Acoustic Beamforming: Mapping Sources of Truck Noise (2009) / Chapter Skim
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From page 12... ...
3.2 Development of Experimental Design 3.2.1 Noise Mapping Technique Development Specific parameters that were considered relative to the beamforming technique developed in this study included the system requirements for heavy trucks (e.g., frequency range of interest, horizontal and vertical spatial resolution required, source to array distance) and design parameters for beamforming measurement application (e.g., number of microphones, array size and geometry, sample size and averaging time, processing software)
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From page 13... ...
The array is mathematically modeled as an acoustic lens that focuses on various points in the vertical plane in order to "scan" for the sources. The narrow band images, determined for 1 Hz band width, were summed to obtain images and sound levels in one-third octave frequency bands.
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From page 14... ...
3.2.3 Balance Between Array Aperture and Spherical Spreading Loss As the truck sources pass by the microphone array, there is a sound level change at the array microphones that is simply due to the spherical spreading loss resulting from the varying 14 Approx. Spiral Angle (b)
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From page 15... ...
15 (b) Ellipse 1(a)
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From page 16... ...
distance. Ignoring possible effects of source directivity, which are probably of little concern in the 250 Hz region of interest, this variation in the sound level provides some localization along the truck.
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is also shown in the table for comparison. 3.2.4 Design Conclusions The following conclusions were drawn from the design analysis described previously for the frequency band of highest A-weighted sound levels of emissions (during cruise)
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From page 18... ...
• The horizontal effective beam width during passby, including both beam width and spherical source spreading loss, would be about 9 ft (2.7 m) for the minor axis [horizontal dimension of 4.1 ft (1.2 m)
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From page 19... ...
. Another channel received a signal from a pair of photocells installed on tripods near the microphone array to register truck passbys.
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From page 20... ...
Only the lower section of the array with 25 microphones was tested using a Mackie Model SRM450 loudspeaker with a pink noise generator, as well as a lawnmower and a gas-powered string trimmer as noise sources in static positions in front of the array. These noise sources were placed alone and in combinations at varying distances from the array, both at on-axis and offaxis locations relative to the array.
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From page 21... ...
These tests were intended to determine the array's ability to identify the truck noise sources (e.g., engine, tires, and exhaust) in comparison with the loudspeaker, as the truck passed by the array at a constant speed.
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From page 22... ...
revolutions per minute, and gear settings used for the truck runs are summarized in Table 2. The next truck tested at the low-speed track was an International® 9200i Eagle truck (2006 model year)
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From page 23... ...
For the stationary tests, LA is the measured time-averaged sound level; for the passby tests, LA is the maximum sound level measured during the single truck run. No tests were performed at this track with a loudspeaker mounted on a truck.
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From page 24... ...
The average overall A-weighted sound levels measured for the stationary conditions on the high-speed track were given previously in Tables 6 through 11 for both the 25 and 50 ft microphone positions. For the passby truck tests, one-third octave band spectra were captured every 0.100 s with a 0.125 s exponential (fast response)
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From page 25... ...
Signals from these microphones were input to a LD 3000 two-channel analyzer for immediate sound intensity measurement in one-third octave bands. To determine the average sound intensity for an area in the plane parallel to the side of a vehicle, the probe was manually swept over specific subareas, as shown in Figures 23 and 24, several times as linear averaging was performed over a period
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From page 26... ...
Tests of the 5900i truck (aggressive tread rear tires) with trailer at the high-speed track.
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From page 27... ...
Sound power levels for subareas and total of the 4400 truck at 2000 rpm. 55 60 65 70 75 80 85 90 95 100 10 0 12 5 16 0 20 0 25 0 31 5 40 0 50 0 63 0 80 0 10 00 12 50 16 00 20 00 25 00 31 50 40 00 50 00 1/3-Octave Band Center Frequency, Hz A -w ei gh te d So un d Po w er L ev el , d BA Upper Cab - 2 Hood - 3 Muffler - 4 Lower Cab - 5 Wheel Well - 6 Total Truck Figure 26.
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From page 28... ...
50 55 60 65 70 75 80 85 90 95 10 0 12 5 16 0 20 0 25 0 31 5 40 0 50 0 63 0 80 0 10 00 12 50 16 00 20 00 25 00 31 50 40 00 50 00 1/3-Octave Band Center Frequency, Hz A -w ei gh te d So un d Po w er L ev el , d BA Wheel Well - 1 Gas Tank - 2 Muffler - 3 Muffler Area - 4 Exhaust Pipe - 5 Exhaust Outlet - 6 Total low compared to the sound power coming from underneath the truck and through the wheel well. Only at very low frequencies of 100 to 160 Hz is there a significant contribution of the exhaust outlet.
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From page 29... ...
Sound power levels for subareas and total of the 5900i truck at 1400 rpm. 50 55 60 65 70 75 80 85 90 95 10 0 12 5 16 0 20 0 25 0 31 5 40 0 50 0 63 0 80 0 10 00 12 50 16 00 20 00 25 00 31 50 40 00 50 00 1/3-Octave Band Center Frequency, Hz A -w ei gh te d So un d Po w er L ev el , dB A Front Wheel Well - 1 Between Fender & Gas Tank - 2 Gas Tank Area - 3 Muffler - 4 Exhaust Outlet - 5 Total Figure 31.
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From page 30... ...
at 1500 rpm. 50 55 60 65 70 75 80 85 90 95 10 0 12 5 16 0 20 0 25 0 31 5 40 0 50 0 63 0 80 0 10 00 12 50 16 00 20 00 25 00 31 50 40 00 50 00 1/3-Octave Band Center Frequency, Hz A -w ei gh te d So un d Po w er L ev el , d BA Front Wheel Well -1 Gas Tank Area - 2 Exhaust Pipe - 3 Exhaust Outlet - 4 Total
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From page 31... ...
In these figures and all other source images presented in this section, numbers in the color bar legend indicate approximately equivalent one-third octave band sound levels in decibels. At 922 Hz, for example, Figure 35 shows elliptical spots whose major and minor axes are complementary to those of the array.
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From page 32... ...
measured and (b) calculated signals for spherical source emission at 922 Hz [source elevation 1.98 m (6.5 ft)
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measured and (b) calculated signals for spherical source emission at 922 Hz [source elevation 1.98 m (6.5 ft)
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from the track, the array will localize and accurately define the magnitude of a stationary source. However, as the following subsection will discuss, truck noise sources are more complex, and that complexity is apparent in the images.
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From page 35... ...
calculated signals for spherical source emission at 922 Hz for two cross-range source locations xs = 0 and xs = 3.05 m [source elevation 1.24 m (4 ft) , offset 7.62 m (25 ft)
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From page 36... ...
measured and (b) calculated signals for spherical source emission at 922 Hz for 20.2 m (66 ft)
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From page 37... ...
Array Y X Z relative array-based acoustic levels are compared to establish the credibility of the array in rank-ordering the truck sound sources. Note that in all cases to be discussed, intensity levels are in one-third octave frequency bands, while the source images are generally in substantially narrower bandwidths: about 64 Hz constant bandwidth is used with the levels adjusted to provide approximate one-third octave band equivalent sound levels.
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From page 38... ...
with engine at 2000 rpm and deactivated spherical source.
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From page 39... ...
Spherical Source Wheel Well Lower Cab Image Intensity Image Intensity Image Intensity Image Intensity 231 250 95 96 <86 87 <86 86 600 630 89 91 <80 87 <80 85 922 1000 80 84 81 85 76 83 1430 1251 to 1600 79 85 77 85 76 83 1938 2000 84 85 77 85 82 83
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From page 40... ...
The array-measured image sound levels are presented as equivalent one-third octave band levels using the relationship where G(f) is the 1 Hz spectrum level, p0 is the reference pressure of 20 µPa, 0.233f is the bandwidth of a one-third octave band at frequency f, and the frequencies are selected to be within the one-third octave frequency bands of the compared intensity levels.
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From page 41... ...
This general observation may provide an important distinction between engine and tire noise sources because the latter should occur at the road surface and appear in the zone within approximately −0.5 m < y < +0.5 m (−1.6 ft < y < +1.6 ft)
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From page 42... ...
Wheel Well Gas Tank Region/ Lower Cab Image Intensity Image Intensity Image Intensity 461 500 72 76 *
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From page 43... ...
The autospectrum at the top of the figure is uncorrected for range and shows a maximum level at about 1.65 s into the run. At any frequency, the sound level varies by about 15 dB through the run.
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From page 44... ...
The spot labeled "T" in the 1400 rpm run is located near the road surface plane and appears to be due to the rear tires. The streaked spots labeled "D" lie below the ground plane between the front and rear wheels and appear to be within 1 dB for the two runs.
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From page 45... ...
the truck noise spectrum. Arrows indicate locations of frequencies (a)
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From page 46... ...
These two images lead to the conclusion that the tire noise is directive slightly to the rear, say about 20 degrees off the perpendicular to the truck side-plane. Similarly, in these views, the sound from rear tires appears to be directive forward.
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. The engine noise or forward tire noise in the two truck runs are at the sound levels of 82 and 79 dB, respectively.
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and (b) in order to illustrate clearly the differences between these noise sources.
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From page 49... ...
Source distribution at 709 Hz for the 9200i Eagle truck: (a) with muffler and (b)
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From page 50... ...
The vertical scans were used for the roadside measurements to provide vertical distributions of A-weighted one-third octave band levels using summations of narrower bandwidth spectra, so the data presented here are preliminary to that application. By bracketing the 922 Hz point of Figures 45 and 46, say, differences between 922 Hz and 937 Hz for this truck were determined to be negligible.
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Vertical scan for the 4400 truck stationary, the engine at 2200 rpm, and the spherical source activated at frequency: (a)
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Note that the depiction of the type shown in Figure 45 would show these other sources at another instant. When the onboard spherical source is deactivated, its contribution disappears from the vertical scan.
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From page 53... ...
3.6.2 Data Post-Processing Algorithm Modifications The data analysis relies heavily on a time-gated FFT technique that provides a detailed frequency–time decomposition of the truck's sound levels and frequency content during its passby. The sound pressure spectra are measured in 1 Hz frequency bands evenly distributed from 0 Hz to the selected upper limit (generally taken to be 3000 Hz)
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From page 54... ...
These vertical distributions are consistent with the two-dimensional beamformed images, as confirmed by evaluations of distributions with the loudspeaker source. A network of codes provided a sequence of steps used in resolving the acoustic images of the passby: display the time records of the passby; truncate the record to roughly 2 s; develop the image functions at 11.7 Hz intervals; and calculate A-weighted one-third octave band levels of image values, reference microphone levels, and vertical distributions of source levels.
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From page 55... ...
3.7.2 Image Results of the Vehicle Passbys Of 100 truck passbys recorded in a single day of data acquisition, passbys for 59 heavy trucks and 4 medium trucks were analyzed and are discussed here for the definition of source levels. Because the objective was to interpret source level images and vertical distributions of the sources, the following requirements were necessary for a passby to be analyzed: the vehicle in the curb lane, significant time–space separation between passby vehicles to allow for distinct one-to-one vehicle–passby identification, a photograph of the vehicle, and nearly constant vehicle speed.
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From page 56... ...
provide ensembles of sound levels by vehicle, one-third octave frequency band, time, and vertical position. These profiles can be used to define maximum levels, mean levels, and sound exposure levels -- all across the ensembles of trucks.
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From page 57... ...
Clearly seen is the correspondence between instances of high sound levels at multiples of approximately 450 Hz that are due to the tires, although high background levels are also seen below 200 Hz. Figure 73 shows the distribution of the one-third octave band A-weighted spectra for all 59 heavy trucks.
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From page 58... ...
Figure 73. One-third octave band sound spectra (in dBA)
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From page 59... ...
The statistical distributions of the sound levels measured across the population of heavy trucks are provided in the form of histograms in Figure 79 for the OASPL at the reference microphone and at the 0.4 m (1.3 ft) height for the one-third octave frequency bands centered at 500, 1000, and 2000 Hz.
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From page 60... ...
60 Figure 75. Source image maps of truck 60 for one-third octave frequency bands from 315 through 2000 Hz.
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From page 61... ...
Vertical distributions for one-third octave band and overall A-weighted sound levels (in dBA) for individual heavy trucks (truck images are to the same scale)
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From page 62... ...
Statistical distributions of the (a) overall sound levels and one-third octave band sound pressure levels (in dBA)
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From page 63... ...
For each figure, the sound levels are determined at the instant of the maximum OASPL at the reference microphone for each medium truck and are evaluated by the microphone array at that instant for each truck; then the mean levels, the highest levels, and the maximum 1 s SELs, respectively, are determined for each one-third octave frequency band and steering elevation for all four medium trucks. The absence of sound sources at the 2 to 4 m (6.5 to 13 ft)
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From page 64... ...
As can be seen in the figure, near the road surface the tire noise 64 Figure 81. Vertical distributions of maximum 1 s sound exposure levels (in dBA)
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From page 65... ...
The mean sound levels for the total heavy truck population appear to be minimally influenced by the exhaust noise sources; however, the maximum sound 65 Figure 83. Vertical distributions of highest A-weighted sound levels in one-third octave bands and of overall sound level (in dBA)
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From page 66... ...
for heavy trucks. The legend on the left identifies the truck ID numbers.
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From page 67... ...
vertical profiles of OASPL for truck 38 in one-third octave bands centered at 250, 630, and 1250 Hz.
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From page 68... ...
The key figures are 76 and 80, which show the distributions for heavy trucks. Figure 76 shows the mean A-weighted spectral and overall levels, while Figure 80 shows the distribution of maximum levels.
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From page 69... ...
. The apparent vertical distribution of truck noise sources can be effectively simulated by a simple system of two uncorrelated sources, similar to that presented in the Traffic Noise Model technical manual (32)
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From page 70... ...
Vertical distributions of source levels for 1000 Hz as measured at 6 m (20 ft) distance for heavy trucks: (a)
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