Acoustic Matters

Acoustic issues are increasingly important with the stringent requirements of BB93 and the dilemma faced by many Designers of the need to have openable windows, which themselves are prone to noise ingress.

It is considered that by drawing air from above roof level, that in itself avoids the problems of traffic noise as compared to low level windows but the Monodraught design, with the relatively large internal ducts in a cruciform design, lends itself ideally to the application of acoustic material

System	Dimension A	Dimension B
800	800 x 800	1100
1000	1000 x 1000	1240
1200	1200 x 1200	1375
1500	1500 x 1500	1585

In January 2005, Monodraught commissioned the Building Research Establishment (BRE) to carry out a series of airborne acoustic tests on the full range of Windcatcher systems, together with various insulation thicknesses.

The BRE established that in its standard format, the style of the Windcatcher system has the effect of reducing noise transmission by 15dB as compared to an open window. However, by incorporating 25mm of acoustic lining to the air paths, a further 11dB was achieved. The chart shows that on larger systems, a greater thickness of insulation foam has the effect of greater attenuation.

Test Number	D n,e,w (C;C tr) (dB)	Construction Details
9	15 (0;-1)	GRP 800 Square system - VCD Open
100	30 (-1;-2)	GRP 800 Square system - VCD Closed
11	26 (-1;-3)	GRP 800 Square system 25mm insulation foam - VCD Open
12	47 (-1;-5)	GRP 800 Square system 25mm insulation foam - VCD Closed
13	24 (-1;-4)	GRP 1000 Square system 25mm insulation foam - VCD Open
14	39 (-1;-5)	GRP 1000 Square system 25mm insulation foam - VCD Closed
15	31 (-1;-5)	GRP 1000 Square system 50mm insulation foam - VCD Open
16	47 (-2;-7)	GRP 1000 Square system 50mm insulation foam - VCD Closed

VCD = Volume Control Dampers
Foam is as follows:
25 mm open cell polyurethane foam (28 kg/m³) lining the trunk, capping and internal airways
OR
50 mm open cell polyurethane foam (28 kg/m³) lining the trunk, capping and internal airways


Side view of Windcatcher	Plain view of Windcatcher

Acoustic pods or other forms of attenuation can also be suspended below the Monodraught terminals to achieve an even greater impact on sound insulation.


Read More >> 3.15MB BRE Acoustic GRP Windcatcher of 29 Jan 2002 600 Report (24^th March 2005)		Read More >> 1.03MB BRE Acoustic Lined GRP 800-1000 Windcatcher Report (29^th Jan 2002)

Conclusion

For a typical classroom served by 1N° GRP 800 square Windcatcher system (or equivalent rectangle of similar cross sectional area), there is a 15dB reduction in noise transfer with an unlined system, (test 9). With the addition of 25mm of insulation (test 11) the reduction increases by 11dB to 26dB.

Further reductions can be made by increasing the insulation thickness to 50mm, but the system size will need to be increased in size accordingly to 1N° GRP 1000 square system. 1N° GRP 1000 square system with 50mm of insulation achieves 31dB reduction (test 15)