Five overhead reciprocating presses with ratings ranging from 40 to 120 tonnes were located within a small batch production press shop measuring 15 m long x 10 m wide x 8 m high. The internal surfaces were acoustically reflective, with a concrete floor, blockwork walls and a steel clad roof. The A-weighted noise level was found to be in excess of 90 dB.
The presses were used to produce quite small batch runs and so had an unusually high down time. As a result, an operator's overall noise exposure was strongly influenced by the noise from neighbouring presses.
It was inappropriate to enclose the presses because of the need to feed strips manually across the tool and to change the workpiece. The reflective finishes within the shop resulted in a low rate (approximately 3 dB) of reverberant fall off between one operating press and an idle press on the other side of the shop.
The presses were positioned very close to the wall. Reflection of sound from the wall acted as a major contributor to the reverberant sound pressure level. As a result, the absorptive treatment was concentrated on the walls. An acoustic lining consisting of 50 mm thick 45 kg/m3mineral wool, retained behind a galvanised perforated steel sheet, was fixed to the walls between 1 and 4 m above the floor. The mineral wool was wrapped in plastic film to keep oil mist out.
Three mobile acoustic screens, 2 m wide and 2.4 m high, were also used to reduce direct line-of-sight radiation between presses. The screens were faced with mineral wool retained behind perforated sheet steel to prevent the reflection of sound energy.
£8000 for the absorptive lining and £2000 for the mobile screens. (1995)
The acoustic cladding to the walls increased the reverberant fall off across the press shop from 3 to up to 11 dB and the mobile screen gave approximately 3 dB additional protection.
Photographs courtesy of John Crane UK Limited. Equipment manufactured by Ian Sharland Limited. Acoustic consultants were Ajax Health and Safety Services.