Powertrain Acoustic Characterization at High ...

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PSA – Peugeot Citroën. ABSTRACT. Predicting the propagation of air-borne noise radiated by the powertrain at high frequencies requires acoustic power data ...
Powertrain Acoustic Characterization at High Frequencies: Acoustic Source Modeling Based on Sound Intensity Measurements Y. BOUSSEAU, L. GAGLIARDINI PSA – Peugeot Citroën

ABSTRACT Predicting the propagation of air-borne noise radiated by the powertrain at high frequencies requires acoustic power data for source modeling. This paper proposes a new model of source based on an acoustic intensity mapping in an anechoic test-bench. Given an energy propagation model, it is possible to back-propagate acoustic intensity data to a model of powertrain, by solving an inverse problem. Application of the method to a 2.0L gasoline powertrain has led to convincing results between 500 and 5,000 Hz, and most of the radiation patterns of the powertrain are well represented in space and frequency.

INTRODUCTION Sound radiation of the powertrain is an important noise source in a vehicle. Interior powertrain noise is due to the combination of structure-borne and air-borne transmission, and it is generally admitted that the crossover between both transmission paths occurs between 500 and 1,000 Hz. New methods exist to predict sound radiation of a powertrain in its compartment, such as Energetic finite elements method (EFEM) or SEA [1,2], and they require acoustic intensity data for surface source modeling. No such model is available at the moment; this is why a new kind of source modeling, well fitted for surface sources, providing quantitative as well as qualitative results was looked forward. This method shows at the same time locations of high acoustic radiation areas, and the actual acoustic power radiated. This paper presents a new model of acoustic source based on sound intensity measurements. Measurement procedure will first be presented; then an inverse illposed problem will be formulated and the main parameters of the inversion will be discussed. To end with, results will be presented for an actual powertrain.

INTENSITY MEASUREMENT OF THE POWERTRAIN The model requires data that represent the propagation of acoustic energy radiated by the powertrain. An acoustic intensity mapping is thus realized in an anechoic test-bench. ACTIVE INTENSITY FIELD PROPERTIES Global power radiated by a source is given by

P=

∫∫ I.dS

[1]

closed surface

Where I is the acoustic intensity vector and where the control surface is outer oriented. Active intensity I describes acoustic energy transfer, and div I = 0. Thus I can be written as:

I = gradφ + rotψ The only pertinent value for the model is the irrotational part of active intensity, and it is necessary to ensure that measurements are not polluted by the rotational part of active intensity, even if measurement points are close to the powertrain surface. Rotational intensity is an unstable vector field, which has zero average. Thus two consecutive averages permit us to minimize the error due to the contribution of the rotational part of active intensity: 1. To avoid fix frequencies, engine speed is not constant, and its oscillations are strictly equivalent for all measurement points. 2. Intensity is integrated over frequency bands.

Practically, sound intensity level in direction of the probe is given by:

Li = 10 log

ℑm(G12 ) ρ 0ω∆r0 I ref

where G12 is the cross spectrum of pressures measured by the two microphones of the intensity probe, and ρ0 is the temperature dependant specific mass of air. THIRD-OCTAVE INTEGRATION Measured intensity is integrated over third octaves for several reasons: To minimize the contribution of rotational intensity, as explained above, Propagation methods used at high frequencies do refer to statistical values, expressed in third octave, This leads to a reasonable quantity of data to be analyzed.

The main limitation is that narrow-band noises -such as turbocharger noise- carry very little energy and will not be considered in this method. A specific signal processing should be developed for such sources. MEASUREMENT SURFACE Location: For practical reasons, measurements have been made over several planes. Their distance to the powertrain surface has to be optimized, knowing that there are several constrains:

gradients. Such conditions are well accounted through a diffuse radiation model. [ref2] The surface is divided into discrete surface sources. It is assumed that each elementary source is independent of each other; sources are uncorrelated and they are characterized by their energy, so that their contributions can be summed at any point. Practically, a 900-element mesh was used (see figure 1); an elementary power source is set at the geometrical center of each triangle or quadrangle.

Measurement surface should be far enough, in order to minimize rotational intensity. Measurement surface should be near the engine in order to improve back-propagation accuracy (see below). Practical difficulties occur near the engine, concerning mainly accessibility, high temperature and high heat radiation. A distance of 8 cm between the powertrain surface and the measurement planes has proved to be a good compromise. Closed surface: Equation [1] shows that acoustic power radiated by the powertrain is given by the flux of active intensity over a closed surface. Thus, the measurement surface should be closed. Practically this is of course unrealizable and some points of the measurement mesh cannot be reached by the intensity probe. Thus the measurement surface in not actually closed and there are some ‘holes’ that cannot be measured. In order to minimize the surface of those holes, attention must be paid for the environment of the powertrain in the test bench to be as discret as possible from an acoustic point of view. Anyway in spite of the holes, it is possible to estimate the powertrain total acoustic power: interpolation over a few points can reasonably be done to compute the powertrain total acoustic power. LP-LI INDICATOR This indicator is defined as the difference of the pressure level to the intensity level in dB. It is useful to check both validity and accuracy of intensity measurements. Applying the proposed method leads to acceptable values for Lp-Li indicator, since Lp-Li