Page 27 - Fall 2005

DINNER CONVERSATION (AN OXYMORON?)
Marshall Long
Marshall Long/Acoustics Sherman Oaks, CA 91423
In a recent popular movie “My Dinner with André,” the two main characters converse onscreen for an hour and a half in a restaurant. It is with good reason that the conversation was filmed in an abandoned hotel without other patrons pres- ent, since in most modern restaurants such an exchange would be difficult at best. Architectural design in general and restau- rant design in particular tends to emphasize the visual. Form follows fashion rather than function in the world of architec- ture. The acoustical features of the built environment are usu- ally noticed only when associated with performance venues. Yet the acoustics of everyday spaces: restaurants, offices, the- aters, sports arenas, and our dwellings affect their use and enjoyment. This is particularly true for the elderly who gradu- ally lose their hearing acuity (due in part to exposure to noise) in the high-frequency ranges so necessary for the understand- ing of speech.
There is an increasing awareness of the importance of noise in restaurants. Michael Bauer, the food editor for The San Francisco Chronicle, publishes a guide to the 100 top restaurants in that area. Reviewers are outfitted with sound level meters to measure background levels and a noise level rating is included in their reviews (1 bell for < 65 dBA, 2 for 65–70, 3 for 70–75, 4 for 75–80, and a bomb for levels above 80 dBA). These rat- ings can be found online by going to the sfgate.com site and following the links to “Food and Dining.” Sadly, only one restaurant on the list received a one-bell rating.
Speech intelligibility
The ability to hear and understand the spoken word, known as speech intelligibility, is an area of great interest in acoustical engineering. It finds application in classroom, auditorium, and office design, as well as in sound system design. And to those of us who enjoy an occasional evening out at a quiet restaurant, where the ability to carry on a conversation with one's dining partner in the presence of the noise generated by other patrons is highly prized, it is of particular significance.
Speech intelligibility, as it turns out, is dependent on the interior design of the restaurant, and can be greatly influ- enced by the choice of surface materials used within the space. Intelligibility depends on the masking effects of extra- neous sounds on the speech we hear. Masking can be caused by noise from background sources or by reflections of the original spoken words. Speech intelligibility is measured in a number of ways, the simplest of which is a test given to sub- jects wherein they are asked to identify words or sentences in the presence of a masking noise. Charts such as those shown in Fig. 1 result, showing the percentage of syllables or sen- tences correctly identified. Our brains are excellent comput- ers and we are more successful in identifying sentences than we are with single-syllable sounds since we use partially understood words to intuit the rest. This is illustrated clearly in the figure. Notice that even when the noise is louder than
Fig. 1. Percent of words and sentences correctly identified in the presence of back- ground noise (Kinsler, et al., 2000) ©John Wiley & Sons, Inc. Reproduced with per- mission.
the signal (the signal-to-noise ratio is negative), we can still understand sentences pretty well.
There is some imprecision involved with the definition of what constitutes signal and what constitutes noise. Clearly, the noise must have a similar spectral content as the signal or else the ratio, which is the signal level minus the noise level in dB, would not be meaningful. For this simple analysis we will con- sider the direct field sound, which travels directly from the source to the receiver (Eq. 1), to be the signal. The farther the receiver is from the talker the lower this level becomes. The original sound energy is distributed over the surface of a sphere expanding away from the talker, a phenomenon called
Dinner Conversation 25