dBx Acoustics

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. Get 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.

dBx Case Studies - Education

There is a proven link between acoustic conditions in schools and educational outcomes. Building Bulletin 93 (BB93) mandates minimum standards in primary and secondary schools for noise levels and room acoustics, as well as acoustic separation between teaching spaces.

The dBx Acoustics team can help you comply with BB93, but our expertise goes even further. We have extensive experience designing environments for pupils with additional needs, including autism and hearing loss, as well as higher education and noisier, practical workshop spaces.

New and refurbished school buildings must comply with Building Regulation E4 and the acoustic performance standards of Building Bulletin 93 (BB93) ‘Acoustic Design of Schools’. Whilst BB93 is not mandatory for higher education establishments, it typically forms the basis of the initial design for such establishments, with modifications as appropriate to allow for specific HE uses. Where projects are being designed with BREEAM in mind, credits HEA05 and POL05 are also relevant.

There are a number of different acoustic aspects which come together to ensure that acoustic conditions in schools are appropriate to support learning, and it’s so important to get it right – studies have shown that educational attainment can be directly correlated to acoustic conditions.

Our involvement often begins at the planning stage with an environmental noise survey, which allows us to advise on ventilation and glazing requirements to control noise ingress to the building. If mechanical ventilation is proposed, if there is an external MUGA, or if community use is proposed, the noise survey also allows noise emission limits to be set to ensure that existing neighbours are not adversely affected by noise.

Internal ambient noise levels in teaching spaces are also affected by mechanical ventilation, and we work with the M&E consultant to specify appropriate noise control measures, such as silencers.
When it comes to the design of the building itself, BB93 requires us to specify partitions and floors to control airborne and impact sound transmission between teaching spaces, based on their relative sensitivity and noise generation characteristics. The detailing of junctions and sealing of any services penetrations is critical in maintaining acoustic separation between adjacent rooms.

Having provided a suitably quiet teaching environment which won’t be adversely affected by activity in other classrooms, our focus moves to room acoustics and control of reverberation. Often this is as simple as specifying the acoustic performance of a suspended ceiling, but for large spaces such as Assembly Halls and Sports Halls, we undertake acoustic modelling to optimise the specification and placement of acoustic finishes. Where an exposed soffit is preferred, we calculate the specification and quantity of finishes, such as acoustic rafts and wall panels to control room acoustic conditions.

Finally, we carry out pre-completion acoustic testing on-site to ensure that all of the acoustic criteria for the project have been complied with on-site.

The dBx Acoustics team also have a particular interest in acoustic design for SEN schools, particularly schools catering to neurodiverse pupils. BB93 specifies design criteria for “children with special hearing and communication needs”, which is intended to include autism, ADHD and auditory processing difficulties, and assists in providing an environment in which speech transmission is clear and effective. The standard does not, however, consider the other acoustic aspects of school life which affect such pupils, including auditory sensitivities and the need to provide spaces to allow a retreat from the noise and bustle of daily school life. Our team’s direct and personal experiences of neurodiversity, both as parents and as individuals, helps us to understand the requirements of individual educational clients, and help guide the design of educational buildings to provide an acoustically diverse and appropriate environment.

","versionString":"wp\/v2\/"}; /* ]]> */