A note on meaningful acoustical parameters for open-air theatres

The acoustics of performance spaces are usually characterized by the reverberation time and a handful of other acoustical parameters defined in ISO 3382-1. However, these parameters have been derived with closed spaces in mind, and it is not obvious that the same parameters are meaningful in an open-air theatre. The lack of late reflections means that the decay curve is often far from a straight line, and the reverberation parameters turn out to be unreliable. Also, parameters that use the balance between early and late reflections are problematic when late reflections are more or less absent. It is necessary to rethink the need for acoustical parameters instead of sticking to the well-established parameters meant for concert halls. The most important acoustical features of a theatre are that speech is sufficiently loud and clear, which can be described by acoustical parameters for strength and clarity. In addition, it is important to avoid echoes, which are more likely to appear in an outdoor environment than in a room. Thus, there is a need for an objective echo parameter. Acoustical parameters that vary strongly with receiver position are not well suited for a global characterization of the acoustics of a space. For this purpose, a parameter for the acoustical efficiency is suggested; it is defined as ten times the logarithm of the total sound energy in the impulse response relative to the energy of the direct sound. The spatial average of this parameter can be used for comparison of the acoustics of different open-air theatres.