BS EN 62489-2-2014 pdf download.Electroacoustics Audio-frequency induction loop systems for assisted hearing
Part 2: Methods of calculating and measuring the low-frequency magnetic field emissions from the loop for assessing conformity with guidlines on limits for human exposure.
4 Situation regarding current standards
Current published and draft IEC standards on EMF exposure can be ambiguous in their guidance on the approach that should be taken by product committees. The differences between the signals that are of concern and those considered in depth in EMF exposure standards are the following:
• wide relative bandwidth, i.e. the ratio of highest to lowest frequency present, 5 kHz and 100 Hz;
• no predominant frequency within the band;
BS EN 62489-2:2014
IEC 62489-2:2014 © IEC 2014 —7—
• rapidly varying amplitude;
• high ratio of peak amplitude to average r.m.s. amplitude (at least 4).
5 Configurations of loops
5.1 Main types of configuration
There are four main types of configuration:
a) large area loops, with the smallest dimension larger than 1 m, usually installed at floor level in a room:
b) medium-area loops, with dimensions of the order of 1 m, often oriented in a vertical plane, installed at service desks and similar positions;
C) small area loops, with the largest dimension less than 1 m;
d) solenoid antennas, including the ear-hook.
NOTE Examples of small-area loops are portable systems. clipboards, neck l00p5, cushion 100p5 (Including those for use in vehicles) end chair loops
5.2 General considerations
5.4 Medium-area loops
For these, there are three considerations.
a) The hearing-aid user is normally at a distance from the loop comparable to its dimensions. Thus the loop current required to produce a maximum r.m.s. field strength of 400 mA/m (in compliance with IEC 60118-4) at the hearing-aid is much larger than the current required to produce it at the centre of the plane of the loop.
b) Nevertheless, the separation ensures that the hearing-aid user is not exposed to the high fields strengths near the loop conductor.
c) However, staff can come into close proximity of the loop conductor while the system is working unless steps are taken to maintain a minimum separation.
NOTE These loops often have more than one turn, so that the loop current can be kept reasonably small.
5.5 Small-area loops
For these, again, there are three considerations.
a) The separation for portable loops is very much greater than the loop dimensions, but for other types, the separation distance can be small or very small unless steps are taken to maintain a minimum separation.
b) The current apparently required is quite large, because of the large separation.
c) Both users and staff can come into close proximity of the loop, even that of a portable system.
NOTE These loops usually have many turns, so that the actual current Is not so large.
5.6 Solenoid antennas
One example that is commercially available is the ear-hook. This device is typically as shown in Figure 1. A very small solenoid is incorporated in the stem of the device.
6.2 Solenoid antennas
There is no simple expression for the field strength at a point outside a cylindrical solenoid. A solenoid may be treated as a stack of loops, or as a magnetic dipole, or the field strength can be calculated by means of a rather complex equation (see [2]).
7 Measurements
7.1 General
In the audio-frequency range, exposure time is irrelevant, because tite predominant physiological effect, if it occurs, is nerve stimulation, which operates over a time-scale of a few milliseconds. It is therefore appropriate to use a quasi-peak measurement of field strength. Furthermore, exposure limits and guidelines are given in r.m.s. values, so the quasi- peak meter should be scaled to read r.m.s. values with a sinusoidal signal. This type of meter. the peak programme meter (PPM), is further described in IEC 60118-4 and IEC 60268-10 (type II) already.
It is also necessary to consider the type of magnetic field pick-up coil or sensor. Sensors may be single-axis, with just one coil, or three-axis, with three orthogonal coils. For use with a PPM, the single-axis sensor is most convenient, and if it is properly constructed, it Is not difficult to orient it for maximum reading, especially as the likely direction of the field can usually be predicted from text-book field patterns.
The first measurement that shall be made is that the field strength is correct at the point or points where it is intended to be 400 mA/rn (or the agreed lower value If adjusted to reduce loudness, as specified in IEC 60118-4).
NOTE IEC 601 18-4 specifies the use of either a PPM or an r.m.s. meter with a 125 ms integration time for the measurement of magnetic field strength However, for the purpose of this standard, the 125 ms Integration time Is Incompatible with the requirement to measure field strengths over limes of the order of a few milliseconds
The Instrument specified for measurements on other equipment and systems, such as In IEC 62233 131, has an averaging lime specified only as an upper limit of 1 s, which is also too slow for the assessment of flelds due to audio4requency signals
7.2 Input signal
The Input signal for the amplifier shall be the simulated programme signal described in IEC 60268-1, with additional filtering, —3 dB at 100 Hz and 5 kHz relative to the 1 kHz level, with ultimate attenuation slopes of at least 12 dB/octave.
7,3 Measuring Instrument.BS EN 62489-2-2014 pdf download.