Schools : Building Bulletin 93 – Acoustic Design of Schools
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Example Calculation - Sound Insulation Between Classrooms
All references (tables etc) within this article are to those contained within BB93 itself. For further information please refer to this document. BB93 is available from the following website: www.teachernet.gov.uk
Figure A4.1 (shown above) shows a secondary school classroom adjacent to a science laboratory, plantroom and corridor. In this example, the designer has decided to build the separating walls between these spaces with masonry. The calculations below are used to determine the specification of the masonry wall (eg mass per unit area, thickness, surface finishes) required to meet the performance standards in Section 1.
There are three walls to consider:
• Wall 1 - between classroom and science laboratory
• Wall 2 - between classroom and corridor
• Wall 3 - between classroom and plantroom.
The performance standards for airborne sound insulation are contained in Section 1. For each of the three walls the following apply:
- The performance standard for Wall 1 is in terms of the weighted BB93 standardized level difference, DnT(Tmf,max),w, in Table 1.2. To determine the blockwork specification, the weighted sound reduction index of the wall is estimated from DnT(Tmf,max),w.
- The performance standard for Wall 2 is in terms of the weighted sound reduction index in Table 1.3.
- For Wall 3, between the plantroom and the classroom, there are no explicit performance standards in Section 1. Therefore the sound insulation between the plantroom and the classroom needs to ensure that the performance standards are met for the indoor ambient noise level in the adjacent classroom (Table 1.1). The sound insulation is calculated using the noise levels of the actual equipment in the plantroom.
Wall 1
From Tables 1.1, 1.2 and 1.5 in Section 1 the minimum performance standards for the airborne sound insulation are:
Classroom to Science Laboratory: 40 dB DnT(0.8s),w
Science Laboratory to Classroom: 45 dB DnT(0.8s),w
As the two room dimensions are similar and the values of Tmf,max are both 0.8 s, the specification for the masonry wall is based on the more stringent criterion, 45 dB DnT(0.8s),w.
As an initial estimate, the procedure described in Section 3.10 can be used to estimate the weighted sound reduction index, Rw, for the separating wall. The first stage is to calculate Rw,est.
Rw,est = DnT(Tmf,max),w + 10 lg (S x Tmf,max / V) + 8 dB
Rw,est = 45 + 10 lg (21 x 0.8 / 168) + 8 dB
Rw,est = 43 dB
To obtain Rw the factor X is added to Rw,est to account for less favourable mounting conditions and workmanship than in the laboratory test. From Section 3.10, X can be estimated to be 5 dB.
Rw = Rw,est + X dB
Rw = 43 + 5 dB
Rw = 48 dB
Therefore, suitable specifications for masonry separating walls and appropriate surface finishes that achieve at least 48 dB Rw can be identified by the designer.
Wall 2
The performance standards for the airborne sound insulation of the corridor wall and door are given in Table 1.3:
Wall between a classroom and a corridor: 40 dB Rw
Door between a classroom and a corridor: 30 dB Rw
In this example there are no ventilators or glazing in the wall. If there were ventilators then they would have to meet the performance standard in Table 1.3. Glazing in the corridor wall does not have a separate performance standard because the performance standard for the wall is for the combined sound insulation of any glazing and the wall. An example of a corridor wall with glazing is included at the end of this appendix.
From Section 3, Figure 3.11, a 44 mm thick timber door with half hour fire rating typically achieves 30 dB Rw if it is "a well fitted solid core doorset where the door is sealed effectively around its perimeter in a substantial frame with an effective stop".
Blockwork Specification
The minimum Rw values for the walls are:
Wall 1: 48 dB Rw
Wall 2: 40 dB Rw
Figure 3.9 can be used to draw up an initial specification for the walls along with laboratory test reports from the manufacturer. For Wall 1 and Wall 2 the following specifications might be proposed:
Wall 1: 100 mm medium density blocks (140 kg/m2) with a 13 mm plaster finish on both sides
Wall 2: 100 mm low density block (70 kg/m2) with a 13 mm plaster finish on both sides
The Rw specification for Wall 1 is only an estimate of the type of separating wall performance needed to achieve 45 dB DnT(0.8s),w and takes no account of flanking transmission which is usually critical in determining the performance. Therefore, at this stage the designer should seek specialist advice from an acoustic consultant to assess whether the proposed combination of separating and flanking walls is likely to achieve the performance standards.
Wall 3
Wall 3 has to provide sufficient sound insulation to ensure that the indoor ambient noise levels in the classroom do not exceed 35 dB LAeq,30min (Table 1.1). As there will be other noise sources contributing to the indoor ambient noise level, the level due to noise transmitted through Wall 3 will have to be significantly less than 35 dB LAeq,30min. BB93 does not recommend a standard method for this situation but one approach is to design Wall 3 so that the noise transmitted from the plantroom is at least 10 dB below the indoor ambient noise level in the classroom. Using this method, the noise transmitted through Wall 3 needs to be less than 25 dB LAeq,30min in the classroom.
The noise transmitted from the plantroom to the classroom depends on the frequency spectrum of the noise in the plantroom and the sound insulation spectrum of the separating wall. For these calculations, plantroom equipment locations and noise emission data are required. Precise equipment details are usually not known until the later stages of a project, therefore generic sound level data are normally used in calculations and the assumptions quoted in the specification.
The calculations are often complex and normally require an acoustic consultant. Guidance for these calculations can be found in the following references: CIBSE (2002). Guide B5: Noise and Vibration Control for HVCA. CIBSE. ISBN 1903287251. Fry, A. ed. (1988). Noise control in building services. Oxford: Pergamon. ISBN 0080340679.
Wall 2 with glazing
Walls between classrooms and corridors may contain glazing, therefore this example is a reassessment of Wall 2 with the following areas:
Area
Masonry 16.6 m2
Glazing 5.6 m2
Door 1.8 m2
Total wall 24 m2
The door is treated in the same way as in the example above with a value of 30 dB Rw.
The combined Rw criterion for the masonry wall and the glazing also remains at 40 dB Rw.
This combined criterion would be achieved if the masonry wall and the glazing each provide at least 40 dB Rw. A masonry wall specification for this has already been described above and there are three glazing configurations given in Figure 3.10, which also provide 40 dB Rw. However, these glazing configurations can sometimes be relatively expensive due to the use of thick and/or laminated glass and/or wide cavities.
An alternative approach is to improve the masonry wall specification to allow the use of another glazing configuration.
From Figure 3.9 the following masonry wall should give at least 45 dB Rw: 100 mm medium density blocks (140 kg/m2) with a 13mm plaster finish on each side
From Figure 3.8, glazing with different Rw values can be assessed to see whether the criterion of 40 dB Rw will be met by the combined value for the wall and glazing. A potential solution would be to use glazing with sound insulation of 35 dB Rw. This is 10 dB lower than the 45 dB Rw sound insulation of the masonry wall. For a glazing area of 25% of the wall area that excludes the door, Figure 3.8 gives a correction factor of approximately 5 dB. The combined Rw is calculated from the Rw for the glazing plus the correction factor, which equals 40 dB Rw. Hence this combination of masonry wall and glazing meets the performance standard. Further reductions in glazing specification could be obtained by reducing the area of glazing or by using a wall with a higher Rw.
