Soundproofing a Restaurant
Soundproofing a restaurant is always a unique challenge as all restaurants are different. Even “chain” operations that look exactly the same, frequently have different acoustical characteristics. Whenever we are called by a restaurant owner to help them with noise levels we follow the same process, described below. Completely soundproofing a restaurant is almost never desired; after all, who wants to enjoy a meal in a place that is quiet as a tomb, but most diners want to be able to carry on a normal conversation with their companion(s), thus restaurant noise is one of the most frequently mentioned components of a restaurant review.
Room acoustics describes the manner in which sound propagates and interacts within a room. On one hand, acoustically “dead” rooms have an abundance of sound absorption within the room, such as heavy carpets, drapes and furniture, which tends to muffle sound. On the other hand, acoustically “live” rooms may hard surfaces that comprise the floors, walls and ceilings that reflect sound. If a room has too much reverberation, the reflected sound will continue to interfere with the incident sound, causing speech to be unintelligible and music to sound “muddy”. Conversation and music will therefore need to be louder, further exacerbating the noise problem. The optimum reverberation time for speech is about one second, and for music, it is about 1.5 seconds. Reverberation times over four seconds are undesirable for either speech or music, and should be addressed with sound attenuation materials. For a restaurant, a reverberation time of about two seconds will provide a higher energy acoustic ambiance without sacrificing speech intelligibility and music clarity.
Sonic-Shield performed acoustic reverberation tests at a restaurant in the morning with the staff present and no guests. This test involved subjecting the room to an acoustic impulse, and measuring the time the acoustic wave decays to -60 dB of the impulse (abbreviated RT60). An example output from the RT60 test is shown to the right.
The second column of the table pertains to the long term acoustic decay, and is of primary importance. As shown, the decay times in the 250Hz and 500Hz bands are extremely long and exceed seven seconds. It is therefore believed that acoustic reverberations at these frequencies are the root cause of the excessive noise at the restaurant, which makes it difficult for guests to hear conversation and music.
In order to determine the required amount of sound absorption material needed to reduce the reverberation time to about two seconds, we performed a Sabine analysis, which relates the room volume to the surface areas and sound absorption characteristics of the floor, walls and ceiling. For this analysis, we used room dimensions of 44’ x 44’ x 15’ tall with a wooden floor, a composite of plywood, drywall, glass and brick walls, and a metal ceiling. By first setting the reverberation time to the measured average of 7.5 seconds, it was possible to estimate the effective absorption coefficients of the floor, ceiling and walls. Once these absorption coefficients were known, it was possible to set the reverberation time to the desired value of two seconds and back calculate the required surface area of sound absorption material (or carpet, if desired).
The measure of a material’s ability to absorb sound is given by the Noise Reduction Coefficient (NRC). An NRC of 0.0 pertains to material like smooth concrete, which will reflect all of the sound. An NRC of 1.0 pertains to a material like Sonic-Fiber, which will absorb 100% of the sound and is the most effective sound absorption material on the market today. Carpet, on the other hand, has a relatively poor NRC of only 0.4.
Our analysis indicates that in order to achieve a reverberation time of 2.0 seconds, 422 square feet of Sonic-Fiber would be needed. Sonic-Fiber is sold in 4’x6’x2” thick sheets, so a total of 18 sheets will be required. These would be hung from the ceiling like banners in a horizontal orientation. The wavelength of sound at 250 Hz is about 4.5 feet, so according to the “quarter wavelength rule”, the absorber material should be hung no less than 14 inches from the bottom surface of the ceiling. If carpet is desired, a total of 1056 square feet would be needed, and since it must be installed on the surface of the floor, it will not be capable of attenuating the noise in the 250Hz and 500Hz bands.
Sonic-Shield offers a full range of technical consulting services, solutions, materials and products to improve the acoustics in restaurants and other interior meeting spaces. Our team of acoustic experts can help you identify noise problems, calculate room reverberation times and assist you in selecting and providing the appropriate materials and products to eliminate, block or absorb unwanted noise. Soundproofing a restaurant is never easy, but with our process sound reduction is always attainable.