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44 tk,log10(Fk),10*log10((GG5( :,:))) (a) -30 10000 -40 8000 -50 6000 4000 -60 Frequency, Hz 2000 -70 0.5 1 1.5 2 2.5 tk,log10(Fk),10*log10((GG5corr( :,:))) 10000 -30 8000 -40 6000 -50 4000 (b) -60 2000 -70 0.5 1 1.5 2 2.5 Time, s Figure 53. Two-dimensional displays of autospectra for a representative sensor in the array: (a) uncorrected; (b) range-corrected. The procedure for analyzing the passby starts with deter- opposite to the vertical array axis "A." A few "spot" areas are mining the resolution at various times during the record. To indicated in the images. The areas designated "F" are located develop a temporal gradient with multiple segments of ade- at the front of the truck, just behind the front wheel and below quate statistical sampling and frequency resolution, a balance the ground plane. The source levels associated with these spots must be made among the number of Fast Fourier Transform differ by roughly 6 dB. The spot labeled "T" in the 1400 rpm (FFT) frequencies (nfft), the number of ensembles (noens), run is located near the road surface plane and appears to be and the number of time segments along the record (nofiles). due to the rear tires. The streaked spots labeled "D" lie below Thus, if the total sample size is N (in the example case, 3 s the ground plane between the front and rear wheels and taken at 6000 samples per second), then appear to be within 1 dB for the two runs. These sources could be exhaust or drive-train noise. As noted previously, sound N = nfft noens nofiles (2) generated in the engine compartment and elsewhere on the In the majority of cases examined during passby, typical truck, when reflected off the ground, appear to the array as values of these parameters are nfft = 128, noens = 10 to 15, sources below ground by 0.5 to 1 m (1.6 to 3.3 ft). Compari- nofiles = 15 to 20 for total sample sizes N up to about 20,000. son of the lower image in Figure 54 with the images in Figure Fewer nofiles allows a greater number of ensembles or greater 49 for this truck stationary--with no tire or drive-train noise frequency resolution, but it gives a much longer time segment and the engine operating at 1400 rpm--shows that, for simi- that averages over too much of a passby. Too high a value of lar frequencies, the levels just behind the front wheel are about nfft provides a narrower bandwidth and associated spectral 75 to 77 dB in each case. During passby at 50 mph (80 km/h), variability. As a practical matter, given the trade-off between however, the other sources dominate over the engine noise. At some spectral smoothing and some time resolution desired, 2000 rpm, the engine noise now competes with the other the above-noted parameter values gave the best results. sources, having increased to about 83 or 84 dB. This increase is consistent with the observation of sources associated with this forward wheel-well acoustic propagation path. 3.5.4.2 Passby Evaluations of the 5900i Truck: Images at other frequencies for the engine speed of 1400 rpm Localization of Engine Compartment are shown in Figure 55, which illustrates the localization of and Tire Noise sources at the front and rear tires. As noted previously in the dis- Passby measurements with the 5900i truck at 50 mph cussion of the array calibrations (e.g., Figure 40), the array will (80 km/h) show the ability of the array to discriminate among successfully localize sources while steering off axis, but the pro- various propagation paths of engine noise and tire noise. Fig- jection of the sound field onto the microphone plane further ure 54 shows truck noise images at 868 Hz for a passby speed elongates and slightly rotates the appearance of the point spread of 50 mph with the engine at 1400 and 2000 rpm at an instant spot function. Keeping these image features in mind, clear con-

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45 (a) Engine at 2000 rpm F T (b) Engine at 1400 rpm D F Figure 54. Source distribution at frequency of 868 Hz of the 5900i truck moving to the right at 50 mph with engine at (a) 2000 rpm and (b) 1400 rpm. (a) (b) Arrows indicate locations of frequencies (c) (d) Figure 55. Source distributions for the 5900i truck moving to the right at 50 mph with engine at 1400 rpm at frequencies: (a) 695 Hz, (b) 868 Hz, and (c) 1346 Hz indicated in (d) the truck noise spectrum.