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46 (a) (b) (c) Figure 56. Source distributions for the 5900i truck moving to the right at 50 mph with engine at 2000 rpm. tributions from the tires are seen at 695 and 1346 Hz; this oper- track (Figure 16) had a side skirt below the cab and a raised ating point [lower engine speed and 50 mph (80 km/h) vehicle roof at the cab top. The version tested at the high-speed track speed] and these frequencies were selected to minimize the (Figure 19) had no skirt and an aerodynamic fairing with an influence of the engine and maximize that of the tire noise. open back was set on top of the cab. This truck was tested The directional nature of these sounds is illustrated in Fig- with a standard vertical muffler installation and with the ure 56 for the same truck speed of 50 mph but the engine at muffler replaced with a pipe. The data presented in the fol- higher rpm of 2000. At the top of the figure is a pair of images lowing paragraphs were collected for the truck speed of 30 to at 868 Hz from a single passby, but with the truck at two dif- 35 mph with the engine at 1850 to 1900 rpm. ferent positions relative to the array. The line labeled "A" in Line plots of autospectra at the time for which the maxi- this figure denotes the location on the truck of the array main mum overall sound pressure level was recorded are shown in response axis, so that the tires are directly opposite the array Figure 57 for each of the three passbys for these truck mod- on Figure 56(a) and just to the right of the array on Figure els. Except for some details in the spectra, the skirted and 56(b). Thus, Figure 56(b) shows just behind the front tire, while on Figure 56(a) the perspective is to the side of the tire. These two images lead to the conclusion that the tire noise is directive slightly to the rear, say about 20 degrees off the per- pendicular to the truck side-plane. Similarly, in these views, the sound from rear tires appears to be directive forward. The images also suggest rearward directivity of the sound from the engine compartment: see the relative levels of the ground reflection in the upper images of the figure. Figure 56(c) was obtained during a passby for this truck on the following day but with different drive axle tires (G167), which are known to be noisier from other research (23, 29). The proof-of-concept testing cannot confirm, however, those results. 3.5.4.3 Passby Evaluations of the 9200i Truck: Frequency, Hz Localization of Engine Compartment and Exhaust Noise Figure 57. Sound autospectra for different tested models of the 9200i truck at 31 mph Figures 16 and 19 show the two body styles of the 9200i with engine at 1850 rpm (dots denote series truck that were tested. The style tested at the low-speed frequencies of images below).

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47 unskirted trucks with a muffler installed provided essentially ground reflection (in both frequency ranges) appears to gen- comparable spectra. With the muffler removed, however, the erally characterize the natures of the received sound levels. measured sound pressure levels increased substantially at fre- Although the source distributions in rightward [Figures quencies below 800 Hz. This frequency range is marked by the 58(b) and 59(b)] and leftward [Figures 58(c) and 59(c)] occurrence of a series of tones (peaks) at multiples of roughly travel are both localizable to the lower cab, the detailed source 90 to 100 Hz: this general periodicity is clear in the sample distributions lack symmetry, as discussed previously for the spectrum shown for the truck with no muffler in Figure 57. 5900i truck. Images of the noise sources of these truck models are pre- With the muffler removed, the expected localization of sented in Figures 58 through 62 for three frequencies denoted sound from the region of the exhaust opening is apparent in by the dots in Figure 57: 655 to 668 Hz, 709 Hz, and 975 to Figures 60 to 62 for the three frequencies, where the sound 980 Hz. These frequencies were selected on the basis of their source maps for the truck with a muffler are shown on the left proximity to the approximately 700 Hz harmonic of the side of each figure and labeled (a), and the source maps for tones. Of note in Figure 58, at 665 to 668 Hz all trucks with a the truck without a muffler are on the right and labeled (b). muffler installed, whether traveling to the right [Figure 58(a) At all three frequencies, the ground reflection of the exhaust and 58(b)] or to the left [Figure 58(c)], have the same char- noise is perfectly clear. Tire noise at the rear tire is apparent acteristic of combined engine and tire noise, with engine in Figure 60(a) with the sound level of 79 dB, but is only 75 noise apparently dominant. This behavior is similar to that in to 76 dB in Figure 61(b). The engine noise or forward tire the 975 to 980 Hz range, as shown in Figure 59. Propagation noise in the two truck runs are at the sound levels of 82 and of sound through the wheel well and around the skirt via 79 dB, respectively. (a) (b) (c) Figure 58. Source distribution at 665 to 668 Hz for various models of the 9200i truck with muffler traveling at 31 to 35 mph.