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In recent years, Nearfield Acoustical Holography (NAH) has been used to identify stationary vehicle exterior noise sources. However that application has usually been limited to individual components. Since powertrain noise sources are hidden within the engine compartment, it is difficult to use NAH to identify those sources and the associated partial field that combine to create the complete exterior noise field of a motor vehicle. Integrated Nearfield Acoustical Holography (INAH) has been developed to address these concerns: it is described here. The procedure entails sensing the sources inside the engine compartment by using an array of reference microphones, and then calculating the associated partial radiation fields by using NAH. In the second part of this paper, the use of farfield arrays is considered. Several array techniques have previously been applied to identify noise sources on moving vehicles.

Numerical acoustics has traditionally been relegated to a prediction only role. However, recent work has shown that numerical acoustics techniques can be used to diagnose noise problems. The starting point for these techniques is the acoustic transfer vector (ATV). First of all, ATV's can be used to conduct contribution analyses which can assess which parts of a machine are the predominant noise sources. As an example, the sound power contribution and radiation efficiency from parts of a running diesel engine are presented in this paper. Additionally, ATV's can be used to reliably reconstruct the vibration on a machine surface. This procedure, commonly called inverse numerical acoustics (INA), utilizes measured sound pressures along with ATV's to reconstruct the surface velocity. The procedure is demonstrated on an engine cover for which the reconstructed vibration had excellent agreement with experimental results.

Over 30 years ago, A. H. Vincent of Westland Helicopters demonstrated that if a structure is excited harmonically, the response at another position (at a fixed frequency) will trace a circle in the complex plane as a result of a dynamic stiffness modification between two points. As either the real or imaginary part of an introduced dynamic stiffness is varied from minus infinity to plus infinity, the structural or acoustic response on any position will map a circle in the complex plane. This paper reviews the basis for this little known principle for vibro-acoustics problems and illustrates the viability for a cantilevered plate example. The applicability of the method is then considered for strictly acoustic systems like intake and exhaust systems. Specifically, it is shown that the response traces a circle in the complex plane if either the real or imaginary parts of the source or termination impedance are varied from minus to plus infinity.

This paper documents an inverse visible element Rayleigh integral (VERI) approach. The VERI is a fast though approximate method for predicting sound radiation that can be used in the place of the boundary element method. This paper extends the method by applying it to the inverse problem where the VERI is used to generate the acoustic transfer matrix relating the velocity on the surface to measurement points. Given measured pressures, the inverse VERI can be used to reconstruct the vibration of a radiating surface. Results from an engine cover and diesel engine indicate that the method can be used to reliably quantify the sound power and also approximate directivity.

Partial enclosures are commonly utilized to reduce the radiated noise from equipment. Often, enclosure openings are fitted with silencers or louvers to further reduce the noise emitted. In the past, the boundary element method (BEM) has been applied to predict the insertion loss of the airborne path with good agreement with measurement. However, an alteration at the opening requires a new model and additional computational time. In this paper, a transfer function method is proposed to reduce the time required to assess the effect of modifications to an enclosure. The proposed method requires that the impedance at openings be known. Additionally, transfer functions relating the sound pressure at one opening to the volume velocity at other openings must be measured or determined using simulation. It is assumed that openings are much smaller than an acoustic wavelength. The sound power from each opening is determined from the specific acoustic impedance and sound pressure at the opening.

Partial enclosures are a very common way to reduce noise emissions from machinery. However, partial enclosures exhibit complex acoustic behavior that is difficult to predict. The boundary element method (BEM) was used to model the airborne path of a partial enclosure. Simulation results were compared to measurement with good agreement. Special attention is given to the determination of negative insertion loss. It is demonstrated that the enclosure insertion loss will be negative at the Helmholtz frequencies for the enclosure.

This paper explores the use of inverse numerical acoustics to reconstruct the surface vibration of a noise source. Inverse numerical acoustics is mainly used for source identification. This approach uses the measured sound pressure at a set of field points and the Helmholtz integral equation to reconstruct the normal surface velocity. The number of sound pressure measurements is considerably less than the number of surface vibration nodes. A brief guideline on choosing the number and location of the field points to provide an acceptable reproduction of the surface vibration is presented. The effect of adding a few measured velocities to improve the accuracy will also be discussed. Other practical considerations such as the shape of the field point mesh and effect of experimental errors on reconstruction accuracy will be presented. Examples will include a diesel engine and a transmission housing.

An advanced computational method is presented for calculating the sound radiated by vibrating engine of arbitrary shape. The method is based on the numerical evaluation of the Helmholtz Integral Equation. In particular an isoparametric element formulation is introduced in which both the surface geometry and the acoustic variables on the surface of the vibrating body are represented by second order shape functions within the local coordinate system. The formulation includes the case where the surface may have a non-unique normal (e.g. at edges or corners). A general result for the surface and field velocity potential is derived. Test cases involving spherical geometry are given for a pulsating sphere and for an oscillating sphere in which the analytical solutions are known. Examples for bodies with edges and corners are shown for the problems of radiation from a circular cylinder and from a pulsating cube.

The source in an intake/exhaust system is commonly modeled as a source strength and impedance combination. Both the strength and impedance are normally measured and measurement accuracy depends on selecting an appropriate acoustic load combination. An incident wave decomposition method is proposed which is based on acoustic wave decomposition concepts instead of an electric circuit analogy providing a more straightforward approach to investigating the effect of acoustic load selection. Based on studying wave reflections in the system, the uncertainty for determining source impedance is estimated.

This paper explores the use of inverse boundary element method to identify aeroacoustic noise sources. In the proposed approach, sound pressure at a few locations out of the flow field is measured, followed by the reconstruction of acoustic particle velocity on the surface where the noise is generated. Using this reconstructed acoustic particle velocity, the acoustic response anywhere in the field, including in the flow field, can be predicted. This approach is advantageous since only a small number of measurement points are needed and can be done outside of the flow field, and a relatively fast computational time. As an example, a prediction of vortex shedding noise from a circular cylinder is presented.

NVH is gaining importance in the quality perception of off-highway machines' performance and operator comfort. Booming noise, a low frequency NVH phenomenon, can be a significant sound issue in a motor grader when it is used under certain operating conditions that cause low frequency excitations to the machine. In order to increase operator comfort by decreasing the noise levels and noise annoyance, both simulation and testing techniques were leveraged to reduce the booming noise of a motor grader. Simultaneous structural/acoustics simulations and experimental modal tests were performed to evaluate this phenomenon. The simulation models were validated using test results and then used to evaluate solutions to this noise problem. Further field tests confirmed the validity of these recommended solutions.

Micro-perforated (MPP) panels are acoustic absorbers that are non-combustible, acoustically tunable, lightweight, and environmentally friendly. In most cases, they are spaced from a wall, and that spacing determines the frequency range where the absorber performs well. The absorption is maximized when the particle velocity in the perforations is high. Accordingly, the absorber performs best when positioned approximately a quarter acoustic wavelength from the wall, and larger cavity depths improve the low frequency absorption. At multiples of one half acoustic wavelength, the absorption is minimal. Additionally, the absorption is minimal at low frequencies due to the limited cavity depth behind the MPP. By partitioning the backing cavity, the cavity depth can be strategically increased and varied. This will improve the absorption at low frequencies and can provide absorption over a wide frequency range.

Microperforated panel (MPP) absorbers are rugged, non-combustible, and do not deteriorate over time. That being the case, they are especially suitable for long term use in harsh environments. However, the acoustic performance is modified when contaminated by dust, dirt, or fluids (i.e. oil, water). This paper examines that effect experimentally and correlates the absorption performance with Maa's theory for micro-perforated panels. Transfer impedance and absorption coefficient are measured for different levels of aluminum oxide and carbon dust accumulation. The amount of dust contamination is quantified by measuring the luminance difference between clean and dirty panels with a light meter. The porosity and hole diameter in Maa's equation are modified to account for dust obstruction. The effect of coating the MPP with oil, water, and other appropriate viscous fluids was also measured. This effect was simulated by modifying the viscous factor in Maa's equation.

The insertion loss of louvered terminations positioned at the end of a rectangular duct is determined using acoustic finite element analysis. Insertion loss was determined by taking the difference between the sound power with and without the louvers at the termination. Analyses were conducted in the plane wave regime and the acoustic source was anechoic eliminating any reflections from the source. The effect of different louver configurations on insertion loss was examined. Parameters investigated included louver length, angle, and spacing between louvers. Based on the analyses, equations were developed for the insertion loss of unlined louvers.