TABLE OF CONTENTS In reverberant sound fields encountered in typical rooms, the decay curves vary throughout the space and it is necessary to determine the spatial average reverberation time. In Section 1.2.6.3 we looked at idealized decay curves in rooms; these were straight lines or smooth curves that were unaffected by random fluctuations or background noise. The ability to gain good estimates of the reverberation time from measured decay curves is determined by the combination of the acoustic system under test, the measurement procedure, signal processing, and evaluation of the decay curve.
There are two main methods that are used to determine the reverberation time in spaces or the structural reverberation time on structures; the interrupted noise method, and the integrated impulse response method (ISO 3382). The derivations in this section tend to refer to sound pressure as this is the most common application; but they are equally applicable to vibration. For accurate sound insulation measurements in rooms, we typically need to determine the reverberation time in seconds to two decimal places. For structural reverberation times the decays are much shorter and three decimal places are often required.
The interrupted noise method uses random noise to create a steady-state level. After a steady-state has been achieved, the excitation is stopped and the subsequent decay of the sound pressure level is recorded. An example decay curve is shown in Fig. 3.56. Compared to the idealized decay curve previously shown in Fig. 1.19, the curve is not smooth;
