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7. Initial Calibrated Results
The results of the TNM Validation Study Phase 1 are now presented for the initial calibrated data in terms of several variables. For the initial calibrated results, the data were calibrated using a reference microphone, as described in Section 6.2. Also, the data were processed according to Section 5, the case where data collected during all wind conditions (all-wind data) are retained. Strong wind data have not yet been eliminated; the results for data captured during limited wind conditions [data captured during wind speeds exceeding ~11 mph (5 m/s) were removed - referred to as the strong-wind-removed data] are presented in Section 8. Results for the calibrated all-wind data indicate how well TNM is computing sound propagation effects, accounting for such things as ground absorption and diffraction.
First, a direct comparison is made between the TNM-predicted sound levels and the measured sound levels (as was also shown for the uncalibrated data). The remaining results are presented in terms of the difference or delta between the TNM-predicted sound levels and measured sound levels as a function of a specific variable. The variables investigated are distance, height, and wind speed and direction. In addition to presenting all the data from all the sites as a whole, the data are also divided into three categories: open area, acoustically soft ground sites {e.g., field grass [effective flow resistivity (F) . 150 cgs Rayls] or lawn [F . 300 cgs Rayls]}; open area, acoustically hard ground sites [e.g., pavement or water (F . 20,000 cgs Rayls)]; and barrier, acoustically soft ground sites. This is done in order to reveal possible site-specific influences on the results. (Measurements for barrier; acoustically hard ground sites were not performed as part of the Phase 1 study; the barrier, acoustically soft ground sites will be referred to as just barrier sites for the remainder of this section.)
Plot and table descriptions that were detailed in Section 6 will be repeated in this section for convenience.
7.1 Direct Comparison of TNM-Predicted and Measured Sound Levels
The first investigation of the results was simply to directly compare the TNM-predicted sound levels to the measured sound levels, as with the uncalibrated results. For presentation, the set of graphs corresponding to the all-wind data results are seen in Figures F.2 through F.6 in Appendix F.
For a direct comparison, the data are plotted with the horizontal axis being the measured sound levels and the vertical axis being the TNM-predicted sound levels. Each 15-minute data block (15-min Leq) is represented as an orange X, where the number of data points is stated in the lower right corner of the figure. A dashed blue line represents the linear fit and solid green lines show the 95 percent confidence band. A solid black diagonal line symbolizes perfect agreement between TNM-predicted data and measured data. Data points that fall above (to the left of) this line indicate over-prediction and points that fall below (to the right of) this line indicate underprediction. The text at the top of the figure indicates the type of site for which the data correspond.
In addition to the graphs found in Appendix F, Table 5 in this section gives numerical values corresponding to the statistical elements of the graphs. In this table, the relation of the linear fit to the line of perfect agreement is examined along with the width of the 95 percent confidence band; values for five variables are stated across the columns. The first two variables concern the linear fit; values for both the average difference and the average of the absolute value of differences are stated. The first variable, the average difference, indicates how well TNM is performing over a broad range of sound levels, combining the over- and under-predictions. The second variable, the absolute value of differences, indicates how well TNM is performing as a function of the amplitude of the over- and under-predictions. This second variable can also indicate the consistency of over- or under-predictions for a range of sound levels. The third, fourth, and fifth variables in the table are the average, maximum, and minimum values of the 95 percent confidence band width, respectively. If all three values are small, and the maximum and minimum values are similar, this indicates that an average of the data shows little variation in amplitude over a broad range of sound levels; as such, a similar data set (sound levels measured and predicted under the same conditions) would provide similar results.
Results
The results are now presented for Appendix F (Figures F.2 through F.6) and Table 5, the data set representing all the wind conditions. The initial calibrated data for all sites (Figure F.2 and Table 5) show that TNM is in excellent agreement with the measured sound levels, the average difference being only -0.8 dB. There is only slight under-prediction across all sound levels, the confidence band width being very narrow, an average of 0.5 dB.
Note: positive values indicate over-prediction; negative values indicate under-prediction.
The results for the open area, acoustically soft ground sites (Figure F.3 and Table 5) indicate excellent agreement between predicted and measured data, with the average difference being - 0.3 dB. There is very slight under-prediction at all sound levels with some variation, the average confidence band width being 1.0 dB, effectively rendering the -0.3 dB average difference statistically insignificant, i.e., there is no statistical difference between the predicted and measured values for the open area, acoustically soft ground sites.
For open area, acoustically hard ground sites, the data are first presented as a group (Figure F.4 and Table 5) then divided into far distance (lower sound levels) and near distance (higher sound levels) data (Figure F.5 and Table 5). As a group, a substantial skew is seen in the linear fit, where the average difference from perfect agreement is 0.6 dB. For near distances, the average difference for the linear fit is only -0.5 dB, with the average confidence band width being 0.7 dB; this indicates excellent agreement. For far distances, the average difference is 2.2 dB, with the average confidence band width being 0.5 dB; this indicates some over-prediction.
Lastly, for barrier sites (Figure F.6 and Table 5), results show good agreement, indicating an average under-prediction of -1.1 dB. The confidence band width is narrow, an average of 0.6 dB, over all sound levels.
Discussion
Overall, TNM is performing very well in a direct comparison to all acoustically clean3 data collected, regardless of the wind condition. The average difference from perfect agreement is less than a decibel. In examining the performance by site type, TNM is performing very well for open area, acoustically soft ground sites; open area, acoustically hard ground sites at near distances; and barrier sites - all within an average of 0.3 to1.2 dB of perfect agreement. The only difference of concern arises for open area, acoustically hard ground sites at far distances [in these cases, beyond 900 ft (~275 m) from the roadway], where TNM is over-predicting an average of 2.2 dB. Please refer to Section 8.1 for a discussion on this over-prediction.
3The highway traffic noise data are not contaminated by other noises. See Sections 3.4 and 5.1 for more details.
7.2 Differences in Sound Levels as a Function of Distance and Height
The second investigation of the results examined the average differences (TNM minus measured) and standard deviation as a function of the distance of the receiver from the roadway or noise barrier and height of the receiver above the ground. It is important to investigate these variables: multiple distances can help determine how far from the road TNM is valid; receiver height above the ground can help validate ground effects (the microphone closer to the ground should be more affected by the ground surface) and can help in examining a noise barrier's shadow zone. For presentation, the set of graphs corresponding to the all-wind data results as a function of distance and height are seen in Figures F.7 through F.12 and Tables F.1 through F.3 in Appendix F.
For these sets of graphs, the data are plotted with the horizontal axis being the distance from either the center of the near travel lane of the roadway or the barrier and the vertical axis being the average difference (TNM minus measured) in sound levels. Also shown vertically is the standard deviation of the data from the average values. A solid black horizontal line at a value of 0 dB for the average difference symbolizes perfect agreement between TNM-predicted data and measured data. Data above this line indicate over-prediction and data below this line indicate under-prediction. The text at the top of the figure indicates the type of site for which the data correspond, with the specific sites listed in the legend. The text also indicates if the data presented are for the 5-ft (1.5-m) height position or the 15-ft (4.5-m) height position. For the tables in Appendix F, values for the average difference in sound levels are presented along with the standard deviation for each microphone location at each site.
In addition to the graphs and tables found in Appendix F, the table in this section, Table 6, gives the values for the average difference in sound levels for each type of site (open area, soft ground; open area, hard ground; and barrier). The averages are given for ranges of distances from the highway or noise barrier; note that only some ranges of distances are covered for each type of site. The data are also divided by the two different heights (5 ft and 15 ft or 1.5 m and 4.5 m), where averages over all distances are given in the right hand column.
Note: positive values indicate over-prediction; negative values indicate under-prediction.
Results
The results are now presented for Appendix F (Figures F.7 through F.12 and Tables F.1 through F.3) and Table 6, the data set representing all wind conditions. The data for the open area, acoustically soft ground sites at the 5-ft (1.5-m) height location (Figure F.7 and Table F.1) show that the average differences between the TNM-predicted and measured sound levels for each position at each site are within about 2.0 dB, except for Site 02MA, where TNM is overpredicting by 2.7 dB at the 200-ft (~60-m) position and under-predicting by 2.5 dB at the 600-ft (~180-m) position. The average difference of all these sites is 0.3 dB (Table 6) and the standard deviations range from 0.3 to 1.5 dB. For the 15-ft (4.5-m) height locations (Figure F.8 and Table F.1) the average differences for each position at each site are within about 1.5 dB, except for Site 02MA, where TNM is under-predicting by 2.5 dB at the 600-ft (~180-m) position, and Site 10CA-open, where it is under-predicting by 4.1 and 3.7 dB. The average difference for all these sites is -0.8 dB (Table 6) and the standard deviations range from 0.1 to 1.5 dB. In examining the different ranges of distances (in Table 6), it is seen that there is no overall trend in variation as a function of distance; with height, the differences are generally less for the 5-ft (1.5-m) position, except in the 501- to 1000-ft (~150- to ~300-m) range, where the differences are about the same for the 5- and 15-ft (1.5- to 4.5-m) heights.
The data for the open area, acoustically hard ground sites at the 5-ft (1.5-m) height location (Figure F.9 and Table F.2) show that the average differences between the TNM-predicted and measured sound levels for each position at each site range from 0.0 to 4.0 dB, the larger differences generally tending to be at farther distances. The average difference for all these sites is 0.9 dB (Table 6) and the standard deviations range from 0.2 to 0.9 dB. For the 15-ft (4.5-m) height locations (Figure F.10 and Table F.2), the average differences for each position at each site are within 1.5 dB, except for Site 17CT, where TNM is over-predicting by 2.8 dB at the 1273-ft (~390-m) distance. The average difference for all these sites is 0.1 dB (Table 6) and the standard deviations range from 0.0 to 0.8 dB. In examining the different ranges of distances (in Table 6), it is seen that there is a trend in variation as a function of distance; farther distances show greater differences, although the differences in the 301- to 500-ft (~90- to ~150-m) range are larger than the 501- to 1000-ft (~150- to ~300-m) range differences. With height, the differences reveal no trend.
The data for the barrier sites at the 5-ft (1.5-m) height location (Figure F.11 and Table F.3) show that the average differences between the TNM-predicted and measured sound levels for each position at each site are within about 2.0 dB, except for Sites 04CT, 08CA, and 09CA, where TNM is under-predicting by about 2.5 to 4.0 dB at most positions. The average difference for all these sites is -1.1 dB (Table 6) and the standard deviations range from 0.2 to 2.7 dB. For the 15- ft (4.5-m) height locations (Figure F.12 and Table F.3) the average differences for each position at each site are within about 2.0 dB, except for Sites 04CT and 09CA, where TNM is underpredicting by about 2.0 to 4.0 dB at most positions. The average difference for all these sites is -0.5 dB (Table 6) and the standard deviations range from 0.2 to 2.0 dB. In examining the different ranges of distances (in Table 6), it is seen that there are no strong trends in variation as a function of distance or height for these sites, except that there is under-prediction in all cases.
Discussion
Where all data are included regardless of wind speed, the results (as a function of distance and height) indicate that the average difference between the TNMpredicted sound levels and the measured data is mostly within 1.5 to 2.0 dB, with several sites' differences being within 1.0 dB. The exceptions are few and occur only at some microphone positions for some sites; discussions regarding these sites will follow. Also, in examining the sites by type, the results do not show any strong trends due to the height of the receiver (microphone) or distance from the roadway, except for the open area, hard ground sites, where the tendency is toward larger differences between TNM-predicted data and measured data at the farther distances [greater than 300 ft (~90 m)].
For the open area, acoustically soft ground sites 02MA and 10CA-open, some under- and overpredictions are observed. Please refer to the discussion in Section 8.2, after the strong-windremoved data results are introduced, for further explanation. For the open area, acoustically hard ground sites 15CA and 17CT, the over-predictions seem to be distance dependent (greater with greater distance). For the barrier sites (04CT, 08CA, and 09CA), there are some underpredictions. Again, please refer to the discussion in Section 8.2 for further explanation.
7.3 Differences in Sound Levels as a Function of Wind Speed and Direction
The third investigation of the results for the all-wind data examined the differences (TNM minus measured) as a function of wind speed and direction. It is important to investigate TNM's performance in terms of wind variables since these are not accounted for in the model; under certain conditions, measured sound levels are affected by the wind, influencing the differences between TNM-predicted and measured sound levels. For presentation, the set of graphs corresponding to the all-wind data results as a function of wind speed and direction are seen in Figures F.13 through F.15 in Appendix F.
For these sets of graphs, the data are plotted with the horizontal axis being the wind speed and the vertical axis being the difference (TNM minus measured) in sound levels. Each data point represents a 15-minute data block (15-min Leq) and is further categorized by wind direction. For characterization of wind direction, the wind component perpendicular to the roadway is specified; the three wind direction categories are up, down, and calm. "Up" signifies an upwind condition (wind blowing in the direction from the receiver to the roadway) at a speed greater than or equal to 2.2 mph (1 m/s); "Down" signifies a downwind condition (wind blowing in the direction from the roadway to the receiver) at a speed greater than or equal to 2.2 mph (1 m/s); and "Calm" signifies that the perpendicular wind component is less than 2.2 mph (1 m/s) in either direction. A solid black horizontal line at a value of 0 dB for the difference symbolizes perfect agreement between TNM-predicted data and measured data. Data above this line indicate over-prediction and data below this line indicate under-prediction. The text at the top of the figure indicates the type of site for which the data correspond, with the specific sites listed in the legend. It should be noted that fewer data points are available for this analysis than for that in Section 7.1 (also presenting 15-minute data blocks) because the wind had to be directionally consistent throughout the 15 minutes; otherwise, the data point was discarded.
In addition to the graphs found in Appendix F, Tables 7 through 9 in this section give numerical values corresponding to the graphs. In these tables grouped by site type, averages for the wind speed are presented for each site, along with the corresponding values for the average difference in sound levels categorized by wind direction. Also, overall averages are given at the bottom of each table for all sites combined. For the wind study, results are not presented as a function of microphone height above the ground; investigation is planned for later phases of the study.
Results
The results will now be described in the order they are presented graphically in Appendix F and in tables in this section. The data for the open area, acoustically soft ground sites (Figure F.13 and Table 7) show that, for the data as a group, there is no strong trend indicated. There is one site, however, that gives the indication of over-predictions for upwind conditions and underpredictions for downwind conditions; this is Site 02MA. Although Site 10CA has upwind data, it does not indicate a trend; it is seen that the wind speeds are high at this site - perhaps there are other effects influencing the results. Averages over each wind condition for all open area, soft ground sites show a -0.1-dB under-prediction for upwind conditions, a -0.4-dB under-prediction for downwind conditions, and a 0.4-dB over-prediction for calm conditions.
Note: positive values indicate over-prediction; negative values indicate under-prediction.
Note: positive values indicate over-prediction; negative values indicate under-prediction.
The data for the open area, acoustically hard ground sites (Figure F.14 and Table 8) show that, for the data as a group, there is no strong trend indicated. The many downwind data points are showing over- and under-predictions and the two upwind data points (for Site 13CA) are showing under-predictions. Notice that the wind speeds at some of the sites are rather high - perhaps there are other effects influencing the results. Averages over each wind condition for all open area, hard ground sites show a -0.8-dB under-prediction for upwind conditions, a 0.4-dB over-prediction for downwind conditions, and a 0.9-dB over-prediction for calm conditions.
Note: positive values indicate over-prediction; negative values indicate under-prediction.
The data for the barrier sites (Figure F.15 and Table 9) show that, for the data as a group, there is a trend indicating that upwind conditions may cause over-prediction by TNM and downwind conditions may cause under-prediction by TNM. Sites 06CA and 11CA show over-prediction in upwind conditions. Sites 08CA, 12CA, and 14CA show under-prediction in downwind conditions. Site 09CA, the only site with upwind, downwind, and calm conditions needs to be examined closely.
As stated earlier (referred to in Section 7.2, described in Section 8.2), Site 09CA shows differences offset in the negative direction. If the predicted 09CA data were to be shifted up 3.0 dB as an approximation to account for additive reflections (see Section 8.2), the data set would show the calm data differences being distributed around the zero line, the upwind differences indicating some under- and some over-predictions, and the downwind differences indicating under-predictions. Averages over each wind condition for all barrier sites show a -0.5-dB underprediction for upwind conditions, a -2.6-dB under-prediction for downwind conditions, and a - 0.8-dB under-prediction for calm conditions. Upon shifting the 09CA averages by positive 3.0 dB, the averages over each wind condition would show a perfect agreement for upwind conditions, a -1.8-dB under-prediction for downwind conditions, and a -0.4-dB under-prediction for calm conditions.
Discussion
For the data representative of all wind conditions, TNM's accuracy seems to be relatively independent of the wind (affected on average less than 0.5 dB) for the open area, soft ground sites; TNM-predicted sound levels are closer to the measured levels in downwind conditions (increasing the accuracy) because of other over-predictions for the open area, hard ground sites; and TNM's accuracy is dependent on the wind (affected on average up to 2.0 dB) for barrier sites.
TNM's accuracy for certain cases should be dependent on wind conditions since the model calculates sound levels for a windless environment. In general, upwind conditions can lower the measured sound levels at the receiver position and downwind conditions can raise the measured sound levels at the receiver position, the effects being greater with higher wind speeds. Refraction caused by the wind can affect both soft-ground attenuation and barrier insertion loss [Beranek 1992]. In addition, over hard ground sites, the sound can be channeled in downwind conditions (raising the received sound levels).
In examining the results as a function of wind, the open area, soft ground data overall indicate neutrality for upwind conditions and some under-prediction for downwind conditions. For Site 02MA, however, it is seen that there is some over-prediction for the upwind condition and some under-prediction for the downwind condition (possibly behaving more like a barrier site because of the large ground undulations). With soft ground sites there are some counteracting effects. Under upwind conditions, the sound can be refracted upward, away from the ground and the microphones (hence reducing the sound levels at the microphones) but would then be interacting less with the ground (hence decreasing the soft-ground attenuation and increasing the sound levels at the microphones). This may be why the overall results indicate that the accuracy of TNM for open area, soft ground sites is independent of wind.
In examining the results as a function of wind, the open area, hard ground data overall indicate some under-prediction for upwind conditions and some over-prediction for downwind conditions. Under upwind conditions, the sound can be refracted upward, away from the reflective hard ground and the microphones (hence reducing the sound levels at the microphones). Under downwind conditions, the sound can be refracted downward then reflected upward, channeling the sound (hence raising the sound levels at the microphones, especially over long distances). The results do not readily support this, although some explanation can be offered. For the downwind results, even though Table 8 indicates a 0.4 dB overall overprediction and not an under-prediction as one would expect, it is important to recall results presented earlier in this section. TNM is tending to over-predict for farther positions at hard ground sites; these over-predictions may overpower the under-predictions from downwind effects, thereby merely lowering the over-predictions. For the upwind results, there are only two data points, under-predicted less than 1.0 dB; no explanation is offered for this. For hard ground sites, results do not indicate that the accuracy of TNM is very affected by the wind, but part of the non-effect is due to general over-prediction at hard ground sites; this is noted for investigation in later phases of the TNM Validation Study.
In examining the results as a function of wind, the barrier data overall indicate some overprediction for upwind conditions and some under-prediction for downwind conditions. Many of the sites indicate this trend, especially the ones where upwind or downwind data, when compared to calm wind data, show a definite increase or decrease, respectively, from the calm wind results (Sites 06CA, 08CA, 09CA, and 14CA in Figure F.15). Referencing back to the distance and height data (Section 7.2), some of the under-predictions that were seen (Sites 08CA and 09CA) may be due to the wind. At noise barrier sites, under upwind conditions, the sound can be refracted upward, making the barrier more effective (hence reducing the sound levels at the microphones). Under downwind conditions, the sound can be refracted downward behind the barrier, making the barrier less effective (hence raising the sound levels at the microphones). Results from the barrier sites indicate that wind is a factor in TNM's ability to predict precisely accurate results. It is seen, however, that the average wind influence is less than 2.0 dB (less than 3.0 dB without adjusting Site 09CA); this is noted for investigation in later phases of the TNM Validation Study.
7.4 Summary of Initial Calibrated Data Results
(Excerpts from Discussion in each of the subsections of Section 7)
Direct Comparison of TNM-Predicted and Measured Sound Levels.Overall, TNM is performing very well for all acoustically clean (see footnote in Section 7.1) data collected, regardless of the wind condition. The average difference from perfect agreement is less than a decibel. In examining the performance by site type, TNM is performing very well for open area, acoustically soft ground sites; open area, acoustically hard ground sites at near distances; and barrier sites - all within 0.3 to1.2 dB of perfect agreement. The only unfavorable trend arises for open area, acoustically hard ground sites at far distances [in these cases, beyond 900 ft (~275 m) from the roadway], where TNM is over-predicting an average of 2.2 dB.
Differences in Sound Levels as a Function of Distance and Height.Where all data are included regardless of wind speed, the results indicate that the average difference between the TNMpredicted sound levels and the measured data is mostly within 1.5 to 2.0 dB, with several sites' differences being within 1.0 dB. The exceptions are few and occur only at some microphone positions for some sites. Also, in examining the sites by type, the results do not show any strong trends due to the height of the receiver (microphone) or distance from the roadway, except for the open area, hard ground sites, where the tendency is toward larger differences between TNMpredicted data and measured data at the farther distances [greater than 300 ft (~90 m)].
Differences in Sound Levels as a Function of Wind Speed and Direction.For the data representative of all wind conditions, TNM's accuracy seems to be relatively independent of the wind (affected on average less than 0.5 dB) for the open area, soft ground sites; TNM-predicted sound levels are closer to the measured levels in downwind conditions (increasing the accuracy) because of other over-predictions for the open area, hard ground sites; and TNM's accuracy is dependent on the wind (affected on average up to 2.0 dB) for barrier sites.
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