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.

5.15 Creative Room Acoustics - Allies and Morrison Offices

Brief
The brief was to provide room acoustic treatments to acoustically soften the reception spaces at Allies and Morrison’s Offices 1

Design Scheme
The challenge here was to provide design options which fit in with the dramatic, hard, minimalistic reception space. Two design approaches were proposed. The first was to added small amounts of treatment in many locations. These were as follows: to the rear of four large cupboards 2, to the display cabinets 3, to the underside of shelves 4, suspended panels fixed into the lighting track 5.

The alternative proposal was to add larger quantities in fewer locations. This included the end wall 6, here a metal panel with a solid foam infill was proposed, picture 7 shows a sample of this product. The second proposal was to add acoustic beams 9 in front of the window, behind the reception 8.

4.1 Subjective Evaluation and Conversion between Rw and Dw

Sound Insulation
Sound insulation describes the reduction in sound across a partition. The sound insulation across a good conventional, lightweight, office to office construction is typically in the order of 45 dB Dw. This means that if the sound level in the source room is in the order of 65 dB, (a typical level for speech) the sound level in the adjacent room, the receiver room, will be in the order of 20 dB (barely audible). If sound levels are increased in the source room to 75 dB (raised voice), sound levels within the adjacent room will also increase to around 30 dB (audible). Sound insulation therefore describes the level of sound lost across a partition and not the level of sound within a room.

Privacy
Privacy describes the perceived sound reduction across a wall. Privacy is a function of both sound insulation and background noise. Background noise is a function of services noise and noise break-in through facades or open windows, vents etc.
If the background noise within a room is increased by 5 - 10 dB, the perceived level of privacy across a partition is also increased by 5 to 10 dB. Therefore, when looking at required sound insulation levels on-site, it is important to consider both the background noise in the receiver room and the sound insulation reduction across the partition.

Subjective Description of Sound Insulation
The table to the right provides an illustrative representation of privacy. This table specifies the two Dw levels of the partition, Column 1. Two levels are provided in this column, one for background noise levels of 35 dBA, 1, and the other for background noise levels of 40 dBA, 2. Please see the text above for an explanation.

Rw (Lab Tested Sound Reduction Index) and Dw (On SIte Sound Reduction Index)
Two parameters are used to describe the sound insulation of a partition; Dw and Rw. Dw represents the sound insulation of a partition on-site. Since these figures describe the final site requirements, Dw levels are specified by clients and Building Regulations. Rw represents the lab tested sound insulation of a product wall type/floor type. Due to flanking and other factors, lab rated sound reduction levels will not be achieved on-site. Conventionally, there is a 5 to 10 dB reduction between a Rw lab tested figure and an on-site Dw figure. The conversion between Dw and Rw is relatively complex and takes into consideration receiver room volume, receiver room reverberation times and the area of the separating partition. The conversion between Rw and Dw should always be calculated.

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.

4.4 Acoustic Performance - Light Weight and Heavy Weight Walls

4.5 Acoustic Performance - Light Weight and Heavy Weight Floor