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Case Study – Installation of Attenuators at Bristol University

28th April 2011BB93, Case Study, Cross Ventilation, NAT Vent Attenuator

MACH Acoustics, in collaboration with Capita Architects have been involved in the design of Bristol University’s Graduate School refurbishment. The need for sustainable and low-energy buildings has led to a natural ventilation scheme being carefully implemented into the design. The scheme involves air being brought into the classrooms through open-able windows and discharged into the main corridor via cross-talk attenuators, as shown within the section below. The stack effect, caused by the height difference between the air in-let windows and the air out-let, naturally drives the hot air out of the building. Cowlin Construction, the main contractor, has now handed over the project to the University and has therefore asked MACH Acoustics to support the final design and installation of the crosstalk attenuators.

Case Study - Installation of Attenuators at Bristol University

The cross-talk attenuators limit the transfer of noise between the classrooms and corridors, therefore maintaining privacy but allowing air flow. BB93 requires ventilators in corridor walls to achieve a weighted element-normalized level difference (Dn,e,W) of 39dB. To achieve this target in the new Graduate School, MACH Acoustics have designed 1500mm long, NAT Vent Attenuators which have been installed into bulkheads between the classrooms and the main corridor.

For the Attenuators to be efficient, they have to be acoustically enclosed so the noise cannot bypass the attenuators. The best solution is therefore to install the attenuators into a pre-constructed duct. The ducts can be made of any material as long as it provides sufficient sound reduction. In this case the ducts have been constructed using plasterboard for cost, time and convenience reasons. The photos below show the different stages of the construction to install the bulkhead and ducts.

Construction of the Duct

The NAT Vent attenuator is a highly efficient attenuator designed specifically for natural ventilation. Its foam design makes it the most flexible and high performance attenuator on the market for this purpose. The NAT Vent Attenuators used in the Graduate School are 1500mm long with a free area of 50%. The photos below show the installation of the foam attenuators into the bulkheads. Each attenuator is made up of 2 rows of 5 foam tiles stacked on top of each other to form the channels. The tiles are then simply slid into the ducts.

Installation of NAT Vent Attenuator

Cowlins Construction completed the installation of the NAT Vent attenuator by means of adding grill to the above units (not pictured). As smoke vents, louvres were fitted at the tapered end of the ducts.

MACH Acoustics specified the partition to perform to a (Dn,e,W) of 39dB. On-site testing following installation resulted in the partition achieving this and conforming to BB93 requirement. Considering the NVA as a separate element, it was shown to perform over it’s quoted attenuation.

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Case Study – London and West London Acadamies

28th April 2011Architecture, BB93, Case Study, Construction, Cross Ventilation, Eco Buildings, NAT Vent Attenuator, Vented Facades

London and West London Academies, two large £30M+ Academy are both designed around a natural ventilation, incorporating cross ventilation. Both schools were required to meet BB93 and BB101 ‘Ventilation of School Buildings’. MACH Acoustics worked with Fosters and Partners to develop a practical solution. Our findings were that the cost of conventional attenuators along with their size and short falls in performance prohibited the use of these devices but allowed for the NVA.

The academies featured many different imaginative implementations of the NVA as a cross ventilator, all venting into a central atrium. Air passed though the facade via actuated windows through bulkheads at ceiling height containing the NVA.

The main principle was to add an attenuator with 25% free area and a face area of 1.5m², with small splitter sizes. The attenuator was to be located above a cupboard spaces as shown below. A flap enable the vent to be opened and closed.

One of the main advantages of the NVA is the ease and simplicity of installation. Once the bulkhead has been constructed, the tessellating foam is arranged into honeycomb pairs by alternating orientation, then simply slid into the duct. Since the foam is malleable and can be cut, fitting around building services or any on-site alternations can be easily achieved. Installation generally takes under one hour – saving money and construction time.

With the window facade bulkhead, we were able to implement cost savings by negating the need for expensive weather louvres, using actuated windows instead. Not only was the cost significantly reduced but the installation was greatly simplified – both points highly valued by the contractor.

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Case Study – Rogiet Primary School

28th April 2011BB101, BB93, Case Study, Cross Ventilation, NAT Vent Attenuator

Rogiet Primary School is a timber frame building, located between the M4, M48 and in close proximity to the intercity train between Swansea and London. Such a site is acoustically challenging before even considering natural ventilation. By performing an environmental survey of the site, MACH Acoustics successfully implemented natural ventilation though careful consideration of building placement and well executed cross ventilation.

Cross Ventilation

The key to naturally ventilating this building was to use cross ventilation rather than single sided ventilation. This design step required the open areas within the facade to be 50 – 75% smaller than required for single sided ventilation – this design change made it possible to use natural ventilation.

Through McCanns (the M&E consultants) a BMS system was used to control the windows rather than to opt for a manually operated system. Windows under manual operation tend to be fully extended/opened to ≈150mm, rather than partially open. A BMS system adjusts the required openings to the exact level, significantly reducing openings by 5-25mm. The result is greatly reduced noise ingress from the building facade.

Design

MACH Acoustics believe that a corridor wall should provide a physical and acoustic separation between two spaces. Working with White Design, Willmott Dixon and McCanns a bulkhead detail was built to incorporate the NAT Vet Attenuator.

The NAT Vent Attenuator is a product specifically designed to achieve BB93’s acoustic requirements and BB101’s ventilation specifications. A 1200mm NAT Vent Attenuator was installed into a bulkhead, formed from plasterboard and a timber. Using a product that was specifically designed to meet the requirements set down by BB93, means that acoustic separation across corridor walls was not comprised below that experienced on schools which do not use cross ventilation.

Rather than fitting standard grill on the classroom openings, a CNC cut alphabet provides the same function whilst enhancing the teaching space – an example of truly bespoke design options that the NVA offers.

Result

The school met all BB93 requirements and BB101 ventilation specifications. Not only this but the finished design was pleasing and in tune with its environment.

Headteacher, Kathryn Evans, said: “There was an echo in the old school building and you could always hear the hum of the motorway outside, but there’s nothing like that now. Our new building is very quiet, and it’s had a big impact on the children – they are so calm.”

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Case Study – Windford Primary School

28th April 2011BB101, BB93, BREEAM, Case Study, NAT Vent Attenuator, Natural Ventilation, Vented Facades

Winford school is located directly under the flight path of Bristol Airport, as such noise ingress is a significant issue. During the design stage of this building several design options where reviewed.

The school is a prime example of the versatility of the NVA and MACH Acoustics. Facade ventilation is incorporated through window and horizontal louvres. Cross ventilation is applied through internal benches and exhausted through atrium vents.

For the design of this building cost was a primary factor. There was a desire to naturally ventilate but under the flight path of Bristol Airport and with cost limits it was a challenge acoustically.

A full natural ventilation strategy was designed in partnership with Stride Treglown Architects. Air would enter the building through openable windows and horizontal louvred ducts – controlled by a BMS – ventilate classrooms and exit through the roof of a central atrium known as the ‘heart’. Cross ventilators were designed between floors, discretely ducted within ground level benches – maximising building usage.

The horizontal facade included thermal dampers, actuated by the building management system (BMS) and featured a 900 bend; reducing the protrusion and increasing acoustic attenuation.

The central ‘heart’ of the building includes a plenum, where the hot air will exit the building. The design included a NVA bulkhead, mounted with BMS controlled actuated windows.

Working with Strides, the flexibility and bespoke nature of the NVA allowed for this design and enabled the same space to include solar thermal panels.

The school was fully compliant with BB93, confirming exceptional acoustic design considering the potential clash between natural ventilation and the busy Bristol Airport flight path.

Importantly, the reception of the building has been extremely positive, with teachers, parents and pupils heaping praise for the spacious and pleasant working environment. Winford’s pupils, who, according to parents are now ‘excited’ to go to school, enjoy day lit, naturally ventilated internal spaces with glazed screens between them so that all activity is in view.

The build received a BREEAM ‘Very Good’ rating and will generate a minimum of 15% of predicted energy consumption, thanks in part to the well executed natural ventilation system.

Click the preview below to download the full case study (pdf).

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Lecture theatre design – Bath University – Part II

22nd November 2010Architecture, Room Acoustics

Maximising Loudness

Speech intelligibility is highly dependent upon the audience being able to differentiate speech from background noise. To enhance intelligibility, speech levels should be maximised and distributed evenly across the seating areas. This is achieved by a combination of methods. First, restricting space increases the amount of sound energy per seat. Secondly, consideration of the length of the hall, to respect distance losses and increase awareness of any visual gestures is advised. Finally increasing the number of early reflections to the audience will increase intelligibility.

Room Shape – Volume

The ideal room volume per seat for lecture theatres is:

Volume / seat 2 – 5 m3

The lecture theatre at Bath University holds 350 people and has a volume of 1400 m3, this equates to 4 m3/seat, thus meeting the above criteria.

Room Shape – Length

Visual gestures made by the speaker can greatly improve the recognition of what is said. Although the benefit is not easily quantifiable, it is generally thought that the maximum distance from source to receiver should be no more than 20m. Firstly because this is the maximum distance a typical person can identify these visual prompts, and secondly, it ensures a good direct sound pressure level; essential for localisation.

The Bath lecture theatre had been designed with a maximum distance of 15m, from source to receiver and, depending on the design of a building, this can limit the audience capacity. However, Bath adopted a fan shaped room to maximise the audience capacity yet maintain the source to receiver distance.

Enhancing early reflections

As has been described, reflected sound plays a critical role in maximising loudness levels to the audience. Ideally, it is important to focus these reflections to the rear of the auditorium, since the direct sound at this point will be reduced due to the decay of sound over distance and the grazing effect of the audience. The method by which to focus these reflections is to correctly angle the surfaces at the front of the auditorium so that sound is directed towards the rear of the audience.

Angling the side walls of a given auditorium is often difficult due to the inherent restraints imposed by the building. Similarly in this case, the layout of the Bath University building is fixed, and therefore the option of

adding angled side walls was not explored further. The angle and layout of the ceiling then became the critical factor affecting the acoustic performance of the theatre.

The initial design of the ceiling was not favourable, the sudden step in the bulkhead and the down stand created by a structural steel, created two acoustic shadows on the audience.

To overcome this, the ceiling height was dropped to make the ceiling flat across the structural steel (green line). An angled ceiling was then introduced (blue). The position was calculated using a geometric method such that the audience to the rear of the theatre would receive two separate early reflections, instead of one; greatly improving the speech intelligibility within the lecture theatre.

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Case Study – Archway Primary School

1st October 2010BB93, Case Study, Cross Ventilation, NAT Vent Attenuator

MACH Acoustics worked alongside B3 Burgess Architects and developed one of the first schools to be cross ventilated post the introduction of BB93. The proposed veneration scheme is shown below, as can be seen to this drawing two rows of classroom block were located either side of a large teaching atrium.

The proposed detail on the first floor was to place the attenuators at and angel following the roof line. As in the case of London and West London Academies, the attenuators were placed over cupboards. The ground floor detail was made simpler since a flat slab was available. The design of the attenuator was boosted by the fact that the air could also pass through a relatively large service void, which further enhanced the acoustics performance of the ventilator.

The result was a naturally ventilated school that still achieved BB93; considered impossible by many before this project. The school had a light and pleasant atmosphere, thanks to the well thought out ventilation strategy and central atrium.

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Site Assessments and Remedial Support

21st September 2010Acoustic Testing

MACH Acoustics have vast experience in providing site reviews/assessments for buildings across all sectors within the built environment. On-site detailing is a crucial aspect in achieving on-site design targets and Building Control requirements are an area which can be overlooked at the design stage. Contractors and site workman are not always aware of the consequences of poor detailing; often running services through separating walls. This can cause huge problems on-site and remedial measures can be difficult to implement.

MACH Acoustics have experienced a huge array of site defects and therefore have a variety of on-site solutions for most site issues. Acoustic site reviews are an essential part of the ongoing design process and allows the acoustician to communicate directly with the site team to ensure that on site design targets are met. On paper designs are not always feasible when it comes to the build stage and mistakes on site are commonplace. Site assessments not only help create a building that is fit for purpose but can also result in significant cost savings as mistakes can be identified and rectified at an early stage.

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Flexible Auditorium

17th September 2010Reverberation Control, Room Acoustics, Variable Acoustics

On a number of projects MACH Acoustics has been requested to provide acoustic consultancy to achieve flexible acoustics within an auditorium. Flexible acoustics is the ability to vary the levels of reverberation within a space to achieve a longer reverberation time for a music and short reverberation time for speech. The table below provides the performance requirements for a range of spaces.

A high reverberation time is required for music, since reverberation will add depth and warmth to the music. On the other hand, a high level of reverberation will have a detrimental effect on speech intelligibility, meaning that a hall specifically designed for music is not suitable for conferences and drama production. Designing an auditorium purely for drama and speech often results is a space where music will tend to sound dead and hollow.

The equation below provides the basic Sabines formula for reverberation. From this equation, it can be seen that to achieve variable levels of reverberation, either the volume of the auditorium and/or the levels of soft treatment within the auditorium, needs to be adjusted.

Due to the impractical nature of adjusting the volume of the auditorium, flexible acoustics is achieved by means of varying the level of soft treatments within a space. More often than not, this is done by means of employing design options similar to those shown below.

The designs above work by exposing or concealing the acoustics absorption in a space, and therefore varying the acoustics performance of a building. The key in providing a hall with flexible acoustics is to determine a method of varying the levels of soft treatment. The auditorium within Slough Libary achieves this very cleverly. A future blog entry will be added showing how variable acoustics was added to this space.

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Creative Forms of Room Acoustic Treatments

17th September 2010Room Acoustics

Different forms of room acoustic treatments and creative designs are vital when creating spaces with a difference. MACH Acoustics were requested by Allies and Morrison to review the acoustics of their offices and reception spaces. For those who have seen these spaces it is clear that conventional forms of soft treatment would clash with the hard, minimalist feel of this building.

Fundamental to MACH Acoustics was to understand variety of materials used within this development which was basically smooth concrete, thick cast iron details, some furniture and only a few other hard elements. Time was also spent understanding the client, Allies and Morrison’s, expectations, desires and needs. The result of our initial acoustics consultancy is presented in the design report which can be found here;
Creative Room Acoustic Treatments

This report illustrates a respect for architecture and the exceptional beauty of this building. The proposed forms of treatment are seen to compliment the concrete and details of this building. To provide design flexibility, two design options where proposed; the first was to apply acoustic treatment in small quantities to a range of objects, including cupboards, display cabinets and other small elements. The second option was to add soft treatment to one or two areas, where these areas had a sufficient surface area. The second half of this report provides a range of visuals to help the design team understand MACH Acoustics concepts.

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Case Study – Dartington School Environmental Noise Break-in

17th September 2010Architecture, BB101, BB93, Eco Buildings, NAT Vent Attenuator, Natural Ventilation, Noise Map, Vented Facades

Environmental noise break-in can be a significant issue when naturally ventilating a building located on a noise site, even if the noise is only moderate. Dartington school, previously reviewed within this blog with regard to the creative room acoustic treatment, also required creative forms of noise control measures to prevent noise break-in to noise sensitive teaching spaces.

The first stage of any noise break-in assessment is to establish noise levels across the site. This was done by carrying out noise monitoring throughout the site, over the full operational hours of the school. During the noise survey addition data was collect with respect to traffic flow levels and other data need to build a noise map of the site. A noise map was used such to assess the effects of the new school and to establish noise levels across all facades.

The result of the original noise mapping exercise is shown above in the left hand image. The results of this assessment indicated that noise levels at the facades to the classroom block where higher than desirable. The solution was to use an acoustic screen and strategically placed earth bund to reduce noise levels at the classroom. The tire wall is shown on the front cover of “BD Reviews” was used to form the required acoustics screen. This design is highly sustainable since these used tires did not go to land fill. Aesthetically there is a strong recycled image and additionally over time the vegetation shown in the image will cover the wall to form a green, live acoustics screen.

A second observation from the above noise mapping exercise was that noise levels at the sports hall were too high to allow for natural ventilation by means of simple openable vents. The solution was to added attenuated vents using the NAT Vent Attenuator. The photo below shows this vent and the diagram provides 3D view through the vent box.

Click the preview below to download the full case study on Dartington

Cross Ventilation

Design

Result

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