Peninsular Acoustics was instructed to carry out a noise assessment to investigate a noise complaint relating to a commercial kitchen extraction system serving a restaurant along South Road, Liverpool. Environmental Health had received complaints from nearby residential properties regarding noise disturbance from the extraction system.
A noise impact assessment was required to determine whether the operation of the extraction system complied with the requirements of BS 4142:2014+A1:2019, which assesses the impact of industrial and commercial sound on residential receptors.
Assessment Approach
Peninsular Acoustics undertook an environmental noise assessment to establish the likely impact of the extraction system on nearby residential properties.
The assessment included:
- Unattended environmental noise monitoring over several days to establish representative background sound levels during both daytime and night-time periods.
- Attended measurements during operation of the kitchen extraction system to identify the sound level and characteristics of the source.
- Detailed calculations based on ISO9613-2 – Acoustics: Attenuation of sound during propagation outdoors – Part 2: General method of calculation, to predict noise levels at the nearest residential façades.
The time history of the continuous measurement position showed clear on / off conditions in relation to the ventilation stack as see in the following chart.
Following assessment at the receiver, the results indicated that the rating level from the extraction system exceeded the background sound level at a nearby receptor by up to +9 dB, representing a Significant Adverse Impact under the BS 4142 assessment methodology.
As a result, mitigation measures were required to reduce the impact of the system and address the concerns raised by Environmental Health.
Recommended Noise Mitigation
Following the noise assessment, Peninsular Acoustics recommended a mitigation strategy to reduce noise emissions to within acceptable levels.
The recommended measures included:
- Acoustic enclosure surrounding the external fan unit to reduce direct noise transmission towards nearby residential windows.
- Inline duct silencers to reduce exhaust noise from the system.
- Acoustic lagging to external ductwork to reduce breakout noise from the metal ducting.
- Vibration isolation mounts to minimise the transfer of vibration from mechanical equipment entering the building structure.
These measures were designed to significantly reduce both airborne noise and vibration associated with the extraction system.
Implementation
The client proceeded to implement some of the recommended mitigation measures, including the introduction of a silencer and acoustic lagging of the exposed ductwork.
Peninsular Acoustics provided technical support during the installation process and undertook verification measurements following completion of the works.
Outcome
Following implementation of these mitigation measures, noise levels from the extraction system were significantly reduced.
The mitigation measures achieved a reduction in sound level of approximately –12 dB LAeq. This resulted in the rating level of the plant in operation being below the background sound level at the residential receptor and demonstrating a Low Impact in accordance with the requirements of BS 4142.
The results were accepted by Environmental Health, allowing the restaurant to continue operating without further enforcement action.
Figure 1: BS 4142 Initial Numerical Impact Assessment
| Assessment Step | Before Treatment | After Treatment |
|---|---|---|
| Specific Sound Level at Receptor, dB LAeq, T | 50 | 38 |
| Acoustic Feature Correction, dB | 3 | 0 |
| Rating Level, dB LAr, Tr | 53 | 38 |
| Background Sound Level, dB LA90, T | 44 | 44 |
| Rating over Background, dB | +9 | -6 |
| Estimation of Impact | Significant Adverse | Low Impact |
