Annual Report 1996-1997
Every year the Institute of Sound and Vibration Research publishes its Annual Report. These are the chapters on ISVR Consultancy Services and the Automotive Design Advisory Unit for the year May 1996 - April 1997.
ISVR Consultancy Services
Manager Mr R A Davis
ISVR Consultancy Services is a self-funding advisory unit which carries out short and medium term consultancy and research for a wide range of clients in the public and private sectors. The unit operates on a commercial basis, with full-time engineering and support staff, and is a Founder Member of the Association of Noise Consultants (ANC). The Unit maintains close links with ISVR Research Groups, and a number of projects are undertaken in collaboration with academic and research staff.
Summary of Activities
Consultancy projects are client-confidential. However, the following general descriptions of projects in progress during 1996-97 illustrate the range of work undertaken.
The assessment of noise from industry, transportation and recreational activities continues to form a significant proportion of the Unit's work. In many cases, these assessments are made in connection with planning applications or potential legal proceedings for noise nuisance. Senior staff are widely experienced in presenting evidence at Public Inquiries and in the courts.
Projects include the assessment of noise from an open-air opera festival, from motor-sports activities including a 'banger racing' track, and from a proposed new narrow-gauge railway in Dorset. Industrial premises and operations investigated included drop forges, foundries, an electricity sub-station, concrete block-making plant, a large gas-turbine generating station, and the proposed re-opening of a disused airfield for the operation of an aircraft maintenance facility.
Noise and Vibration Control Engineering
Several projects have involved the practical application of noise and vibration control techniques. These include design support to a major supplier of acoustical packages for gas-turbine powered generator and compressor sets, the redesign of vibratory conveyors used in the food industry, and the specification of enclosures for a mobile aircraft refuelling rig and for plastics extruders. There has been a significant increase in the number of marine projects: the unit has advised on noise reduction on a cruise liner, a chain ferry, a Majorca-based luxury motor yacht, and is also involved in the rebuild of the classic J-class yacht 'Velsheda', currently in progress in Southampton.
Structural Dynamics and Computational Acoustics
A major study has been carried out to develop an analytical model of airframe noise generated by large commercial jet aircraft. Airframe noise is now becoming a significant factor in determining noise levels in the approach condition. The model has been validated using data from scale tests in an anechoic wind tunnel. An experimental and theoretical investigation was performed on a small commercial turbo-prop aircraft to develop interior trim modifications to reduce interior noise.
ISVR Consultancy Services has considerable experience in the application of ISVR-developed software for the prediction and optimisation of the performance of reactive silencers. This has been used for the design of a novel silencer intended to resolve a low-frequency noise problem from a large diesel-powered generator station, where space constraints ruled out a conventional solution.
The Unit continues to provide input to the European Space Agency. ISVR Consultancy Services was a sub-contractor to the NIMBENTOM (Noise Identification Models and Benchmark Testing of Mechanisms) study undertaken by MMS France. The objective was to continue work to classify the vibration characteristics of mechanisms and to develop adaptive filters to permit automatic recognition of mechanisms and monitor their condition on in-orbit spacecraft. This work was performed by Dr S J C Dyne of the Signal Processing and Control Research Group. Additionally, a number of revisions and updates have been prepared for the ESA Structural Acoustics Design Manual, and a 'user manual' for the application of Statistical Energy Analysis (SEA) in non-aerospace areas was prepared under the ESA Transferable European Technologies (TEST) Programme.
Test and Experimental Work
ISVR Consultancy Services operates and manages the main ISVR test chambers, including the recently-refurbished large anechoic chamber, which is increasingly used for product testing as well as for undergraduate and postgraduate research.
High intensity testing of aerospace components continues to be the major element of testing work. A series of tests on spacecraft antenna dishes is in progress. With the continuation of the EFA ('Eurofighter') project, a number of tests have been carried out on airborne stores equipment at test levels between 140 and 160 dB.
A number of laboratory tests to determine sound power levels using reverberant and anechoic room methods have been performed on equipment such as shipboard machinery, computer peripherals, air-conditioning units and cellular telephone base stations. Field tests have been carried out to determine the sound insulation of walls and floors between dwellings, and to measure the noise levels of a range of devices, from forestry saws to locomotive horns. The reverberant chambers are also used for the measurement of sound absorption coefficients to ISO-354, and for transmission loss testing to ISO 140. A number of tests have been carried out on absorbent-faced highway noise barriers.
Other experimental projects have involved the monitoring of ground vibration from transportation and construction works, the investigation of wind-induced oscillation of a new type of overhead electricity supply cable system, and the 'forensic' examination of tape recordings to identify any evidence of tampering and to provide transcripts of poor quality recordings in connection with police disciplinary proceedings and criminal cases.
Speech and Communications, Hearing Conservation
A major project was carried out for an aerospace company to assist in the specification of passenger public-address systems for a twin turbo-prop aircraft with different accommodation layouts for up to 50 passengers. Measurements were carried out for a number of different cabin configurations, on the ground and in the air, to determine the Speech Transmission Index (RASTI) of the PA systems and to demonstrate that the performance met the appropriate Civil Aviation Authority Specification 15, which is a certification requirement.
A study, jointly funded by the DTI and the British Toy and Hobby Association, has been started to investigate the exposure of children to noise from toys. The next version of the European Standard on toy safety, currently in draft as prEN71-1, will implement specific noise limits on different types of toy, with the objective of protecting children's hearing. The project involves measuring the noise emission from a wide range of toys, advising on appropriate test methods and advising on appropriate noise limits, taking into account typical play durations and considering the applicability to children of generally accepted damage-risk criteria, which are based largely on retrospective studies on an adult population.
The Unit is regularly instructed by solicitors in connection with employee claims for noise-induced hearing loss. Many of these claims relate to noise exposure in manufacturing industries, although an increasing number involve wearers of headsets used for communications or for monitoring test signals. Special techniques, involving the use of a 'Kemar' manikin or miniature microphones under the headset, have been developed for these investigations. A number of claims refer to single short-duration noise exposures from sources such as personal-attack alarms.
Other activities and developments
The Unit is in the process of updating and re-issuing a series of technical information sheets covering the major consultancy activities. These are also available on the ISVR Consultancy Services web site, which can be found at http://www.isvr.co.uk/.
The use of Statistical Energy Analysis for predicting noise levels within vehicles and from machines and structures demonstrates considerable potential, and a number of commercial SEA-based software packages are now available. ISVR Consultancy Services has made a new collaborative agreement with Vibro-Acoustic Sciences and Litton Data Systems to market and support the AutoSEA package in the UK.
Davis, R. Review of Detailing for Acoustics (Lord, P. and Templeton, D., 3rd edition, London, E. & F.N. Spon, 1996). In: Journal of Sound and Vibration, 194(5), 1996, 781-782.
Ferguson, N.S., Fahy, F.J., Gardonio, P. and Pinder, J. N. Modelling of the acoustic protection given by a space launcher payload fairing. Proceedings of Internoise 96, Liverpool, 1996, 1, 537-540.
Ferguson, N.S., Finder, J.N., Fahy, F.J. and Gardonio, P. Noise transmission through payload fairings: An investigation of the sensitivity to point mass loading. Proceedings of the Conference on Spacecraft Structures, Materials and Mechanical Testing, Noordwijk, 1996, 227-235.
Lower, M.C. and Bagshaw, M. Noise levels and communications on the flight decks of civil aircraft. Proceedings of Internoise 96, Liverpool, 1996, 1, 349-352.
Lower, M.C., Hurst, D.W. and Thomas, A. Noise levels and noise reduction under motorcycle helmets. Proceedings of Internoise 96, Liverpool, 1996, 2, 979-982.
Ratcliffe, K. Amplified music within residential buildings. Proceedings of the Institute of Acoustics, 17(5), 1995, 191-5.
Smith, M.G. Validation of an SEA model of the Olympus satellite. Proceedings of the Conference on Spacecraft Structures, Materials and Mechanical Testing, Noordwijk, 1996, 723-730.
Automotive Design Advisory Unit
Manager: Mr ] D Dixon
In recent years the topic of NVH (Noise, Vibration and Harshness) has been in the spot light, receiving high priority in the development of new vehicles. However, there is increasing awareness in the industry that the customer is now becoming generally content with the current levels of refinement and that demand is switching to other areas such as cost and fuel economy. This has therefore shifted the bias from seeking new levels of refinement to retaining the current level, but at greatly reduced cost and weight. Although all the same techniques and methodologies are used for these new objectives, there is a possible danger that NVH could revert to playing its old role as a rearguard action. To avoid relapsing into this reactive approach, many manufacturers are putting considerable effort into creating processes for predicting NVH characteristics at the earliest possible time in the development of an engine or vehicle. Since design detail is far from fixed at this preliminary stage, simplistic predictive methods are appropriate, and there is now considerable renewed interest in the empirically-based noise-modelling techniques developed by the ISVR over past years. These models, combined with SEA and broad-mesh FEA, allow the designer to carry out meaningful “attribute trading” whilst attempting the total engineering compromise.
Another recent change in the automotive industry is the move by major manufacturers to devolve design responsibility for components to the relevant suppliers. This has resulted in considerable interest from such suppliers for rapid noise and vibration training, together with specific up-front design advice regarding their products.
The only change in staff in the past year was the departure of James Lalor who has taken up a post in a legal advice organization in Sheffield. We thank James for all his help at Chilworth and wish him well for the future.
The continued investment in in-house research coupled with global cost cutting within the industry has resulted in a trading deficit for 1996/97. However, it is believed that in the near future the Unit will begin to benefit from this self-funded research and, with an increasing order book, next year gives promise of a return to a trading surplus.
Summary of Activities
All four engine test cells at Chilworth have been fully utilized throughout the year, mainly running diesel engines of both car and truck size. One area of great activity is front-end drive systems. In-depth fundamental studies into chain noise have been going on throughout the year, leading to a far better understanding of the generation process as well as to the formulation of novel control measures for certain aspects of chain noise.
In a similar way, gear-impact noise is being studied at a fundamental level with a newly-commissioned gear-noise test facility. Belt noise is also being investigated, using sophisticated instrumentation developed in-house which allows the front-end accessory drive run to be studied in the vehicle during road test. This, in turn, has revealed some interesting belt-slip phenomena during gear change and start up.
A major study into the generation of turbo-charger blade-pass noise has resulted in novel noise control features being developed on the test bed, followed by successful evaluation in the corresponding vehicles.
The free-field exhaust noise rig together with the two free-field chassis dynamometers have been used extensively for developing measurement procedures for intake and exhaust shell and orifice noise, as well as evaluating novel concepts in silencers and catalyst cans.
New methods for measuring tyre noise have been developed, together with procedures for deriving noise transfer functions from tyre to passenger compartment. By combining results from these two techniques, impressively accurate tyre noise contributions are being predicted.
The numerical analysis group have split their efforts between the two ends of the modelling spectrum. Using models with very fine, and questionably excessive, mesh density, they have been carrying out a highly detailed optimization of a lightweight prototype engine looking for robust reductions of fractions of a decibel. In contrast, they have also been carrying out conceptual studies on very crude large-mesh models showing noise reductions of 5 - 6 dB through macro-optimization. It is generally believed that the correct level of development modelling is somewhere between these two extremes. However, all levels of modelling have greatly benefited from the recent purchase of a Silicon Graphics Origin 200 computer which has reduced optimization time by a factor of around 15.
Design and manufacture
A new design of V Engine Noise Simulator (ENS) has been designed and manufactured for an American customer. It was agreed by all concerned that this glorified 10-channel 1.5 kW Hi-Fi was the best simulator to date.
A broadband omni-directional sound source has been under development throughout the year. The requirement for creating high levels of sound omni-directionally from 100 to 10 000 Hz has proved to be most challenging; a concept involving three separate driver units is showing great promise.
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