One study compared fixed and Dynamic FM performance for hearing aid users, while the other made the same comparison with cochlear implant recipients.
In the hearing aid study, conducted by Dr. Linda Thibodeau, both adults and children (ages 11 tol5) were used as subjects, and both objective and subjective measures were taken. The objective comparisons were conducted in a large classroom, with the interfering noises - ranging from 54 dB to 80 dB - emanating from four speakers located in the corners of the room.
The FM microphone was placed six inches from another loudspeaker located in the center of the room. The goal was to create a situation where someone would talk into an FM microphone while the listener was surrounded by changing levels of ambient noises. For the subjective comparisons, the subjects tried both the fixed and the adaptive systems at home and during activities in a public aquarium.
As expected, the objective results show no difference between the two systems at low noise levels. This is because the adaptive feature is not triggered until the background noise level exceeds 57 dB. But as the noise level increased, the speech perception scores increased with the adaptive system only, reaching a maximum difference of about 50 percent at the higher noise levels.
Across all noise levels, the average difference between the fixed and the adaptive systems was about 25 percent. Subjectively, too, most or all (depending upon the activity) of the subjects preferred the adaptive (dynamic) over the fixed system under all the listening conditions; for example, during classroom activities, in a public aquarium, at home, etc.
Cochlear Implants
In the cochlear implant study, the subjects were divided between users of Advanced Bionics and Cochlear Americas devices. The authors (Jace Wolfe and five colleagues) remind us that cochlear implants include speech processing features not found on hearing aids; the implication is that results obtained with hearing aids are not necessarily applicable for cochlear implant users.
The investigators note the possibility that these speech processing features may interact with the operation of the dynamic FM in noise so as to eliminate potential benefits. Since each of these implants incorporate different speech processing strategies, it was necessary to compare performance with the implants as well as to investigate the general effects of the dynamic versus the fixed FM.
As with the hearing aid study, the subjects were tested in a large classroom under conditions that replicated a typical difficult listening situation for a person with hearing loss (for example, being surrounded by noise while the FM microphone was located about six inches from the source). The noise levels ranged from moderate (but below the trigger point for the dynamic FM) to quite loud (where the adaptive feature kicked in).
The results of the first experiment indicated that the dynamic feature significantly improved speech perception over a fixed gain system, particularly at the higher noise levels, but only for those wearing Advanced Bionics implants. For users of Cochlear Americas implants, the dynamic feature provided almost no improvement in speech perception scores at the higher noise levels.
The researchers then conducted another experiment, in which they used a different pre-processing strategy that the Cochlear Americas implant offers. In this second experiment, the implants were tested while the wearers activated the ASC (Auto Sensitivity Control) speech processing strategy.
In this second experiment, the speech perception scores were far superior to those obtained with the same implant in the previous study, and quite comparable to those that had previously been achieved by the Advanced Bionics users. The authors, therefore, recommend that in using an FM, recipients of a Cochlear Americas device enable the ASC strategy, possibly in combination with the ADRO (adaptive dynamic range optimization) strategy.
Insofar as the main purpose of the study is concerned, i.e., comparing fixed and dynamic FM systems, the findings clearly indicate that in the presence of high levels of sound, the dynamic FM provided superior speech perception with both implants.