Noise, Vibration and Acoustics Consultancy from the Institute of Sound and Vibration Research
Written by Bob Davis
Original version © 2002, ISVR University of Southampton. All rights reserved.
Three pilot projects were part-funded within the overall Project. Other projects were proposed but were not taken up by the companies concerned, because of financial or other constraints.
The projects were intended to demonstrate specific principles, and to assess the effectiveness of remedial measures which in some cases have not been systematically applied and evaluated elsewhere. They were not necessarily expected to provide a complete solution to a noise problem, and it was anticipated that some would reveal practical drawbacks which would limit their usefulness.
This is a foundry close to a residential area. The nearest houses share a common boundary with the yard area, beyond which is the finishing building, where castings are fettled and inspected. This building is of part-brick construction, with steel sheeting to upper walls and roof. There are three doors giving access to the yard. The finishing building also serves as a despatch area: castings in bins or on pallets are loaded on to vehicles in the yard by forklift truck, using one of the doors. The general layout is shown below.
An initial inspection revealed a number of noise sources, audible in the yard. All were intermittent in nature and most were caused by metal-on-metal impacts:
Average noise levels in the yard area, close to the boundary, were sometimes as high as 70 dB(A) when doors were open, and 60 dB(A) -62 dB(A) with doors closed. The noise was distinctive, with repeated impact noises (crashes and rumbles).
A package of works was carried out to demonstrate the effects of various noise reduction measures. The main objective was to reduce the number of metal-to-metal impacts involved in the various materials handling operations, and to apply treatments to reduce impact noise at source or by locally enclosing particular operations. Some means of limiting the time for which doors were left open was also considered essential: the existing doors were manually-operated and tended to be left open for long periods when forklift trucks were loading a vehicle in the yard.
The rapid-action doors have proved to be particularly effective and practicable, reducing door open times to a minimum. The finishing area does not generate significant heat, and there is local air extract from fettling benches, and no adverse effects on building ventilation have been observed.
The lining treatment to hoppers and inspection chutes reduces maximum noise levels (Lmax) from component impacts by 10 - 15 dB(A). The material is extremely durable and has reasonable low-friction properties, although castings sometimes have to be 'assisted' down the chutes from the inspection benches. The main drawback is cost, around £100 per square metre.
Noise from wheelabrator loading and unloading operations has been reduced by 5-7 dB(A) Lmax.
The bin-lifter device was effective in eliminating a tipping operation but lack of floor space precluded the widespread use of these devices, without major changes to layout.
Overall, average noise levels at the boundary have been reduced by around 5 dB(A), and the noise 'peaks' associated with the various impact sources are subjectively far less apparent. The major remaining source is impact noise from loading the hoppers at the fettling benches. Further work is in progress to construct a partial wall between the fettling and inspection areas to limit the spread of noise from this area into the inspection area, and to outside.
The various measures, although intended to demonstrate reductions in external noise, have also significantly reduced work-area noise.
|Contractors/suppliers:||Clark Door Limited|
|The Noise Control Centre|
|Polyurethane Products Ltd.|
This is a large site with two forge shops. The larger shop houses 6 hydraulic and gravity forging hammers and 3 forging presses. Billet heating is mainly by electric induction. There are two bar cropping machines in an adjoining shop. There is a history of complaints from residents who live 60 - 100 metres away from the large forge, mainly concerning noise in the early morning.
The forge building is mainly of corrugated sheeting on a steel frame, with some lower walls of brick. The roof sheeting incorporates translucent sheeting, to provide some natural light, and has open roof shutters and full-length ridge openings for ventilation. Access is through roller shutter doors. The layout is shown on the sketch below.
The main noise sources were identified as follows:
Previous noise surveys revealed average (Leq) typical noise levels of 60 - 62 dB(A) at the nearest houses. The impulsive character of the noise was clearly a major feature. Noise levels inside the main forge are 94 - 102 dB(A).
The high external noise levels result from the high noise levels inside the main forge and the poor sound insulation provided by the building. The feasibility of enclosing individual forging units, to reduce noise within the forge, was assessed. This approach was judged impracticable because of problems of access for production and maintenance. There was no prospect of reducing noise from the hammers and presses at source. The more obvious but potentially the most costly approach was to improve the sound insulation of the building.
Following preliminary costing exercises, this was the course of action adopted.
The building modifications (cladding and doors) were carried out successfully with little or no disruption to production.
Installation of the forge ventilation system involved some out-of-hours and weekend working for access to install high-level ductwork. It became apparent the ventilation was inadequate to maintain reasonable working temperatures close to the forging units during warm weather. This problem could not be overcome by local 'man-cooler' fans or portable evaporative cooler units. It was resolved by the addition of a powered extract system, together with uprating the supply fan, to provide an air change rate of 10 changes per hour. The retention of adequate ventilation to control internal temperatures is clearly a major concern, since to obtain significant improvements in sound insulation it is necessary to close off any unsilenced openings in the building.
Measurements after completion of works demonstrated that with doors closed, noise emitted from the forge building and cropper bay had been reduced to less than 48 dB(A) at the nearest houses, a reduction of about 14 dB(A). Actual noise levels remained above 50 dB(A), because of the steady noise from the heat exchanger fans. However, with these fans running the impact noise from the forging units cannot generally be distinguished, and the 'masking' noise they create is probably beneficial.
Compressor noise is not generally detectable beyond the site boundary.
|Contractors/suppliers:||UK Industrial Roofing Ltd.
|Clark Door Ltd.|
|Amber Doors Ltd|
This is a foundry with housing in close proximity on two sides. There is a history of noise complaints, most of which have been resolved by major works - installing a new cupola, improving buildings, and re-locating the stockyard. Some problems remain, including noise from the two shot-blast machines in the finishing shop, which is a lightweight building close to the boundary. It is sometimes necessary to work these machines during the evening, and the loud crashes during loading and unloading can be audible in residential areas (although it may be masked by noise from passing traffic). Within the finishing areas, short-term average noise levels during loading and unloading the shot blast machines are around 100 dB(A). There is also a more general problem of noise from materials being dropped into stillages, particularly in open areas.
The shot blast machine enclosures reduced noise levels from these machines, during loading and unloading, by 10-12 dB(A). However, maintaining this reduction has proved difficult because the enclosures, particularly the doors, suffer regular impact damage from forklift trucks. This is to some extent the result of limited access round the machines. made even more limited by the enclosures. More robust enclosures and local barriers would alleviate this problem, but probably not resolve it (and barriers round the enclosure would further restrict access).
The quiet stillages reduced maximum impact nose levels, when dropping a single casting, by up to 15 dB(A) Lmax. Average noise levels Leq,10s when dropping a load of scrap into the stillages were reduced by 7 - l0 dB(A). The quiet stillages are also noticeably quieter when carried on a forklift truck, unloaded - the characteristic metallic rattle at the stillage bounces on the forks is noticeably dulled. There is clearly scope for further development, and for consideration of adoption of a 'quiet' design as standard. The most practicable alternative to the standard single-skin construction is the double-skin spot-welded type. This would seem to have no significant disadvantages (apart from first cost) compared with the standard stillage. Other types are likely to be less robust. The 'woven' type, although very effective, appears more liable to damage and to components becoming 'hooked' into the mesh.
The double-skin technique can also be used effectively on chutes and tables which are subject to metallic impact. This form of construction is unlikely to be as effective as a resilient lining in reducing impact noises, but it has the advantage of being suitable for dealing with hot components.
|Costs (typical)||Shot blast machine enclosures: £20k|
|Quiet stillages (each) £200 - £300
|Contractors/suppliers:||The Noise Control Centre|
The CD or video tape which accompanies this handbook gives further information about these proje
Original version © 2002, ISVR University of Southampton. All rights reserved.