Concert halls need 1–2 seconds of reverberation balancing loudness against clarity

Evidence

Reverberation time (RT or RT60) is measured as the time for a 60 dB reduction in amplitude. There are other versions of RT measurements aimed at clarifying specific performance aspects. Performance halls need reflections to keep loudness high throughout a large audience, but enough acoustical damping must be present to prevent the reverberation from interfering with the intelligibility of voices or clarity of the music. It is a difficult compromise. Typical values for RT in performance venues are in the range 1 to 2 seconds, opera houses to concert halls, optimized for the music most commonly performed. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1232

The first approach, exemplified by the Birmingham Symphony Hall, is to physically add volume to the space via the use of controllable doors or shutters. If the volume added has little or no absorption, the effect of this is to increase the volume of the space without altering the absorption. As we have already seen from equation 6.19, this will have the effect of increasing the reverberation time. In the case of the Birmingham Symphony Hall, the volume of the space can be increased by up to 50% this way. Theoretically this could increase the T60 by the same factor. However, in practice the increase is less than this, due to the absorption in the added volume. The advantage of this type of approach is that, theoretically, it can uniformly cover the whole frequency range with its modification of T60. However, there are disadvantages. First, the expense is high, as a significant amount of extra volume has to be applied. Second, properly coupling the extra volume into the space is difficult to achieve. If it is done wrongly, one can have a double slope reverberation fault, as discussed in Section 6.1.15. Or the extra space can have little measurable effect if not coupled strongly enough. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1920

Categorization

• DEFINE: Defines RT60 precisely as time for a 60 dB amplitude reduction, with concrete typical values for performance venues.
• corroborated: 2 independent passages support this claim — Toole, Floyd E, Sound reproduction- the acoustics and…, 1232; David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1920