Installation Europe has published (Sept '07) an overview of loop system basics written by Julian Pieters, Ampetronic's Managing Director, giving a thorough overview of the key considerations for anyone involved in loop systems.
Please download the full article via the link on the left, or you can read the full text (without graphics) below.
Keep in the loop!
Audio Induction Loop Systems are increasingly found as a part of AV and electrical installations around the world. These systems can provide a significant benefit to the hard of hearing and are often the best solution for providing hearing accessibility across a wide range of environments. As legislation increasingly requires service providers to ensure accessibility, induction loop systems are becoming a necessary part of any audio system or speech communication. However there are many considerations in making sure that a loop system is both beneficial and can perform well enough to meet required performance standards.
What is hearing loss?
Experiencing hearing loss can be a traumatic and isolating experience, one which affects up to 15% of our population. If you suffer from hearing loss you will have a reduced ability to understand what you are hearing, particularly speech. Firstly it becomes very difficult to distinguish the sounds that you want to hear from surrounding background noise. Secondly as hearing loss advances you lose your ability to hear the high frequency sounds that make speech intelligible – speech becomes muffled, incoherent and impossible to understand.
For the many people who have the misfortune to experience hearing loss, their condition can change life dramatically. It can become impossible to interact normally with other people, either socially or in public places such as shops and banks, and impossible to enjoy a visit to the theatre or to watch a film, in other words to lead a normal life.
Assistive listening and induction loop systems
There are solutions available to help of course. Many people who experience high levels of hearing loss wear hearing aids of some form to help improve their situation; however hearing aids only form part of the solution. Even with a hearing aid, background noise is very disruptive and has a strong impact on intelligibility. To allow hearing aid users to hear clearly in noisy public environments, assistive listening systems need to be installed, systems that are often part of an AV system aimed at extending accessibility to the hard-of-hearing. These systems are so beneficial to the hard of hearing that they are required to be provided by legislation in many developed countries around the world.
In Europe directive 2000/78/EC requires member states to adopt legislation to prvent disability discrimination. Each country is putting in place legislation to meet these requirements, for example in the UK we have the Disability Discrimination Act (2004) which mandates accessibility solutions wherever it is reasonable to provide them, and Part M of the Building Regulations which require assistive solutions in new buildings. In the US the Americans’ with Disabilities Act (ADA) calls for suitable assistive listening systems to be installed in places of worship, theatres, movie theatres, conference and meeting rooms, and indeed anywhere where sound amplification is used.
An assistive listening system will transmit sound to a receiver that allows the user to hear that sound very clearly and effectively, removing background noise completely. Systems include infra-red or FM radio transmitters with dedicated headsets that can be provided to inpidual users, or induction loop systems that use a magnetic field to transmit sound directly to the users’ hearing aids. Induction loops are the most widely used systems in many parts of Europe and are becoming increasingly popular in the US, largely because of the direct use of the hearing aid which creates many benefits:
- Hearing aids tailored to the inpidual user, ensuring the best compensation for their specific hearing loss
- Infra red and FM system headsets require the user to ask – this is discriminatory and relies on the hard of hearing person to find and request a receiver
- Hired headsets are often not used due to concerns over hygiene and the social stigma of having to wear a headset in a public environment
- Induction loop systems require no receivers to be provided, substantially reducing the maintenance and management costs
- A loop system is often a completely invisible installation with little or no maintenance cost
- Induction loop systems can be used where the user is just passing through, such as in transport systems, where receivers could not be provided
How do loop systems work?
So what are loop systems? An induction loop system transmits an audio signal directly into a hearing aid via a magnetic field, greatly reducing background noise, competing sounds, reverberation and other acoustic distortions that reduce clarity of sound. The magnetic field is created by driving a loop or series of loops with a specialised audio amplifier. The diagram **** shows how a typical system is configured.
Loop systems can be provided in a very wide range of environments. Because the user is always carrying their receivers with them in their hearing aids, a loop system can prove useful in almost any environment that the hard-of-hearing inpidual might be. The table **** shows some of the many environments in which induction loops can be found.
Considerations for a successful loop system
For anyone considering a loop system, there are several issues to check before proceeding.
Will it help?
While seemingly obvious, many assistive listening systems provide no benefit. To be useful the loop system must be able to provide better separation of the signal audio from background noise. This means there must be suitable audio input, from an AV system or direction microphone, that will separate out background noise and pick up the required sound. If there is no background noise and the normal audio is clear then an assistive listening system is likely to be of no benefit.
Simple perimeter loops can interfere with other loop systems up to 4 times the width of the room away in any direction. It is important to check whether there area any other systems or planned systems in the area. The spill from a loop system can be dramatically reduced – down to 1.5m from the edge of the loop – by using a special low spill design which some manufacturers can provide.
Magnetic fields can be distorted by metal structures. Most modern buildings have substantial metal structural components and particularly metal floors and ceiling grids can have a very strong effect on loop performance. Where metal is present it is common that the amplifier needs to be 2 to 3 times more powerful to compensate. Often the loop layout needs to use smaller loop segments to provide a flat un-distorted field over the required area. Losses tend to reduce the critical high frequencies more, so amplifiers need to have frequency compensation devices to give flat frequency response.
Many different loop layouts can be used to give a particular shape and evenness to the magnetic field – the colour graphs show some of the many different types of system available. Systems providers should be able to select and design the appropriate loop layout if they know enough about your application.
Where to lay the cable?
Practical installation of loop cable requires careful evaluation of the site. In some environments it may not be possible to run cables on the floor, so ceiling voids might be an option either above or below the volume of use. Special cables can be buried within concrete screeds, or directly buried in ground outside the building. It is also necessary to avoid other signal cables, ensuring separation of at least 600mm where cables are parallel.
There is a recently revised international performance standard for induction loop systems IEC 60118-4:2006. This seeks to ensure that all installed loop systems provide a useful output, and only applies to the complete installed system, not to inpidual pieces of equipment. The contractor or installer is usually responsible for ensuring compliance to this standard. In outline the loop system must provide:
- Even field strength – capable of 400mA/m RMS, with variation of +/- 3dB over the entire volume in which the hearing aid user may be
- Flat frequency response - +\- 3dB over the volume, often requiring the system to have special frequency compensation capability to adjust for frequency dependent losses when metal structures are present
- It is also necessary to ensure that existing background magnetic interference is not present, or is more than 32dB below the signal strength.
Loop system providers should design a system to meet these requirements and provide a certificate of conformity for the installed system.
One of the great benefits of loop systems is the near invisible installation – no-one needs to know that they are there or that they are being used. However the hearing aid users do need to know that a loop system is installed! Good clear visible signage is very important to let people know that a system is available for them and must be provided for in the system specification.
Training & Maintaining
The most common cause of loop system failure is poor use or insufficient maintenance. Too often systems are installed without consideration of those who must look after and maintain the systems. There must be someone nominated to be responsible for (and trained to) regularly inspect and adjust any loop system. Every installation must have at least a basic listening device to allow the system to be checked, with a field strength meter available for larger or critical systems. Training, support or a maintenance service should be part of the package provided by any loop system provider.
How do induction loops work?
Audio Inputs (1), either from an existing audio source such as a P.A. system or from dedicated microphone inputs feed an audio signal into an Induction Loop Amplifier (2). The amplifier drives a current into a Loop (3) or series of loops. As the current flows through the cable it creates a Magnetic Field (4) in the required area – careful loop and amplifier design ensures that the vertical component of the field is even and free of dropouts and dead zones wherever the user might be. Inside most Hearing Aids (5), a small coil known as a Telecoil (6) picks up the magnetic field signal, which is amplified into a high quality audio signal delivered directly to the ear of the hearing aid user.
Where can induction loops be used?
There is a surprisingly wide array of solutions available for installing and using induction loops in many different environments. Often the provision of loop systems for the benefit of the hard-of-hearing is limited simply because architects, consultants or end customers are not aware of the range of solutions available. These are just some of the environments in which loop systems are currently used successfully:
- Airports; Stations; Ticket counters; Elevators; Help points; Car park access points
- Taxis and private cars; Minibuses; Coaches; Trains; Trams; Boats
- Theatres; Cinemas and concert halls; Stadia and sports venues; Places of worship; Conference and lecture halls
- Counters; Intercoms and entry-phones; Drive-throughs; Information points; Help points
- Meeting rooms; Video conference facilities; Desks and offices
- TV rooms; Phones; Inpidual car systems
- Lecture halls; Classrooms
- Public address systems; Voice alarm systems; Help points; Door entry systems