MODELLING | VENTILATION AND ACOUSTICS Figure 1: Relationship between heat gain and acoustic performance Standard window * Solar reduction Glazing fraction Typical Min Max 40% 25% 60% Baffle window Glazing fraction Typical Min Max 50% 30% 90% Total g-value glazing 0.60 0.40 0.65 Total g-value glazing 0.40 0.25 0.50 SR* open window 12 dB 10 dB 15 dB SR open window 16dB 14dB 19dB Light transmittance 0.80 0.40 0.80 Light transmittance 0.70 0.40 0.70 is a higher air pressure, because additional airflow will be forced through. A smaller vent will, of course, have an inherently higher acoustic performance. In practice, crossand stack ventilation make better use of temperature differences across vents because of air buoyancy, which increases the pressure across vents and so draws higher flow rates. Along with fan-assisted ventilation, they can be seen as a type of noise control, therefore, as smaller openings in windows are needed compared with single-sided ventilation, which has lower pressure across vents. Site and building orientation heat and acoustic gains Standard openable windows offer minimal solar protection and minimal acoustic resistance An alternating baffle placed partially over an open window reduces the transmission of sound through the opening. It also gives the faade thermal protection Alternating window Typical Min Max Glazing fraction 40% 25% 60% Total g-value glazing 0.50 0.25 0.50 Box window Typical Min Max Glazing fraction 90% 70% 100% Total g-value glazing 0.50 0.12 0.30 SR open window 16dB 14dB 19dB SR open window 18dB 16dB 21dB Light transmittance 0.70 0.40 0.70 Light transmittance 0.80 0.40 0.80 A fully baffled window offers a similar performance to that of an alternating baffle. Covering the whole window, however, has the benefit of reduced thermal transmittance and some acoustic enhancements The increased glazing transmits more heat into the building, but the external glazing reduces this by controlling solar transmittance. The larger external baffle has the greatest benefit in respect to acoustics Various methods to improve acoustic performance while meeting ventilation requirements are discussed below, and there is an explanation of how the modelling of sound can accurately predict the effectiveness of sound-reduction methods. Ventilation efficiency as a form of noise control The flow across a vent is fixed by ventilation rates needed to offset building heat gains. Openings can be decreased in size, however, if there It is common to do an orientation heat impact assessment during the early design stages of a building. Typically, south-facing windows have higher heat gains, so are harderto acoustically attenuate than north-facing ones with lower heat gains because larger vent sizes are required to offsite heat gains. In the same way, noise levels (acoustic gains) to faades vary depending on whether they have direct line of sight to a noise source or whether they are in acoustic shadows. Enhancing g-values and the acoustics of windows A g-value of 1 represents full transmittance of solar radiation, while 0 represents a window with no solar energy transmittance. In practice, most g-values will range between 0.2 and 0.7, with solar-control glazing having a g-value of less than 0.5. Put another way, the g-value times the area of the glass, plus solar orientation, are key factors affecting the total heat gain upon a space. So the g-value is critical when considering a buildings heat gain and its acoustic design; lower heat gains mean smaller openings, with inherently higher levels of acoustic resistance, can be used to ventilate spaces. It is worth considering the g-value as a form of noise control, therefore. Figure 1 shows how window designs that reduce heat gain have a better acoustic performance. Thermal mass and solar shading can also be used as an effective form of noise control, as shading/ cooling reduces either the heat gains or the ventilation to offset these heat gains (see Figure 2). So these design options should be considered when buildings are on noisy sites. Acoustic shading In the same way that light shadows can be used to reduce heat gains, acoustic shadows can be used to control noise ingress. Acoustic shadows are more complex than light ones as the wavelength of sound is massive compared with that of light. This means sound tends 50 May 2019 www.cibsejournal.com CIBSE May19 pp49-50, 52 Acoustics ventilation modelling.indd 50 26/04/2019 17:13