CI-81-12-3 -- Prediction of Noise Levels from Ceiling Air Grilles in Large Low-Ceilinged Rooms PDF

One of the persistent problems in acoustics has been the prediction of sound pressure levels within rooms. The conventional approach to solving this problem has relied on the concept of a reverberant field, assumed to be uniform throughout the room. However, it has been well known for years that there are many conditions under which this assumption is violated, and where the predictions of sound pressure levels are correspondingly difficult. Patches of highly absorbent material in an otherwise highly reflective environment, for example, can cause problems with prediction methods. Another situation which causes difficulty is rooms which are "disproportionate" that is, rooms whose dimension ratios fall outside of the 1:1 - 5:2 limits which are necessary to produce a reasonably diffuse field. There are many examples of such rooms. Large, open-plan offices are a common example, as are many factory spaces. This paper will be concerned with the prediction of sound pressure level within such a disproportionate room. It will be particularly concerned with the prediction of sound pressure levels caused by ceiling air conditioner vents in large open-plan offices.

Three approaches will be discussed to the problem of predicting sound pressure levels in large open-plan offices. All three are based on computer calculations. The first two require large computer facilities, and are presented here primarily for comparison purposes. The third method departs from the diffuse field assumption and treats the room as a pair of acoustically absorbing parallel planes. This method is simpler, easier to program, and gives results which appear consistent with measurements in large, low-ceilinged rooms.

Each o£ the three prediction methods is derived from geometric acoustics reasoning. The "ray," or geometric acoustics, approach, and the wave approach each have their strengths and limitations. While there is much to be learned from the wave approach, this paper will be concerned with the geometric acoustics approach and the methods based on it.