THEATERS AND AUDITORIUMS are intended for performances and events where speech is the most important signal of interest. For this reason, speech intelligibility is the single most important acoustic parameter to consider. There are entire textbooks devoted to this type of venue and we don’t pretend to get too deep into the details, but will give you a general overview and recommendations that will serve to inform the design process.
Theaters and auditoriums often use amplified sound but, since it is not the purpose of this text to discuss sound system design, we will discuss how to make the best passive acoustic decisions when designing this type of venue.
Amplified sound will aid in the intelligibility of speech, and this can be enhanced with architectural elements. However, too many reflective surfaces will inevitably end in higher reverberation times that will have a detrimental effect on speech intelligibility, so the process to design good acoustics for a theater is a careful balance between absorptive surfaces and carefully placed sound reflectors.
10.2.1 Reverberation times
Reverberation times in a theater need to be short enough to protect speech intelligibility. Figure 10.1 presents some optimum reverberation times for speech relative to room volumes and use of the space. It is not the intention to have a very dry space, since this type of venue will usually have music as a secondary activity (e.g., opera or musical theater, where speech intelligibility is the priority, but music quality is also desirable).
10.1 Optimum reverberation times for speech by room volume
10.2.2 Clarity, C50
Speech clarity (C50) is a measure of the ratio between early and late energy. This means that it gives an idea of how the balance is between the helpful early reflections and the late reverberation. It is called C50 because it defines the limit between early and late reflections as 50 ms, which, as discussed in Chapter 3, is the point at which our brain starts being able to separate individual reflections.
The higher the value for C50, the more clarity, and so the better speech intelligibility the room will have. In Eq. A.26 (in Appendix A) we explain how to calculate C50.
Definition, (D) There is a similar parameter that is used in the acoustic analysis of auditoriums and it is called definition (D). It is the ratio between the energy arriving during the first 50 ms and the total energy:
10.3.1 Recommended room volumes