Why do traditional opera houses work so well for opera?

Abstract

Most computer models of room acoustics assume geometric acoustics (as if sound behaves like light). This has assisted our understanding of how room shape (fan shape versus rectangular, for example) can affect the acoustics of concert halls. When one applies geometric acoustics modeling to the traditional opera house, typically all one finds is a single ceiling reflection. The geometric approach holds little promise for understanding the magic of opera house acoustics. One of the acoustical attributes ignored by the geometric model is edge diffraction of sound. Our research has shown edge diffraction to be essential in predicting the superior acoustics of the traditional opera house. We have developed an alternative to the geometric acoustics model based on edge diffraction. In a two-dimensional representation, the locus of points with equal delay time is an ellipse; in three dimensions, the locus is the volume of revolution of an ellipse. By studying where this ellipsoidal volume intersects with the balcony fronts, we can locate where, at any given time, the edge-diffracted sound is coming from. This model is applied to LaScala, and a ‘‘movie’’ is developed to show how the balcony front edges contribute to the sound that is heard.