Alan Nethersole, senior acoustic consultant with Lee Cunningham Partnership, argues that noise control has moved forward, yet duct-attenuator performance is still rooted in the past
THERE have been fears in the noise-control industry about the accuracy of published data on the performance of duct-attenuators.
Being able to test duct-attenutor performance more accurately - at a new test facility - may go a long way to assuaging those fears.
The current test standard for duct-attenuators is BS EN ISO 7235:2003 Acoustics - laboratory measurement procedures for ducted silencers and air-terminal units - insertion loss, flow noise and total pressure loss.
Initially published in 1995, the 2003 version incorporated a number of major improvements.
Despite this, many UK attenuator manufacturers still provide data based on BS 4718:1971, which does not have the same extent of strict validation or degree of accuracy.
Test facility
Two years ago Lee Cunningham Partnership was commissioned by its sister business, Caice Acoustic Air Movement, to build an ISO 7235 test rig and, with technical assistance from Dr Ray Kirby of Brunel University, this was commissioned at the end of 2007. Lee Cunningham is now in the process of gaining UKAS accreditation for the test facility.
The facility recently tested duct-attenuators from each of the four leading manufacturers in the UK. Each attenuator was identical in size and had the same internal splitter and airway configuration. All four performed similarly and fell short of the performance claimed by the manufacturers.
It may, therefore, seem surprising that we don't get more noise problems on site because of under-performance of duct-attenuators. However, three factors come into account here:
-The acoustic calculations used to determine the required duct-attenuator insertion-loss performance are probably safe
-Further safety margins are then added before selecting the duct -attenuator
-Commissioning of noise levels after project completion is not always carried out and minor errors of 3-4dBA would not be sensed by the occupier
How can tested duct-attenuator performance be different from published duct-attenuator performance?
This latest test standard is more accurate and has managed to address a number of measurement issues that could lead to inflated performance. A loophole has also been closed in the insertion-loss calculation which, again, could lead to unreliable data.
It is impossible to test the full range of duct-attenuators and, therefore, interpolated or extrapolated data is used accordingly. However, unless the rules for expanding the data are applied properly, then performance across the range can be wildly adrift.
Manufacturers have tended to adjust their duct attenuator performance upwards, as they know the calculations are safe and problems rarely occur.
We believe it is time for the noise-control industry to move forward and for all attenuator manufacturers collectively to improve the accuracy of their published duct-attenuator performance data.
Lee Cunningham Partnership can help by making the test facility available to attenuator manufacturers. This would be particularly useful for smaller companies, as the cost of building a facility of this kind is prohibitive. Although we would provide a technician to give guidance on the testing procedures, each company would be able to undertake their own testing.
Improved confidence
In addition to static insertion losses, the test facility will provide dynamic insertion losses, aero-dynamic pressure losses, and flow-generated sound-power levels at a number of forward- and reverse- airflow rates for each attenuator.
Lee Cunningham can also provide guidance on how to accurately interpolate and extrapolate test data from a relatively small sample of attenuators across an attenuator range.
This could lead to improved confidence in the industry that attenuator performance data can be trusted within acceptable tolerances, wherever you source the product.