BS EN 60118-13-2011 pdf download.Electroacoustics Hearing aids Part 3 : Electromagnetic compatibility ( EMC ).
In 1994. the European hearing instrument manufacturers association (EHIMA) undertook a series of measurements to establish a basis for measuring the effects of interference on hearing aids, and for quantifying a practical limit of immunity. In Australia similar work was undertaken at around the same time. This work concentrated on providing the basis for measuring and specifying what is now known as the bystander compatibility. At that time, the issue of user compatibility and the need to deal with it was limited by the lack of knowledge on the subject and the low use of digital wireless devices in most countries.
However, the rapid growth in the use of digital wireless devices produced an urgent need to address the issue of the hearing aid wearer who wanted to use a digital wireless device. Work on this problem commenced in the USA in 1997 and led to proposals for methods of measurement for both hearing aids and mobile phones. This work led to the ANSI C63.19 [4J standard, which provided the impetus for further work in Europe to evaluate the proposals.
A.2 Test methods
The EHIMA GSM project was set up to establish a test environment enabling the GSM interference problems to be addressed. The projects final report [2] presents the results of the development phase. It also includes results from other investigations. The relevant parts of the project are summarised below.
Five hearing aid types were selected for a laboratory investigation, representing different electroacoustic characteristics, interference levels and interference spectra. The overall input related interference level (OIRIL). expressed as SPL, in decibels, was chosen to characterise the interference performance of the hearing aids.
First, the aids were tested acoustically according to IEC 60118-0. To enable the metallic ear simulator to be removed from the RF-field, coupling between the hearing aid and ear simulator was modified by using 500 mm long tubing. Relatively large variations in the acoustical effect of this modification were seen. This means that the hearing aid gain should be measured for the individual hearing aid under test in the determination of OIRIL.
The hearing aids were then exposed to a simulated GSM RF-fleld in an RF anechoic room, placed in a position corresponding to normal use. A test signal having a peak field strength of 10 V/m was used. This corresponds to a digital wireless device having a power of 8 W at a distance of 2 m, or a 2 W device at a distance of I m. The frequency spectrum of the interference signal at the orientation causing maximum interference was determined. The input related spectrum was then calculated by subtracting the hearing aid gain, and finally the OIRIL was determined.
A.4 Test field strengths — Bystander compatibility
To be able to suggest realistic field strengths for testing hearing aids, i.e. field strengths which simulate situations where the hearing aid user is disturbed by a nearby person using a digital wireless device, a number of points should be taken into account.
Firstly, the proposed test procedure is based on a number of worst case considerations:
— The maximum interference level is found in each of four different orientations of the hearing aid relative to the disturbing field, and out of these four maxima the highest is used to characterise the interference level in the hearing aid.
— If compliance to the standard is documented, the maximum interference level within a wide frequency band is used to characterise the immunity of the hearing aid in the frequency band, even if the maximum interference level is only obtained at a single frequency.
NOTE As the immunity test is performed En a broad frequency band, the frequency with the worst case interference will seldom coincide with an actual carrier frequency.
— The field strength corresponding to maximum transmitting power is used, despite the fact that digital wireless devices only transmit with maximum power in certain situations (battery fully charged, large distance between the digital wireless device and the base station).
Secondly, another practical circumstance should be noted: Users of digital wireless devices will probably tend to obtain as much privacy as possible and thereby increase the distance to nearby persons as much as possible. The field strengths given for bystander compatibility in Table 1 of Clause 4 of the main body of the standard correspond to a theoretical protection distance of approximately 2 m for any digital wireless device.
A.5 Field strengths — User compatibility
As a follow-up on the EHIMA studies concluded in 1995, a project funded by the European Union ISIS programme was conducted in 1999. This project Hearing aids and mobile phones immunity and interference standards — HAMPIIS “ was conducted to establish specifications for a revision of this part of IEC 60118 regarding criteria for hearing aid wearers to use digital wireless devices themselves. A test method proposed in the ANSI C63.19 standard with nearfield illumination of hearing aids using a dipole antenna was verified during the project. Nearfield illumination of hearing aids using a dipole antenna (or a digital wireless device) was found to be valuable during design and development of new hearing aids, where it can bring valuable information. An example of a suitable test arrangement using a dipole antenna is therefore given in Figure A,2.BS EN 60118-13-2011 pdf download.
BS EN 60118-13-2011 pdf download
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