Large Area Assistive Listening Systems: Review and Recommendations

2.1.2.1 Summary of IEC 118‒4 (1981) Standards

With a source of 1000 Hz equal to the long-time average level of the speech signal applied to the input of the system, the resulting field strength within the loop shall average 100 mA/meter +/- 3 dB. This level should not go below 70 mA/meter or above 140 mA/meter. These values apply to the vertical component of the field strength inside the area enclosed by the loop, measured 1.2 meters above the floor. Allowing for the 12 dB peaks occurring in speech signals, peak field strength may reach 400 mA/meter. The frequency response shall be 100 Hz - 5 kHz +/- 3 dB. The document includes a statement that in schools for hearing-impaired children it may be desirable to boost the low frequencies to compensate for the decrement in the low frequency response often produced by the inductive process. The degree of such a boost, and whether and under what circumstances it should take place, is therefore a local option.