Page 54 - Special Issue

Acoustic Comfort in Restaurants
Figure 3. Average absorption coefficients associated with various amounts of absorbing materials in restaurants. C, ceiling; W, wall; color blocks, average absorption coefficient of the actual surface (ᾱ); vertical bars, ranges of measured α values. Averages for ᾱ are 0.12, 0.20, 0.27, 0.38, and 0.44, respectively.
in conjunction with up to 30% of the wall surfaces, will result in an ᾱ of 0.38. And treating 80% or more of the ceiling sur- face and over 30% of the walls will result in an ᾱ of 0.44. In most restaurants, it is difficult to treat more surfaces than this due to the number and locations of windows, lighting fixtures, mechanical ducts, etc. Accordingly, the ᾱ of a restaurant typi- cally tops out around 0.44.
The higher the average absorption coefficient, the more sound will be absorbed by the room surfaces. So, if a restaurant is to offer a quiet, subdued environment, where people can talk quietly, it is important to use larger amounts of sound- absorbing material on the available ceiling and wall areas. If a venue is to have a more “energetic” feel, less sound-absorbing material should be used but that material should be used in strategically placed areas that have the potential to get louder than others.
Using no absorbing material in a restaurant, however, often results in acoustic environments that are uncomfortable and become excessively loud, even in places that want a more
“energetic feel.” This is typically why noise is considered one of the chief complaints of patrons of restaurants. Many restau- rants have no acoustic material; they have hard floors, with painted gypsum board walls and ceilings. Some restaurants have acoustic ceiling tiles, but older establishments may have
254 | Acoustics Today | Suprminmge2r0201,9Special Issue Reprinted from volume 15, issue 2
“refreshed” the space with a new coat of paint, so the ceiling tiles are often old and painted. Unless the tiles were spray painted with nonbridging paint, the paint seals up the sur- face of the tiles and essentially reflects sound back into the room, making the tiles that would previously absorb sound actually reflect it.
Technical Analysis of Intelligibility in Restaurants
The speech transmission index (STI) is “an objective mea- sure used to predict the intelligibility of speech transmitted from talker to listener” (British Standards Institution 2011, BS EN 60268-16) or how well speech is heard and under- stood from one person to another. The STI was calculated in 13 untreated restaurants that suffered from poor acoustic environments (Siebein and Siebein, 2017). The STI values
for the untreated restaurants ranged from 0.49 to 0.75 when unoccupied. To give context, according to BS EN 60268-16, an STI of 0.50 is considered the target value for voice alarm systems (a life safety system designed to provide spoken emergency alerts in a building but may also include back- ground music or other nonemergency signals); a STI of 0.58
is considered a “high-quality public address (PA) system”; an STI of 0.70 is considered “high speech intelligibility”; and an STI of 0.76 or greater is considered “excellent intelligi- bility but is rarely achievable in most environments.” This may be why in these 13 restaurants, one could clearly hear conversations from other diners at other tables when the restaurant was minimally occupied.
The STI for patrons sitting at the same table should be maintained as high as possible in all situations to optimize the ease of communication among those diners. Assum- ing no other patrons are in the area and the background noise is quiet, the STI value here will be higher, meaning that speech will be more likely to be understood. It is desir- able to maintain high STI values for this situation under all conditions. The STI from locations across the room should be minimized under all conditions to limit the buildup of noise that would reduce the STI across the table. Generally, the more sound absorption present in the room, the higher the STI value.
However, the background noise level also has a significant relationship to the STI. In a dining or social space, average background noise levels of 77 dB(A) (Scott, 2018) are often found. The “background noise” is the voices of all the other diners speaking at their tables reflecting across the room