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INDUCTION LOOPS ARE A COMMON AND CHEAP WAY OF IMPROVING SOUND

TECHNICAL & PRACTICE

This latest article in our series of NBS Shortcuts looks at how designers can cater for hard-of-hearing building users with the specification of acoustic loops and other systems.

The placing and operation of acoustic loops in public buildings needs careful consideration. Incorrect specification can result in an unsatisfactory audible environment for the hard of hearing as well as the possibility that electronic equipment, within range of the acoustic loop's magnetic field, may be adversely affected.

In 2005, a design collaboration between the Royal National Institute for Deaf People (RNID), Blueprint magazine and creative agency Wolff Olins, was set up with the aim to convince the public that hearing aids, re-branded as 'hearwear', should become as stylish as designer glasses. By making people realise that ungainly, badly flesh-toned plastic earpieces are a thing of the past, the designers hope to encourage some of the nine million UK residents who are hard of hearing to use hearing aids.

At present, only two million UK residents own hearing aids and just 1.4 million admit to using them regularly. There is still, it seems, something of a stigma associated with hearing loss. A bit like bifocals, it tends to suggest that you are getting on a bit.

Over the years, deaf and hard of hearing children have begun to be educated in mainstream, rather than 'special' schools, and the British Association of Teachers of the Deaf (BATOD) has assessed that these pupils account for about 75 per cent of all deaf children of school age. One of the simplest methods to enhance their ability to hear the lesson clearly is to reduce background noise by siting the classroom away from playgrounds, busy circulation spaces and communal areas; improving acoustic separation and keeping the classroom occupants quiet.

Coincidentally, there is a growing Health and Safety campaign by the National Union of Teachers, pointing out that teachers are more likely than most other occupational groups to consult doctors about voice disorders. The Health and Safety Executive (HSE) and the General Teaching Council of Scotland have both confirmed that teachers are more likely than other professionals to be absent from work with voice problems. As a recommended solution, microphones, which have been used in schools in the United States since the early 1970s, may find their way into UK classrooms. According to the British Association of Audiological Scientists (subsequently merged and renamed the British Academy of Audiology), amplified sound combined with low background noise can have more of a beneficial impact on students' hearing ability than fine-tuning the reverberation times of fittings and finishes.

AMPLIFIED SOUND The 'sound-field system' comprises a teacher's headset and transmitter, an amplifier and a number of audio speakers to provide an equal distribution of sound whichever way the teacher or pupils are facing. This amplified audible sound system is an improvement on hard-of-hearing children having to wear a headset in class, but this may still be necessary for those with significant hearing loss. The sound-field system uses a wireless link between the microphone and amplifier, operating on VHF, UHF Radio or infrared frequencies, and is relatively simple to set up.

INFRARED SYSTEMS While a sound-field system may be suitable for some school and lecture situations, it is not always desirable to improve audibility by magnifying the sound for everyone. Infrared systems (available with a stereo facility) use invisible infrared light to transfer sound from the 'radiator' (a transmitter linked to a speaker's microphone and amplifier, for example) to receivers' headsets. It may be necessary to have several radiators, depending on the size and shape of the venue. In general, infrared systems are directional, which means that headset wearers may lose sound quality if they turn their heads. Care should also be taken to ensure that infraredabsorbent wall coverings and furnishings are not used.

AUDIO FREQUENCY INDUCTION LOOP SYSTEMS One of the more common - and cheapest - methods to improve the sound for targeted users is the use of Audio Frequency Induction Loop Systems (AFILS). Approved Document M - 'Access to and Use of Buildings', 2004, states that AFILS are needed in the somewhat ill-defined 'reception points' of buildings other than dwellings. In 1985 the International Federation of Hard of Hearing People (IFHOH) adopted the induction loop icon as the official symbol for AFILS. These signs should identify the presence of the system and prime locations for reception. Ideally, approved signage should be displayed if reception is not possible.

An acoustic loop is simply a coil of electrical wire, preferably solid rather than stranded, connected in a loop to the output device (an amplifier linked to a microphone, radio, etc).

Essentially, an audio-frequency electric current circulates in the wire, varying with the speech or music played through the amplifier; this, in turn, generates a magnetic field within the loop. The alternating magnetic field replicates the input sound modulation and this can be picked up within range by suitable receivers and reconverted back into sound. Receivers in hearing aids must be tuned to 'T' for 'telecoil' to enable the wearer to pick up the useful component of the magnetic field, and once tuned in, he or she can hear without having to rely on visible paraphernalia that would draw attention to their deafness. In normal situations, hearing aids are tuned to 'M' for 'microphone', which allows the wearer to pick up normal close contact sounds; and for those with hearing aids without an M-T switch, a special adaptor is required to benefit from acoustic loops.

When setting out an induction loop circuit, consideration must be given to the listening plane, which lies at the median level of listeners' ears, about 1.2m above finished oor level. Loops are often embedded in the walls but as the field gets stronger the nearer to the loop, this can cause disturbances to hearing aid wearers if they are seated around the perimeter wall. Placing the loop in the slab or the ceiling/void is acceptable although this requires more current for the same field strength. Wherever the location, all systems must be satisfactorily designed to provide what BS 7594 calls 'a sufficiently (but not excessively) strong, and uniform (useful) component of magnetic field within the required working area'.

In a horizontal loop the magnetic field is in the vertical plane and lines up with the vertical magnetic pick-up coil in the hearing aids of the audience. If the loop is situated in a hospital, where the hearing aid wearers may be lying down for considerable periods of time, the loop should not be horizontal as it is the perpendicular component that is important. Similarly, in large auditoria, several loops will be required at various tier heights to ensure that each storey benefits, although there must be no magnetic interference between each loop. While the RNID suggests that in domestic conditions loops are not badly affected by having to lay the perimeter wire around obstructions, in larger applications the effect on the magnetic field caused by a nonuniform layout should be carefully calculated.

The system must be designed and installed for maximum efficiency: BS 7594 states that it must not produce 'an unacceptably extended coverage area which could cause interference with other systems or compromise confidentiality'. Solid (not twisted-core) wires with a core diameter ranging from 0.5mm to 2.5mm, which complies with BS 6500 and is chosen to suit the specifics of the room layout, can be housed in plastic tubing for added protection.

However, these must not be laid within the vicinity of other horizontally laid electrical wiring, as this will cause magnetic interference and consequently create an annoying background hum in the ears of hearing aid wearers. Similarly, if there are metal, or other conductors, in the area surrounding the loop (metal-stud walls and steel columns, for example), they will also give rise to poor levels of reception.

Conversely, the magnetic field can spread beyond the designated room, meaning that those with telecoil functions on their hearing aids can pick up conversations outside the area; this can be irritating for the wearer, but also worrying for the speaker, especially if it is a home-based system where you expect private conversations to stay private. Often, multiple-loop systems laid into the floor slab are sufficient to combat this, but there are a number of proprietary systems that deal with this issue. Professional design guidance and installation as well as thorough precompletion certificate testing is essential to minimise this problem - but you may want to consider an infrared system if the speech is likely to be of a personal nature, as in doctors' surgeries. After installation, the system should be calibrated and its controls 'locked' to prevent unauthorised or accidental alterations to the system. It is wise to provide accessible junction boxes for maintenance, which should be minimal. On completion, the person responsible for the building should be given full instructions on how to use the system.

LEVELS OF DEAFNESS:

The 'Decibel hearing level' (dBHL) indicates a person's hearing relative to accepted standards for normal hearing. For example, 50 dBHL marks a hearing loss of 50 dB.

Mild hearing loss (25 - 40 dBHL).

Difficulty in following speech Moderate hearing loss (41 - 70 dBHL).

Difficulty in noisy situations; ability to use telephone with ampli-cation

Severe hearing loss (71 - 95 dBHL).

Lip-reading is an essential skill. Difficulty hearing amplified telephones. Text messaging is increasingly used Profound hearing loss (96+ dBHL).

Hearing aids are of little or no help.

Lip-reading, signing and/or writing and texting may be the main communication tools used

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