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``````Analysis of over 20 dining spaces that suffered from sufficient acoustic issues to drive the owners to call in an acoustician had a variety of materials in the rooms, with some establish- ments having almost no sound-absorbing surfaces (meaning hard floors, walls and ceilings), whereas others had some absorbing materials, as shown in Figure 2.
The rooms that had absorbing materials but still needed inter- ventions often had the absorbing material on the floor in the form of carpet. Some rooms had heavy carpet and drapes along the windows, yet still resulted in unsatisfactory acous- tic environments. Where the absorbing material is installed matters! Certainly, adding absorbing material anywhere will cut down on the amount of reflected energy in the room (to some extent), but putting it in areas where sounds are more likely to actually interact with the material will result in a more favorable acoustic environment. As more absorbing material is added to the space, the ᾱ gets higher.
It is also very important to think beyond the simple equa- tions that we use to calculate “bulk” performance variables such as the average absorption coefficient and consider the above discussion. Once while teaching a design workshop in Mexico City, an architect asked if it was possible to solve the noise issue in one of the most famous restaurants in the city in a simple way. The architect wanted to know if put- ting acoustical treatment under all the tables and chairs would solve a “big” noise problem. The entire workshop group talked through this proposed solution and came to the conclusion that the likely outcome would be a 1 dB level reduction because no more than one-half of the floor area could be covered, and even if this was done, the likelihood of getting sound up under the tables and chairs was not so good.
The untreated rooms that required acoustic treatment had an ᾱ of 0.12, with a range of 0.05 to 0.23, as seen in Figure 3, red, as the “untreated” option. Adding absorbing materials to 50% of the walls or ceiling resulted in an ᾱ of 0.20 (which is typically considered the “break point” between an acous- tically live room and a room that begins to absorb sound).
This step might be considered a “first pass” in attempting to control excessive reverberation.
However, if a more subdued environment is desired, adding sound-absorbing material to more than 50% of the wall or ceiling surfaces will result in the next tier of treatment, which has an ᾱ of 0.27. Treating 80% or more of the ceiling surface,
Figure 2. All of these restaurants suffered from acoustic defects and all having a varying amount of absorbing material. Top: Heavy carpet, drapery, and upholstered seating. Center: Mainly reflective materials. Bottom: Acoustic ceiling tile (in grid).
Spring 2020S, uSpmemciaerl I2s0s1u9e | Acoustics Today | 253 Reprinted from volume 15, issue 2

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