INDUCTION LOOP HEARING AID PA SYSTEMS
It is estimated that approximately two million people in the UK have hearing aids that are capable of receiving audio frequency induction loop transmissions.
Inductive loop amplifiers are a must for modern PA installations to give equal access to information and announcements to people with hearing disabilities. The Disability and Discrimination Act (1995) states that it is unlawful for service providers to discriminate against disabled people by providing them with a lower standard of service, or service on worse terms for reasons relating to their disability.
How do induction loop hearing aid systems work?
Induction loop amplifiers allow audio signals to be transmitted using a magnetic field to a suitably equipped listener. A magnetic field is created when current flows though a wire - in induction loop systems the wire is normally installed as a loop around the perimeter of the area. A small coil within a hearing aid or other suitable receiver then receives this magnetic field and converts it to audio at a suitable level for the listener.
The loop itself is simply an insulated wire installed at a low level, around the skirting board for example.
Hearing aids designed to receive induction loop signals generally have a switch marked 'M' for microphone and 'T' for telephone (Sometimes a combined position is provided). In the 'T' position a pickup coil, originally designed for picking signals from a telephone earpiece, is used to receive audio from induction loop systems.
The ideal signal strength for an induction loop system is such that a person switching between 'M' and 'T' would not notice a difference between the sound of someone speaking normally and a signal transmitted from the loop. To help in determining the perfect level it is often a good idea to engage the services of a hearing aid user - after all they are the ones that the system will most benefit. To enable a person with normal hearing to test your inductive loop system installation, there are simple loop receivers that work with standard audio headphones.
What cable do I use for induction loop systems?
It is important that the cable used to form the loop is capable of carrying the current that the amplifier produces. The table below is a guide to the maximum loop length for each cable thickness.
- Total loop length: Up to 60m use 1.0mm² Ø loop cable
- Total loop length: Up to 110m use 1.5mm² Øloop cable
- Total loop length: Up to 200m use 2.5mm² Ø loop cable
- Total loop length: Above 200m use 4.0mm² Ø loop cable
Where do I lay the induction loop cable?
Simple induction loop systems in a single room or hall are called perimeter induction loops. The cable starts at the induction loop amplifier and is laid around the perimeter of the venue and then back to the amplifier. The cable should be laid at approximately 14% of the shortest loop dimension, above or below ear level (the listening plane). See example below:
Listening plane (ear-level) for a seated person: approx 1.2m
Venue dimensions: 10m x 20m
Loop cable height calculation: 14% of 10m (shortest dimension) = 1.4m above or below 1.2m
In the above example where most listeners are seated, the loop cable would be located at 2.6m above the floor - (1.4m above the listening plane of 1.2m)
For more complex loop systems and loop array layouts, please seek professional advice from a qualified installer in your area.