Heat Pumps Today

www.acrjournal.uk/heat-pumps 17 T E C H N I C A L Richard A Collman BSc (Jt. Hons), CEng, MIOA, Tech IOSH is Managing Director of Acoustical Control Engineers Ltd and Acoustical Control Consultants Ltd having joined the company in the 1980s and has specialised in the measurement and assessment of sound from industrial and commercial plant for over 35 years. He pioneered the use of digital instrumentation for short duration consecutive logging techniques. As an expert on sound from refrigeration and air conditioning plant he represented the Institute of Refrigeration on BSI committees responsible for various acoustic standards. Mike Hewett MIOA is Principal Acoustician with Acoustical Control Consultants Ltd having joined the company in 2021 bringing more than 30 years’ experience of Acoustic consultancy. Mike’s particular expertise is in the assessment, prediction and control of noise and vibration from structures, plant and equipment. He is a former examiner for the Noise Control Engineering module of the IOA Diploma and a former Secretary and Chair Noise and Vibration Engineering specialist group. Graph 1 shows the time varying levels of overall sound and the three most significant one third octave bands measured in an apartment at one of the premises during various stages of the work. The first series was measured once the initial vibration isolation work had been completed. Although this had achieved some improvement, the overall level remained very high, with significant amplitude modulation, particularly in the 500 Hz band. This was when the ground loop pipe concreting was identified as a significant transmission path. The pipes were therefore cleared, following which, the temporal modulation virtually ceased leaving the overall level steadily around 35dBA, due primarily to the 100 Hz band. Further adjustments were made to the vibration isolation systems which reduced the 100 Hz level by around 7dB. The corresponding overall dBA level fell but was then more aected by residual sound so did not show as great a reduction. Finally, one further fixing was found and removed providing a further improvement. It should be noted that the final 100 Hz band shows some temporal amplitude modulation, although the level is suciently low that this is not significant. Other sites At another site, there remained a problem after similar work had been completed. Initial investigations had already included checking that pipework outside the plantroom was not a significant 5 10 15 20 25 30 35 40 45 50 55 60 65 00:00:00 00:00:05 00:00:10 00:00:15 00:00:20 00:00:25 00:00:30 00:00:35 00:00:40 00:00:45 00:00:50 00:00:55 00:01:00 00:00:00 00:00:05 00:00:10 00:00:15 00:00:20 00:00:25 00:00:30 00:00:35 00:00:40 00:00:45 00:00:50 00:00:55 00:01:00 00:00:00 00:00:05 00:00:10 00:00:15 00:00:20 00:00:25 00:00:30 00:00:35 00:00:40 00:00:45 00:00:50 00:00:55 00:00:00 00:00:05 00:00:10 00:00:15 00:00:20 00:00:25 00:00:30 Sound pressure level (dB L A re 20 µ Pa) Elapsed time (s) Graph 1 Time varying levels of overall sound and the three most significant one third octave bands measured in an apartment at one of the premises during various stages of the work. 160Hz 315Hz 500Hz dBA Vibration isolation installed (315 & 500 Hz modulation driving overall level) Fixing found & removed (160 & 500 Hz furhter attenuated, some amplitude modulation now apparent in 160 Hz but of little subjective significance) Vibration isolation further adjusted (160 Hz attenuated, residual sound significant component of overall level) Concrete cleared from around Ground Loop pipes (315 & 500 Hz significantly attenuated, 160 Hz driving overall level) transmission path. It was then found that a steel column within the plantroom may have been compromising the attenuation system’s performance. The concrete was broken out from around the column and the level and character of sound in the apartment became demonstrably suitable. However, the resident remained dissatisfied. It was identified that sound from the plant was most noticeable close to one wall, so the wall lining was removed and replaced with one that was decoupled from the structure, providing a further slight improvement for the resident. At a dierent site, although the sound level was low, its tonal nature meant it was still slightly intrusive, although the resident was happy with what had been achieved. Further testing showed that the GSHPs were generating standing waves within the plantroom, resulting in relatively high levels of tonal sound that was then breaking through the ceiling/floor slab into the apartment above. Rather than trying to increase the slab’s sound reduction index, a better solution was to install some strategically placed absorption within the plantroom to reduce its modal response. This achieved a further improvement as predicted. Some of the residents are still disturbed by sound which has been reduced to well below standard criteria (BS 8233, NANR45 etc). This may in part be due to the very low residual sound levels in the flats but also to a degree of hyper-sensitivity and sensitisation in the residents. As the isolation/attenuation schemes were installed, adjusted and refined the sound reductions achieved were significant and most of the residents felt that the results were satisfactory. However, inevitably at some of the sites, plant noise was still present to some degree. Options for further reductions, within the constraints of the sites, are limited as there are potential transmission paths and issues which cannot be practicably addressed. For example; there is a possibility that there are further transmission routes between the underground ground loop pipework and the building. The pipes may pass close to, or touch the foundations, or vibration may even pass through the ground. Access to these pipes is not possible. There are similar potential problems with working on pipework in inaccessible parts of the building. There is therefore a practical limit to what further reductions can be achieved. Figure 13 Graph of progress in sound reduction