Summary
A loop maker's explanation of how metal in a building weakens a hearing loop's field, makes it uneven and dulls its high frequencies, with its examples of how large the loss can be.
Key findings
- Lists three effects of metal within the loop area: a weaker field, an uneven field across the listening plane, and losses that grow with frequency, so that sound through the loop becomes muffled and indistinct.
- Says only metal forming an electrically closed path matters, and that conductivity, not magnetism, drives the loss, so copper and aluminium cause more loss than steel.
- Gives losses as usually up to 3 dB per octave, and says a metal loss corrector is needed to restore the flat 100 Hz to 5 kHz response the loop standard requires.
- Calls a loss of more than 15 dB at 1 kHz in the centre of a perimeter loop over a reinforced concrete floor not unusual, and says losses above 25 dB at 1 kHz have been known.
- Argues that more amplifier power is no remedy: in its example, a loop needing 8 A peak without metal would need 45 A, and the field would still be uneven because the loss varies across the area; it says such rooms need special designs such as a phased array.
Limits
A manufacturer's page, without measured data, sites or methods behind its examples, and promoting its own correction circuits, arrays and design service. The page shows no date (its metadata gives first publication on 5 April 2018 and a last change on 30 October 2020).
Read the original
www.ampetronic.com/info-centre/learning-zone/metal-structures/effects-on-loop-performance
Cited in
Cite this source
Ampetronic. Effects of Metal Structures on Inductive Loop Performance. Ampetronic (learning zone web page). n.d. https://www.ampetronic.com/info-centre/learning-zone/metal-structures/effects-on-loop-performance/ (accessed 9 October 2026).
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