PROLOOP FIELD STRENGTH METER PLUS The PROLOOP FSM Plus accurately measures the magnetic field strength of induction loop systems, according to IEC 60118-4:2006 and BS 6083, Para. 4. Designed for professional use, PROLOOP FSM Plus delivers reliable RMS values on the output level, response frequency, Automatic Gain Control (AGC) function, distortion and background noise in the tested induction loop. In addition, it enables the user to make an acoustic evaluation of the sound via earphones.