Large Area Assistive Listening Systems: Review and Recommendations

2.1.2.2 Additional Proposed Specifications (by Oval Window Audio)

1. Loop Wire Installation: In order to maximize signal strength and uniformity, the loop wire shall be installed either at floor or ceiling level. At least 80% of the installed loop should be free of the influences of metal, either in front of, or immediately behind the wire.

2. Field Strength: As per IEC 118‒4, with the additional condition that the measurements be "A" weighted to disallow the influence of inaudible low frequency power line electrical noise.

3. Input Signal Compression: To compensate for fluctuating signal input levels, an automatic gain control, signal compression, and/or adjustable non-distorting peak limited must be employed at the input of the system (Recommended compression ratios: 4:1 for music, and up to 20:1 for speech).

4. Frequency Response: Frequency response measurements conducted with an "A" weighted field strength meter must be corrected to compensate for the substantial low frequency roll off characteristic of this weighting network.

5. Ambient Electrical Interference: Sources of electromagnetic radiation that may interfere with the proper functioning of an induction loop system include: faulty fluorescent lighting, light dimmers, electrical wiring, TV and computer monitors, surge protectors that are in close proximity to the loop system. An on-site evaluation of ambient electromagnetic noise should be performed before a loop is installed in order to identify and resolve electromagnetic interference. Ambient electromagnetic noise should not exceed 25 mA/meter or –12dB ("A" weighting) re: 100 mA/meter as measured at any seat within the looped area. If potential sources of noise cannot be reduced or eliminated, then the use of a loop must be ruled out.

6. Signal Spillover: When adjacent areas are equipped with an induction loop, signal spillover must not exceed 12.5 mA/meter, or –18 dB ("A" weighted) re: 100 mA/meter as measured at any location within an adjacent loop. Listening tests should also be performed to determine if signal spillover is audible.

7. System Signal-to-Noise: The electrical signal to noise ratio of the loop amplifier output (measured directly, not inductively) at 1000 HZ must be at least +30 dB at an output level sufficient to deliver mA/meter as per IEC 118‒4 specifications.

8. Distortion: With an input signal of 1000 Hz and the system adjusted for an output of 100 mA/meter, harmonic distortion must not exceed 3%.