
THE OUTREACH PLATE AUDIO INPUT EXTENSION SYSTEM The Outreach Plate audio input extension system comprises a range of wall, ceiling and desk mountable single gang audio input plates specifically designed to increase the audio input capability of an induction loop system. Covering the most common variants of audio connector, they work by mixing the signals from various audio input sources into one balanced line level input which can be fed into the line input of a compatible amplifier. Mountable on 25mm single gang back boxes, each plate features a built-in mixer, pre-amp, input level control and balanced output. Typically, up to 10 Outreach plates (any mix) can be daisy-chained to one balanced line level input with cable lengths of up to 100m easily achievable using standard two-pair audio cable with no recognisable degradation of audio signal quality. Each outreach plate requires four wires, two balanced line (A+, A–), one ground (0V) and one power connection (12-32 V DC regulated). To facilitate their straightforward connection, most PDA Range amplifiers are fitted with an Outreach connection socket as standard. INDUCTION LOOP CABLE There is nothing electrically special about induction loop cable. Almost any stranded or solid single core cable with tough insulation can be used, provided it is not liable to break (to minimise the chance of it shorting to earth and damaging the amplifier’s output) and is of the appropriate gauge and DC resistance (0.5 to 1 Ohm). If purchasing a PDA Range induction loop kit, always use the loop cable supplied with the kit. If purchasing an amplifier that does not include loop cable, refer to the loop cable selection chart in our Guide to AFILS. MICROPHONES Electret microphones have the highest tolerance of magnetic feedback and are therefore recommended for induction loop systems. Dynamic microphones should not be used as they contain moving coil magnets which can be affected by the magnetic field generated by the loop. Phantom power is provided on all PDArange amplifiers, so electret microphones with internal batteries are not necessary. The PDA Range includes a wide selection of electret microphones, please refer to our Guide to AFILS for further information.
We recommend all induction loop systems are tested using our ‘FoSmeter H’ loop listening device (order code AHHM/H) and a set of 32 Ohm headphones (order code HEAD1). For compliance with BS7594 (the code of practice for audio-frequency induction loop systems), we recommend you also check the system using a pink noise generator as the system’s audio source. BS7594 recommends what the magnetic field strength over a covered area should be and we believe the most efficient way of ensuring this is achieved is to measure the field strength of a steady output. Unlike music or speech (both of which provide a variable sound output from which it is virtually impossible to obtain an accurate RMS reading), pink noise has an equal and constant average amount of energy content per octave of bandwidth. When fed into loop system, pink noise therefore gives a constant field strength which can be easily and accurately measured using a magnetic field strength meter. The following PDA RANGE test equipment is available:- • FoSmeter H Combined magnetic field strength meter & loop listener (part no. AHHM/H) • FoSmeter+ Combined pink noise generator & magnetic field strength meter (part no. AMPN) • FoSmeter Magnetic field strength meter only (part no. AHHM) • PNGN Pink noise generator only (part no. PNGN) • AL1 3.5mm jack (for PNGN) to 3.5mm jack lead • AL2 3.5mm jack (for PNGN) to double phono lead • AL3 3.5mm jack (for PNGN) to bare ended lead • HEAD1 Headphones for use with FoSmeter H
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