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7 · Exhaust pipes exhaust configuration, and pavement type can all be deter- · Engine block mined by using typical statistical passby test methods. Other · Intake key parameters, such as tire type, engine configuration, vehicle · Fan loading/operating condition, and general state of mainte- · Tires nance and/or modification, cannot be obtained in uncon- · Aerodynamics trolled roadside testing. The state of compliance with the federally regulated noise emission levels also cannot be deter- The relative contributions of these sources vary with vehicle mined. For these reasons, evaluation of truck noise sources type and operating condition, and (for tire noise) the type of must include a combination of controlled passby and random, pavement. Most sources are fairly localized, although installa- statistical passby data. tion details (i.e., engine compartment configuration) can affect the effective position and size of mechanical sources. Tire and 1.3 Source Identification Methods aerodynamic noise are distributed, with tire noise near the ground along the length of the vehicle. During development of EPA's heavy truck noise regulations Source distribution is important in several ways: (12), extensive studies were conducted on truck noise sources. Source identification was typically obtained by combinations · Source height is a major parameter in the design of roadside of near-field measurements, component wrapping, removal of barriers. components, and substitution of components (13). Stationary · Source height is an important parameter for calculation of tests (idle-max-idle and dynamometer) and moving tests at the effect of the ground on sound propagation. various power settings (including coastby) further mapped out · Truck dimensions are not small compared to standard ref- the contributions of various sources. That type of testing pro- erence measurement distances of 50 ft (15 m)--typical U.S. vided good results. For example, Wyle's light vehicle source practice--or 25 ft (7.5 m)--typical European practice. study (14) mapped source and subsource characteristics over Measurements conducted by Wyle during development of a full range of power and revolutions per minute (rpm), and roadside test procedures (11) showed that propagation of yielded designs that were demonstrated to work on the road truck noise at distances less than 50 ft (15 m) does not nec- (15). Such testing is, however, labor intensive, and source analy- essarily follow the 6 dB per distance doubling rule expected sis was performed on a relatively small number of vehicles. for simple sources. Substantially higher productivity can be achieved with remote sensing methods, such as acoustic holography, beamforming, The relative contribution and spatial distribution of truck and acoustic intensity measurements. noise sources can be quite complex and display significant A number of studies reported in the current literature have variation from truck to truck, from site to site, and with truck addressed localizing noise sources on moving ground vehicles. speed due to the following factors: Much of this work initially addressed high-speed trains. More recently, applications to motor vehicle passbys are appearing. · Exhaust system sources including the outlet and sound radi- A technique that has been proven successful in identifying ation from the muffler and exhaust pipe shells depend not and quantifying motor vehicle noise is the mapping of sound only on the location and orientation of the system (e.g., ver- intensity. Typically, this technique is applied in a laboratory tical or horizontal) but also on the amount of deterioration setting such as a chassis dynamometer where the sound inten- or modification. sity probe can be placed near different source regions of the · Powertrain noise including radiation from the engine block, vehicle. The resultant data can then be processed into sound cooling fan, and intake will depend on powertrain configu- intensity contour maps to assess source regions. Although this ration as well as its state of maintenance and loading. methodology is commonly used in a stationary laboratory · Tire noise will depend strongly on tire type and on pave- setting, it has also been successfully used to map the source ment type with either of these contributing differences of regions of moving tires (16). For controlled vehicle testing, up to 10 dB or more. this method could be extended to mapping the source regions · The presence of exterior shrouds and air deflectors may of an entire vehicle. Such mapping would provide a direct also affect aerodynamically generated noise. linkage to the sound intensity method of tirepavement noise measurements currently being applied in the California Because of all these parameters, it is important to have more Department of Transportation (Caltrans) work and the Federal information about each specific truck being evaluated than Highway Administration (FHWA) studies. The method could that which can be observed in typical uncontrolled passby not be employed in uncontrolled situations, but was used in measurements along the side of a roadway. Vehicle speed, this study as a more sophisticated controlled test technique,
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8 supplementing the classic near-field, component wrapping, the source plane (acoustic plane wave assumption). For the and component substitution methods. more complex spherical case, path lengths are assumed to be Another microphone array method used for source identi- different and are accounted for properly by including spheri- fication is acoustic holography. This method is also typically cal divergence. Because of the large physical size of the source used in stationary laboratory test conditions. However, it has plane for trucks, spherical beamforming which tracks the vehi- been used to identify source regions of tirepavement noise cle during the passby is likely the only appropriate approach under on-road operating conditions. In this application, the to consider. acoustic array is suspended along side and moving with the test Based on the analysis of existing literature, research, and tire (17, 18). Using a combination of near-field acoustic holog- current practices, as well as the study team's experience in the raphy and far-field calculations based on Helmholtz's integral preceding truck noise studies, beamforming is considered the equation, it is possible to obtain a complete description of the most promising noise mapping method for trucks. It is capa- sound field of the source, where both magnitude and phase of ble of mapping both vertical and horizontal noise source dis- the sound pressure field are known at any point. The results of tributions and implicitly carries with it spectral information these works clearly indicated the source regions and compared about sources under actual operating conditions. Unlike the well with similar localization studies performed using sound sound intensity and acoustic holography techniques, this intensity. One of the features of acoustic holography is that the method is also capable of identifying and tracking large mov- plane of the source distribution can be propagated very close ing sources during uncontrolled vehicle passby testing. to the source or farther away from the source, to typical passby Although beamforming techniques are promising for local- measurement distances. However, unlike beamforming, the izing truck noise sources under passby conditions, several array used for acoustic holography must be physically as large issues needed to be resolved before this technique could be as the source region of interest. In addition, acoustic hologra- selected for the truck application, namely: phy is not suitable for passby applications when the array is sta- tionary and the source is moving. As with sound intensity, this · The spatial resolution of the technique for the frequencies method could not be employed for uncontrolled passby test- of interest for truck noise. In the successful applications in ing, although it may be useful for some aspects of source local- the literature, the revealing "pictures" of sound are typically ization under controlled conditions. higher in frequency, above 1500 Hz. In part because of the The more compelling method of localizing sources has been longer wavelengths and the limitations of the array, at fre- the application of acoustic beamforming, as detailed by Crewe quencies below 1000 Hz the source regions may appear et al. (19). Beamforming techniques in a horizontal direction quite large and source identification uncertain. have also been employed in French research specifically on · Source-to-array distance. For controlled tests, distance can truck sources (6). Acoustic beamforming uses an array of be optimized to be relatively close to the vehicle. For road- microphones to focus measurement on a specific point on an side measurements, practical issues of safe access, ambient imaginary source plane near the actual source. This focus conditions, and not distracting drivers limit how close the point is electronically swept across this plane, and the noise array can be positioned to the lane of travel. level is determined. In more advanced approaches, which · Practical concerns such as the effect of large vehicle wakes, should be used in the case of moving vehicles, the source plane random turbulence, and other background noise. moves with the vehicle and points are scanned over the time the vehicle goes by. Such an advanced approach requires that A study (20) performed by Illingworth & Rodkin, Inc. (I&R) the algorithm accounts for both the moving source plane and under contract to Caltrans in 2005 focused on the ability of Doppler shifting of the sound during the passby. In this man- beamforming technology to localize sources on different types ner, several slices in time can be evaluated near the position of of trucks under several modes of operation. The results of this interest, such as the time when the maximum passby noise work addressed many of the application issues mentioned pre- level occurs. Unlike the more simple methods that consider viously. The data from the Caltrans project was used to opti- only one instant during the passby, beamforming, by tracking mize the methodology with regard to the number of array the source and averaging over several instances, has a more microphones needed and the configuration of the array itself. accurate result. Because of tracking capability, this type of The signal processing required for the beamforming algorithm beamforming can also be exploited to evaluate source direc- was developed by Dr. William Blake for both the Caltrans tivity as the vehicle approaches and recedes from the measure- study and this NCHRP study and includes spherical diver- ment position. Beamforming algorithms can also be provided gence, source tracking, and Doppler shift, as described in more for either spherical or planar cases. For the planar case, the detail in Section 3.2.1. array is assumed to be far enough from the source plane so A significant part of the demonstration of a relatively new that there is no path length difference for different points on technique such as beamforming is to demonstrate that it yields