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Summary of  IEC 118-4 (1981) Standards:

With a signal of 1,000 Hertz at a level equal to the long-time average of the speech signal applied to the loop system's input, the resulting field strength inside the area(s) enclosed by the loop(s) shall average 100 mA/meter +/- 3 dB, with 70 mA/meter as the minimum level and  140 mA/meter as the maximum level, measured 1.2 meters above the floor.  Allowing for 12 dB peaks occurring in speech, peak field strength may reach 400 mA/meter.  The frequency response of the induction loop system shall be 100 Hz-5 kHz +/- 3 dB.

Additional Specifications Suggested by Oval Window Audio (rev 8/03):

1.  Loop Wire Installation:  In order to maximize signal strength and uniformity, the loop wire shall be installed either at floor level or at a ceiling height not to exceed the manufacturer's recommendations.  At least 80% of the installed loop wire should be free of the influences of metal, either in front of, or immediately behind the wire.

2.  Field Strength: 
Pure tone: As per IEC 118-4 with the additional conditions that the measurement be "A" weighted to disallow the influence of inaudible low frequency power line electrical noise, and that the measurement at the center of the loop(s) be 100 mA/meter +/- 3 dB at a height of 1.2 meters, with an input signal of 1,000 Hertz.
Speech: Using unweighted/flat frequency response setting of measurement equipment, speech test signals must average + 3 dB at the center and throughout the listening area, with signal peaks not exceeding +12 dB re: 100ma/meter.

3.  Input Signal Compression:  To achieve a long-time average field strength of      100 mA/meter, and to maximize signal intelligibility and listener comfort regardless of fluctuating signal input levels, an automatic gain control, signal compression and/or adjustable non-distorting peak limiter must be employed at the input of the system.  Recommended compression ratios:  4:1 (music), up to 20:1 (speech).

4.  Frequency Response:  The frequency response of an induction loop system shall comply with IEC 118-4 specifications, as confirmed with a field strength meter.  Frequency response measurements conducted with an "A" weighted field strength meter will need to be corrected to compensate for the substantial low frequency roll off characteristic of this weighting.  The use of a graphic equalizer is beneficial in certain situations where the environment is affecting the system's frequency response, and/or the listening audience consists of people who have specific needs and/or preferences.

5.  Ambient Electrical Interference:  Sources of electromagnetic radiation that may interfere with the proper functioning of an induction loop system include:  faulty florescent lighting, light dimmers, electrical wiring, TV and computer monitors, surge protectors and radio transmitters that are in close proximity to the loop system.  When possible, an on-site evaluation of ambient electromagnetic noise should be performed before a loop system is installed in order to identify and resolve any potential sources of interference.  Using a field strength meter, ambient electromagnetic noise should not exceed -20 dB ("A" weighted) re:  100 mA/meter, as measured at any seat within the area that is to be enclosed by the loop.   Depending on the location, intensity and frequency of the electrical noise, and the potential for remedying the problem, the use of an induction loop system may have to be ruled out.  Using an induction receiver or telecoil equipped hearing aid, listening tests may be required to determine if the frequency(ies) of the electrical noise fall within the frequency response range of hearing aids.  For example, induction receivers and most hearing aids do not reproduce sound below 100 Hertz.  As a result, 50/60 Hz. power line noise should not cause a problem, even though it may be measurable on some field strength meters, unless higher (audible) harmonics of the noise are present.

6.  Signal Spillover:  In the event adjacent areas are equipped with induction loop systems, signal spillover must not exceed 12.5 mA/meter, or -18 dB ("A" weighted),  re:  100 mA/meter, as measured at any location within any of the induction loop(s) in use.  Using an induction receiver or telecoil equipped hearing aid, listening tests should be performed with all systems in use in order to determine if the signal spillover is audible and/or distracting.

7.  System Signal-to-Noise:  The electrical signal to noise ratio of the loop amplifier output (measured directly, not inductively) at 1,000 Hz. must be at least +30 dB (unweighted) re: to the system's internal noise at an output level sufficient to deliver 100 mA/meter as per IEC 118-4 specifications.

8.  Distortion:  With an input signal of 1,000 Hz and the system adjusted for an output of 100 mA/meter, harmonic distortion must not exceed 3%.

9.  Listening Tests:  a regularly scheduled listening check of all assistive listening systems and their associated receivers is strongly recommended, preferably before each use.