Page 29 - Fall 2005

Conclusion
To an acoustical engineer the solution to this problem is straightforward. More absorption means a quieter restaurant. People like quiet restaurants. So use more absorption.
To the architect or restaurant designer there is a different equation. The item that gets the architect more work, praise and accolades is a great-looking picture of the restaurant in an architectural magazine. Hard-edged structures with strong design elements look “cool” even if the seats are uncomfortable and the interiors are noisy. Acousticians must help fulfill the architect’s visual goals to get acceptance of a solution to the noise problem. We must give architects and designers good-looking absorptive materials with hard edges that can be painted any color and made into any shape desired. We are slowly improving in these areas with perfo- rated metals, absorbent plasters, more interesting ceiling tiles, and fabric-wrapped panels, but there is more we can do. We hope that architects and interior design professionals will discover that good design is not simply visual. Ultimately it is easier to design new materials that solve the architects’ prob- lems than it is to design new architects.AT
References
Long, Marshall (2005). Portions of this article are included in the author’s book, Architectural Acoustics, to be published in 2005 by Elsevier, Inc. and are reproduced with permission.
Kinsler, Lawrence E., Frey, Austin R., Coppens, Alan B., and Sanders, James V. (2000) Fundamentals of Acoustics (John Wiley & Sons, Inc. New York).
berant noise level drops to 66 dBA (2 bells), which is just low enough to carry on a cross-table conversation. At an adjacent table 3 m (10 ft) away, the direct field level from our conver- sation is about 54 dB and so it is not understandable. Off-axis directivity losses also may provide some additional isolation.
What we see from these relatively simple calcula- tions is that unless we add absorptive treatment with an area at least equal to the restaurant ceiling area, when the room is full of patrons, conversation across a table will be difficult and the background noise level will be uncomfortable. Second, even when we add this amount of absorption, the environment is not so dead that conversations are easily over- heard at a neighboring table. More formally, these two condi- tions can be stated as follows.
where N is the number of simultaneous talkers (or tables) in the room and At is the absorptive area per table. The signal- to-noise ratio is the difference between these two equations
To ensure adequate communication for a cross-table dis- tance equal to rs we apply the condition that Lsn > - 6 dB. This leads to the requirement that the amount of absorption per table in terms of the cross-table separation distance must be
To ensure privacy between tables, we apply the condition that the signal-to-noise ratio Lsn < - 9 dB. This leads to the requirement that the amount of absorption per table, in terms of the separation distance rt between tables, be limited to
For a talker-to-listener distance of 1 m, our analysis sug- gests at least 6.3 sq m (68 sq ft) or more of absorption per table. If we treat the ceiling with a highly absorptive materi- al, the minimum spacing between tables becomes about 2.5 m (8 ft), based on filling the room evenly. At that distance the maximum allowable absorption from Eq. 8 should be no more than 20 sq m (215 sq ft) per table. Normally we design based on Eq. 7 since the requirement in Eq. 8 is easily met. If the cross-table distance is greater than 1 m then the amount of absorption must be increased accordingly.
Marshall Long received a
B.S.E. from Princeton University
in 1965, attended the Univer-
sity of Grenoble in France and
the University of Madrid in
Spain in 1966, and received
M.S. and Ph.D.degrees in engi-
neering from UCLA in 1971.
While still a graduate student,
he founded his own consulting
firm now in its thirty-fourth
year. Marshall Long/ Acoustics
specializes in architectural acoustics, audiovisual design, noise and vibration control, and other technical areas related to acoustics. He enjoys sailing, judo, soccer, reading, and writing, and is living happily ever after with his family in Sherman Oaks, CA.
Dinner Conversation 27