Reverberation Time vs. Decay Time

Every space has a “decay time” or “persistence” of sound after interruption of the source. But, this is not necessarily reverberation. In most cases it is just a train of specular reflections. Even if the reflections arrive outside of the ear’s integration time (from many directions) and last for several seconds, they are not necessarily reverberation.

A reverberant field is uniform throughout the space, like coffee or tea mixed with water. Specular reflections are seat-specific and their effect can vary wildly from seat-to-seat.

The reverberant field can be estimated by simple equations that treat the room like a large discharging capacitor or decaying LED. Specular reflection estimates, on the other hand, require construction of the room geometry with a modeling program (or even a physical scale model), and the selection of specific seats for calculation.

An acid test for reverberation vs. specular reflections is the Schroeder integration curve. Reverberant energy produces a fairly straight line out past 300 ms or so (Figure 1). Specular reflections produce jaggies or stair-steps  (Figure 2).

Here’s is why the distinction matters.

If your STI predictions assume a reverberant field when the energy is not reverberation (most spaces), they can be wildly inaccurate. So, there is a catch-22 here. We can dumb down the design approach (statistical formulas) to allow the EC or Fire folks to predict the STI, but this results in STI scores that may have little to do with the performance of the finished system.

This initiates a blame game that can involve lawyers.  pb