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CONTENTS

Introduction ........................................................... 2

The aim of this report ........................................ 2

Scope of the report ............................................ 2

Credits and Thanks ............................................ 2

Glossary.................................................................. 3

Part 1: A User Oriented Perspective ..................... 4

Initial considerations .............................................. 5

Identifying the system requirements ................ 5

Portable versus installed systems...................... 6

Technology comparison at a glance .................. 6

Management of Devices ........................................ 8

Instruction ......................................................... 8

Training .............................................................. 8

Hygiene .............................................................. 8

User Information and Signage ............................... 8

User considerations ............................................... 9

Wearable device types ...................................... 9

Wearable device visibility .................................. 9

Financial Considerations ...................................... 10

Initial Installation Cost ..................................... 10

Subsequent maintenance cost ........................ 10

Choosing an Assisted Listening System ................ 12

Considerations ................................................. 12

Part 2: A Technical Perspective ........................... 13

Hearing Loss and Correction ................................ 14

Sound loudness versus frequency ................... 14

How hearing loss affects the user ................... 14

Correction of hearing loss ............................... 15

Assisted Listening Systems in Detail .................... 16

Wi-Fi Systems ....................................................... 17

Overview .......................................................... 17

Infrared Systems .................................................. 18

Overview .......................................................... 18

Modulator ........................................................ 18

Emitter ............................................................. 19

Receiver ........................................................... 19

Potential problems .......................................... 19

RF Systems ........................................................... 20

Overview .......................................................... 20

Range and overspill.......................................... 20

System components ........................................ 21

Inductive Loop Systems........................................ 22

Overview .......................................................... 22

Loop installation types ..................................... 23

Portable and local installation types ............... 25

Performance and standards ............................ 26

Specifying a Loop Amplifier ............................. 26

Measuring performance .................................. 27

Maintenance .................................................... 29

Hearing Reinforcement In Teaching Spaces

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INTRODUCTION Aside from the requirements related to various legislation such as the Disability Discrimination Act 1995 and

subsequently the Equality Act 2010, there is a need to ensure that both new and existing hearing reinforcement

installations are not only fit for purpose but also offer the best user experience possible. Currently, although

various types of assisted listening system are available it is sometimes unclear which is the most suitable for a

particular room type or use case. Additionally, it is often unclear how to best utilise a particular system to give

optimum performance and endurance; many existing installations fall short in these areas.

Depending on source, it is widely thought that between 10-20% of the UK population suffers from some form of

hearing impairment. Although the number of users with significant enough impairment to require an assisted

listening system will be smaller than this figure, this still means a significant number of the higher education

community would actively benefit from systems to assist their hearing.

THE AIM OF THIS REPORT The aim of this research is to inform the community as a whole, and in particular enable the IT, Audio Visual and

Estates departments of institutions to make informed decisions on how to progress with implementation of new

or improvement of existing hearing reinforcement systems in teaching spaces. Not only towards choosing the

best type of system for their needs, but also to ensure that the installation enables it to give the best coverage

and user experience possible.

Through this report, the team aims to provide a concise insight into the specification and installation of assisted

listening systems of various types, presenting the information in a clear, non-technical format.

SCOPE OF THE REPORT The team have adopted a technology-centric rather than manufacturer-centric approach to the project.

Although numerous manufacturers exist for each type of device, the team have found that aside from some

individual features, the performance and quality of systems made by reputable manufacturers are broadly

similar, and that the choice of which equipment to install lays mostly with the type of room, budget and use case

envisaged.

For each technology type, devices from numerous manufacturers were evaluated and the findings for each type

of system scored versus competing technologies.

CREDITS AND THANKS The team would like to thank James Bottrill from Ampetronic for loan of equipment for evaluation and for

technical advice, and for giving us permission to use the images related to induction loop spill, shown later in

this report.


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GLOSSARY CHANNELS

The number of monaural audio signals that a system is capable of transmitting simultaneously. Most

use cases only require one channel; however, multiple channels are useful in environments where the

audio program is transmitted in various languages.

OVERSPILL

Any portion of a system’s audio signal that audibly spills outside the intended coverage area of the

system, for example the boundaries of the room where the system is installed. Overspill has two

implications; it may pose a confidentiality concern and additionally may interfere with similar systems

in adjacent rooms.

T-SWITCH

A setting fitted to some hearing aids that enables the device receive an audio signal wirelessly from an

inductive loop, either in the form of a wearable pendant or a full room system.

WEARABLE DEVICE

A device that has to be worn by a user in order to receive the audio data from an assisted listening

system. Often in the form of a pendant worn around the user’s neck, the device may connect to the

user’s hearing aid or a pair of headphones.


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PART 1: A USER ORIENTED PERSPECTIVE


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INITIAL CONSIDERATIONS

IDENTIFYING THE SYSTEM REQUIREMENTS Choosing the correct assisted listening system requires identifying the most important requirements of the

usage scenario and of the user, and finding a suitable compromise that satisfied both sets of requirements while

still being practical and realisable within reasonable cost. For example, the average user may desire a system

with crystal audio quality, 100% coverage, 100% uptime and which works perfectly with all types of user hearing

device. In reality, finding a system with all of these features is not feasible and even satisfying just some of the

requirements would result in a system that was prohibitively expensive to implement and maintain.

The bottom line is that the chosen system should satisfy as many of both the user and management preferences,

while providing adequately low cost per head, while also providing sufficiently high quality of service and

usability.

REGULATIONS AND THE LAW The law (notably in the form of the Disabilities Discrimination Act 1995 and the Equality Act 2010) states that

‘reasonable adjustment’ should be made to cater for hearing-impaired users, but stops short of specifying what

form those reasonable adjustments should take. In some instances, existing assisted listening systems may have

been installed purely to meet regulations and to enable a room to be ‘signed off’, with the result that they may

barely meet minimum regulations but offer a very poor user experience.

In short, an assisted listening system should be designed from the outset to enhance the experience of the

hearing-impaired user as much as possible and not be used just as a means to meet regulations.

USER PERSPECTIVE The typical user is focussed solely on the performance and versatility of the system and is not concerned with

the technical aspects of the system. In this case, we find that the most important aspects of the system are based

around the more humanistic elements of the system. From a poll of a group of hearing-impaired University of

Essex community members, feedback identified the following considerations from the user’s standpoint as the

most relevant, listed in order of importance:

System coverage

System usability

Wearables visibility

System reliability

Sound quality

INSTITUTION PERSPECTIVE The institution’s requirements are broader than the user, encompassing both all user requirements necessary

to maintain adequately high quality of service, in addition to delivering a system that provides an optimum trade-

off between cost, reliability and quality.

Therefore, the ideal assisted listening system from the institution’s perspective is one that is cost effective in

both initial installation and subsequent maintenance and which provides a reliable user experience with a

sufficiently high level of reliability; reliability doesn’t necessarily correlate with system cost, however sound

quality and coverage may.


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In summary, from the University of Essex’s standpoint, the most important considerations in order of importance

are:

Initial Cost

Maintenance Cost

Reliability

Minimal disruption during install and repair

PORTABLE VERSUS INSTALLED SYSTEMS Aside from the various technology types available, there are two broad types of systems; those that are installed

as part of the room’s infrastructure, and those that are deployed to rooms as required in the shape of a portable

device. Making an informed choice on which of these to choose requires some thought as to the nature of the

number of teaching rooms and the comparative number of users. For instance, an institution with relatively few

teaching spaces but with a comparatively large number of students, the cost of installing an inductive loop

system which would work with most hearing aid devices would prove vastly more cost effective than deploying

portable devices and having to furnish every hearing impaired person with a wearable device. Conversely, in an

institution with a large number of teaching rooms and comparatively less students, it may make better economic

sense to deploy portable devices as required rather than fit installed systems to every teaching room where

each installation may only see very infrequent use.

TECHNOLOGY COMPARISON AT A GLANCE Several assisted listening technologies exist, each with their own advantages and disadvantages. Each

technology type will be described in more detail later in the report, but below is a brief summary:

INFRARED Zero spillover through boundaries without line of sight

Easily expandable to cover large areas

Simpler installation in large or complex rooms versus inductive systems

Not affected by the structure of the surrounding building

Ability to carry multiple audio channels at the same time

User requires wearable receiver

Requires line of sight contact between transmitter unit and user

RF Potentially high sound quality

Large coverage range

Able to transmit multiple channels simultaneously

Overspill not controllable

Only portable type systems commonly available in Europe


Cannot be moderated


WI-FI Potentially high sound quality

Capable of transmitting multiple channels simultaneously

Secure and mediated through password authentication

Variable latency depending on network infrastructure and user device


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Requires user to have headphones & Wi-Fi capable device, e.g. smartphone or tablet

INDUCTION The only technology which does not require any user worn equipment; hearing aid T-Switch compatible

Only able to transmit one channel of audio at a time

Overspill problematic to control

Potentially disruptive to install

Performance potentially degraded by structural construction and contents of the room

Easily damaged during subsequent room works

ASSISTED LISTENING TECHNOLOGY CROSS COMPARISON

Induction Infrared RF Wi-Fi

Install Cost

Maintenance

Robustness

Relative Sound Quality

Stereo Audio Capability

Directly T-Switch Compatible

Maximum Number of Channels 1 32 100 Unlimited

Containable within Room Area


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MANAGEMENT OF DEVICES Most types of assisted listening system will require some form of user device to be worn and provision should

be made to manage both the wearable device and any associated consumables. The preferred solution is to

stock the items at a central location, preferably at a student or IT helpdesk, where staff are available to assist

the user and ensure that the devices are maintained in working order.

INSTRUCTION In cases where wearable devices are required as part of the assisted listening system, provision should be made

to provide clear instructions to both inform users of the type of system installed and notify them where the

required wearable devices can be loaned from. Although most user devices are straightforward to use, clear and

simple instructions should be supplied with the device to enable the user to operate it unaided. The instructions

should also provide simple troubleshooting instructions in the case the user encounters any problems using the

device.

TRAINING Staff at the location where the equipment is loaned from should be adequately trained so that they can both

instruct users how to operate the wearable devices and regularly check the devices to ensure that they are

operational. Ideally, a basic functional check of a wearable device should be performed each time it is returned

by a user, before it is put back into stock; in most cases, a check of battery life and a brief visual inspection of

the device to check for damage or missing parts will be sufficient.

HYGIENE Where devices incorporate earbud type headphone earpieces or similar, these should be treated as disposable

consumables and an adequate supply should be kept in stock. Any wearable device should be kept clean, for

example by wiping contact areas over with disinfecting wipes each time the device is returned, before placing it

back into stock.

USER INFORMATION AND SIGNAGE A large requirement of an assisted listening system is adequate signage to both inform the hearing impaired that

the system is available, and to inform them of the type of system, the

requirement and location of any wearable devices. Additionally, the signage

should provide clear and concise user instructions where required, for

example where an inductive loop system is compatible with a user’s own

hearing equipment through a ‘T-switch’ setting or similar.

The signage should be placed in a prominent position to maximise its visibility

to users. In the case of a counter loop system, the signage should likewise be

placed in a prominent position, for example within the user’s normal field of

view when standing or seated at the counter.

Example of basic induction

loop signage


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A plan indicating the area coverage of the system and the name of the person responsible for the system’s

operation should also be displayed either in close proximity to, or ideally as part of, the sign itself.

A sign or indicator lamp to indicate when the system is active may also be provided, but this is not required as

part of any regulations.

USER CONSIDERATIONS Several factors exist which could adversely affect user experience and thus lower adoption of any assisted

listening system that requires wearable devices. When specifying a system that requires user-wearable items,

consideration should be given to the following:

WEARABLE DEVICE TYPES Thought should be given to the user of the assisted listening system; most technology types require the user to

wear a receiver device, usually in the form of a pendant or necklace device. No matter which type of system is

used, the wearable device performs a similar function, receiving the signal transmitted by the system and

converting it into a suitable form to be output as an audio signal to the user.

The receiver device outputs the processed audio signal either to a pair of earphones or to the user’s own hearing

device through a local induction loop, utilising the ‘T-switch’ setting on the user’s own device.

Receiver devices with earphones work satisfactorily in most situations but are not suitable in cases where the

user has anything more than moderate hearing loss or has a more complex form of hearing loss that affects the

mechanism of the ear. Hygiene is also a concern with earphone type devices; ear bud components can be treated

as disposables, however this will add significant maintenance cost overheads and may not prove a satisfactory

solution.

Receiver devices that output sound program via a localised induction loop worn around the neck are preferred

where possible. This type of device negates most of the hygiene concerns of the earbud type as it is worn only

around the neck, however it does require the user to have their own ‘T-switch’ compatible hearing aid or

cochlear implant.

WEARABLE DEVICE VISIBILITY From the user’s point of view, the main drawback with all wearable receivers is that they are visible to some

degree and require the user to request to be furnished with the devices; some users may feel uncomfortable

with this and decline to use the system.


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FINANCIAL CONSIDERATIONS Cost of an assisted listening system can broadly be split into three sections:

Equipment cost

Installation cost

Maintenance cost

INITIAL INSTALLATION COST

EQUIPMENT COST The actual equipment cost will vary widely, not only dependent on the type of system used but also depending

on room layout and size. For example, a larger room will require a more powerful inductive loop driver or a

higher number of infrared emitters

INSTALL COST Inductive loop systems will present the highest installation costs because of the added complexity of installing

the inductive loop. Because the loop will be distributed around the room, during installation access will need to

be gained to ceiling/wall/floor spaces in order to install the loop(s).

SAMPLE REAL WORLD PER-ROOM COSTING The following table is based on the cost of the University of Essex purchasing various types of system from a

normal supplier channel and installing the system in a single teaching room. Installation cost is based on the

projected timescale and price of employing two of the university’s Audio-Visual team to install the system, based

upon the hourly chargeable rate of the staff and the estimated installation duration, not including any extra

work (for example, removal of existing installations and making good thereof).

SUBSEQUENT MAINTENANCE COST Maintenance comprises the costs of periodic system testing and servicing and the cost of servicing/replacing

consumables (for example, earpieces and batteries on wearable receivers).

Item QTY Price EA Subtotal Item QTY Price EA Subtotal

Ampetronic MLD7 Amp. 1 £1,580.00 £1,580.00 Ampetronic D7-2 Dante Amp. 1 £1,860.00 £1,860.00

Copper loop foil (100M) 3 £180.00 £540.00 Copper loop foil (100M) 3 £180.00 £540.00

Installation (costed hourly) 16 £30.00 £480.00 Installation (costed hourly) 50 £30.00 £1,500.00

TOTAL £2,600.00 TOTAL £3,900.00

Item QTY Price EA Subtotal Item QTY Price EA Subtotal

Sennheiser SZI 1015 Radiator 2 £780.00 £1,560.00 Sennheiser SI 1015 Modulator 1 £575.00 £575.00

Sennheiser EKI830 Receiver 4 £105.00 £420.00 Sennheiser SZI 1025 Radiator 2 £780.00 £1,560.00

Installation (costed hourly) 8 £30.00 £240.00 Sennheiser EKI830 Receiver 4 £105.00 £420.00

TOTAL £2,220.00 Installation (costed hourly) 12 £30.00 £360.00

TOTAL £2,915.00

600m sq room space (approx. 22m x 22m)

Seating capacity approx. 60 people

Assuming 7% requirement for assisted hearing devices

All prices are correct at time of publication (September 2017) and are ex. VAT

INDUCTIVE PHASED ARRAY LOOP - NORMAL INDUCTIVE PHASED ARRAY LOOP - NETWORKED AUDIO

INFRARED ARRAY (SINGLE CHANNEL) INFRARED ARRAY (DUAL CHANNEL)


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Inductive loop systems are physically more complex and ideally require a periodic check of performance to

ascertain that the system is still functional and within specification; a periodic simple ‘pass/fail’ functional check

is sufficient, with a more comprehensive performance test less frequently, perhaps annually.

CONSUMABLES Falling under this category are items associated with any wearable receiver/headset devices required by the

system, for example batteries and headset earpieces. Although the base material cost of these consumables

may be low, consideration should be given to management of the loan and routine maintenance of wearable

receivers.

LOSS AND DAMAGE Inevitably, wearable devices will occasionally be lost or damaged and this presents an added maintenance cost

in the form of parts replacement.


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CHOOSING AN ASSISTED LISTENING SYSTEM

CONSIDERATIONS Choosing the best type of system for a particular use case depends on numerous factors. Considerations such

as the quantity of rooms versus the number of anticipated users, the density of rooms and building structure

should all be considered when choosing the most suitable type of system to use.

When choosing which type of assisted listening system to install, it may help to start with the following

questions:

Is confidentiality/zero overspill of the signal into adjacent rooms a required feature?

Is the room one of multiple installations within a small area?

Are multiple audio streams likely to be transmitted in the room?

Are the users in a fixed (e.g. seated) position, or are they likely to move about?

Does the room have line of sight into areas covered by adjacent installations?

Suggested decision process for assisted listening system selection


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PART 2: A TECHNICAL PERSPECTIVE


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HEARING LOSS AND CORRECTION

SOUND LOUDNESS VERSUS FREQUENCY Sound encompasses both loudness and frequency; the spectrum usually regarded as being audible by the human

ear ranges from 20 Hz to 20 KHz, although in reality factors such as age and damage will limit this, most

commonly at the higher end of the range.

In addition to frequency, the sound pressure level (loudness)

of a sound also plays a large part in how audible it is to the

listener. The loudness of sound is commonly represented

using the decibel scale; as a general reference, 0dB

represents the softest sound audible by a perfect human ear,

normal conversation is around 60dB, 100dB is

uncomfortably loud and anything louder than 120dB most

likely being painfully loud for most listeners.

Speech resides in a more specific band of frequencies,

spanning from roughly 100 Hz to 5 kHz depending on the

sound being spoken.

HOW HEARING LOSS AFFECTS THE USER Using normal age-related hearing loss as an example, gradual degradation response of the ear may go unnoticed

by the person, although they will gradually experience more difficulty understanding speech, especially in more

dififcult environments; for example, in crowded rooms where there is a lot of background noise or where the

sound source or orator is placed further away.

There is often an incorrect assumption made that hearing-

impaired listeners simply require all sound be amplified. In

reality, this is not the case; often, hearing loss occurs at a

particular band of frequencies and simply amplifying

everything will make the frequency ranges unaffected by the

hearing loss uncomfortably loud while still not addressing

the volume imbalance between these sounds and those the

user struggles to hear.

Additionally, different causes of hearing loss tend to affect

different bands of frequencies; for example, while age-

related loss tends to affect higher frequencies, damage-

related loss (caused for example by continued exposure to

loud sounds over extended periods) will tend to cause a loss

centred on the frequency envelope of the sounds which caused the damage. Other factors may cause an

altogether different hearing loss characteristic.

An example of damage-related hearing loss

versus frequency

Sound signatures of common sounds


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CORRECTION OF HEARING LOSS As hearing loss characteristics vary so widely, there is no ‘one size fits all’ solution to compensate for hearing

loss or impairment of users and a system which aims to simply make all sounds louder will not be satisfactory

for the majority of users. Often, simply boosting the overall volume level of the entire audio spectrum may

actually result in a degraded user experience, possibly worsening the user experience instead of improving it.

The ideal solution is to compensate for the actual hearing loss across the frequency spectrum rather than simply

boosting the volume level at all frequencies. The majority of modern hearing aids are already tailored to the

user’s individual hearing loss profile and it makes sense to utilise this tailored compensation. The ideal solution

is for the assisted listening system to use this customised compensation and deliver sound through the user’s

own hearing aid, rather than through separate headphones.

Results of incorrect (left) and correct (right) hearing loss compensation


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ASSISTED LISTENING SYSTEMS IN DETAIL


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WI-FI SYSTEMS

OVERVIEW This system uses either a local wireless access point or the room’s existing Wi-Fi infrastructure to transmit the

audio signal to the user’s device. Commonly, the audio is

received via a client app that the user installs on their own

smartphone or tablet that enables them to receive the

transmitted audio program directly through their own

device.

Wi-Fi hearing reinforcement systems are commonly

available in versions that either integrate into existing

network infrastructure or else offer a dedicated wireless

access point that is isolated from the room’s existing

network system.

Although Wi-Fi-based systems offer advantages in terms

of initial installation cost, audio quality and channel

quantity, noticeable latency that varies depending on the

user’s device is often a problem. Latency-related

problems should become less of a concern as technology progresses, however for now latency should be

considered as a significant setback to a Wi-Fi system.

ADVANTAGES Flexible implementation; installed into existing Wi-Fi infrastructure

User can connect through their own smart device

DISADVANTAGES Network security considerations

Requirement for user to install 3rd party app

Requires user to have their own smart device and headset

Variable latency depending on user device and network topology

Sennheiser MobileConnect Wi-Fi system (image

courtesy of Sennheiser UK)


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INFRARED SYSTEMS

OVERVIEW Infrared systems transmit audio signals wirelessly through air in the form of light of a frequency that is outside

of the visible light spectrum. Infrared systems are only able to transmit signals to targets within line of sight; this

may be either an advantage or a disadvantage depending on the implementation.

Infrared systems are ideal in implementations where any amount of spillover is forbidden; as they require line

of sight with the receiver, the signal cannot possibly pass through solid walls into adjacent rooms.

One advantage with infrared systems is that they are mostly ‘plug and play’; in most cases, the system will

require no configuration, in comparison to an inductive system which will most likely require adjustments to

accommodate factors such as the metal content of a room.

ADVANTAGES Zero spillover through walls

Easily expandable to cover large areas

Simpler installation in large or complex rooms versus inductive systems

Not affected by the structure of the surrounding building

Ability to carry multiple audio channels at the same time

DISADVANTAGES User requires wearable receiver

Requires line of sight contact between transmitter unit and user

An infrared system requires several components; depending on the size and complexity of the system, each of

these components may be separate or included into the same unit.

Infrared Neck Loop

...

Hearing Aid

Analogue signal (wired)

Infrared (wireless) Inductive (wireless)

Speaker s microphone

Infrared Modulator/

Radiator

MODULATOR Converts one or more channels of audio input into a format suitable for wireless infrared transmission. Many

larger systems are capable of handling up to 32 separate channels of audio; some systems are capable of

handling stereo signals, and do this by combining two separate channels.


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EMITTER An array of infrared light devices that transmit the signal

wirelessly through the air. Depending on the size of the

installation, one single emitter may be sufficient – on larger

installations, multiple ‘slave’ emitters may also be required, but

adding these is usually straightforward. On smaller systems, this

component is often part of the same unit as the modulator.

RECEIVER A module that is held or worn by the user to receive the

transmitted infrared data and convert it back to an audio signal.

These devices often take the form of a pendant worn around the

user’s neck; the audio signal is either then passed directly to the

user’s own hearing aid (through the T-switch setting) or

alternatively output to a pair of headphones.

POTENTIAL PROBLEMS Although infrared hearing systems are capable of functioning well in many environments, some potential

problems may occur and consideration should be given towards these.

EMITTER AND RECEIVER CONTACT Infrared systems require ‘line of sight’ contact between emitter and receiver to function correctly. Although in

some instances enough infrared light may actually be reflected from other surfaces for the receiver to function

correctly when out of line of sight of the emitter, the reception in these situations will be sporadic and should

not be relied upon. Additionally, other factors may affect system performance, such as the composition and

colour of the room’s walls; some colours will absorb more infrared light. 200Because of these considerations,

care should be taken to optimise the quantity and position of emitters to ensure that line of sight contact is

ensured whenever possible.

When designing the installation, care should be taken to consider the layout and use of the room. If the audience

are expected to remain seated and facing in one direction, emitters placed only along the front facing wall may

be sufficient; however, if the room layout is more fluid, emitters should be placed equally around the perimeter

to ensure maximum system coverage.

Additionally, in some cases the user-wearable receiver device may become obscured under the wearer’s

clothing. Although the device may possibly still function, the amount of infrared light received will be much

reduced and therefore the quality and level of the received signal may be reduced.

OVERSPILL THROUGH WINDOWS Although the infrared signal is effectively contained by opaque surfaces, the signal will spill through transparent

objects such as windows and glass partitions. Therefore, overspill into adjacent areas is possible where line of

sight persists through open doorways or transparent surfaces and in this case, an alternative type of hearing

reinforcement system may be required.


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RF SYSTEMS

OVERVIEW These systems allow for signal transmission up to a range of approximately 150m, without the line of sight

requirement of infrared; the resulting audio signal is potentially high quality and not affected by the structure

of the building.

The downside to an RF system is that it is not limitable to a specific area; although the structural components of

a building (walls, framework etc.) will attenuate the signal to an extent, it is not possible to actively limit the

signal’s coverage to a particular boundary. Although this may raise a confidentiality concern, it does not

necessarily mean that RF systems in adjoining rooms will interfere with each other; some RF systems offer

multiple channels and it may be possible to arrange locally grouped installs across separate channels to prevent

interference.

Because of these drawbacks, RF assisted listening systems are common in countries where buildings tend to be

spaced further apart (such as the US), but are not commonly used in Europe.

ADVANTAGES No line of sight requirement

Potentially high sound quality, not influenced by building structure

Can transmit multiple audio channels at the same time

DISADVANTAGES Not possible to limit coverage to specific area


RF-based assisted listening systems are capable of providing high quality sound output over multiple channels.

FM Transmitter

...

Hearing AidAudio signal (wired)


FM (Wireless)

FM Receiver

Inductive Neck Loop

Inductive (wireless)

Headphones/ Headset

RANGE AND OVERSPILL The maximum range of an RF system varies from manufacturer to manufacturer and most devices have

switchable power levels, with the maximum transmission range for most systems being up to 150 meters.

On many systems, the power of the transmitted signal is adjustable and this has a direct effect on the range of

the device, however it is not possible to limit the signal to a specific area (e.g. the confines of a room).


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Because of these constraints, large-size installed RF type hearing systems are not common in countries such as

the UK where there is a trend towards closely grouped clusters of teaching spaces.

SYSTEM COMPONENTS RF systems are available in both room installation and portable system types; in both cases, the system

comprises a transmitter/receiver setup.

Most devices have multiple selectable channels to enable multiple programs to be transmitted within the same

room without risk of interference; this functionality is also useful to enable transmission of multiple audio

streams in the same space, for example multiple-language versions of the same program.


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INDUCTIVE LOOP SYSTEMS

OVERVIEW Inductive loop systems transmit audio signals wirelessly through a process known as electromagnetic induction.

Induction is the only technology that enables the user to receive the transmitted signal directly through their

existing hearing aid without requiring any extra equipment (for example, a wearable receiver pendant).

Directly T-Switch compatible; no user wearable equipment required

Requires complex configurations to minimise spillover

Can only transmit one single channel per installation

May potentially interfere with sensitive audio installations

Potentially high install cost of inductive loop

Loop potentially vulnerable to wear and tear

SYSTEM TOPOLOGY

Amplifier & Loop

...

Hearing Aid

Analogue signal (wired)

Inductive (wireless)


Irrespective of the type of system, the overall system layout will be similar; the source audio (derived for example

from a microphone or pre-recorded audio program) is input to a driver or amplifier unit which amplifies the

signal to a sufficiently high magnitude current to allow the signal to radiate from the room’s loop.


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LOOP INSTALLATION TYPES

PERIMETER LOOP The perimeter loop is the simplest type of induction loop

installation, however is only suitable for smaller areas (less than

400m2). The installation utilises a single loop placed around the

perimeter of the intended area, making a more cost-effective

system that is simpler to install.

While adequate for small spaces, this method allows the loop

signal to spill widely outside the room’s boundaries and may

exhibit a drop in response towards the centre of the area if

employed in larger areas.

Summary

Straightforward installation

Less prone to accidental damage than floor loop installations

Large amount of uncontrolled spill outside the loop area

Voids in coverage appear if used in larger rooms

ARRAY LOOP An array loop is suitable for larger rooms where the area

coverage of a perimeter loop is insufficient. This type of

installation comprises longer single loop run, arranged into an

array of rectangular loops. The loops must be laid in a ladder

pattern to ensure that the direction of current in the edges of

adjacent loops flows in the same direction; otherwise,

cancellation of loop response in these areas will result. The image

to the left shows the preferred layout to prevent adjacent loop

cancellation in an array installation.

A further enhancement of the array loop is the phased array; this

utilises two electrically separate but physically overlapping

induction loops to reduce overspill outside the intended

reinforcement area. A correctly implemented phased array system is able to reduce overspill to as little as 1.5

meters.

CEILING MOUNTED ARRAY

The loop can be attached to the ceiling surface within the room. This approach makes the loop less vulnerable

to damage caused by foot traffic and room maintenance. This mounting method is only suitable for rooms with

even, low ceilings.

Loop is less prone to physical damage than floor-mounted type

Can be hidden behind suspended drop ceiling

Relatively easy access for maintenance

Unsuitable for ceilings which are uneven or which have heights above 4 meters

Unsuitable for rooms with tiered floor level


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FLOOR MOUNTED ARRAY

The more common method of installation, the loop fixed to the floor surface below the floor covering. Flat

copper tape can be used as an alternative to traditional round wire to help minimise the profile of the loop and

help prevent damage being caused by footfall or by room maintenance operations such as carpet fitting.

Attaching the loop to the floor surface however does have the advantage that the loop is at a more constant

distance to the wearer’s hearing device, ensuring a constant signal level.

Suitable for rooms with high/uneven ceilings

Constant distance from user’s hearing device

Loop is prone to physical damage from traffic and room maintenance

Potentially difficult to access for maintenance

CHOOSING A SUITABLE LOOP LAYOUT The choice of loop layout depends on the size of the room and the density of surrounding rooms. The main rule

of thumb is that a perimeter loop is only suitable for a room with a floor area smaller than 50m2; for anything

larger, a loop array is the only suitable choice.

LOOP ARRAY INSTALLATION IN MULTIPLE STOREY BUILDINGS In environments where multiple loops are required across two or more storeys, vertical loop overspill may

become a problem. Although the signal from a loop will be attenuated to some degree as it passes through the

structure of the building, enough signal may pass through the floor or ceiling medium to be received at an

audible level by a user in the adjacent room. In this situation, the preferred solution is to separate the respective

loops as far as possible by installing the lower room’s loop in the floor and the upper floor’s loop in the ceiling.

This layout will ensure that inter-loop interference is as low as possible.

PHASED ARRAY AND LOW SPILL LOOP INSTALLATIONS In environments with high-density inductive loop installations, horizontal overspill between adjacent rooms on

the same floor may become a concern. This overspill can be reduced through use of a more complex layout

comprising a pair of loops, the signal to each loop being 90° out of phase. The loop amplifier for a phased array

system is in many ways just a pair of normal induction loop amplifiers, however the second amplifier contains

circuitry to give the required 90° phase shift on the second loop array; attempting to use two normal hearing

loop amplifiers to power a phased array will not have the intended effect.

INDUCTIVE LOOP DECISION TREE

START

Room Size

Perimeter Loop Array Loop

< 50m2 > 50m2

Location of Adjacent Rooms

Dual (Phased) Array

Single Array

Dense Sparse


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A loop will audibly spill outside the intended coverage area by several times the loop’s width; this effectively

makes single perimeter loop installations unsuitable for use in most educational environments, where multiple

teaching or seminar rooms are often situated in close proximity.

Illustration of perimeter induction loop spill; horizontal on left, vertical on right (images reproduced courtesy

of Ampetronic)

Overspill can be contained horizontally on one or more edges of the area by means of either a simpler

‘cancellation loop’ for containment on one edge, or a more complex ‘low loss’ or ‘low spill’ phased array loop

arrangement for complete containment to within 1.5m of the horizontal loop boundaries.

Design and successful implementation of a low spill array is a complex task that requires computer modelling,

requiring the assistance of an experienced hearing loop installer.

PORTABLE AND LOCAL INSTALLATION TYPES

PORTABLE LOOP SYSTEMS This consists of a portable device that is stood on a table or desk to enable a single user to communicate with

another party. Although portable systems do serve a useful purpose in a temporary ‘one on one’ meeting room

or interview scenario, they are not suitable for deployment in a teaching room or seminar environment.

In use, the device is placed on the desk or table facing directly in front of the user; devices often have a built in

microphone for the other party to speak into, although some devices are also capable of connecting to a

separate microphone.

SUMMARY

Truly portable device; no installation required

Requires user to identify themselves as hearing impaired to request equipment to be present

Very directional and positional area coverage

Many devices have ‘auto power-off’ functions to save battery life and need to be manually switched on

before each use


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COUNTER LOOP SYSTEMS This installation type is essentially a small, localised system that is designed for installation in environments such

as counters and reception desks. Most systems are supplied with a small, pre-formed loop that gives a small

area coverage and often allow integration with a two-way intercom, to allow installation in environments such

as cash desk and security kiosks where there is a glass screen between staff and customer.

SUMMARY Suitable for a small area, where the user is likely to remain in one position

Very small coverage area

PERFORMANCE AND STANDARDS As inductive loops are more prone to performance variations than other types of assisted listening systems,

more validation is required to ensure an adequate level of performance in terms of quality of service.

Standards such as BS EN 60118-4 specify the performance requirements for an inductive loop type hearing

reinforcement installation. From this specification, the notable performance requirements are:

Frequency response between 100Hz-5KHz must be within +/- 3dB, relative to the response at 1KHz Field strength of 400mA/m, within +/- 3dB Background noise (i.e. the interference from other sources, including other nearby loop installations)

must be lower than -32dB

Essentially, these requirements quantify system performance in three areas:

The frequency response of the system must be relatively flat across the section of the audio spectrum

used for speech

The loop must radiate a field of adequate level to allow the user to hear the program at a comfortable

volume level

The level of any background noise must not be so high as to affect the ability of the user to understand

the transmitted signal.

SPECIFYING A LOOP AMPLIFIER The major specifier for loop amplifiers is the amount of current the amplifier is able to output into the loop.

LOOP CURRENT VS. FIELD INTENSITY Electronics is bound by Ohm’s law; a hearing loop is a single conductor and Ohm’s law states that the current

throughout the length of a conductor is constant; so if for example an amplifier outputs 5 amps into a loop, the

current in the loop will be 5 amps throughout the length of the loop.

Although loop current will be constant throughout the loop, the field strength picked up by the user’s hearing

device will decrease as the distance between the conductor and receiver increases; therefore, an increase of

loop conductor current will be required to maintain the correct magnetic field strength as the distance between

loop conductor and user receiver increases.

Larger installations require a trade-off between array loop dimensions and the total conductor length. Making

the loops in the array smaller will decrease the current required (as the distance between user and loop

conductor will often be smaller), however the total length of the loop will be longer and therefore the loop

amplifier will need to be capable of producing a larger output voltage. Conversely, arranging the array with larger

loops will decrease the total conductor length that will reduce the amount of voltage required from the loop


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amplifier but the maximum distance between loop conductor and user will be higher and therefore will require

a larger current to produce the required field strength.

MEASURING PERFORMANCE To ensure that a loop installation meets regulations and provides adequate quality of service, background noise,

field strength and frequency response need to be checked with the aid of a suitable measuring device. Although

system measurement is possible through over means, it is recommended that the installer use one of the

commercially available measurement devices to ensure that the measured data adheres to the accepted

regulations and standards.

The sequence of measurements should be performed in a specific order to correctly qualify and calibrate the

system for optimum performance and adherence to the required regulations, in the order shown in the following

flowchart:

Suggested hearing loop measurement and qualification sequence

Although the overall test sequence is standardised, the individual requirements of each individual test (for

example, the type of signal) depends on the brand/model of test equipment used. The qualifier should refer to

the instructions for the test equipment used, however the following sections give an overview of the reasoning

for each test and the basic specifications.

BACKGROUND NOISE Background noise may be in the form of overspill from adjacent loop installations, or from other external sources

of electro-magnetic noise, i.e. hum from mains wiring or induced noise from other electronic devices.

Background noise should be measured with the loop in question not transmitting any signal, but with

surrounding loops active and transmitting a signal; this ensures a ‘worst case’ measurement is taken.

Adjust Magnetic Field Strength

Measure Frequency Response

Frequency Adjustment Required?

YES

Commission the System

Test Magnetic Signal Strength over Entire

Coverage Area

Meets Specification on all

Signals?

NONO

System Certificated

Suitable Site for Inductive System

YES

Measure Background Noise

Background Noise Less Than

-32dB?

Investigate and Rectify Source of Noise

NO

YES

0dB Field Strength Attainable?

YES

Rectify Inadequate Field Strength

NO


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Measurements should be taken from various points within the area; the worst measurement (highest dB figure)

recorded as the final test result.

MAGNETIC FIELD STRENGTH If the background noise level is found to be acceptable, it next needs to be ascertained whether the system is

able to provide adequate field strength within the desired coverage area. This consists of taking measurements

from various points around the area. The signal required for this test will depend on the brand and model of test

equipment used but it usually either a sine wave of around 1 KHz or a pink noise signal.

The goal of this measurement is to ascertain that the loop and amplifier are able to provide adequate field

strength to enable a sufficient volume level of signal to be received by the user. If this test proves satisfactory,

subsequent measurements and calibration can take place.

FREQUENCY RESPONSE Assuming that the system is capable of providing adequate field strength, the next step is to measure the

frequency response to ensure that the system is able to provide a flat, even response across the frequency

spectrum. To meet regulations, the frequency response must be within +-3dB of the response level at 1 KHz at

all places within the loop’s intended area.

Frequency measurement requires a signal with a broad frequency response to be transmitted through the loop;

the type of signal depending on the measuring equipment in use. The installer then walks around the area while

holding the measuring device and notes any deviation of the measured frequency response; the worst noted

measurement is recorded as the test result for the system.

There are two major reasons why the frequency response of the system may not be flat; metal losses and

amplifier overload. Metallic room contents or building constituents will absorb the higher frequency

components of the transmitted signal, effectively reducing the high frequency response of the system, reducing

both audibility and potentially causing the system to fall outside specification requirements. Many loop

amplifiers offer a solution to metal loss in the form of ‘metal loss compensation’ – often in the form of an

adjustment that boosts the amplifier’s response at higher frequencies to compensate for the losses.

Additionally, it is possible

that the loop amplifier may

simply be unable to drive

the loop adequately and as

a result, the system

response may drop

noticeably at higher

frequencies. In this case, if

adjustment via the

amplifier’s metal loss

compensation proves

insufficient, the only

solution is to investigate the problem further and possibly replace the loop amplifier with one more suited to

the size of loop installed in the room.


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SYSTEM MAINTENANCE Inductive loop systems inherently require more periodic maintenance than other types of assisted listening

system. Problems may arise from unseen damage caused to the loop by room building works or general wear

and tear, and although loop amplifiers tend to be very reliable, there is a chance that they may develop a fault.

Although an inductive loop system does not require ‘servicing’ as in routine replacement of parts, it is important

that a periodic check is made by a trained person to ensure that the system is still both functional and performing

to specification. The BS standard states that checks should be made at regular intervals, utilising a portable

receiver equipped with a field strength display graduated in dB and a headphone output to allow the person to

listen to the program being played over the loop to verify that the audio output quality is acceptable.

A Field Strength Meter (FSM), the same device used to initially set up and calibrate a loop system, is the ideal

device to use to perform any periodic testing. In this case, it is preferred to test the system as per the original

calibration and setup procedure, i.e. measuring first field strength and then frequency response at various points

throughout the coverage area. Any degradation in performance should be noted and investigated as over time

this will likely cause the system to fall outside specification.


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