2.1 Attenuation Incorporated into a Facade

One of the main difficulties in designing low energy buildings can be the prevention of noise break-in via vented facades. This chapter looks at a range of options and details which can be used to prevent environmental noise break-in from motorways, dual carriageways, trains, aeroplanes, inner city noise and other noise sources.

To overcome this issue an attenuator is selected and incorporated into the facade. This attenuator is typically combined with a damper such to control the flow of air into the building with a weather louvre being used externally to provide the weather protection. MACH Acoustics describes this combination of units as the ‘NAT Vent Box’. The outline schematic of this system is shown below. The damper can take the form of a thermal volume control damper, open-able vents within the facade, thermal insulated doors etc.

2.3 Locating Inlet Vents and Cross Vents as a Noise Control Measure

Locating Air Inlet Vents
The orientation of a building has a significant impact upon noise levels at the different facades of the building. It is often the case that facades on the opposite side of a building to a significant noise source will be considerably lower than those on the noisy side of the building.
By orientating the building and by placing non-critical spaces on the noisy side of a building, it is possible to form a good acoustic buffer. In these instances, cross vent can be used where the air intake is placed on the quiet side of the building. Cross ventilation to an atrium or circulation zone is then used to provide the air extract. Alternatively, single sided ventilation could be used for sensitive spaces on the quiet side of a building.
1 shows sound levels around a building in the form of a noise map. The classroom on the far side of building to the A40 are vented using openable windows.

1

Cross Vent to Assist with the Prevention of Noise Break-in
In instances where a building is located on an exceptionally noisy site, cross ventilation can improve the feasibility of natural ventilation. Cross ventilation has an important advantage over single sided ventilation, in that air inlet vents can be between 25% to 75% smaller than those required for single sided ventilation. This significant reduction in vent size helps considerably in preventing noise break-in, as smaller vents restrict the passage of sound into a building.

2.4 Bench Seating

Adding attenuation to the vented facade of a single storey building is comparatively easier than a multistory building, since it is often possible to extend the building envelope, accommodating the additional depth of the NAT Vent Boxes.

In the case of single storey buildings, it is common to maintain a simple vertical thermal line, by placing the thermal damper into the line of the facade 4. The acoustic attenuation in this instance is placed on the external side of the building. This design approach has been implemented by MACH Acoustics on several projects.

The NAT Vent Box has been installed under bench seating, flower boxes, play boxes, small steps in the facade and other elements. These units have all been used to hide and accommodate the additional facade depth often required when naturally ventilating a building on a particularly noisy site.

A second advantage of single storey buildings is the potential to incorporate the NAT Vent Box above or within roof lines, above corridors, over storage rooms and other areas.

2.5 Window Details

The details used to incorporate the attenuation box into a seat or play box are very similar to those used when incorporating the NAT Vent Box into the facades of buildings. As noted, it is often easier to extend out the facade line of single storey buildings such to accommodate deep attenuators. This in turn means that it is seen as possible to provide natural ventilation irrespective of noise levels. The illustration below 1 was used to control noise break-in to a sensitive office space in close proximity to a major motorway.

Installation of the Attenuator
Forming the NAT Vent Attenuator by tessellated, W-shaped foam blocks, means that this product can easily be dropped into a timber enclosure or metal duct work. The W-shaped tiles compress and can be cut to any size; hence these units are extremely easy to accommodate into the facade of a building 2

2.6 Window Systems and Curtain Walling

The thermal damper is one of the main factors affecting the cost and depth of the NAT Vent Box. Replacing the damper with an openable or motorised vent/window, eliminates both the thermal damper and weather louvre from the box make up. This typically reduces costs by ≈50% and can reduce its depth by ≈225mm.
Facade and window manufacturers can easily accommodate openable vents in curtain walling or window frames. Placing the NAT Vent Attenuator directly behind an open vent, provides a simple, cost effective design solution for preventing noise break-in.

High level air inlet
In the case where noise levels are exceptionally high, for example due to motorway noise, flight paths or inner city noise, the depth of the attenuator is required to be increased. The additional depth of the NAT Vent Box can be accommodated by using a high level bulkhead 11.

2.8 Internal Sliding Doors and Windows

The presented illustrations show alternative arrangements 9 – 13. In this case, the NAT Vent Attenuator is placed on the outside of a thermally insulated door or sliding window. The key advantage of this scheme is that it again eliminates the need for a thermal damper. Additionally, it can often be easier to accommodate the NAT Vent Box outside of the thermal line.

2.9 Double Facade

Double facades can be used to control environmental noise break-in without the need for acoustic attenuation. When using a double facade, air enters the building through conventional open windows. The acoustic protection is achieved by acoustically screening these windows by means of a secondary facade. Air enters the void between the two facades via a gap at the bottom of the outer, secondary facade. The edges of the secondary facade are typically taken back to the primary building envelope. Attenuation may be required at the air inlet between the two facades. The advantage of this type of facade is the fact that simple openable windows can be used. It is also possible to form buildings with an interesting and unique appearance.
The drawbacks are clearly cost and space and for these reasons this type of noise control measure is somewhat uncommon.
It is also important to note that the acoustic separation between two rooms can be compromised when windows are open. Acoustic splitters or absorption may be required.

2.10 Secondary Facades - External Ventilation Shafts

An alternative to secondary facades is to use external chimneys. This scheme uses very similar principles to that of a double facade; the difference being that the external chimneys are only used over the ventilation openings. This arrangement clearly has cost and space saving advantages over that of double façades.

A second architectural advantage is that it is possible to provide an animated facade. Forming the chimneys from glass or other translucent materials, allows interesting designs in the form of graphics to be incorporated within the chimneys, adding further interest to the facade of the building.

One of the drawbacks of this design is that acoustic treatment may be required within the chimneys to prevent the spread of sound along its length. This may be required to maintain the acoustic separation across floors. If this is the case, acoustic art work could be used to give the architectural design and also to provide the acoustic absorption within the chimneys.

2.13 Screening under overhangs and above roof

The scheme below provides three design options incorporating acoustic screens into the facade of a development. In these instances, the air inlet vents are acoustically screened by baffles which break the line of sight to a given noise source. The acoustic screens are, in this instance, created by extending parts of the facade or adding panels to the facade such to cover the air inlet vents.

Option 1 - Overlapping Façades
With a perpendicular air inlet to the facade, this design provides an ideal screen to a noise source propagating from the left-hand side of the building.

Option 2 - Solar Shading and Acoustic Screening
Here a solid transparent screen incorporated into the solar shading, provides acoustic screening to a noise source directly in front of the building.

Option 3 - Photovoltaics used as Acoustic Screens
Photovoltaics provide acoustic screens in this instance. The photovoltaics are used to provide solar shading, power and acoustic attenuation, all within the building’s facade. Off-setting the photovoltaics and placing the air vents directly behind these panels provide high levels of acoustic resistance.

1.2 Acoustics and Vented Facades

When naturally ventilating a building on a noisy or moderately noisy site the acoustic design of the facade becomes fundamental. The ability to provide high levels of sound resistance within a limited depth is often a requirement for an attenuator in the facade of a building. Due to the honeycomb structure and the performance of the acoustic foam, the NAT Vent Attenuator provides an exceptionally slim line attenuator with an outstanding acoustic performance. The size and depth of the NAT Vent Attenuator is dependent upon two main factors;

1 The free/open area specified by the M&E consultant/engineer. This governs the required face area of the attenuator and its percentage free area. A large face area will transfer a greater level of sound into a room, hence the depth of the attenuator is required to be lengthened such to resist the increased level of sound power.

2 The second factor affecting the depth of the attenuator is the required level difference between the environmental noise and the required internal noise level. The greater the difference, the longer the attenuator. To design and test the NAT Vent Attenuator, MACH Products has an in-house calibrated test rig to BS EN ISO 7235:2003.  This test facility enables MACH Acoustics to design and test a range of options during Design Stages C, D and E of a project.  The NAT Vent Attenuator is then manufactured to fit into a given location, as well as meeting the buildings ventilation and acoustic requirements.

See Chapter 2 and our website, www.machproducts.co.uk for further details.

Introduction

MACH Acoustics is a specialist acoustics consultancy providing acoustic design across all buildings types. Our aim is to become one of the leading acoustic consultants in the field of sustainable and creative acoustics. Our ethos is to constantly develop our understanding of acoustics and other disciplines, including architecture, building services and structures. Through consultation, research and imaginative thinking, we find creative acoustic solutions to the challenges we are presented with.

Each blog post will be an excerpt from the MACH Acoustics book – “Sustainable Acoustics – Sustainable Acoustic Scheme Designs from MACH Acoustics”. The posts will be categorised into the relevant chapters as they appear in the book. The purpose of this book is to highlight some of the unique designs employed by MACH Acoustics and teams we have worked with. Inspiring concepts, challenging schemes and fresh design approaches are presented across all aspects of building design. This book hopes to help design teams think greener and to achieve sustainable designs. This book also aims to act as a reference document, including performance standards, rules of thumb and standard forms of construction.

The book begins with an introduction into The NAT Vent Attenuator, a flexible product developed by MACH Acoustics on the back of extensive research. This product allows the flow of air whilst restricting the passage of sound, hence it can be used to achieve cross ventilation whilst complying with BB93, HTM, BREEAM and other privacy requirements. The flexibility of the NAT Vent Attenuator means it can also be used to enhance the performance of vented facades such to accommodate noisy sites.

Chapter 2 looks in detail at one of the most important and challenging aspects of low energy buildings; acoustic facade design to prevent noise break-in. Chapter 2 looks at three areas; attenuated facades to multi storey buildings, single storey buildings, and double and screened facades.

Since the introduction of stringent performance requirements, cross ventilation has become more difficult to implement. Cross ventilation is highly effective and more cost efficient than single sided ventilation. Chapter 3 therefore looks at design options for cross venting to atriums/corridors, as well as single ventilating stacks feeding multiple floors.

Sound insulation is an important aspect of any building’s design. Chapter 4 provides a range of performance requirements, standard acoustic details and sustainability methods of providing sound insulation.

Creative room finishes are an important part of building design. Chapter 5 therefore provides a complete range of alternatives to ceiling tiles and standard forms of room acoustic treatments. To supplement this information, a series of case studies and green practical solutions are presented.

Open plan is an important aspect to teaching and office accommodation. Valuable design roles, schemes and design concepts are therefore addressed within Chapter 6 of this book.