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Calculation of facade sound insulation

The Apex method for calculating the façade sound insulation is in accordance with BS 8233 and the principles of BS EN 12354-3.  Our paper and poster presentation, published in the Proceedings of the Institute of Acoustics and available here, explains the derivation of the basic façade sound insulation equations presented below.

The equations may not be applicable in all situations; consideration is also given to factors such as the shape of the external façade, the proposed mounting position of trickle ventilators (proximity to other surfaces, both internally and externally), and the potential performance of other building envelope elements such as the wall and roof.

The following equation is used to calculate the partial sound level in a room due to sound penetrating a façade element (such as glazing) that is characterised with a sound reduction index and element area:

(Equation 1)

For small façade components such as a ventilator, characterised with an element sound reduction index, the partial sound level in a room is calculated using:

  (Equation 2)

In the above equations,

  • L2 (dB) is the partial sound level in the room due to sound through the specified façade element;
  • L1, ff (dB) is the external free-field noise level at the position of the façade;
  • R (dB) is the sound reduction index of the façade component;
  • S (m2) is the area of the façade component;
  • V (m3) is the room volume;
  • T (s) is the room reverberation time; and,
  • Dn,e (dB) is the element-normalised sound level difference of the component.
  • All internal and external noise levels are A-weighted.

From ISO 16283, the reverberation time is typically 0.5 seconds across the relevant frequency range for a furnished living room.  It is therefore considered appropriate to calculate the standardised internal level – that is, referenced to a notional reverberation time of 0.5 seconds across the frequency range, for both living rooms and bedrooms.  Glazing areas and room dimensions are typically taken from the architect’s drawings.

Sound penetration is calculated through the weakest façade elements, usually the vents and the glazing, and then combined in each frequency band to give an overall internal level from the external sources by these routes.

Calculations are typically performed in the five octave bands between 125 Hz and 2kHz, as indicated in BS 8233, but the frequency range considered in the calculations may be extended if there is significant spectral content outside this range.  A single-figure calculation based on the weighted sound reduction index with a suitable spectrum adaptation term (e.g. Rw + Ctr) may be appropriate where the incident noise spectrum is well approximated by the normalised road traffic spectrum described in BS EN 1793-3.

Calculations are based on manufacturer’s test data for example glazing and ventilator products which together achieve the required internal room criteria.  Acoustic performance specifications for façade elements are determined based on an analysis of the incident noise spectrum and the spectral sound insulation performance of a range of glazing and ventilator products from our database of manufacturers’ test data.  The in-situ performance of a facade may be undermined by other sound paths, such as poor sealing around or between elements.

An example façade sound insulation calculation is shown below.

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Apex Acoustics Ltd Design Works, William St, Gateshead, Tyne and Wear NE10 0JP Head office: 0191 620 0750 Midlands: 0115 708 0750 info@apexacoustics.co.uk

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