HLS-2D Metal Loss Test Procedure (document UP17940-4)

Summary

A loop maker's site-survey procedure for measuring how much a building's metal weakens a hearing loop's field, with its rules for choosing a perimeter loop or an array.

Key findings

• Describes a pre-design site test: a temporary test loop is driven at a known current, and field strength, frequency response (100 Hz, 1 kHz, 5 kHz) and magnetic background noise are read at 1.2 m above the floor, at least 1 m from the wire, across the structural elements of the area.
• Treats a perimeter loop as suitable when no spill control is needed and the highest and lowest readings differ by less than 6 dB; with more than 6 dB of variation it says a perimeter loop could not meet IEC 60118-4 and recommends testing a figure-of-eight array with segments 2 to 4 m wide.
• Where spill must be controlled for confidentiality or because of adjacent spaces, it limits array segments to 2.0 to 2.5 m wide; it adds that a floor-level perimeter loop wider than 15 m in its narrower dimension varies by more than 6 dB even with no metal present, and that elevating the loop or using its MultiLoop array system corrects this.
• Says its network drivers can correct metal loss by up to 4 dB per octave (enough for a 5 kHz reading down to -9 dB relative to 1 kHz) at the cost of up to 4 dB more voltage headroom, that metal loss is usually greatest in the middle of a perimeter loop, and that if the loss slope varies by more than about 3 dB between test points, smaller loop segments may be needed.

Limits

A manufacturer's procedure built around its own products and design software; the thresholds are its rules of thumb, not independently validated, and the document gives no data on typical metal loss in buildings. It prints no publication date; its declaration-of-conformity page is dated March 2022.

Read the original

www.ampetronic.com/wp-content/uploads/2019/05/UP17940-4-HLS-2D_Metal_Loss_Test_Procedure.pdf

Link checked 9 October 2026. We link to the original and do not host copies.

Cited in

• Auracast, hearing loop, infrared, RF or Wi-Fi: which suits which room?Disputed
• How is an installed hearing loop checked against IEC 60118-4, and how often does it pass?Partly answered
• Where do hearing loops fall short: metal, spill, noise and upkeep?Partly answered

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