More Physical Approach
It gets worse. Low intelligibility scores, which indicate serious problems, usually provide little or no information on the nature of these problems.
Sometimes one or more physical problems are apparent in such cases, but are these really the causes of the poor performance?
Often, the only way to be sure is to correct the problems and see if that improves the scores.
Of course, this may be completely impractical, and in fact, there may be multiple problems, some masking others, so that correcting the most obvious might accomplish nothing useful.
A much more practical approach might be to identify exactly which physical factors adversely affect speech intelligibility, and how, and calibrate physical measurements to subjective effects.
If this were accomplished, then not only would meaningful test methods be available, but effective design criteria could be established to predict results and avoid problems in the design stage.
Some significant work has already been done in this area, with results pointing to the ratio of direct to reflected (or reverberant) sound being the most important factor.
Bob Thurmond is principle consultant with G. R. Thurmond and Associates of Austin, Texas.