Abstract
This paper provides theoretical estimates of the signal-to-noise (S/N) ratio advantage that can be gained from a radio microphone (FM) system used with an environmental (EV) microphone, compared with the S/N ratio delivered by a conventional hearing aid alone. These estimates show that the S/N advantage gained is a function of the speaker/listener distance and the FM/EV gain difference, the greater the distance and the closer the FM gain is to the EV gain, the greater the S/N advantage. The implications for choice of FM and EV gain when fitting FM systems are discussed. The two goals of equating the FM and EV outputs and maximizing the S/N ratio are mutually incompatible. It is concluded that the FM gain should be set to be 10dB less than the EV gain unless the situation can be closely monitored, in which case the FM gain should be set as high as possible consistent with loudness tolerance and acceptable levels of distortion.
Decisions about whether to use the (FM & EV) mode and about the optimum EV/FM gain difference are complicated because they will depend on the circumstances, the listening environment, and the maturity and hearing loss of the hearing-aid wearer. This highlights the need to give older students information about the situations in which the different modes of operation will be most beneficial together with advice on the type of situation in which they may find it helpful to increase or decrease the volume control of the FM receiver and the EV channel. Audiologists using speech perception tests to demonstrate the benefit of an FM system should be aware of the effect that the speaker/listener distance may have on the system's apparent performance.
Seewald and Moodie (1992) discuss the many electroacoustic factors that should be considered when selecting, fitting and monitoring FM systems and the EV/FM gain difference is just one of these factors. It is however an important one when the system is going to be used with both FM and environmental microphones active because the potential benefit of the FM system to improve the S/N ratio is reduced when it is used in combined (FM&EV) mode. The greater the reduction in the FM gain to compensate for the increase in input at the FM microphone the less effective the system will be in improving the S/N ratio. This poses a dilemma when it comes to choosing the FM and EV gains in the (FM&EV) mode. The only protocols that have been specifically designed with this problem in mind are those of Rowson (1994) but further research is needed in to their practicality and effectiveness.
When FM is active, strategies that reduce the EV gain compared with that of the conventional aid alone either are still at the research stage or have clear disadvantages.