During the current lockdown situation, we thought it may be of interest to share a case study from one of our recently completed noise control projects.
NCSL were commissioned by a pub in the North West following a complaint of noise from a neighbouring property. The pub had recently upgraded the kitchen extract system in order to provide increased airflow to the kitchen areas. Due to the coincidence of the new extract system along with the complaints, this was believed to be the source of the issue.
The complainant was observing a low-frequency hum, clearly audible when the pub was open. In conjunction with the local council, it was estimated that the hum was centred around the 125 and 160Hz one-third octave bands. The most significant time of disturbance was indicated to be early in the morning (when the pub was opening) and late at night (just before the pub closed), because of the relatively lower background noise levels in the area.
From this information, NCSL devised a schedule of testing.
The first task was to understand the level of disturbance, and if basis for a complaint existed. This was conducted using a method based upon the BS4142 protocol which requires the measurement of the background sound level and comparing this to the ambient sound level when the noise source is active. In order to complete this, NCSL took background sound level measurements before the pub opened and after closing, and measured the ambient sound (with the noise source active) during the interim period. From this data, it was shown that a significant noise impact was present and the complaint was legitimate.
The next stage of the investigation was to understand the source of the noise. It was believed to be due to the extract system, but we have seen many instances where significant time and money has been spent on a potential noise source, only to find that it was not the actual cause of the disturbance.
NCSL analysed the acoustic data taking during the background and ambient sound level measurements and found a significant tonal component at 141.5Hz. This correlated with the details provided by the local council and the complainant in terms of frequency range and time of the disturbance.
Now that the specific frequency had been identified, it was possible to track down the source.
Close proximity acoustic measurements were taken at the terminations of the ventilation system ducts (of which there were three), and it was discovered that a strong tonal noise at 141.5Hz was clearly present. As an overcheck, the ventilation system was deactivated and the tonal noise was eliminated.
So, NCSL had defined the level of the noise impact, the frequency range of interest and the source. The final step was to find a solution to reduce the noise level.
The first issue was understanding which of the three vent terminations was the biggest contributor to the noise, or weather all three contributed equally. In order to define this, two measurements were undertaken. Firstly, acceleration measurements of the ducts to define the most significant structure-borne radiating surfaces.
Secondly, a coherence measurement between each vent termination and the complainant premises (coherence measurements allow us to define the amount of causation between a source and receiver). From these measurements, one duct in particular was identified as the most significant contributor by far.
In order to minimise cost to the client, it was recommended that palliative treatment was applied to this vent only at first, before a re-assessment at which point treatment of the other two vents may be required.
The treatments recommended were three-fold. Firstly, a muffler system should be installed within the identified vent. Secondly, the identified termination should be re-oriented so that it faced empty fields rather that domestic premises. And thirdly, a timber fence should be installed around the terminations to act as a barrier while not restricting air flow.
Once these measures were in place, a second measurement was conducted at the complainant premises and feedback was sought from the complainant and the local council. It was agreed that a significant improvement had been achieved and that no further work was necessary.
As you can imagine, this was a great result for the pub owner (our client) who had managed to fix the issue quickly and relatively cheaply with assistance from our acoustic consultants.