Methods to quantify opera houses acoustics—Changes to why, what, and how?

The historical and current reasons for quantifying opera house acoustics (the why?) are reviewed. The emerging shift from quantitiative assessment (by measurement) to qualitative assessment (by auralization) is discussed. The key characteristics of the acoustics of opera houses (the what?) are then considered. Assessment of a theatre with an audience present is suggested to be more important than unoccupied measurements. This has implications for the development of assessment methods. The suitability of classical room acoustics parameters—mostly derived from concert hall studies—is debated (the how?). Possible new assessment techniques are emerging. Characteristics not addressed by the classical parameters include the early (ensemble) and late (room) support provided to the singers and the onset threshold of image shifting of the sound of instruments within the orchestra pit. The assessment of loudness balance between the stage and the pit requires refinement and development. A difficulty with the assessment and comparison of opera houses is the effect of the stage settings on the acoustics.