Rolls-Royce Noise Measurement System
A new noise measurement system using state-of-the-art technology has recently been installed at Rolls-Royce's site at Hucknall, near Derby. Supplied by Brüel & Kjær, the new installation enables noise measurements and analyses to be made simultaneously on up to 124-channels - the data is used to confirm compliance of Rolls-Royce engines to international noise standards and for extensive R&D investigations.
Hucknall's History
In 1927, Rolls-Royce decided to carry out its own flight development, but the airfield originally used was a long way from the main Derby factory, and it was too small. A suitable permanent site was necessary and Hucknall, some 15 miles (24 km) from the Rolls-Royce factory, was chosen.
In December 1934 two hangars were erected and Hucknall became Rolls-Royce's flight test centre. In1971, after nearly 40 years in this role, it was decided that test flying would be transferred to Filton, near Bristol, but Hucknall remained as a test site for Rolls-Royce aero engines.
Test Facilities
Using a wide range of technical expertise and facilities, the Test Facilities Team of about 100 employees carry out a variety of tests that are vital to the integrity of Rolls-Royce aero engines throughout their development and certification programmes.
Rolls-Royce recently decided to replace its No. 11 Test Bed noise measurement system. The Test Bed, originally built in 1983, is designed to test engine noise and performance characteristics for all Rolls-Royce aero engines. It is the only rotating test bed in Europe and has been used to noise certify all Rolls-Royce civil aero engines.
Michael Clarke, a Chartered Engineer who has worked at Rolls-Royce for 12 years, was appointed as the Project Manager.
Mike Clarke explains, "Noise measurement is vitally important. Our aim is to have as quiet an engine as possible. This benefits airframe manufacturers, airline passengers and the public in general. The noise measurement tests are carried out for both certification of each engine type and for our own extensive R & D investigations".
Mike continues, "The new noise measurement system is a significant investment for Rolls-Royce. It must be reliable, stable, accurate and require minimum maintenance. We've used Brüel & Kjær analyzers, sound level meters, transducers and other products for many years and Brüel & Kjær offered us a cost-effective and efficient package. As the world leaders in the manufacture of aero engines, it was natural for Rolls-Royce to award this turnkey contract to the world leaders in noise measurement systems".
The No 11 Test Bed is impressive! The whole structure weighs hundreds of tons but can be rotated through 360 degrees by a hydraulic motor. The position of the engine under test can therefore be adjusted according to the prevailing wind direction. It can take as little as two hours for an engine to be installed on the test bed.
Noise Measurement System - an Overview
Engine noise is collected by:
- 144 Brüel & Kjær far-field microphones - placed in pairs at ground level at 5 degree intervals around the perimeter of the test bed
- 48 Brüel & Kjær near-field microphones - placed at predetermined intervals at ground level, close to the engine
- 128 pressure transducers - mounted on the engine
Switching units are used to select up to 128 channels to be recorded from the 320 available transducers. Placed in the underground room are:
Eight Brüel & Kjær Type 2694 16-channel signal conditioning amplifiers
Four Sony® SIR-1000 32-channel DAT recorders
Brüel & Kjær 32-channel PULSE multi-analyzer - enabling 32 channels of CPB or FFT data to be viewed in real-time
Two computers - one is the PULSE master system controller; one is a data concentrator used to collect weather data
Various channel switching units
Brüel & Kjær calibration switch unit
Certification Testing - R & D Investigations
Noise measurement testing is basically of two types. The first is certification testing where the data is used to prove to authorities such as the CAA or FAA that the engine does what it should and performs as specified. Rolls-Royce's No. 11 Test bed is fully approved.
Mike Clarke explains, "Our aim is to achieve at least a 95% confidence level and we take three sets of data. We use 20 power points, from idle to full thrust, and record data for 35 seconds from each power point. Each total test takes about three hours. Other types of noise testing are for R&D investigation".
About twice as many R&D investigation tests as certification tests are performed. All noise measurement data is stored in Rolls-Royce's central database.
Mike Clarke concludes, "Airline customers and passengers worldwide rely on these essential tests to ensure the certification and integrity of Rolls-Royce aero engines. The testing process is constantly being developed and, for example, in the future we intend to measure on up to 124 channels using pressure transducers. We will use the data to identify the detailed source of the noise using modal analysis techniques, and to continuously develop and test those features to reduce the overall engine noise. In the end, this will benefit everyone."
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