Large Area Assistive Listening Systems: Review and Recommendations

4. COMPARISONS BETWEEN ALS

4.1 Research

There have been four published research studies that have examined the relative performance of different ALS compared to when the signals are delivered through a PA system. In the Bankoski & Ross (l984) study, nine hearing-impaired adults ages 48 to 84 years were administered a speech intelligibility test through a PA system in a large auditorium under two listening conditions: one in their usual mode (monaural or binaural) and one with each of two different FM systems. Their scores in the non ALS condition was compared to the scores obtained by 146 normal hearing college students in the same auditorium.

The results showed that the scores for the hard of hearing group, with either of the FM systems, were significantly higher than those obtained in their usual listening condition (an average of about 45% without an ALS and 70% with an ALS). The scores of the two different FM systems were essentially similar. What was interesting about these results was that with an ALS, the older hard of hearing subjects scored as well or better on speech perception measures than did the young normal hearing college students. The students achieved average scores of about 65% in the auditorium, which varied considerably depending upon their specific location in the auditorium, compared to an average of 90% with headphones in the clinic. What these results indicate is that even the normal hearing group could have used auditory assistance in this particular auditorium.

Three other studies compared the speech perception scores of different ALS to those obtained via a PA system. Nabelek, Donahue, and Letowski (1986) compared the performance of IL, FM, and IR systems to a PA system in a medium size classroom. There were four groups of subjects: 10 young adults wi th normal hearing, 10 hearingimpaired with mild-to-moderate hearing loss not using hearing aids, 9 elderly people with hearing loss "typical" of their age, and a group of 10 hearing aid users with moderate hearing loss. Speech perception scores were obtained under two S/N conditions (+8 dB and + 20 dB) with all listening systems.

The results showed that for both S/N conditions, the speech perception scores obtained with the IR, IL, and FM systems were superior to those achieved with the PA system (Nabelek, Donahue & Letowski l986). The differences between the scores with the PA system and the three ALS widened as the listening conditions became poorer. Minor differences were found between the FM, IR and IL systems, but these were not considered to be clinically significant.

In a companion study, Nabalek and Donahue (l986) compared speech perception scores obtained with a PA system to those obtained with an FM and an IR system in a large auditorium. They reasoned that the results obtained in a classroom may not be applicable to a larger listening situation. Five groups of subjects were tested. In addition to the four types tested in the earlier study, they added a group of normal hearing nonnative listeners. Two listening positions were selected, one representing a favorable seating location (10th row center) and a poor one (under a balcony overhang).

The results corroborate those obtained in the earlier study. Speech perception scores achieved with either the FM or the IR system were significantly higher than those attained with the PA system, while differences between the FM and IR system were minor and not clinically significant. The biggest differences were found with the hearing impaired and non-native listener groups. The study did show that, as did Bankoski and Ross (l984), that the ability to comprehend speech by normal hearing people, particularly non-native normal hearing people, can be affected in large auditoriums but can be improved with an ALS. Even a "favorable" seating location is no guarantee of a superior speech perception. In this study, people in the "poorer" location did better while listening to the PA system than those in the "better" location. This fact helps explain the huge demand for IR receivers by normal hearing people in some legitimate theaters, as reported in the manufacturer's focus group. In other words, the quality of the acoustical conditions in large-area listening environments should be a universal concern, and not just something that affects only people with hearing loss.

In the most recent study, speech perception scores obtained with four types of ALSs were compared to scores achieved when no system was being used (Noe, Davidson, & Mishler l997). The four systems were IR, IL, FM and an FM sound-field system (This latter is basically a PA system employed with an FM rather than a hard-wire microphone being used.) Two groups of subjects were tested: a group of 10 listeners with normal hearing and 18 listeners with sensorineural hearing loss, half of whose hearing losses included a central aging component. Testing took place in a medium-size classroom, with the understanding that this would be less of a listening challenge for the subjects than would occur in a larger venue. The hard of hearing listeners were tested using the ALS with earphones and coupled inductively (via a neckloop) to their ITE hearing aids. As in the previous studies, both the normal hearing and hearing-impaired groups benefitted from the use of an ALS (FM, IR, or IL). The only exception was the lack of improvement shown by the normal hearing subjects to the FM sound-field system (but these scores were already very high). Generally, the hearing-impaired subjects preferred the FM to the other ALS systems, though differences between them appear minor compared to when no ALS was being used.

One other purpose of the study was to compare ALS performance with and without inductive coupling for persons with hearing loss. The results showed poorer scores when the ALS was inductively coupled to personal hearing aids than when headphones were used with the ALS receivers. The investigators attribute this to the higher quality signal the ALS receiver delivered when coupled to a headset, as opposed to a neckloop/telecoil combination. They reiterate the point made many times by other investigators (and reviewed earlier in this paper) that telecoil performance with a neckloop is a significant factor that must be considered when inductively coupling an ALS to a hearing aid. The authors also point out that the use of an amplified telecoil provides a significant improvement in hearing aid performance compared to conventional telecoils. They point out the necessity to evaluate telecoil performance whenever an ALS is being coupled inductively to a personal hearing aid and the need to compare which option (unaided with headset or inductively with hearing aid) would provide a hard of hearing person with superior listening performance.

4.2 Functional Comparisons

The advantages, problems and limitations of IL systems were reviewed above, in the description and issues concerning such systems. While these systems have a continued contribution to make in providing auditory access for people with hearing loss, they rarely appear in direct competition with IR and FM systems in terms of new installations. Unless more hard of hearing people can be convinced (or their hearing aid dispensers) to include telecoils in their hearing aids, this situation is unlikely to change. This section, therefore, will concentrate on comparing the advantages and disadvantages of FM and IR systems.

It is easier to ensure that a transmitted signal is adequate and equal throughout any size venue, including outdoors, with an FM rather than an IR system. An FM transmitter can be hooked into the sound system and, in most instances, that appears to be the extent of the necessary "installation". FM systems, however, are susceptible to outside radio interference or may cause such interference to other nearby users of radio equipment. There appears to be an adequate number of channels available to ensure coverage in multiplex cinemas or multiple adjacent auditoriums. Since the FM transmission is not contained within the facility, an FM ALS is not appropriate if privacy is a concern.

An IR signal, on the other hand, stays contained within a facility, thus ensuring privacy. Any number of systems can be used in adjacent auditoriums without interference or spillover. Most venues with IR systems currently use the same sub-carrier (95 kHz), thus providing compatibility between different locations. However, this is an informal practice, and other sub-carrier frequencies can and are now being used in some places. The use of other and different sub-carrier frequencies would require either that receivers be designed only for particular transmitters, or that multiple-frequency IR receivers be deployed.

IR signals are affected by sunlight and, less severely, by fluorescent lights. It is possible to install more emitters in a facility, and overcome all but the most unfavorable natural or artificial light conditions (like direct sunlight). Ensuring adequate light coverage in a facility requires a skilled installer, a fair amount of "trial and error", and a willingness to verify the field strength at all likely seat positions in a facility. IR systems are considered "line of sight", in that the "eye" of the receiver must be in line with an emitter. However, because of the undefined light reflection patterns in different rooms, this may not be strictly applicable; in some rooms and at some use positions, strict "line of sight" reception is not necessary. Reception, therefore, may be uncertain and has to be empirically determined in each venue. Covering the light receiving diode with clothing (as in a pocket) or turning one's back to an emitter will most likely interfere with reception.

IR receivers tend to be simpler than FM receivers, though this may be less true as additional sub-carrier frequencies become used. In this case, multiple frequency IR receivers may become common. In some places and for some populations, this can be a problem; simpler is better. FM receivers vary from a fixed, single frequency reception to units with the capacity to detect many ALS channels. Both FM and IR receivers can be used with the same types of coupling possibilities, including personal hearing aids.