Wavelet Transform to Index Road Vehicle Vibration Mixed Mode Signals

Goods and products transported by road are subjected to vehicle vibration which, without proper protective packaging, can suffer damage. To reduce shipment costs, protection has to be optimised to limit product damage occurrence while keeping packaging weight and size to a minimum. Optimisation is realized by simulating the vibration of transport vehicles. To achieve an accurate simulation, each vehicle vibration mode has to be modelled. These include: the nonstationary random vibration induced by road roughness and speed variations, the transient vibration created by road surface aberrations and the harmonic vibration created by the vehicle engine and drive train. Identifying and indexing these mixed-modes within complex road vehicle vibration signals is essential to define the severity and occurrence of the different modes in order to develop an accurate model. This paper shows that indexing can be performed using the orthogonal wavelet transform such as Daubechies 10. Assuming that each mode is preponderant in different analysis scales, the wavelet coefficients can be used to perform the indexing. This allows more sensitive mode detection and a more precise time indexing thanks to the multi-resolution nature of the wavelet transform compared to other time-frequency analysis methods.

Vibration Control of Variable Thickness Plates With Embedded Acoustic Black Holes and Dynamic Vibration Absorbers

In the past decade, plate-like structures embedded with one or more acoustic black hole (ABH) features have been developed as a promising passive approach for structural sound and vibration control. In this study, the concept of combining dynamic vibration absorbers (DVAs) and the ABH effect is proposed to further improve the vibration control effectiveness of a variable thickness plate. A finite element (FE) model is developed to analyze the vibration response of a plate embedded with both ABHs and DVAs under point force excitations. To demonstrate the effectiveness of different vibration control approaches, the vibration responses of plates of uniform thickness, variable thickness embedded with ABH features, variable thickness embedded with both ABH features and damping layers, and variable thickness embedded with both ABH features and DVAs are compared experimentally. It is shown that, in the frequency range considered in the current study which is up to 6.4 kHz, the uniform plate presents high average velocity response level. On the other hand, although 11.5% lighter, the variable thickness plate integrated with both ABH and DVA features results in the lowest response level. Results in this study demonstrate the potential of combing DVAs and ABHs together as an effective lightweight noise and vibration control approach.

Field Comparison of IEC 61400-11 Wind Turbines — Part 11: Acoustic Noise Measurement Techniques, Edition 3.0 and Edition 2.1

Following the release of Edition 3.0 (2012) of IEC 61400-11 Wind Turbines – Part 11: Acoustic noise measurement techniques, there has been a lot of interest as to how analysis results differ from methods stipulated in Edition 2.1 (2006). This paper provides a detailed review of the differences between Edition 3.0 and Edition 2.1. An analysis is provided on differences in evaluation of the apparent sound power level and tonal audibility between both versions of the measurement standard.

Two-Dimensional Arbitrary Shape Acoustic Cloaks Composed of Homogeneous Parts Realized by Layered Structures

Acoustic cloaking is an important application of metamaterials and has received much attention since it was first proposed. Due to the extreme properties of the cloaks produced by previous methods, they are difficult to fabricate. In addition, cloaks with arbitrary shapes are more favorable in applications but are difficult to realize. Therefore, it is important to present a method for designing arbitrary shaped cloak with attainable properties. In this paper, a technique for realizing cloaks with arbitrary shapes is presented by dividing the cloak into finite parts. Transformation acoustics is used to derive the properties of each part of the cloak. With appropriate mapping relationships, the properties of each part are anisotropic but homogeneous. Layered structures are adopted to approximate the anisotropic properties within each part. Full wave simulations are conducted to validate this technique. The method can be used to design cloaks with arbitrary shapes, which perform well within certain frequency limits. It provides an easier way to fabricate cloaks with arbitrary shapes.

Aeroacoustic Source Separation on a Full Scale Nose Landing Gear Featuring Combinations of Low Noise Technologies

The reduction of noise generated by aircraft at take-off and approach is crucial in the design of new commercial aircraft. Landing gear noise is significant contribution to the total noise sources during approach. The noise is generated by the interaction between the non-aerodynamic components of the landing gear and the flow, which leads to turbulence generated noise. This research presents results from the European Clean Sky funded ALLEGRA project. The project investigated a full-scale Nose Landing Gear (NLG) model featuring the belly fuselage, bay cavity and hydraulic dressing. A number of low noise treatments were applied to the NLG model including a ramp door spoiler, a wheel axel wind shield, wheel hub caps and perforated fairings. Over 250 far field sensors were deployed in a number of microphone arrays. Since technologies were tested both in isolation and in combination the additive effects of the technologies can be assessed. This study describes the different techniques used to quantify the contribution of each technology to the global noise reduction. The noise reduction technologies will be assessed as a function of frequency range and through beamforming techniques such as source deletion.

Determination of the Bearing Quality by Means of Vibroacoustic Response

A signal of mechanical and acoustical vibration generated during a bearing’s quality testing contains variety of useful information about their operational conditions. At defined testing speeds, the bearings generate a periodical and a non-periodical vibroacoustic signal of different intensities, which are dependent on the quality of bearings and/or their damage type. A dynamical response of the tested bearings recorded in time characterizes also the nature of the unwanted noise of bearings. Therefore the dynamical response of the maximum acceleration recorded in time is one of the major criteria of bearing quality evaluation in terms of noisiness. The second criterion is a determination of statistically significant frequency interval for the equivalent acceleration value expressed in decibels. Consequently, from the effective acceleration value, the bearing quality can be determined in terms of its vibration severity, as well as its noise level. This objective methodology substitutes the evaluation bearing quality by means of measurement of the vibration acceleration on the given test device and simultaneously evaluation of the noise quality and its intensity auditorily. Verifying the proposed methodology 100 % conformity was achieved between the methodology currently used and the new methodology, which eliminates the subjective quality evaluation of bearings auditorily.

Characterization of Low Noise Technologies Applied to a Full Scale Fuselage Mounted Nose Landing Gear

The negative impact of aircraft noise includes effects on population’s health, land use planning and economic issues such as building restrictions and operating restrictions for airports. Thus, the reduction of noise generated by aircraft at take-off and approach is an essential consideration in the development of new commercial aircraft. Among the different aircraft noise sources, landing gear noise is one of the most significant during approach. This research presents results from the European Clean Sky funded ALLEGRA project, which investigated a full-scale Nose Landing Gear (NLG) model featuring the belly fuselage, bay cavity and hydraulic dressing. Tests were performed for a variety of wind speeds and yaw angles. In this paper, a characterization of the noise generated by the full-scale Nose Landing Gear (NLG) model is presented and the different techniques used for characterizing acoustic sources on the NLG are described. The landing gear noise source is characterized in terms of OASPL, directivity, source spectra, PNL and PNLT. A comparison between the NLG with and without the application of low noise technology is presented.

Experimental Analysis of Vibration and Radiated Sound Power Reduction Using an Array of Acoustic Black Holes

The Acoustic Black Hole (ABH) has been developed in recent years as an effective, passive, and lightweight method for attenuating bending wave vibrations in beams and plates. The acoustic black hole effect utilizes a local change in the plate or beam thickness to reduce the bending wave speed and increase the transverse vibration amplitude. Attaching a viscoelastic damping layer to the ABH results in effective energy dissipation and vibration reduction. Surface averaged mobility and radiated sound power measurements were performed on an aluminum plate containing an array of 20 two-dimensional ABHs with damping layers and compared to a similar uniform plate. Detailed laser vibrometer scans of an ABH cell were also performed to analyze the vibratory characteristics of the individual ABHs and compared with mode shapes calculated using Finite Elements. The diameter of the damping layer was reduced in successive steps to experimentally demonstrate the effect of damping layer distribution on the ABH performance. The experimental analysis demonstrated the importance of low order ABH modes in reducing the vibration and radiated sound power of plates with embedded ABHs. The results will be useful for designing the low frequency performance of future ABH systems and describing ABH performance in terms of design parameters.

Study on Driving Point Power Flow as Power Dissipation in Discrete Vibratory Systems

Mechanical power flow into a discrete system is formulated as power dissipation in the system using driving point advantages. Firstly, the complex-valued power flow into a system is defined, and it is shown that the real part (or active power) corresponds to the power dissipation, and the imaginary part (or reactive power) contains the Lagrangian energy. To represent the power dissipation in the system, all the power flows into the system must be counted. Otherwise, the value of power flow may become negative, and its physical interpretation may be troublesome. One of the main advantages of the formulated power flow is that to estimate the power dissipation in the system, only the power flows into the system are necessary. In other words, only the driving point power flows into the system are needed, and no information inside the system is required. Next, to estimate interfacial force/moment for the power flow into a sub-system, the two alternative indirect methods are presented. It is shown that these are exact methods utilizing the responses and the frequency response functions at the driving points only. This is another remarkable characteristic of the driving point. A 7 degree-of-freedom system is employed as an example case, and the presented formulations are confirmed computationally.