Proceedings of the Institute of Acoustics

 

Towards a simplified model for partially open windows – connecting acoustic and thermal comfort

 

J Harvie-Clark, Apex Acoustics, Design Works, William Street, Felling, Gateshead, UK
J Hill, Apex Acoustics, Design Works, William Street, Felling, Gateshead, UK
J Batten, Apex Acoustics, Design Works, William Street, Felling, Gateshead, UK
L Pereira, Apex Acoustics, Design Works, William Street, Felling, Gateshead, UK
J Healey, Peninsular Acoustics, 214 Scott House, Gibb Street, Birmingham, UK
J Howell, Peninsular Acoustics, 214 Scott House, Gibb Street, Birmingham, UK
G Fusaro, University of Bologna, Via Zamboni 33, Bologna, 40126, Italy

 

ABSTRACT

 

Traditionally, the internal environmental quality (IEQ) aspects of acoustic and thermal comfort have been considered and designed separately. However, where occupants rely on opening or closing windows to achieve these dimensions, they are not independent in use. To consider both simultaneously, it is crucial to have a simple model of a partially open window that can predict both acoustic and thermal conditions internally. CIBSE TM59 defines the assessment method for adaptive thermal comfort. In the thermal model the ventilation performance of open windows can be described with an ‘Equivalent Area’ (EA). We propose to use the EA to calculate the façade sound insulation based on a simple assessment according to Annex D of ISO 12354-3, i.e. as a façade element with an area of EA and zero sound insulation. This study presents field measurements of sound insulation according to ISO 16283-3 with road traffic sound sources, to evaluate the acoustic uncertainty of using the EA to calculate internal sound levels. Results show encouraging correlation between measurements of element-normalised level difference, D_n,e,w + C_tr and calculations based on the EA. This model is considered adequate for acoustic design purposes, and represents a significant simplification compared with previous proposals.

 

1 INTRODUCTION

 

The English government has introduced a Building Regulation [0] to mitigate overheating in new residential buildings. Approved Document O [0] provides guidance on the Regulation. Approved Document O describes how windows cannot be assumed to be open during the night-time period if internal noise levels exceed guideline values. This means that an acoustic assessment and an overheating assessment are both required to assess the indoor environmental quality (IEQ) conditions simultaneously. Currently, the open area of the window considered in the acoustic assessment and overheating assessment are defined in different ways. This paper proposes a solution to this problem, to facilitate ease of communication between acoustic and thermal modellers.

 

1.1 Open Area Terminology

 

‘Free area’ when applied to the description of a façade opening, such as a partially open window, is an ambiguous term without consistent definition, despite its widespread use. For example, Sharpe et al [0] presents six different methods of attributing a value of ‘free area’ to an open window.

 

In England, Approved Document O of the Building Regulations presents a description of flow performance termed the Equivalent Area (EA), which is the area of a circular hole in an orifice plate that passes the same volumetric air flow as the element or flow device in question, for the same pressure difference. Appendix D of Approved Document O provides reference tables to calculate the Equivalent Area for a given window opening width, height and opening angle and an online calculator tool is available [0].

 

Guidance produced by the Association of Noise Consultants [0] on demonstrating compliance with Approved Document O proposes the use of an ‘Acoustic Open Area’ (AcOA), defined as the measurable, cross-sectional, geometric area. The AcOA is proposed as the lesser of two potential areas. These are:

 

• The sum of the rectangular area at the base (of a top-hung window) and the two triangular areas formed on each side of the opening light, Equation (1).

• The width times the height of the opening in which the opening light sits, Equation (2).

 

The total AcOA is therefore given by the lesser of areas from Equation (1) or Equation (2) based on the dimensions shown in Figure 1.

 

 

Figure 1: Concept of ‘acoustic open area’ shown shaded, with the window opening light opening out of the page

 

This means that for small angles of opening, the area of the opening rectangle and triangles limits the AcOA, until these areas equal the width x height of the opening in which the opening sits. This is a simple flat plane geometrical model of the opening light.

 

1.2 Theoretical assessment of façade sound insulation

 

The sound insulation of a building façade against outdoor sound can be calculated according to BS EN ISO 12354-3 [0]. Annex D of that standard suggests that for small openings, a global indication is given by treating the opening as an element with negligible sound reduction. This results in an element normalised level difference as shown in Equation (3).

 

 

Where S_open is the area of the opening in m² and A₀ is the reference equivalent sound absorption area, 10 m².

Where the value for element-normalised level difference, D_n,e is the same in each frequency band, as implied in Equation (3), the single-figure weighted value, D_n,e,w has the same value and the spectral adaptation term, C_tr has a value of zero. Hence D_n,e = D_n,e,w = D_n,e,w + C_tr .under this assessment.

 

The Association of Noise Consultants guidance [0] uses the AcOA as the opening area S_open for the calculation of the sound insulation. Comparisons are presented here using both the AcOA and EA as the opening area S_open.

 

2 FIELD MEASUREMENTS

 

A range of sites were selected that were exposed to steady continuous road traffic noise, such that the requirements of ISO 16283-3 could be conveniently satisfied. A count was made of fifty vehicles passing the site, to determine the minimum time period for measurements at each site. This varied between 30 seconds and a few minutes for the different sites. Sound measurements were made simultaneously outside, between 1 and 2 m from the façade with a fixed microphone, and inside, using a manual swept microphone technique. Measurements were made in frequency bands (1/3 octaves with the exception of site 6 in 1/1 octaves) over the relevant frequency range to determine the façade level difference, D_nT,2m,w + C_tr for each window position. Reverberation time measurements were made with the window closed in accordance with BS EN ISO 3382-2. The key parameters of each site are shown in Table 1, with features illustrated in Table 2.

 

The window opening extent was measured with a tape measure; the dimension measured was the distance that the opening light moved to the opening position, disregarding any overlap from the depth of the window frame. The measured dimension illustrates the distance between the opening light closed position and its open position. The measurement is considered to be consistent with the concept of a partially open window as a flat rectangular plane hinged in a flat plane opening, as described in the Equivalent Area calculator [0]. This concept disregards the thickness of the opening light and window frame, and any effect of the reveal. These features may affect both the airflow and sound insulation performance in different ways. The window was opened in 50 mm increments, up to 400 mm if possible. This leads to a variety of window opening angles and open areas between the sites. Measurements were also made of the room volume.

 

Table 1: Summary of room parameters

 

 

Table 2: Example illustration of key features of measurement sites

 

 

3 RESULTS

 

The Equivalent Area and Acoustic Open Area are calculated as described in Section 1.1. There are then three values of level difference determined for each window position:

 

• Measured

• Calculated based on the AcOA

• Calculated based on the EA

 

 

Figure 2: Comparison of calculated and measured sound insulation

 

The window opening position is provided in terms of an opening angle, calculated from the simplified model of an opening light as a flat rectangular pane. A graphical representation of calculated sound insulation values against measured data is shown in Figure 2 along with linear regression lines.

 

4 DISCUSSION

 

Overall, the results show reasonable agreement between the modelled and measured values, on average, although there is quite a significant uncertainty for any individual data point. A comparison of the coefficient of determination (R²) for a linear regression of the calculated values against the measured values, and the Root Mean Squared Error (RMSE) between the calculated and measured values is shown in Table 3.

 

Table 3: Linear Regression R² and RMSE

 

 

Comparing the RMSE of the EA to the RMSE of the AcOA, an increase of 0.03 dB, or 1.6 % is observed. The coefficient of determination is increased from 63 % to 66 %. It is considered that within the context of the calculations being performed, this difference is relatively minor, such that whilst the AcOA provides a slightly better fit to the measured data, overall uncertainty in calculations is not likely to be significantly affected where the EA is used in place of the AcOA.

 

Given there is a significant practical advantage in calculating façade sound insulation based on the EA, as this aligns with the performance parameter in the thermal (aerodynamic) model, it greatly facilitates the exchange of model attributes with the overheating modeler. The most significant advantage is that in the design process to assess thermal and acoustic compliance with guidelines, both disciplines use the same values to assess the performance of a partially open window. The use of the EA also overcomes any need to know the window dimensions and angle of opening – all the details of the façade are bypassed in the models.

 

Many modellers and other practitioners find the combined assessment of thermal and acoustic compliance is complicated; basing the sound insulation on the EA simplifies the process, and reduces the risk of greater discrepancies between acoustic modelling assumptions and thermal modelling assumptions.

 

5 FINAL COMMENTS AND CONCLUSIONS

 

It is suggested that the sound insulation of façade openings may be based on either the Acoustic Open Area (AcOA) or Equivalent Area (EA) without introducing significant additional uncertainty, based on the preliminary measurements presented. There are significant practical advantages to the use of EA over AcOA in the modelling for new buildings. Further work is required to expand the data set to cover a wider range of site conditions encountered in practice. The performance may also vary with sources other than road traffic, such as aircraft sound which is incident on an open window from a different angle.

 

Future studies could implement the method with numerical analysis based on Finite Element Method (FEM) in order to further investigate the dependency on parameters such as window dimensions, opening angle, room dimensions and source position. Through a parametric analysis it may be possible to review a sufficient amount of data to review statistically the two methods involving the AcOA and EA, and reduce the uncertainty using this simple method.

 

6 REFERENCES

 

1. The Building Regulations 2010; Statute Law Database, 2010.
2. The Building Regulations 2010 – Overheating Mitigation 40B; England, 2010.
3. P. Sharpe, B. Jones, R Wilson, C Iddon, What we think we know about the aerodynamic performance of windows, Energy and Buildings , 2021.
4. UK Government Discharge Coefficient Calculator available online: https://www.gov.uk/government/publications/classvent-and-classcool-school-ventilation-design-tool.
5. The Association of Noise Consultants, Demonstrating Compliance with the Noise Requirements of Approved Document O, 2024.
6. BS EN ISO 12354-3 Building Acoustics – Estimation of Acoustic Performance of Buildings from the Performance of Elements. Part 3: Airborne Sound Insulation against Outdoor Sound, 2017.