Background
Griffiths Green Enterprises Ltd, located in rural Shropshire, operates a biomass boiler plant converting poultry manure into energy. As the plant was a relatively unprecedented piece of machinery, the operational profile presented unique acoustic challenges.
Following the installation of this plant at Oaklands Farm, concerns were raised by a nearby residential property regarding excessive noise levels. Anderson Acoustics was commissioned to assess the noise impact and recommend mitigation strategies to ensure compliance with relevant standards and to address community concerns.
Challenge
Initial assessments identified that noise emissions from the plant, particularly from the control room extract fan and auger pipes, were audible at the nearest noise-sensitive receptor, approximately 310 meters from the plant.
The cumulative rating level of these sources exceeded the background sound level by up to 11 dB, indicating a significant adverse impact according to BS 4142:2014+A1:2019. The challenge was to reduce plant noise to levels that would not adversely affect the local community, while maintaining full operational capacity.
What We Did
Comprehensive Assessment:
- Conducted both short-term attended and long-term unattended sound level measurements at the site and at the receptor location;
- Identified the main noise sources: the control room extract fan (with a low-frequency tonal component) and the auger pipes (producing a grinding sound due to ash build-up);
- Used 3D noise modelling (CadnaA) to simulate the propagation of noise and evaluate the effectiveness of potential mitigation measures.
Mitigation Strategy:
- Recommended the installation of a circular in-duct attenuator on the extract fan to reduce noise at the source;
- Proposed acoustic encasement of the auger pipes with dense barrier lagging and steel cladding to achieve at least a 5 dB reduction;
- Explored the feasibility of an earth bund as a physical barrier. Modelling showed that an 8.5 m high bund would provide a perceivable reduction in noise at the receptor;
- Advised redirection of the extract fan away from the receptor and installation of the earth bund along the site boundary to obscure line of sight.
Implementation:
- The earth bund was constructed along the eastern boundary;
- The extract fan was redirected from facing west (toward the receptor) to facing south;
- Post-mitigation noise surveys were conducted at proxy locations due to access constraints.
Result
The post-mitigation assessment demonstrated a measurable reduction in noise levels:
- Daytime noise levels at the receptor were effectively 2 dB lower, and night-time levels 1 dB lower than pre-mitigation measurements, after accounting for distance corrections;
- 3D noise modelling confirmed these reductions, showing the earth bund and fan redirection were effective.
