When designing multi-purpose spaces there is often conflict between the acoustic design requirements for each intended use. For example, one acoustic parameter which is often in conflict is the reverberation time. Rooms intended for speech communication (lecture halls, video conference rooms etc.) require relatively short reverberation times. Conversely, rooms intended for music performance need much longer reverberation times.
Usually, a compromise must be made, whereby the ideal acoustic environment for one use is forfeited to create a reasonable environment for all intended uses. A well-known phrase springs to mind – ‘Jack of all trades, master of none’.
For some multi-purpose spaces, however, variable acoustics can be used to provide the best acoustic environment for each use. This is usually done by making a temporary physical change to the space to alter the important acoustic parameters, such as reverberation time. Typical options for variable acoustics include:
- Changing the volume of the space;
- Changing the amount of sound absorptive material within the space;
- Changing the number of seats within the space; or
- Coupling the space with reverberation chambers.
Alternatively, a non-physical option could be used for variable acoustics such as an electroacoustic system which artificially adds reverberation to the output of the loudspeakers.
An excellent example of the use of variable acoustics through physical means is at the Milton Keynes Theatre. This theatre hosts many different types of performances, including:
- Orchestral concerts; and
- Variety & light entertainment.
Each of these performance types have different optimum acoustic environments within the theatre. For example, drama performances benefit most from a space with high clarity and a reverberation time of 1 second. For opera it is preferable to have clarity and reverberation balance with a reverberation time between 1.2 and 1.7 seconds. A longer reverberation time is best for orchestral concerts, ideally between 1.2 and 1.7 seconds.
At Milton Keynes Theatre, the solution to varying the acoustic environment is through changing the volume using a movable ceiling. The ceiling can be moved up to 10 m from its highest to its lowest position, which takes around 2 minutes and significantly changes the reverberation time. To minimise the chance of (unwanted) acoustic coupling between the theatre and the space above the ceiling (i.e. to prevent the space above the ceiling from influencing the theatre’s acoustic environment), the gap around the edge of the ceiling has been kept as narrow as possible. Any residual effect caused by the gap around the ceiling is minimised by high performance sound absorptive material covering the surfaces of the space above.
During the design of the theatre, a 1/50 scale model was built to help optimise the geometry of the theatre and control early reflections (50-80 milliseconds) which are important for good speech clarity. Some of the important design features which were enhanced through testing of the scale model includes:
- The ceiling shape;
- The shapes and angles of the balcony fronts; and
- The amount of rear wall diffusion needed to limit auditory focusing.
The sound absorptive properties of the proposed seating were tested in a laboratory, with and without an audience, to accurately predict the effect that the seating would have on the acoustic environment with different levels of occupancy.
Milton Keynes Theatre’s success with variable acoustics has prompted others to replicate it. A similar design is now in place in the Netherlands.
In summary, multi-purpose venues frequently opt for a compromise, using a fixed acoustic solution to create reasonable conditions for all uses. A more sophisticated solution for these spaces is the use of variable acoustics to provide the optimal acoustic environment for each use. Milton Keynes Theatre is an excellent example of how a multi-purpose space can use physical means to vary the acoustics to good effect. This example illustrates that design conflicts between different uses can be overcome when the acoustic environment is considered from an early stage in the project.
What has happened?
BRE Global have published the new technical manual for the 2018 scheme which sets out a revised environmental performance standard against which new buildings in the UK can be assessed for a BREEAM New Construction rating. Existing projects that are registered under the 2014 scheme are not required to meet the 2018 scheme criteria for BREEAM certification, however any individual or company wishing to register a new development for BREEAM certification will be required to do so under the 2018 standard.
How have the performance standards changed?
There are two categories which include acoustic performance standards:
- Health and Wellbeing
Let’s discuss each in turn.
Health and Wellbeing
For industrial, retail, prisons and other building types there are two credits available under the 2014 scheme. The first credit is for achieving the sound insulation and internal ambient noise level standards. The second credit is for achieving the reverberation time standards.
In the 2018 scheme there are three credits available, one for each of the acoustic principles: sound insulation, internal ambient noise levels and reverberation times.
For residential institutions (short-term and long-term stay) there are up to four credits available under the 2014 scheme depending on by how much the sound insulation performance is better than the Building Regulations minimum performance. One credit can be awarded for performance values 3 dB better, two credits for values 5 dB better and four credits for values 8 dB better.
In the 2018 scheme there are still up to four credits available but only the first two credits relate to sound insulation. One credit for values 3 dB better than Building Regulations and two credits for values 5 dB better. The third credit can be awarded for achieving the internal ambient noise levels standards. The fourth credit can be awarded for achieving the Building Regulations requirement for sound absorption in common spaces (e.g. corridors and entrance halls).
To be awarded the credit under the 2014 scheme, noise from building services installations (as measured or predicted at the nearest noise-sensitive development) must be no greater than 5 dB above the background noise level during the day and no greater than 3 dB above the background noise level at night.
In the 2018 scheme the credit can only be awarded where noise from building services is at least 5 dB lower than the background noise level during the day and night.
What does this mean for new buildings registered under the 2018 scheme?
In the Health and Wellbeing category, there are now three credits available for industrial, retail, prisons and other buildings types (without the need for bespoke performance requirements). This brings these building types in line with education, healthcare, office and law court buildings, and means the acoustic criteria is consistent across all these building types.
For residential institutions the inclusion of criteria for internal ambient noise levels and sound absorption in communal areas acknowledges the importance that these criteria have in the achievement of an acoustically comfortable environment for building users. Indeed, the attainment of appropriate internal ambient noise levels have long been required by Local Planning Authorities when granting consent for new domestic buildings. Similarly, the requirement for sound absorption in communal areas of residential buildings has been part of the Building Regulations for many years.
In the Pollution category, the change amounts to a 10 dB ‘swing’ from 5 dB above background (daytime) for the 2014 scheme to 5 dB below background for the 2018 scheme. However, the criteria in the 2014 scheme is effectively redundant and the credit awarded almost by default, since the BREEAM requirement is far less onerous than typical Local Authority planning requirements to control noise emissions from building services installations. By contrast, the criteria in the 2018 scheme is more in line with typical planning requirements, which can range between 0 dB (equal to background noise) and 10 dB below background noise. This means that the Pollution category now reinforces the good practice of minimising potential impacts that noise emissions from a new building may have on its neighbours.
In summary, the changes to the Health and Wellbeing acoustic criteria should allow a more holistic assessment of acoustics for all building types, while the changes to the Pollution acoustic criteria should give property developers even more encouragement to ensure that noise from building services is minimised. Therefore, it is likely that most will indeed see the changes as a step in the right direction with an increase in the perceived value of BREEAM certification for local authorities and developers alike.