and increased signal strength. There are many different ways to configure conductive loops to give different patterns of magnetic fields and solve different technical problems such as the presence of metal structures. Other equipment within the magnetic field Audio induction loops create relatively high magnetic field levels. Other equipment must be designed and installed to work properly within this field. The most common cause of problems is earth loops, where different pieces of equipment are connected together by signal wires, but powered from different power sockets in different parts of the room or building. The combination of the mains earth and signal earth creates a receiving loop that produces an interference signal proportional to the area within the earth loop. Various steps are used to prevent interference on audio and video equipment. Powering signal sources and output devices from the same mains circuit to prevent formation of an earth loop; shielded cables or signal isolators may be used. Technical standards An objective of the field strength requirements of standards for AFILs is to make the perceived loudness of sound from the loop the same as from the microphone in the hearing aid. This is the basis of the average field strength of 100mA/m used to generate today's performance standards around the world. IEC 60118-4 (formerly Britain's BS 6083 part 4, also known as EN 60118-4) is now the main specification for international use. This is based on the principle that the long term average of the field strength at a typical listening location must be 100mA/m, +- 3 dB. To determine this long term average requires a measurement over 60 seconds or more. The standard therefore sets a more pragmatic requirement for determining the short term peaks of the signal. Short term peaks need to be 12 dB (x4) higher than the long term average, based on the fact that peaks of speech are approximately 12 dB higher than the long term average level of speech. Therefore, an induction loop system or AFILS must be capable of delivering field strength peaks of 400mA/m +- 3 dB (280 to 560mA/m). Peaks must be measured using fast RMS measurement (125ms averaging time). In addition the IEC 60118-4 standard sets limits on acceptable background noise, and requires that the system delivers frequency response of +- 3 dB from 100 Hz to 5 kHz relative to the field strength at 1 kHz. All measurements