3.3 Location and Cross Talk Depths

When installing cross talk attenuators, there are numerous locations and arrangements which can be used. Some of these arrangements have been illustrated in 1 to 9. As shown, the sizing, depth and height of the attenuators is typically proportional to the free area of the unit.

Options 1 - 2, If a long attenuator is feasible, 50% free area units are recommended, which results in a longer unit.

Option 3, one of the most popular methods is to install the NAT Vent Attenuator within a bulkhead. The bulkhead is either located within the cellular spaces or under a walkway within a circulation zone, or a combination of the two. The depth of the bulkhead is proportional to the free area of the unit, 50% free area units may require a depth of 1200mm with a height of around 250mm. 20% free area attenuators will only need to be 600mm deep, in turn this unit will require a height of 625mm.

Option 4, an alternative to option 3. In this instance, the bulkhead is only 900mm deep, a 37% free area unit has been used with a height of 340mm

Option 5, a second alternative to option 3, 600mm deep bulkhead, attenuator 30% free area, height of 625mm

Option 6 a further variation on option 3, the NAT Vent Attenuator has been split to allow the passage of services.

Option 7 to 9, Cellular offices/medical rooms require a significantly smaller unit due to reduced air flow rates. A wide range of options are therefore available.

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.

2.6 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.

2.10 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.

1.3 Acoustics and Cross Ventilation

It is generally accepted that cross ventilation is the most effective form of natural ventilation. Acoustics plays a key role in the design of a cross ventilated building as air must flow freely through the building whilst maintaining privacy across partitions. To allow cross ventilation and maintain privacy, cross talk attenuators are required within partitions adjacent to circulation spaces.

One of the key design benefits of the NAT Vent Attenuator is the simple implementation of cross ventilation through a corridor wall, while still maintaining the acoustic integrity of these partitions. Furthermore, this product enables cross ventilation to vertically stacked rooms, vented through a single stack. In other words, vertically stacked spaces no longer require independent chimneys to maintain the acoustic separation between rooms, resulting in a significant recovery of floor area and a considerable cost saving.

One of the drawbacks of ventilating through the corridor wall is the requirement for an exceptionally large bulk head, such to accommodate large, heavy attenuators. The NAT Vent has been designed to provide exceptional levels of cross talk separation. MACH Acoustics has undertaken extensive research to understand the required levels of acoustic separation across these partitions. Depending upon the air flow and the required level of acoustic separation, the NAT Vent can be as slim as 600mm deep. In some instances, this is required to be increased to 1200mm, depending on the required acoustic performance.

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.