Committees responsible
Foreword
Code of practice
Section 1. General
1.0 Introduction
1.0.1 How to use this code of practice
1.0.2 What is an audio-frequency induction-loop
1.0.3 Advantages of AFILS (for assisted hearing)
1.0.4 Limitations
1.0.5 Specialist advice
1.0.6 Safety aspects
1.0.7 Conforming to existing performance standards
1.1 Scope
1.2 References
1.3 Definitions
1.4 Signs and symbols
Section 2. Guidance on purchasing an AFILS or placing a
contract for its installation, or obtaining independent
assessment of the performance of an installation
2.1 Technical advice
2.2 Choosing the system supplier
2.3 Contractual provisions
2.4 Limitations of an AFILS
Section 3. Recommended procedures for planning and
designing an AFILS
3.1 Planning and project management
3.2 Site assessment
3.3 Design
3.3.1 General
3.3.2 Symbols
3.3.3 Design definitions
3.3.4 Basic theory
3.3.5 Relationship between the requirements of
BS 6083: Part 4: 1981 and the characteristics
of hearing aids and speech signals
3.3.6 Impedance of the loop
3.3.7 Selection of the method of driving the loop
3.3.8 Voltage-driven loop
3.3.9 Voltage-driven loop with high-level equalization
3.3.10 Voltage-driven loop with low-level equalization
3.3.11 Current-drive and intermediate-drive amplifiers
3.3.12 Use of transformers
3.3.13 Effects of building construction
3.3.14 Electromagnetic compatibility (EMC)
3.3.15 Tone signals
3.3.16 Equalization, other than for compensating loop
impedance characteristics
3.3.17 Multiple loops
3.3.18 Protection of loop conductors
3.3.19 Automatic gain control (AGC), compression,
limiting, noise gating and voice control
3.3.20 Signal-to-noise ratio
3.3.21 AFILS for purposes other than assisted hearing
3.3.22 Flow-chart for loop design and final amplifier
requirements
3.4 Final amplifiers and integrated amplifiers
3.5 Preamplifiers and mixers
3.6 Signal sources
3.7 Safety and reliability considerations
3.8 The role of the System Designer in commissioning
3.9 Designing for monitoring and maintenance
Section 4. Recommended procedures for installing AFILS
4.1 Installing the loop
4.2 Installing the microphone(s)
4.3 Installing the amplifier, electronic equipment
and induction-loop monitor receiver
4.4. Safety and reliability considerations
4.5 Provision of information
4.6 Provision of spares
Section 5. Recommended procedure for commissioning AFILS
5.1 Recommended commissioning procedure: minimum
operating performance
5.2 Recommended commissioning procedure: equipment
audit, setting up and testing of the system
Section 6. Recommended procedures for operating AFILS
6.1 Provisions of information for users, management
and operators
6.2 The hearing aid user
6.3 The site management
6.4 Training operators and staff
6.5 The operating technician or operator
6.6 Checking the AFILS after room configuration
changes
Section 7. Recommended procedures: monitoring/maintenance
7.1 Induction-loop monitor receiver
7.2 Use of the monitor receiver
7.3 Maintenance of the equipment
7.4 Provision for spares
7.5 Personnel considerations
7.6 Maintenance manuals
7.7 Documenting of information
Annexes
A. (informative) National organizations
B. (informative) Guide to the preparation of an AFILS
specification
C. (informative) Explanation of the basis of the
design equations given in section 3
D. (normative) Induction-loop monitor receivers
E. (informative) Explanation of the specification and
measurement of magnetic field strength of
induction-loop systems
F. (normative) Design of small multi-turn loops (for
neck loops and ticket-office (counter) systems)
G. (informative) Procedure for setting up a single
AFILS
H. (normative) General guidance on induction loops
for ticket office (counter) windows
Tables
C.1 Values of Ho/Hoz
C.2 Class 5 flexible annealed copper conductors for
standard single-core and multi-core cables
C.3 Class 1 solid annealed copper conductors for
single-core and multi-core cables
C.4 Ratio of approximate to exact inductance
C.5 Values of L/Z/
Figures
1. Symbol for use on AFILS circuit diagrams
2. Sign for display in premises to indicate that an
AFILS is installed and for AFILS equipment
identification
3. Sign to show seating areas where AFILS reception
is not satisfactory
4. Pictorial view of magnetic field (lines of force)
in a plane through one axis of a rectangular loop
5. Directional response of a hearing aid telecoil
6. Variation of the strength of the perpendicular
component of the magnetic field across an axis of
a rectangular loop, with listening height as
parameter
7. Variation across the median of a square loop of the
perpendicular and coplanar components of the
magnetic field at a height ratio hn=1
8. Variation across the diagonal of a square loop of
the perpendicular and coplanar components of the
magnetic field at a height ratio hn=1
9. Variation with frequency of the logarithm of the
impedance of loops of constant inductance L, with
the loop resistance R as parameter
10. Circuit diagram of a 'high-level' equalizer (for
insertion between the amplifier and the loop)
11. Preferred loop layout in a steel-framed building
12. Methods of breaking eddy current paths in metal
structures
13. Filter for attenuating VHF signals picked up by the
loop
14. Methods of 'breaking' loops into which interference
voltages could be induced by an AFILS
15. Multiple loop layouts
16. Variation across an axis of the system, at a height
ratio of 0.32, of the component of the field
strength, for a loop system similar to that shown
in figure 15c, compared with the corresponding
variations for a conventional single loop
17. Flow chart to determine the design of AFILS loops
and the final amplifier needed
18. Horizontal directional response of an omni-
directional microphone with its axis vertical:
decibel scale
19. Directional response of a cardioid microphone:
decibel scale
20. Directional response of a supercardioid microphone:
decibel scale
21. Directional response of a hypercardioid microphone:
decibel scale
22. Directional response of a highly directional
shotgun (rifle) microphone where f = 250 Hz and
barrel length is 275 mm
23. Directional response of a highly directional
shotgun (rifle) microphone where f = 2.5 kHz and
barrel length is 275 mm
24. Directional response of a highly directional
shotgun (rifle) microphone where f = 10 kHz and
barrel length is 275 mm
25. Relative operating distances of directional micro-
phones for equal direct-to-reverberant signal ratios
26. Two methods for positioning microphones on a
conference table
27. The 3:1 ratio for microphone positioning (normal)
28. A reduced 3:1 ratio using angled microphones
29. Illustration of good and bad microphone placements
30. Optimization of the height of the microphone above
the table
31. Example of a notice to hearing aid users
C.1 Diagram for calculating magnetic field strength
C.2 Diagram for calculating magnetic field strength
at the centre of a rectangular loop
C.3 Diagram for calculating magnetic field strength
at an arbitrary point
C.4 Diagram for calculating cos phi and sin phi
C.5 Diagram for calculating magnetic field strength
at point (x,y,z)
D.1 'Normal' frequency response: target curve and
tolerances on response
D.2 'EQ' or 'wideband' frequency response: target
curve and tolerances on response
List of references