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Charity Spotlight 2020

Carterhatch AA is pleased to support the following charities in 2020:


A charity that brings together businesses and individuals from across the property industry to support life changing projects for young people facing homelessness.


This charity reduces food waste by redistributing surplus food to charities who produce meals for those in need.


An initiative of CARE International UK, this charity fights poverty by lending money to poor entrepreneurs in the developing world. This allows some of the world’s poorest people to work their way out of poverty by starting small businesses.

Whitechapel Mission

A volunteer led charity that supports homeless people in London.

How to avoid the need for mechanical cooling systems

How to avoid the need for mechanical cooling systems

A common way to prevent excessive or prolonged high temperatures in buildings is to use an active cooling system, such as:

  • mechanical ventilative cooling – where fans introduce external air to the building to give a cooling effect


  • comfort cooling – where a mechanical system cools the air in the building to a specific temperature.

The drawback of these active cooling systems is that they increase the energy use of the building, resulting in greater whole-life costs.

What are the sustainable alternatives to mechanical cooling systems?

The typical passive method of cooling is to use open windows. However, for buildings located in noisy environments (e.g. near major highways, railways, flight paths etc.) opening the windows to the extent required to give a cooling effect would also introduce a significant amount of external noise to the building. This could result in unacceptably high internal noise levels.

Here are four options for passive cooling that also reduce the level of noise introduced to the building:

  1. Balconies

    Balconies with solid balustrades can shield open windows from sources of external noise. Sound absorptive material on the underside of balconies can be used to attenuate noise reflected towards windows beneath balconies.


4 to 10 dB improvement in noise reduction compared to open windows alone.

  1. Plenum windows (noise reducing windows)

These are dual windows with absorption lined cavities and staggered openings.

Plenum Windows

4 to 11 dB improvement in noise reduction compared to standard open windows.

  1. Acoustic vents/louvres

These are large ventilation openings formed of acoustic louvres or absorption lined plena/ducts. Some acoustic vents have a small built-in fan to boost the air flow rate.

Acoustic Vents

4 to 16 dB improvement in noise reduction compared to standard open windows.

(Photo: TEK LTD)

  1. A combination of methods

Combining balconies that provide a shielding effect with either plenum windows or acoustic vents/louvres can result in a much greater level of noise reduction than would be achieved with any single method.

8 to 26 dB improvement in noise reduction compared to standard open windows alone.

In conclusion, it may prove more sustainable and cost-effective to explore the possibility of innovative passive cooling solutions than it would to specify an active cooling system.

It is time to change the way in which we design residential developments

The current situation

The general comfort of a building’s occupants is dependent on factors such as indoor air quality (i.e. ventilation), thermal comfort and acoustic comfort. These factors are interdependent but are typically assessed independently by different designers and based on different assumptions regarding use of the building. For example, it is not unusual to find that a noise assessment concludes that windows need to be closed to achieve reasonable internal noise conditions, whilst the overheating assessment calls for open windows for reasonable thermal conditions.

Excessive internal temperatures are partly explained by the increased airtightness and enhanced thermal insulation of modern homes. An unintended consequence of this is that the dissipation of excessive heat can be difficult. Other contributing factors include global heating (i.e. climate change) and the urban heat island effect.
It has become more and more common to encounter buildings in which occupants feel the need to open windows to regulate internal temperatures to a comfortable level. By doing so, however, those occupants may be exposed to levels of noise which the acoustic design of the building intended to avoid. This results in dwellings in which occupants may choose between having acoustic comfort or thermal comfort but cannot have both simultaneously.

This situation is at odds with the concept of placing the occupants’ health and wellbeing at the centre of housing design. Therefore, an integrated approach to residential building design is needed.

The new approach

The Association of Noise Consultants and the Institute of Acoustics jointly published the Acoustics, Ventilation and Overheating Residential Design Guide (‘AVO Guide’) in January 2020 to help acousticians conduct noise assessments with due regard for ventilation and mitigation of overheating.

The AVO guide recognises that the acoustic requirements of building designs should be integrated with the ventilation requirements to provide homes that are sustainable and achieve good indoor air quality, thermal comfort and acoustic comfort. This is especially important where the increased demand for new housing results in building closer to highways, railways and flight paths than would otherwise be desirable.

In pursuit of this objective, the AVO Guide helps acousticians to:

  • Determine the appropriate internal noise levels, due to transport noise sources, under different ventilation conditions;
  • Determine the appropriate internal noise levels, due to mechanical ventilation systems serving the dwellings, under different ventilation conditions;
  • Assess how the ventilation strategy impacts on the acoustic conditions;
  • Assess how the strategy for mitigating overheating impacts on the acoustic conditions;
  • Develop options to suitably control external noise ingress in conjunction with adequate ventilation and mitigation of overheating;
  • Develop options to suitably control noise from mechanical services in conjunction with adequate ventilation and mitigation of overheating; and
  • Apply a consistent and practical approach to assessing noise impacts under different ventilation and overheating conditions.

This new approach requires closer collaboration between acousticians, overheating risk analysists, MEP engineers, building physicists and the wider design team.

The next steps

In time, local planning authorities may demand a holistic approach to residential design (such as that encouraged by the AVO Guide) for all new developments, but perhaps building designers should not wait for this to become mandatory. Better quality housing, for the benefit of the occupants, may be incentive enough.

Charity Spotlight 2019

Carterhatch AA is pleased to support the following charities in 2019:


An initiative of CARE International UK, this charity fights poverty by lending money to poor entrepreneurs in the developing world. This allows some of the world’s poorest people to work their way out of poverty by starting small businesses.

National Autistic Society

The leading UK charity for autistic people and their families.

Chosen in memory of Dr Robert (Bob) Peters 1944 – 2019. Bob was one of the most widely recognised professionals in UK acoustics, through his teaching, research, consultancy and Institute of Acoustics activities over a career spanning more than 45 years.

Multi-Purpose Spaces: How variable acoustics can overcome design conflicts

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:

  • Drama;
  • Musicals;
  • Opera;
  • Ballet;
  • 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.

Movable Ceiling

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.
Balcony Fronts

Balcony Fronts

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.

Theatre Seating

Theatre Seating

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.