Gareth Townley Cordant Services Portland House Oak Green Stanley Green Business Park Cheadle Hulme Cheshire SK8 6QL
Our ref: P16-563-L01v1 25 November 2016
By email only:
[email protected] Dear Gareth, Re: Fox Restaurant, The Square, Hale Barns P15- Extract Ductwork Noise 069 Further to your recent instruction, I am writing with the findings of our investigation into break out of extract duct noise into apartments from the service riser. The noise assessment was requested following complaints from the tenants of apartments which adjoin the services riser on the 1st and 2nd floors of the building (i.e. Apartments 10 and 20). The service riser only supplies the Fox Restaurant unit on the ground floor. Hepworth Acoustics Ltd visited the building on 17th November 2016. We inspected the service riser on the 1st, 2nd and 3rd floors of the building. At this stage no access was available to either of the apartments on the 1st and 2nd floors. The service riser has ductwork for the kitchen extract and the pizza oven flue. The main source of noise inside the riser is the kitchen extract duct. Noise levels from the extract duct were measured inside the service riser on 1st to 3rd floors, which were similar on each floor. The measurements were taken with a B&K 2260 ‘Type 1’ sound analyser (serial no. 02467016). The analyser was check calibrated before and after the measurements with no variance noted. The results are shown in Table 1. Table 1: Noise level inside service riser from Extract Ductwork Octave Band Centre Frequency (Hz)
A
63
125
250
500
1k
2k
4k
84
86
81
72
65
60
54
76
Sound Insulation between the riser and the apartments Based on the as built drawings for the apartments (Leach Rhodes Walker Architects project no. 6926 drawing no. L(42)02 rev R), we understand the wall between the riser and the apartments is a twin stud metal frame construction consisting of 2 layers of 15mm British Gypsum ‘Soundbloc’ and 1 layer of 12mm Cement Particle
board on 100mm METSEC stud, 2 layers of 12.5mm ‘Soundbloc’ and 1 layer of 19mm British Gypsum plank on 60/70 Gypframe metal stud with 100mm ISOWOOL APR 1200 inside the cavity. Total width of wall construction is 377mm. However, during our site visit it was noted that although there appear to be areas of these walls where at least 2 layers of plasterboard are in place on the riser side of the wall. There are also significant areas where there is only one layer of board with visible gaps between the joints and little or no mastic used to seal these gaps. It was not possible to confirm if plasterboard was a British Gypsum ‘Soundbloc’, plasterboard plank or wallboard product or the thickness of these boards. Moreover, the actual construction of the rest of the separating wall could not be seen/checked. Calculations of the sound insulation performance of the wall construction shown in the as built drawings and also assuming that the riser leaf of the wall construction is only 1 layer of plasterboard (as it appears on site) have been carried out using Marshall Day INSUL sound insulation prediction software. This assumes that the rest of the separating wall construction is as per the drawings. The difference between the sound insulation performance of these two wall constructions is as much as 10 dB. This would indicate that the sound transmission from the service riser to the adjacent apartments (with the wall construction as it appears on site) is around 30 dBA. The noise level in the apartment maybe even higher as the gaps between the joints on the riser will further reduce the sound insulation performance of the wall. Furthermore, this does not take into account the influence of structure borne transmission of noise from service riser to the adjacent apartments. It was noted on site that although the extract fan unit in the 3rd floor service riser was mounted on anti-vibration mounts, other areas of the ductwork are directly mounted to the wall which is perpendicular to the party wall. Therefore, we recommend that ideally the wall is constructed as shown in the as built drawings. If this is not feasible then the gaps between the joints should be fully sealed up with plasterboard pieces with a significant overlap. The ductwork should be effectively wrapped with an acoustic lagging material, we recommend CMS Danskin L10 SuperLag AcoustiSheild (10kg/m2). This will not provide the same improvement as building the wall per the as built drawings. Further to these works (either constructing the wall as per the as built drawings or sealing up gaps and lagging the ductwork) the ductwork should also be mounted on anti-vibrational mounts. Service Riser Access Doors The existing access doors from the communal areas to the service riser on 1st to 3rd floors are 54mm solid-core hardwood construction with seals around the sides of the door. It was noted on site that in some areas around the door frame the seals were not flush with the door edge. In order to improve the performance of these doors, the existing seals should be made good i.e. to ensure there is an effective seal between the door frame and door edge all the way around. Also, the doors could be fitted with effective threshold drop seals. These measures could improve the performance by as much as 10 dB. Yours sincerely for Hepworth Acoustics
David Thurstan BSc MIOA Senior Consultant
