BS4142 Noise Impact Assessment & Noise Control - Case Study 1
Noise Impact Assessment to BS4142 and Noise Control
Noise Impact Assessment
Recently, NCSL acoustic consultants were involved in a project involving a retrospective Noise Impact Assessment (BS4142) and subsequent investigation and noise control proposals.
The project in question was conducted in the Lake District, where the small villages are known for tranquillity and low background sound levels. In the village in question, the water supply is provided by a water pump system which is fed by a small water treatment plant.
As the water pump system runs continuously, a complaint of noise had been made by a local resident who’s property was in close proximity to the treatment plant and water pump.
Our acoustic engineers were commissioned to undertake a noise impact assessment as the first stage of the project. This was needed to define if the noise complaint was legitimate.
As per the process defined within BS4142, a background sound level survey was conducted at the complainant premises for 24-hours. This initial survey was conducted with the water pump shut-down, with a second noise survey conducted with the pump operational. This allows for the background noise level to be defined, as well as the specific sound level of the pump.
These noise measurements resulted in a background sound level of 24dB(A) and a specific sound level of 42dB(A).
BS4142 Criteria
Noise Impact Assessments to BS4142 also allow for a penalty to be applied for tonal and impulsive sounds. In this case, a penalty of +4dB was applied for the tonal sound emitted by the water pump system.
This resulted in a rating level of 46dB(A) – which is 22dB above the existing background sound level. BS4142 suggests the following:
- In instances where the rating level exceeds background noise level by +10dB, this is an indication of a significant adverse impact, depending upon the context.
- In instances where the rating level exceeds background noise level by +5dB, this is an indication of an adverse impact, depending upon the context.
- The lower the rating level is relative to the measured background sound level, the less likely it is that the specific sound source will have cause and adverse noise impact. Where the rating level does not exceed the background sound level, this is an indication of the specific sound source having a low impact, depending on the context.
Clearly in this case, the complaint was legitimate, based on the noise impact assessment in accordance with BS4142.
The next stage for our noise consultants was to assist the client find the cause of the noise and a potential solution.
Firstly, acoustic measurements were undertaken to define the frequency range of interest. This was completed with assistance from our friends at Acoustic Research Consulting (ARC). ARC conducted simultaneous acoustic and vibration measurements of the pump systems – defining a noise frequency of 100Hz. This is a very narrow tonal peak, with no significant harmonics.
As the water pump system was actually made up of 2 individual pumps, we looked at the phase relationship between the two. The plot below shows the acoustic phase, as measured by surface accelerometers.
As you can see, the pumps are radiating noise in-phase, which is likely to result in increased noise output. NCSL suggested that the phase of the pumps should be staggered (180 degrees) in order to take advantage of phase-cancellation. However, this was not possible due to the electrical circuitry.
Acoustic Materials
Acoustic testing was undertaken with acoustic lining material introduced around the pump systems. This did reduce the noise emissions by approximately 6dB, however concerns were raised about sufficient airflow to the pumps.
Vibration Isolation
Our vibration experts also ran trials with the introduction of vibration isolation materials (rubber mounts) beneath the pumps. This effectively provides a resilient layer and isolates the pumps from the floor. This made negligible improvement, and it is therefore likely that the noise transfer is not structure or ground borne.
Enclosure Vibration
Surface vibration measurements were taken of the plastic housing which encases the water pump. This shows that the casing has high vibration levels at 100Hz and is likely radiating noise.
Due to the high vibration levels of the housing, it was decided to undertake modal testing of the structure. Modal testing allows acoustic engineers to define the natural frequencies of structures and the corresponding mode shapes. Where a natural frequency and a forcing frequency clash – this is known as a resonance condition and high noise levels will result.
As shown in the colourmap below, the structure exhibits modal peaks around the 100Hz frequency band. This is likely to be a key cause of the noise output, as a resonance condition exists.
Noise Control Solutions
At this stage it was clear as to the main cause of the noise issue and the probable acoustic transfer path. The pumps, radiating noise in-phase were generating an acoustic pressure field within the enclosure. The enclosure was then set into vibration (easily, due to the modal response at 100Hz) and noise was then re-radiated into the environment.
This theory was easy to prove out, with acoustic measurements taken with and without the enclosure fitted. Although it is counter-intuitive, the noise level actually reduced by 9dB when the enclosure was removed. Proving that the theory proposed by out acoustic engineers was accurate.
With this finding, two solutions were proposed. Firstly, the modification of the existing enclosure, introducing bracing and stiffening features in order to increase the natural frequency and remove the resonance condition. This improved the output noise by 3dB, but still not enough to meet the requirements of BS4142.
The second solution was a re-design of the enclosure. NCSLs acoustic consultants worked closely with the client to design a high-performance acoustic enclosure, including material specifications and closure details. Internal acoustic materials were also specified by NCSL engineers. Once constructed, a second noise impact assessment to BS4142 was undertaken at the complainant premises. This time, the specific sound level was measured at 22dB(A) – a 20dB reduction with the new enclosure design.
Due to the inaudibility of the pumps at the complainant premises, the +4dB correction could also be removed. This resulted in a difference between the background sound level and the rating level of -2dB. The result of this BS4142 Noise Impact Assessment was therefore acceptable, and the planning application was approved by the local authority.
If you feel that NCSL can help with your noise impact assessment, or other acoustic consultancy services, then please feel free to reach out.