5.2 Performance Standards – BB93, HTM and BREEAM

Performance Requirements
The function of a space governs its acoustics requirements. Spaces which need to be quiet and where speech intelligibility is important require a low reverberation time.

BB93 ‘Acoustics Design for Schools’
Table 1.5 of BB93 provides a comprehensive list of performance requirements for educational spaces. This table is used as a benchmark for many buildings including multifunctional buildings and higher educational facilities.

BREEAM Offices
At present BREEAM Offices does not provide performance requirements with respect to room acoustics, therefore the 1 second reverberation time BB93 requirement for offices is commonly used.

HTM - Health Technical Memorandum Acoustics
HTM states that ‘Sound-absorbent treatment should be provided in all areas (including all corridors), except acoustically unimportant rooms (storerooms etc), where cleaning, infection control, patient-safety, clinical and maintenance requirements allow.

Acoustically-absorbent materials should have a minimum absorption area equivalent to a Class C absorber (as defined in BS EN ISO 11654:1997) covering at least 80% of the area of the floor, in addition to the absorption that may be provided by the building materials normally used. If a Class A or B absorbent material is used, less surface area is needed.

Acoustic absorption is likely to be needed in large open spaces such as atria, particularly in localised areas.

MACH Acoustics advises that an acoustic consultant be appointed to undertake a detailed assessment if an alternative to ceiling tiles is to be used.

5.3 Estimating Levels of Room Acoustic Treatments

The key to understanding the required level of room acoustic treatments is to study the relevant equation in section 5.1. To implement this equation, MACH Acoustics provide an excel spreadsheet 1 which can be acquired by email from ze@machacoustics.com. This spreadsheet can be used to find the exact required levels of room acoustic treatments. Estimating levels of room acoustics treatments such to approximate the required level of soft treatment, four factors need to be considered:

1 Required reverberation time
2 The average ceiling height
3 The floor finish
4 Added acoustic treatment (acoustic ceiling tiles, acoustic ceiling panels, acoustic wall panels...)

The tables below 3 to 5 present the amount of total absorption required, as a percentage of the floor area, to control the reverberation time based on the four factors above. The three different tables are provided for hard floor finishes, industrial carpet and an industrial carpet placed on an industrial underlay.

Example 1 - Carpeted Office, Ceiling Height of 2.8m and Required RT of 1 Second
From 4 the required levels of surface treatment are found by multiplying the floor area with the required percentage, for example

         28% * 60m2 = 0.28 * 60 = 16.8m
         16.8m2 of 100% acoustic absorption is therefore required within this 60m2 space.

Correction for Material Selection
The acoustic absorption of finishes is between 0 and 100% absorption, therefore a scaling factor is also needed for a given finish. As noted, materials are often rated between A and E, the scaled factors for these materials is therefore given below.

A = 1.25 * surface area of finish - see section 5.4
B = 1.42 * surface area of finish - see section 5.5
C = 2.00 * surface area of finish - see section 5.6

Example 1 - Continued
Perforated plasterboard with a Class C rating is proposed for the soffit finish. As such 33.6m2 of perforated plasterboard is required to achieve a reverberation time of 1 second.

More than One Finish
If more than one finish type is being proposed please see BB93, example Option C.

Example 2 BB93 Primary School Classroom
60m2 Classroom with a fl oor to ceiling height of 3.2m.

BB93’s reverberation time target is 0.6s

The floor is carpeted; hence 69% of the floor area is required to be treated.

        0.69 * 60 = 41.4m of Treatment is required

Option A - Class A ceiling tiles are proposed to be used, therefore 51.75m2 of treatment is needed. This figure is less that the floor area; hence a plasterboard border could be used.

Option B - Class B suspended rafts are proposed, the required area of the rafts is therefore 58.8m2.

Option C - Class B suspend rafts, in combination with 10m2 of Class A wall panels (10/1.25=8, 41.4-8=33, 33*1.42=46.9), therefore 46.9 m2 of suspend rafts are required.

Option D – The classroom ceiling height is dropped to 2.4m, Class B suspended rafts are proposed, the required area of the rafts is therefore 40m2.

4.3 Performance Specifications

This illustration presents the required performance standards for partitions to meet BB93, HTM and BREEAM office requirements for a range of cellular spaces.

The provided performance targets are given in terms of the Rw levels. Assumptions relating to room sizes, floor to ceiling height, room acoustic finishes and other factors have been made during the conversion between Rw and Dw levels specified BB93, HTM and BREEAM. These assumptions do not apply to all developments; hence this information should be used as guidance only. Please consult with an acoustic consultant for accurate levels.

3.2 Size and Acoustic Performance Requirements

The length of the NAT Vent Attenuator is a function of the required acoustic performance and the free area requirements for ventilation. If a large free area is needed, the depth of the cross talk attenuator will need to be increased. This increase in depth is required to balance against the increase in sound transmission as a result of a larger face area.

The free area of the NAT Vent Attenuator is typically between 20% and 50%. The calculated pressure drop through this product is minimal due the low air speed experienced with natural ventilation. 20% free area attenuators are used in cases where there is a limited depth for the attenuator. The drawback of this configuration is that a large face area is required to maintain the same free area specified by the M&E engineer. In this instance, the cross talk attenuator typically runs the width of the classroom, office or medical room.

The acoustic performance of the NAT Vent Attenuator is rated between 34 dB Dne,w and 39 dB Dne,w. Through research, it is seen that cross talk attenuators with an acoustic resistance of 34 dB Dne,w provide an equal performance to that of a solid partition containing an acoustically rated door (30 dB Rw). BB93 requires 39 dB Dne,w across a vent within a corridor wall, due to this limitation of the door this is seen as an over specification.

1.1 Introduction

The NAT Vent Attenuator has been designed to overcome the clashes between natural ventilation and acoustics. The NAT Vent Attenuator allows the fl ow of air into and through a building, whilst preventing the passage of sound. This product can easily be incorporated into the facade of a building, allowing for natural ventilation on all sites, irrespective of environmental noise levels. The NAT Vent Attenuator can also be used to prevent cross talk issues when implementing cross ventilation to atriums/corridors. This product can be made to comply with BREEAM, BCO, HTM, BB93 and other standards. 

The concepts and design specifi cations for this product have come from MACH Acoustics consulting experience. Our experience has shown that current products have a limited acoustic performance, are exceptionally infl exible, costly, and are lacking in technical innovation. Through frustration, knowledge and insight, the NAT Vent Attenuator has been designed and produced by MACH Products. The NAT Vent Attenuator is formed from W shaped tiles manufactured from acoustic foam. These elements are then tessellated and stacked together 11, 12 to form the NAT Vent Attenuator 13. The result is a fl exible product which can be fitted into bulk heads to allow for cross ventilation, or incorporated into the facade of a building to prevent noise break-in. 

The key features of the NAT Vent Attenuator are its patented technology based around the honeycomb structure, its novel W shaped splitter arrangement and the materials from which it is made. A simple manufacturing process delivers a cost effective, lightweight product, which is exceptionally flexible and therefore can be made to fit into a wide range of spaces and locations. The NAT Vent Attenuator is designed and manufactured bespoke for each project and tested using MACH Acoustics in-house test facilities 14 meaning that the acoustic specifi cation of this product meets the exact project requirements.

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.