Acoustical performance criteria and environmental specifications are often part of building contract documents. Governmental agencies, lending institutions, owners, and tenants frequently require objective standards of performance.
To a large degree, acoustical criteria are subjective or are based on subjective response to acoustic parameters. This complicates attempts to provide objective specifications, but long experience has permitted acoustical experts to determine broad classes or ranges of criteria and standards that will produce satisfaction in most instances.
It is important to remember that one-point differences are normally insignificant in acoustical criteria. Usually, a tolerance of 21â„2 points from a numerical value is acceptable in practice.
Tables 11.31 to 11.33 list some of the more common criteria for ordinary building spaces. They should, however, be used only as a guide. Obtain specific data or requirements whenever possible.
Acceptable Background Noise Levels. Steady, constant, unobtrusive sound levels that normally occur in typical rooms and that are acceptable as background noise are indicated in Table 11.31. For specific applications and in acoustically critical spaces, specific requirements should always be determined.
[Note: Levels are given in dBA, easily obtained numbers with simple measuring equipment (Art. 11.78). When noise criterion (NC) values are specified, they can be determined by subtracting about 7 to 10 points from dBA values; that is, dBA 50 NC 40 to NC 43.]
Use of electronically generated masking sound, usually by means of speakers located above the ceiling, has become commonplace in open-plan spaces as a means of establishing a uniform background level. However, it is an expensive and complex procedure and should be handled only by acoustics experts.
Sound-Transmission-Loss Requirements. Acoustical performance requirements of sound barriers separating various occupancies may vary widely, depending on the particular needs of the particular occupants. Typical requirements for common occupancies in normal buildings are shown in Table 11.32.
In highly critical spaces, or where a large number of spaces of identical use are involved (as in a large hotel or multiple-dwelling building), it is always advisable to obtain expert acoustical advice.
Impact Isolation Requirements. Because the only standard test method available is controversial, impact isolation performance specifications are only broad, general suggestions, at best. The values in Table 11.33, however, are reasonably safe and economically obtainable with available construction systems.
Acoustical Absorption Requirements. It is ironic that requirements for the most widely used acoustical materials are the least well defined. So-called optimum reverberation time requirements (Fig. 11.99) are a reasonably safe specification for most rooms, but many additional requirements are often involved.
In general, where noise control is the significant requirement, the equivalent absorption of a full ceiling of acoustical tile providing a noise-reduction coefficient of about 0.65 to 0.70 is adequate. This absorption may be provided by carpet, furnishings, or other materials, as well as by acoustical tile. In many instances, however, no absorption at all should be applied to the ceiling of a room, because the ceiling may be a necessary sound reflector.
For important projects, always obtain the advice of competent acoustical consultants.