Bengt-Inge Dalenbäck (bid@catt.se)
CATT
Mariagatan
16A
S-E41471
Gothenburg
SWEDEN
Room acoustic prediction software are now often considered as standard tools but basic
engineering principles and techniques remain as important.
These programs are tools and will not solve an acoustican's problem, the acoustician still has to come up with a sound design idea, based on knowledge and experience, and the prediction program can then, within its limitations, offer an independent check of how well a given design may work. A good description of these software is "qualified discussion partner". This paper will list some engineering techniques that sometimes seem forgotten once a prediction software is used. One example, that relates to that the standard measurements of absorption coefficients are discussed again, is that in some types of rooms it is not the absorption coefficients that are most crucial to estimate (once a basic estimate has been made) but the scattering coefficients. On the other hand there are also special cases where nearly everything hangs on an accurate estimate of the absorption coefficients. Further, there is an increasing need to emphasize and teach the limitations of geometrical acoustics when these software have become common since many times the expectations have become unrealistically high. One misunderstanding is that if a sufficiently detailed model is made, and if a sufficiently high number of rays are used, the results will always be accurate. Computers and algorithms may get faster, and more complex models can be handled, but the limitations of geometrical acoustics stays the same and including a few wave-related phenomena will have limited impact on the overall results. Examples will be shown of how a too detailed model will lead to bad results. Unfortunately, marketing statements such as "highly accurate" can be found also regarding geometrical acoustics based software and that further increases the risk for this misunderstanding, especially for new users. Software like this can, for many reasons, never be "highly accurate" but applied correctly they can be sufficiently accurate to be very useful tools in room acoustics. The accuracy of room acoustic measurements is naturally also a part of the discussion, since they are used to estimate the prediction accuracy, and common measurement and evaluation problems will be exemplified.