BS 7594:1993

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

Provides recommended design and measurement methods for use by local authorities and administrators of public buildings and sports stadia, purchasers, designers, manufacturers and installers, for the correct installation of AFILS with an acceptable performance level and good reliability for use by persons who are hard of hearing.