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Jargon Buster – Reverberation Time

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.

Controlling Reverberation

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.

New European HQ for SAP

dBx Acoustics worked with TTSP architects and project managers Storey on the fit out of two floors in London’s iconic ‘Scalpel’ building. The project created an activity based, agile workspace for the new European HQ for enterprise software company SAP.

The client provided in-house US acoustic design standards to us as our starting point. As the standards vary from UK terminology in several subtle ways, our first task was to review the requirements and convert them into a specification that could be used to tender the fit-out using a UK contractor.

During the work we identified areas where the specification was inconsistent or unclear and helped the client to develop a more coherent and well-defined set of acoustic requirements.

The fit-out included open plan office areas, a suite of meeting rooms, breakout and cafe space and quiet working areas. Speech privacy between meeting rooms was initially challenging due to the landlord’s limitations on connections to the mullions and the desire to have an open ceiling with suspended rafts. Good design and detailing ensured that, upon commissioning, the required level of speech privacy was achieved.

As SAP’s European HQ, the fit-out also includes an open plan area which can be used as a presentation space for external clients. Excellent speech intelligibility throughout this area was a key requirement and an acoustic model of the space was created to establish the most effective quantity and placement of acoustic treatments.

The project was completed in Spring 2021. You can find out more about the HQ here.

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