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INDUCTION LOOPS - BS7594:2011

Published 24/08/2012 11:00

INDUCTION LOOPS - BS7594:2011

BS7594:2011 is the Code of Practice for audio-frequency induction-loop systems (AFILS Systems), commonly known as Induction Loops, Hearing Loops or Deaf Loops.

BS7594:2011 replaces BS7594:1993 and provides clarification to the existing BS EN 60118-4:2006 Electroacoustics - Hearing aids. Induction loop systems for hearing aid purposes standard. It relates specifically to the correct deployment, usage and maintenance of induction loop systems in the UK.

Induction Loop systems should conform to the UK Equality Act 2010 (incorporating DDA Act 1995), Building Regulations Part M1 : Access to and use of buildings (2004), The Care Standards Act (2000) and BS 8300 (2009).

The aim of the Standard is to ensure that induction loops are fit for purpose, functioning in a correct manner and of real benefit to the users. Compliance with the Standard is achieved by the correct deployment, setup and maintenance of induction loop systems, and this compliance is recorded in a system log book along with the relevant test certificates.

BS7594:2011 Includes requirements that the induction loop system is:

  • Fit for purpose
  • In working order
  • Has a field strength of 400mA/m
  • Has a frequency response of ±3dB from 100Hz to 5kHz (referenced against 1kHz)
  • Background noise level is < -32dB (referenced at 400mA/m)
  • Is regularly tested and correctly maintained
  • Is the responsibility of a nominated person
  • Is operated by trained people Has detailed commissioning and handover information such as;
    • Certificates for the design, installation, commissioning and acceptance
    • Log book
    • Training Record
    • Induction Loop System Maintenance

The standard can be split into 5 sections:

  • Purchasers of audio-frequency induction-loop systems (AFILS Systems)
  • Designers of AFILS
  • Installers of AFILS
  • Commissioning, Maintaining and Testing
  • Day to Day operations

The major changes and updates which can affect system design can be summarised as follows;

AFILS System Design

The AFILS should be designed and assessed by a competent person and then given a classification:

Class A1

  • Portable, Hand held or self contained systems.
  • Used as a temporary measure only for one to one communication.
  • These shouldn’t replace fixed counter loop installations.
  • For example; a portable system that a doctor may carry when making house calls.

Class A2

  • Small counter and signage systems.
  • Small coverage where user is in a fixed location. Usually only covers very small areas of a couple of metres maximum.
  • For example; a reception desk, counter or help point.

Class A3

  • Perimeter loops for covering a single volume where other AFILS are not present and where overspill is not a concern.
  • For example; a church.

Class A4

  • Multiple loop systems.
  • These can be used to control the horizontal and vertical field produced by the system.
  • They are used to overcome spill issues and systems that are affected by frequency loss due to metal. The consultation of a specialist designer should be sought.
  • For example; classrooms or cinemas with multiple AFILS, an auditorium with a box office AFILS in the same building. Reinforced concrete construction or an office space with a raised steel computer floor.

Class A5

  • Systems designed for purposes other than assisted listening.
  • For example; stage direction or observational counselling. 

Class A6

  • Personal loop systems either hand held or worn on the body.

Class A7

  • For special applications that aren’t catered for in the above.
  • For example; lifts

Microphones and Other Signal Sources

There are numerous calculations and recommendations made in the document for reverberation and noise relating to microphones.

When designing an induction loop system the placement of the microphones is critical in ensuring that the system provides more intelligible audio than the hearing aid can on its own. If you don’t get this correct then the system will fail.

For example, microphones should be located so as to have a clear “view” of the source they are intended to cover. They should be located away from grilles, fans, strong air currents and other potential sources of acoustic noise pick-up e.g. theatre lighting and video projectors.

The further the microphone is located away from the sound source, the weaker the required signal becomes. This effectively reduces the signal-to-noise ratio and also reduces the direct-to-reverberant sound pressure level ratio. In order to maintain an acceptable signal to noise ration, the maximum microphone distance should be limited to approximately 6-10m.

If connecting to an existing sound system it’s important to ensure that the feed to the induction loop system will not be altered when the loudspeaker volume is altered. It is also important to take the feed prior to any room equalisation for the sound system.

Installation Practices

Under current IEEE wiring regulations (17th Edition), loop cable is classified as class 2A and, as such, must be sited at least 600mm away from telephone, mains and control cables. In most installations, loop cable is usually run around the perimeter (edge) of the room.

The field strength in the plane of the loop (the height at which the cable is positioned) varies greatly so it is best to install the loop above or below the listener at floor or ceiling height.

The loop field will not be as strong but it will be much more even and provide better results.

Try to locate ceiling loops approximately 1.2m above the listener’s head (listening height with the hearing aid user sitting or standing is normally 1.2 to 1.8m from the floor).

Structural steel, in particular, large sheets of metal (solid or perforated) such as metal suspended ceilings, can absorb the magnetic field resulting in uneven coverage or dead spots. To avoid the magnetic field being absorbed by structural steel, if possible keep the loop about a metre from large uprights. If there is a steel-reinforcing grid in the floor, either put the loop in the ceiling or, if it must go in the floor, install the loop in plastic conduit as far above the grid as possible.

Commissioning

At commissioning, the entire system should be inspected and tested to ensure that it operates satisfactorily and that, in particular:

  • Magnetic field strengths and signal-to-noise ratios conform to the requirements of BS EN 60118-4:2006.
  • An acceptable level of intelligibility is achieved throughout the useful magnetic field volume
  • No changes to the building since the time of the agreed design have compromised the conformity of the system to this standard
  • Mains power supplies are inspected as far as reasonably practicable to ensure compliance with the recommendations of Clause 15
  • All relevant documentation (Clause 18) has been provided to the user or purchaser.

Maintenance & Servicing

On a weekly basis, the responsible person should ensure the satisfactory operation of the AFILS by use of a test signal and either a fixed loop monitor receiver or a portable field strength meter.

It is essential to supplement this with a listening test using real speech into the microphone(s) to ensure that the microphone(s) are working correctly and amplifier control settings have not been inappropriately changed.

During periodic visits, at intervals not exceeding 12 months, the following inspection and test of the system should be carried out by a competent specialist contractor:

  • Examine the system log book
  • Visual structural inspection
  • Report made of any changes and Responsible Person informed
  • Check visual indicators and controls
  • Any manufacturer specified procedures
  • Physical equipment inspections
  • Field strength tests
  • Frequency response
  • Signal to noise ratio
  • Documentation updated, inspection and service certificate added to log book

Please don’t hesitate to contact us for further details and advice.

Details provided above are not comprehensive; they are designed to give an overview only.  No responsibility can be accepted by CDS Security & Fire for any misinterpretation of an instruction or guidance note or for the compliance of the system as a whole. Copies of BS 7594 can be purchased from the British Standards Institute (www.bsi-global.com).

 


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