Introduction

In the summer of 1992, the client requested an investigation into environmental noise emissions from one of their many Service Centres. Adverse local community reaction had been receieved from nearby residents at night prompting the involvement of regulatory authorities to request immediate company response to the complains. Unfortunately this coincided with seasonally high night time background noise levels which had frustrated their own investigations of the problem.

The client produces rolled profile steel sheeting, and rolled structural members to fulfil individual customer orders. The sheeting is roll formed from flat strip, and guillotined to length. The cut sheets are stacked, strapped, and dispatched to the product loading area.

Investigations - Field Measurements

It was found on visiting the site that as evening closed in, the local noise level would rise due to the noises of crickets and cicadas. The cutting of profiled sheet could be clearly heard, but was undetectable as a single value Aweighted noise level. The Aweighted noise levels from equipment operating in the plant were below the A-weighted background noise levels due to insects.

Due to potential impulsive and tonal environmental noise components, it was decided that real time frequency analysers were required to investigate the plant noise emissions.

Third octave measurements were made at close proximity to each item of equipment during the day. Later that night a second set of measurements were made from the rear boundary of the nearest residences. Multiple measurements were made of the general insect noise when the plant was silent, and again when plant activity could be perceived. Spectra were captured whenever the real time analysers showed a clear indication of plant noise emissions within the general insect noise.

Back at the office, the spectra were analysed to extract the plant equipment noise spectra from the background insect noise. Generalised insect noise spectra was developed from the field measurements to match the noise levels occurring at the time of the individual equipment noise events. "Clean" equipment spectra were obtained by computer manipulations of field and equipment noise spectrums.

The equipment noise spectra taken in the plant were modified for distance, ground, and air absorption. These were then compared to the "cleaned" spectra to confirm identification.

It was found that in the regions of the spectra obscured by insect noise the contribution to the overall A-weighted noise level was negligible, hence it was possible with confidence to provide the client with a list of noise contributions for each major item of equipment, and to assess the actual exceedances the plant exceeded its operational criteria.

Without the use of real time frequency analysers it would have been impossible to extract the individual equipment noise events from the rapidly fluctuating background noise of insect. Using conventional equipment, measurements would have either, had to have been delayed a further three to four months until seasonal insect abated, or the plant would have had to have been computer modelled.

Investigations - Noise Modelling

During the investigations it was noted that closure of the ten heavy vehicle roller doors facing the local community, "fully" sealing the building, did not reduce the perceived noise at the residences. Logic would dictate that there should have been a reduction of at least 10dB, but this did not occur.

Complete structural building plans were examined, and a crude computer noise model was constructed for the building to provide guidance as to the best methods of controlling the escaping noise. This model was based on the third octave noise levels measured within the building, and estimated transmission loss values for each element of the building. The computer model was run for a range of sceneries, from the current situation, through a range of potential modifications to both the building and to operational procedures.

The computer model showed that the acoustic energy was escaping almost equally via the roof vents, the vehicle doors, and the translucent wall and roof panels. There had not been any intergration of acoustic acoustic considerations into the original overall structural design. The critical importance of acoustic designs at initial design stages of a project to avoid costly and difficult operational rectifications became very apparent when the client was informed of the retrofitment and necessary building changes to reduce the environmental noises to a satisfactory level.

Conclusions

The computer model and the field measurements clearly showed that the process equipment noise from the plant exceeded the local noise criteria. It indicated that even if the entire building were to be sealed, and all panels upgraded to the most sound resistant wall panels, the noise emissions would still be well over the local noise criteria.

Noise control will be required on each of the six sheet forming machines as well as the building. Structural changes will be required for each machine, the building, and a complete rethinking of product handling techniques.

The client was presented with a strategy of dealing with the total noise reductions by a combination of building modifications, machinery modifications, and alterations to product handling.

The client is currently slowly implementing the above strategy.

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