APPLICATION STORY
     

Bruel & Kjaer Pulse System helps TRL's White Van Man get to grips with mysteries of tyre/road noise. If you're cruising along at 60mph (95km/h) on any of Britain's motorways or trunk roads in the future and a large 7½-tonne white truck looms large in your rear view mirror, don't worry. It's not necessarily white van man with a bad case of road rage. It could well be the Transport Research Laboratory's latest mobile research laboratory for investigating tyre/road noise. Here, Martin Armstrong, sound and vibration specialist from Brüel & Kjær, describes how TRITON, as the truck is known, is helping TRL's scientists to better understand the relationship between tyre noise and road surface texture. TRITON is one of the world's most advanced systems for monitoring noise emitted from tyres as they run over the road surface. The present method of assessment relies on the statistical pass-by (SPB) method which measures, at the roadside, individual pass-by noise levels of a large number of vehicles travelling through a fixed test area. TRITON uses the proposed ISO close proximity (CPX) method for testing. This involves measurement of actual tyre noise using a microphone mounted close to a specified test tyre running in the wheel track.
Typically, the microphone needs to be located approximately 200 to 400mm from the tyre sidewall. However, to produce high precision and repeatability, it is necessary to isolate the microphones from external noise sources such as the truck engine, other tyres, other vehicles and so on. Consequently, TRITON features a specially designed body that incorporates an anechoic chamber enclosing the test tyre and an array of microphones.
The anechoic enclosure comprises a double skin structure constructed from 2mm thick steel plate with 100mm thick absorbent foam, a second steel plate and further 100mm foam inner liner. This chamber can be hydraulically lowered so that the test tyre and microphone set up are virtually sound isolated during testing. The chamber is so effective that TRITON measures signals of 100dB from the tyre/road interface with engine and background noise limited to just 60dB. Based on a 10-tonne drawbar Leyland DAF truck, TRITON is fitted with an upgraded 5.88 litre Euro 2 turbo diesel that generates 158kW (215bhp) at 2,500rpm enabling motorway speeds to be maintained with ease. The vehicle has been "down-plated" to 7½-tonnes for operational purposes so it can legally travel at 70mph (110km/h) on motorways and trunk roads. It is actually capable of running at 80mph (130km/h) on TRL's own test track. The major benefit of TRITON is that measurements can be carried out at real traffic speeds and at any location along virtually any road. TRITON has accurate on-board positioning equipment that allows measurements to be made along a series of short trial lengths of road surface to investigate the variations in noise over long stretches of road. Tests have shown the repeatability of such measurements over short segments. This ensures that these measurements can easily be repeated, as the road wears and with a range of tyres, to build up better knowledge about how the noise is generated. TRITON also allows the angle of the test tyre to be adjusted to simulate different cambers and toe-in conditions. Additional measurements are made of road surface and tyre surface temperatures and internal tyre cavity pressure. Inside the anechoic chamber, a total of seven Brüel & Kjær microphones capture noise emitted close to the tyre. These are linked to a Brüel & Kjær PULSETM multi-purpose analyser. The eight-channel PULSE system measures and analyses each of the seven microphone inputs with the eighth channel used for triggering purposes. The trigger sensor is positioned on the side of the vehicle and detects reflective markers placed at regular 10m or 20m intervals at the side of the road under test. Each trigger point initiates the next test section so that new measurements are being made as the PULSE analyser is calculating noise measurements from the previous test. The entire system is linked to a 400MHz Pentium II PC with 128MB RAM that is used to control operations.
In the sophisticated on-truck control room, TRITON gathers huge amounts of data which, in time, will enable TRL to help improve the design of both tyres and road surfaces. Steve Phillips, TRL's head of tyre/road and vehicle noise studies, commented, "We are currently evaluating the initial data from TRITON to determine the best way to present data and implement possible changes so that safety is not compromised. Even with the power of Brüel & Kjær's PULSE system it will take a long time to investigate all the data and all the possibilities because we are well aware that, in the case of both tyres and road surface technology, changes made to reduce noise could adversely affect road holding performance and safety." He continued, "With TRITON though, we can now measure a wider range of road surfaces under a wider range of conditions and over much longer distances. All this can be done in real conditions without requiring any special precautions and, especially, without causing unnecessary disruption to other road users, a factor that keeps TRL in favour with local police forces throughout the country." "TRITON will be monitoring noise routinely around the trunk road network and we anticipate that it will help us to unravel the mysteries of the complex relationship between tyre noise and road surface texture so that we can, perhaps, all look forward to whisper quiet motorway travel in the not too distant future." he concluded. TRL's TRITON demonstrates the power and flexibility of B&K's PULSE multi-channel, multi-purpose analyser. The standard sound and vibration analysis software has been enhanced with additional functions for capturing the noise profile over given short road lengths. As B&K's chosen measurement platform, PULSE's large range of standard applications and accessories is still growing, making any PULSE system a secure investment. On top of this, Brüel and Kjær also provides tailored solutions for PULSE. The PULSE analysis software comprises three powerful analysers as standard; narrow band FFT, fractional octaves and overall level. These analysers can all be run in real-time and in parallel on multiple channels (multi-analysis). This feature is unique to PULSE and reduces measurement time while also improving the quality of data collected. In the PULSE analyser system the front-end is separate from the PC. This ensures that low level signals can be conditioned and digitised before transfer to the Analysis Engine software in the PC. With PULSE, the connected PC does all the analysing, doing away with the need for a dedicated analyser. Analysis Engine software in PULSE takes full advantage of the processing power of today's PC processors using it for real-time signal processing. PULSE is based on the reliable and familiar Windows NT environment providing tight integration with Microsoft Office applications. Additional features include multiple 2D and 3D displays, drag-and-drop reporting in Word and dynamic linking to Excel. Control programs for system integration can be written in a variety of industry standard languages including Visual Basic, C++ and Delphi working with PULSE's extensive ActiveX interface. PULSE applications currently available include order analysis, operational modal, sound intensity, noise source identification, sound quality, sound power, PULSE bridge to MATLAB, PULSE bridge to ME'scope, Vold-Kalman order tracking filter, Modal Test ConsultantTM and Time Capture. _________________________________________________________ For more information contact:- Alan Gibson at Bruel & Kjaer Tel: +44(0) 1438 739000 E-mail: info@bkgb.co.uk Website: www.bksv.com January 2002
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