The technical definition of reverberation time (RT) is “the time it takes for sound in a room to decay by 60 dB”. For the layman, that’s not hugely useful. So what is reverberation time? Well, it’s that characteristic of a room which makes it feel “echoey” or, at the other end of the scale, very “dead”.
Imagine spaces as different as a cathedral (St Pauls’ in London is reputed to have a reverberation of 11s at some frequencies), and a typical living room (which might have an RT of around 0.5-0.7s). Think what happens when you are decorating, and remove all the soft furnishings and carpet from a room – that change in how it sounds is due to the increase in reverberation.
Why does it matter? Well, different reverberation conditions are appropriate for different uses. For good speech intelligibility, a relatively low RT is desirable. For music, a longer RT is great – that’s why singing in the bathroom is always so satisfying. Getting the wrong RT for the use of the space, and it can significantly impact the usefulness of the room as well as occupant comfort.
Reverberation occurs when sound energy is repeatedly reflected from hard surfaces; the way that is is controlled is by introducing acoustically absorptive surfaces to stop these reflections and soak up the sound energy, much like a sponge soaks up moisture.
Selecting these surfaces is not simple, however. Different materials will help to control different parts of the frequency spectrum, so for example, a thin surface like carpet will be of little benefit in absorbing bass notes. The thickness and porosity of the material used, as well as the use of air cavities behind the absorption, are specified by acoustic consultants in order to precisely “shape” the sound within a room.
For simple spaces (low volume and simple shapes), there are straightforward formulae which we can use to predict reverberation time. In larger spaces, diffusion (the scattering of sound from different surfaces) also comes into play. It’s in these situations that a consultant is likely to suggest a computer model of a space. Although some clients shy away from this perceived complexity, modelling can be a quick and cost-effective way to determine the placement of acoustic treatment – it means that the absorption is placed exactly where it is needed, and where it will be most effective.