Annual report 2005
Every year from 1963 to 2010, the Institute of Sound and Vibration Research published its Annual Report. This is the chapter on ISVR Consulting for May 2005 - December 2005. This period is shorter than usual to align the annual rport with the calendar year.
ISVR Consultancy Services
Manager Mr S J C Dyne
ISVR Consulting is the commercial consultancy division of the department, predominantly serving public sector and industrial clients. The work is mainly in the automotive and aerospace sectors but the unit is working increasingly with the marine and chemical industries, and is active in building and architectural acoustics, litigation (particularly hearing loss claims), occupational noise assessments (especially where headsets are worn such as in call centres) and industrial noise control. The unit maintains close links with the ISVR research groups and a number of projects are carried out in collaboration with academic and research staff. Our website, www.isvr.co.uk, includes full details of the range of services and facilities available.
This report discusses three areas of activity in the reporting period: airframe noise, noise insulation testing in compliance with Schedule E of the Building Regulations and measurement of noise emissions from chemical plant stacks.
The aerodynamic noise produced by the airframe of aircraft in the landing configuration is a major contributor to the overall noise of modern aircraft at approach, and one of the dominant sources of this airframe noise is the landing gear. Having participated in a number of studies to understand and control the mechanisms of landing gear noise generation, including the development of 'advanced low noise gears' as part of the EC SilenceR project, which gave up to 6 dB noise reduction relative to equivalent standard gears, ISVR Consulting were provided with further funding from SilenceR to carry out wind tunnel testing of a quarter-scale model of an advanced main gear. The aim was to understand the complex flow around the gear, to interpret this in terms of noise mechanisms, and then to achieve still higher noise reductions.
The model gear was installed in the University’s 7’ x 5’ wind tunnel, and a wall mounted array of 54 microphones was used to assess the noise benefit of changes to the configuration of the bogie. The model was designed to allow for the variation of three major parameters; the bogie angle, the lateral wheel spacing, and the longitudinal wheel spacing.
A number of ways of achieving significant noise reductions were identified, with the most successful configuration having an increased front wheel spacing and a -10° toe down bogie angle to shield the rear wheels, and a porous fairing between the front wheels to shield the front axle. Reductions of 6 dB - 10 dB in the noise levels of sources in the bogie region were achieved over a wide frequency range. Further details are available in AIAA paper 2006-2626.
The tests demonstrated that there is a substantial benefit in making fairings porous, allowing some flow to pass through to avoid the high-speed deflected flow produced by solid fairings that can generate additional noise sources within the gear. The reduced pressure drop behind a porous fairing also means that rear components of the gear are shielded more effectively.
As a result of this successful initial experimental programme, a number of new contracts for low noise design and testing of landing gears have been awarded. The use of the wind tunnels for other types of aerodynamic noise control experiments is now also expanding, with recent contracts undertaken on the control of wind noise generated by architectural features on buildings.
Schedule E Sound Insulation Testing
For new or converted buildings that contain party walls, e.g. flats, semi-detached houses or terraced houses, it is a requirement of the building regulations that the party walls and floors meet or exceed prescribed sound insulation levels in order to protect the residents from the noise from neighbours. To achieve this, the builder may either build to a set of pre-defined ‘Robust Details’, which involves registering and assessing the building process, or the builder may use alternative construction methods that are then tested to ensure that the sound insulation requirement is satisfied.
Once built, a portion (typically 10%) of the party walls and floors are tested. The test comprises two elements; for the walls and floor an airborne sound insulation test is carried out by creating and measuring high noise levels on one side of the party wall/floor and measuring the resulting noise levels on the receiving side of the wall/floor. In addition, for floors, an impact test is carried out in which a series of known impact forces are applied to the floor and the noise levels produced in the downstairs room are then recorded.
Having gathered this information, as well as the relevant background noise levels and the reverberation times of the receiving rooms, a calculation is made to assess the sound insulation performance of the floors and walls which is then compared with reference minimum standards. A certificate is issued which the builder provides to the building control officer provided the building has adequate insulation. If, however, the floor/wall does not pass the test, assistance can be provided to determine the cause of the failure and to advise on suitable remedial measures.
In-duct measurements of sound power
The measurement of the noise emitted by hot exhaust gases has been a topic of interest for the gas turbine industry for many years. An International Standard (ISO 10494) provides a method for deriving the sound power level of the exhaust-outlet noise from measurements taken 1 metre from the stack tip at inclinations to the axis of the stack. Gas turbine manufacturers are also interested in the un-silenced exhaust noise, so that the acoustic insertion loss of exhaust silencers can be verified. This presents severe problems because gas turbines are hardly ever run without some silencing. One way to obtain this information is to measure noise in the duct upstream of the silencer. However, this may be a hostile environment, often with temperatures that are beyond the range of commercial sensors.
Measuring noise inside ducts is challenging because the fluid is in motion. Fluid movement along the duct produces turbulence, which is a non-propagating pressure fluctuation arising from the large-scale rotational motion in the fluid. An acoustic signal is a compressible pressure fluctuation that propagates in the fluid at a velocity dependent on the characteristics of the fluid and is, effectively, independent of the velocity of the fluid. When noise occurs in a turbulent fluid the problem is to separate the fluctuating particle velocities associated with the incompressible turbulent flow from those associated with the compressible turbulent field.
Attempts to measure the in-duct sound pressure level have used various designs of probes. The method adopted by ISVR Consulting uses two probes, positioned axially in the duct. The probes extend through the duct wall and incorporate microphones fitted to a T piece in the pipe-work outside of the duct which is a less hostile environment.
The method was used to measure exhaust stack sound power in a petrochemical plant following laboratory tests in the ISVR acoustic chamber suite. For the laboratory tests, a source of known sound power was placed in a reverberation chamber. A duct (Figure 2) connected this chamber to a second reverberation chamber and the source sound power was then determined using the probe method with flow in the duct. The measurements of source sound power were compared with (i) the sound power measured according to ISO 3744 and (ii) a single in-duct microphone measurement. Results, shown in Figure 3 below, demonstrate the applicability of the method. Note that the single microphone measurement produces higher estimates than the other methods, illustrating the contamination effect of turbulent flow in the duct.
Lawton, B.W. Variation of young normal-hearing thresholds measured using different audiometric earphones: Implications for the acoustic coupler and the ear simulator. International Journal of Audiology, 44, 2005, 444-51.
Rawlinson, R.D., Alberola, J. and Joseph, P. Reducing noise from an oil refinery cat cracker. Internoise 2005 - The 2005 Congress and Exposition on Noise Control Engineering, 7-10 August 2005, 9pp.
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