Considerable thought is now being given to the problems of the acoustic qualities of buildings and although much of this work has been directed towards improving the sound quality of buildings intended for speech and music, the control of noise has also received attention. The results of this work can be made use of in food service installations where noise can be a problem, and the reduction of loudness levels is desirable.
In general the kitchen tends to be a noisy area because of the nature of some of the work carried out involving the use of machinery, and because the demands of hygiene dictate the use of hard shiny surfaces for walls, floors and working surfaces and of metal pans and vitreous china. The areas where the problem is most acute are the dishwashing room, the pan washing area and the dining room. The last of these creates a problem on two counts. When the dining room is in use the sound level of conversation often rises to a high pitch to overcome the noise of the clatter of dishes and cutlery. The intensity of sound, which is measured in decibels (one decibel is the lowest sound intensity the average person can hear close to the ear), is considered a nuisance when it is in excess of 40 decibels. Secondly, when the room is used for speeches after a formal dinner extraneous kitchen noises may penetrate into it and make hearing difficult.
Some knowledge of the fundamental concepts of the behavior of sound waves and of the underlying nature of noise is necessary for an understanding of the problem and of the solutions for improvement.
The sound received by our ears from a given source reaches us in two ways, firstly direct from the source and secondly by reflection from nearby surfaces. Surfaces partly reflect and partly absorb sound waves and the proportion varies from one surface to another depending on the nature of the surface, the method of construction and also on the frequency or pitch of the sound waves. The reflected waves do not reach the ear at exactly the same time as those from the direct source, but merge in with the continuous waves and give character to the sound. They also add to the noise level. They are known as reverberant sound, and the time taken for a sound to die away completely after the original source has ceased is known as the reverberation time. This is controlled by the volume of the room and by its capacity for sound absorption and can vary from half a second in a room at home to twelve seconds or more in a large cathedral.
Some practical measures for dealing with noise
When dealing with the problem of noise in a room it is necessary first of all to calculate the reverberation time. This can be worked out from the formula: T=(0.05V/A). where T is the reverberation time in seconds, V is the volume of the room in cubic feet and A is the total absorption in the room in sq. ft. units or sabines. Tables are available giving absorption coefficients of various materials at different frequencies (low—125 cycles per sec., medium–500 cycles per sec. and high-2,000 cycles per sec.-4,000 cycles per sec.). It is not necessary for the food service manager to make these calculations but it is interesting to notice the effect of different surface treatments on this figure. Thus in the middle range of frequencies the absorption coefficient for concrete is 0.02 and for carpet on a solid concrete floor 0.3 or 15 times as great, a fact which explains the noticeable noise reduction in a carpeted room. The thought of carpets in a kitchen or even in an institutional dining room would have seemed a wild one in the past but as already mentioned in Chapter 1 it is with the development of new fibers today not beyond the bounds of possibility. Indeed in a number of university cafeterias and in some kitchens in the U.S.A., acrylic carpets have already been fitted. Such carpets are very hard wearing, and cleaning and maintenance do not present any great problems. They can be cleaned regularly with a vacuum cleaner and when necessary scrubbed with a mechanical machine.
A point to note in the use of sound absorbing materials is that the maximum benefit is obtained if they are placed as near as possible to the sound source so as to intercept the maximum arc of direct sound. For a generally distributed sound the ceiling is the area most usefully treated, but unless it is a low one it is advisable also to treat parts of the walls to diminish sustained reflection between them. Another point worth noting is that materials vary in efficiency for different frequencies. Thus porous materials absorb mainly at the higher and resonant panels mainly at the lower frequencies.
Porous absorbents in use for sound absorption include acoustic plaster, acoustic tiles of soft fiber board and asbestos, foamed polyurethane, glass and mineral wool, felts, curtains and other soft furnishings. The absorptive quality of porous plaster is dependent on the open nature of the small holes contained in it and if these are filled in with decorative treatments such as paint, their value for sound absorption will have been impaired. This should not be the case for proprietary tiles with drilled holes and a mainly non-porous surface. The effectiveness of these tiles for sound absorption lies in the fact that sound passes into the holes and is absorbed by the walls which have a porous nature.
Two objections are often raised to the use of these absorbent materials, namely that they collect dust making them difficult to clean, and that they may add to fire risks because they are combustible and help to spread flames. The first of these is difficult to counter except by expensive treatments or by the use of porous plaster with distemper decoration. The second can be overcome by the use of an appropriate impregnation treatment.
Other practical measures for reducing noise include the choice of equipment which operates quietly, the use of trucks and trolleys with rubber bumpers and tires, suspended ceilings with a dead air-space above, the isolation and closing off of noisy areas and the training of employees to work quietly.