Dr. Ross on Hearing Loss
Classroom Sound-Field Systems
by Mark Ross, Ph.D.
Much has been written about classroom sound-field amplification systems since they were first introduced about twenty years ago. As their potential contribution to children in classrooms becomes increasingly clear, there now appears to be a developing professional consensus regarding their efficacy. Indeed, those professionals who use and/or evaluate classroom sound-field system view them as one of the most significant technical developments that have taken place in schools in recent years. In actuality, however, they do not represent a technical breakthrough as much as a special application of a commonly used device.
Classroom sound-field systems are basically Public Address (PA) systems with the inclusion of a wireless microphone. As the teacher talks into the microphone his/her voice is transmitted to a specialized receiver/amplifier that is connected to, or physically a component of, a loudspeaker assembly. The loudspeakers may be located in the ceiling, on the walls around the room, or at the room corners. Whatever arrangement is used, the purpose of the system is to ensure that the teacher's voice is clearly audible above the background sounds at all instructional locations within the room. Research has shown that in the average classroom, the teacher's voice usually arrives at the children at a level only 6 dB or so above the background sounds. The extra 8 or 10dB of amplification provided by a classroom sound field system is sufficient to ensure a more suitable speech to noise ratio.
The rationale underlying these systems is very simple: how well children hear a teacher affects how well they learn. The more they can hear, and the less they have to strain and guess, the better chance they will have of learning their lessons. The hearing "boost" offered by these systems seems to be most advantageous for children with mild hearing losses, but has also been shown to help children with special auditory needs (e.g. children with central auditory processing problems, attention deficit and learning disorders, and second language learners). Basically, what they do is take some of the auditory strain out of the auditory learning process.
The initial generation of sound-field systems employed an FM radio signal for the wireless microphones. This is still widely utilized and, for some types of situations, this is the system of choice. In the past few years, Infra-Red (IR) classroom amplification systems have been introduced. Their advantage lies in the fact that they preclude interference between classrooms or from outside radio sources. This is because, unlike FM systems, the IR signal is contained within the classroom. Some systems come with an additional "pass around" microphone designed to be used by the children as they recite, ask or answer questions, etc. Teachers report that one of the bonuses of this arrangement is a much more orderly class. The children have to wait to talk until they have the microphone (something that teacher's love!).
Classroom sound-field systems are not intended for children with moderate hearing losses or greater, for whom a personal FM system would ordinarily be more appropriate. These children would still benefit more from a personal FM system than from a classroom system. The use of a classroom system also does not eliminate the need for appropriate acoustical treatment in classrooms. These systems do not work too well in noisy and reverberant environments.
Since the first research project on these systems some 20 years ago, there have been more than 50 additional studies testifying to their efficacy. The "special" children in the amplified classrooms attain higher speech perception and spelling scores, attend to the teacher for longer periods of time, and score higher on academic achievement tests than children in non-amplified classrooms. Evidence is continuing to accumulate regarding their positive impact upon literacy, academic accomplishments, and classroom behavior. There is less "acting out" or "tuning out" problems. Classroom systems have been shown to be a cost-effective way of minimizing special education referrals and services. In one large school district, such referrals dropped to about half compared to what they were before the systems were introduced in most elementary classrooms.
While classroom sound-field systems were initially designed for the "special needs" children, experience has demonstrated that the normally hearing children in the classroom are also benefiting. Presumably, while these children have no difficulty understanding the teacher in an unamplified classroom, they are nevertheless able to do this with much less effort and more certainty in an amplified classroom. Clearly, even normally hearing children have to hear well in order to learn well!
As an additional bonus, the teachers' responses to the systems are almost uniformly positive. They appreciate being able to teach all day without straining their voices. This is not a trivial advantage. In one large scale study, it was found that 20% of the teachers suffered from some sort of active voice pathology, with 70% reporting voice problems in the past that caused them to miss work and/or that impaired their teaching effectiveness. In two laryngological practices, teachers were the most frequent occupation identified, representing proportions of 20% and 16% of the total caseload. So while classroom systems were designed to help "special needs" children, and there is much evidence to support this goal, it is apparent that the benefits to teachers should not be overlooked.
The potential market for classroom sound-field systems is enormous and more and more companies are introducing new systems, or modifying their older ones. In the most recent Educational Audiology Conference, leading manufacturers report that they are not only exploring current issues related to installation and application, but are also looking at such future interesting developments as "bending wave panels and reflect technologies" that may significantly reduce speaker wiring and simplify installation. One company is now marketing a single speaker system that they claim can present an equal sound field through an average classroom. Refinements of existing systems are a frequent occurrence.
One recent development in classroom amplification systems has targeted the human element as well as the variability of background noises in the classrooms. That is, teachers do not talk at exactly the same loudness level all during the day, and there may be periods when, on a temporary basis, additional unwanted noise enters the classroom. In the usual system, when this happens, there is no change in the degree of amplification provided by the system. The signal from the teacher simply drops (if he/she talks softer) or gets buried somewhat in the noise (if background sounds significantly increase). The application of an "ambient noise compensation" (ANC) system in classrooms, previously used only in the professional sound-reinforcement industry, is designed to rectify this situation.
The ANC system, developed by Oval Window Inc., employs digital signal processing to make appropriate decisions about the signal-to-noise status of the sound-field system. It can be used with any type of classroom sound-field system, FM or Infra-Red. A "sense-microphone" is located some six feet away from one of the sound-field system's loudspeakers. The function of this "sense microphone" is to monitor and compare the loudspeaker's acoustic output with the electrical input signal from the teacher's microphone. Using this information, the ANC automatically adjusts the gain of the sound-field system up to 12 dB to compensate for loudness changes in the teacher's speech or the ambient noise levels. Basically, what it is doing is ensure a constant signal-to-noise ratio at the children's desks. The potential contribution of the ANC system was demonstrated in several listening studies that compared it to a sound-field system lacking an ANC. Overall, somewhat better performance on speech recognition tests were obtained with the ANC system than with conventional sound-field systems.