- Degrades magnetic field strength.
- Produces uneven magnetic field strength across the loop plane.
- Frequency dependant losses. High frequencies (ie intelligibility) are attenuated more than low frequencies. The effect is for the audio received through the loop to sound is as though you had placed a thick blanket across your hi-fi speakers - muffled and indistinct.
The main consideration is the amount of metal structures containing closed paths within the area covered by the loop. Any metal structures that do not form an electrically closed path within the loop can usually be ignored. This means that heating pipes, radiators, metal windows and isolated reinforcement bars within concrete tend not to cause problems.
A common misconception is that only ferrous metals create a problem. This is not true as both copper and aluminium cause signal loss. Since these materials have a lower resistance, the loss would be greater. In short it is the conductivity of the metal, not it’s magnetic properties, that is relevant here. Losses are usually up to 3dB/Octave with a lower corner frequency typically between 0.1Hz and 100Hz. As the losses increase with frequency, a metal loss corrector will be needed to flatten the response. The standard covering induction loops (IEC118-4) requires a flat frequency response between 100Hz and 5kHz.
When metal is found, the losses incurred are often so great that simply using a larger amplifier will not suffice. (Losses of more than 25dB @ 1kHz have been known!) In such cases a simple perimeter loop cannot achieve acceptable performance, so special loop design (eg phased array) and equipment are needed. Given the construction details, Ampetronic can estimate the required equipment and suggest a suitable loop design approach. For larger projects, a site visit may be advisable to measure the loss experienced in practice. Please contact Ampetronic for assistance in all these situations.
To illustrate why simply using more current is not an effective solution, consider the following example. It is not unusual for a reinforced concrete floor to suffer more than 15dB (@ 1kHz) of loss in the centre of a perimeter loop! Imagine such a loop required 8A peak without loss; you would need to use a loop current of 45 Amps to reach the target field strength! Even if it was this was feasible it would still not be possible to make the field adequately even across the loop as the loss would vary across the area.
The effects of metal structures on inductive loop performance can be seen using the HLS-2D Metal Loss Test Kit, to deliver a preset output level to give consistent test results. Combined with the Loopworks Measure R1 or Ampetronic FSM, provides an ideal aid to quick and simple site assessments.