CLARIFICATIONS on FM systems and magnetic loops in children

Faced with the demand for information from parents, specialists and Administration in an increasing way; Of the drawbacks of using magnetic loops or FM systems combined with loops or inductive loops in the school environment

And today’s provision of advanced technology, where the concept we have been applying since 1987 (FM systems) is changing considerably, we consider it necessary to clarify certain technical issues that are currently discouraged the use of systems Magnetic loop in the child population.

Since our service we have been conducting research and advice since 2002 on the use of Modulated Frequency Systems technology in children. We have collaborated with entities such as the Catalan Department of Education and the University of Alicante to carry out different studies. The results of this work can be consulted at

Regarding the technical characteristics, personal FM systems can now be electrically coupled (by connecting the receiver through the direct audio input of the hearing aid or cochlear implant processor) or through an electromagnetic signal (using an inductive loop).

In simple systems the transmitter transmits in FM and the receiver coupled to the hearing aid or processor of the IC sends the signal directly to the hearing aid or processor of the IC, in which case the signal must undergo minor modifications, less than 2dB to be considered a transparent system. In the combined systems the transmitter transmits FM to the receiver and this, through electromagnetic signals sends through the loop or inductive loop the signal to the telephone coil of the hearing aid or IC processor. It is demonstrated that the conversion of signals causes the system to lose its transparency, increasing the gain in central frequencies, even surpassing in some cases the optimal parameters, and decreasing it in high and low frequencies, which does not make it suitable for severe / deep losses. In addition, as can be seen in practice, by placing hearing aids or processors in the mode where the induction coil is activated we transform it into a receiver of a large variety of electromagnetic waves, which inevitably adds noise to the system and is a source of complaints by Part of adult users. Considering that young children are sometimes unable to notify when they are receiving interference, the use of this type of system is discouraged.

On the other hand, the individuality of the FM systems according to the characteristics of the impedance of the hearing aids and processors of the ICs is essential to ensure that the FM signal is higher than that which is input by the microphones of the primary amplification system . This gain can not be adjusted in the induction loops. In any case, the increase in FM volume is general and does not allow individualization for each ear, which is a disadvantage since not all children need the same gain for both ears.

In each of the three brands of IC of the market and in each one of its models it is essential to make such modifications that in no case and in any manufacturer is determined individually. This led, in its day, to the realization of a standard of adjustment of systems of frequency modulated where it is tried to give knowledge of the degree of advantage that each receiver must use depending on the model of Cochlear Implant. The adjustment must be individualized, and the benefit must be verified in a particular way in each child. In the case of non-performance of these checks in the adaptation of the FM systems, the good result of any of the frequency modulated systems is compromised, not being subject to the equipment or technology, but to the necessary professional adjustment. The FM should not be thought of as a simple amplifier or as a radio that is turned on and used by the user without any adjustment. For many years this concept of the system was taken, which led to the storage of equipment purchased by the Administration, since they did not provide any benefits.

On the other hand, in the last years the technology has advanced in such a way that the capacity of speech perception has been improved in noisy environments in situations where FM systems of fixed gain did not confer benefit if it was not isolating the child from the environment, Ie mute the microphone of your hearing aid or your processor is proven that when the ambient noise exceeds 60 dB SPL – something very frequent in the school classroom – an FM system adjusted to a fixed gain significantly decreases its performance.
Dynamic gain systems automatically increase their volume in the face of increased background noise by providing greater intelligibility than fixed gain. This technology has been available since 2007 only through Dynamic FM technology. It should be noted that both the transmitters and the receivers of this technology are compatible with transmitters and receivers of technology of fixed gain.

In schools where there are several children with FM systems the limitation on the amount of frequencies available for legal use can be an inconvenience. For these cases it is recommended to use ROGER systems that allow the transmission of reliable broadband audio, not by FM system, but by ISM band of free use. This system ensures the elimination of interference as it has intelligent adaptive protocols that constantly search for free bands. The delay time is minimal – which is not the case with standard 2.4 GHz technology – and the system is adaptive gain – increases the gain in the face of increasing background noise and does not transmit any signal when the person carrying the Microphone is not talking so the noises in the system are eliminated.


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