Hearing aids and room acoustics By Arthur Boothroyd 1 Why should I be concerned about room acoustics? Isn’t acoustics like the weather— everybody talks about it but nobody does any- thing about it? You should be concerned because most listening takes place in rooms with bad acoustics. This is bad for people with normal hear- ing and even worse for hearing aid users. What’s more, unlike the weather, room acoustics is not entirely beyond your control. Hearing professionals can address bad room acoustics in the selection and adjustment of hearing aids and accessories. They can also counsel and instruct patients in the control of listening environments, in the effec- tive use of their aids and accessories, and in techniques for dealing with the situations in which these steps are not enough. In controlled listening situations such as classrooms and lecture halls, hearing professionals can even play a major role in promoting and cre- ating good acoustics. To understand the effects of room acoustics on hearing aid performance we need to review four topics: speech acoustics, noise, reverberation, and distance. 2 Uh-oh! Did you say speech acoustics? I thought I made the vocational choice between hearing and speech years ago. I have never understood the aversion of hearing professionals to speech science. Of all the signals that a hearing aid is designed to handle, speech is by far the most important. Surely, a thorough understanding of that signal is an essential tool for hearing profes- sionals. 3 Okay! Okay! So, what should I know about speech acoustics? And don’t call me Shirley! We often specify speech by a single decibel value, but this is not enough. Speech is a rapidly varying signal containing many acoustic cues that are distributed unevenly across frequency, amplitude, and time. The average dB level is the result of summing across fre- quency and averaging over time, often in ways that are not made clear to the reader. For a full understanding of the effects of hearing loss, amplification, and room acoustics on speech perception, we need a more complete description, beginning with the way average amplitude changes with frequency. This is known as the long-term-average-spec- trum or LTASS. 4 Isn’t this already incorporated into prescriptive algorithms? It often is. But this LTASS is averaged over many talkers. It is an average of an average. The LTASS received at the hearing aid in everyday life varies with the characteristics, effort, and distance of the specific talker, and with the reverberant properties of a specific room. Persons with a limited dynamic range of hearing may have difficulty dealing with this vari- ability, even if an LTASS-based prescription has placed them in the right ballpark. 10 The Hearing Journal Page Ten October 2003 • Vol. 56 • No. 10 A common finding in hearing aid research is that it is difficult to predict real-world hearing aid benefit and satisfaction on an individual basis using speech intelligibility test results obtained in the clinic. Among the many factors that impact on this outcome, the acoustics of the patient’s everyday lis- tening environments is one that contributes significantly. Room acoustics concerns the way sound is propagated and perceived within an enclosed space. Room acoustics can be influenced by resonant modes, diffraction, and diffusion. Reverberation time is the para- meter commonly used to describe room acoustics, although background noise and distance from talker also are important fac- tors in determining if communication is suc- cessful. The best possible directional- microphone/digital noise-reduction hear- ing aids may provide little or no additional benefit in some environments. It’s important, therefore, to understand the relationship among common speech signals, different listening environments, and hearing aids. That’s just what our Page Ten article for this month is all about. It’s not everyday that we can find a Page Ten author who is a “Distinguished Profes- sor Emeritus” (City University of New York), “Scholar in Residence” (San Diego State University) and “Visiting Scientist” (House Ear Institute). If you’re guessing that those credentials suggest that he also writes a good article, you’re right. Arthur Boothroyd, PhD, has pub- lished extensively on the effects of hearing loss on development, with special emphasis on speech perception, its assessment, and its enhancement with hearing aids, cochlear implants, and tactile aids. His current research deals with remote and beam-forming micro- phones as accessories to hearing aids. Born in England, Arthur spent his early professional years working in electronics research and as a high school physics teacher. He become interested in audiol- ogy as a result of having a deaf son. He has spent most of his career at Clarke School for the Deaf in Northampton, MA, and at the City University of New York. You’ll enjoy reading this Page Ten arti- cle, as Dr. Boothroyd clearly outlines the nature of the problem, and provides a few solutions. As Arthur explains, knowledge of the interaction between room acoustics and speech understanding with hearing aids will assist you in your “counseling, instruc- tion, demonstration, and coaching” of patients. Hopefully, improved patient ben- efit and satisfaction will result. Gus Mueller Page Ten Editor Page 10 Boothroyd