In 2015 dbx Acoustics Ltd was approached by Renewables First to assist with a contentious planning application for an Archimedes screw micro generation hydro scheme in Eaton Socon, Cambridgeshire. The applicant wanted to install a turbine to generate power for their home and the adjacent pub, but based on their observations of the twin screw in nearby Bedford, local residents raised concerns about the noise impact of the scheme.
dBx undertook a thorough survey and investigation to ensure we could accurately model and predict noise from the proposed turbine including the low frequency ‘slap’ of water exiting the base of the screw. We made measurements at turbines across the country (including Bedford) in different flow conditions in order to obtain typical sound power data in third octave bands for both the screw and the machinery house, as well as reviewing the limited manufacturer’s data available.
A 72 hour noise survey was carried out at and around the proposed site, to ensure the most peaceful periods of the existing ambient noise climate were captured. This allowed for a ‘worst case’ assessment of potential noise impact as residents are more sensitive to any new noise source when other sources – such as traffic, commercial use and plant – are at their quietest. The survey included measurements at the residential facades and amenity areas during both weekday and weekend periods.
Next we created a model of the site and surroundings. This was made as accurate as possible, with topography, buildings of the correct height, areas of acoustically reflective surfaces (tarmac, water) and acoustically absorptive surfaces (grass, woodland) included. The proposed turbine and engine house was modelled in position, as a sound power source based on the data gathered from other similar sized turbines. Separate sources were modelled for the end of the turbine and its length, as well as for the machinery at the turbine head.
The aim was to carry out a BS 4142 assessment for noise from the turbine as received at the residential properties, demonstrating the rating level for noise from the turbine would be no more than 5 dB below the night time background noise level at the receptors. The rating level included corrections for ‘acoustic features’ and in this case it was decided to apply a +9 dB correction for the repetitive ‘thumping’ of water exiting the turbine.
To control noise to within the agreed limits, it was necessary to introduce mitigation measures to the design. This included a concrete wall to the west of the turbine enclosure, extending into the mill pond beyond the end of the screw, to provide additional screening to properties to the west. Unlike the other turbines observed during our investigations, the decision was made to fully enclose the screw, and a sound insulating sandwich construction was developed to control noise breakout while allowing access for maintenance.
Finally, an innovative rubber curtain design was developed to cover the opening at the base of the turbine and provide additional screening for water impact noise. This was designed to allow debris to pass through while remaining closed for most of the time in order to contain noise.
With these measures in place, it was possible to demonstrate that the BS 4142 rating noise level for sound emitted from the proposed turbine would be at least 5 dB below the lowest background noise level at the residential properties. As such no adverse noise impact is predicted.
To provide additional comfort, dBx Acoustics carried out a frequency analysis of the measured and predicted noise to demonstrate there would be no particular frequency content to the sound at the receptors. Finally, our report included detailed advice for the Client on the proposed mitigation measures, including the sound insulation performance of the enclosure, plant housing, and curtain.