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Results

Kmax

The two sets of sound measurements of Superior's Kmax on the Moon 25 timber sale are shown in appendix 2. On April 19, we took one set of readings at one log landing site and on April 20 we took another set of reading at a second log landing. At the second location we took readings in both a west-east orientation and in a northwest-southeast orientation (see figure 3a in Methodology).

The average maximum readings of noise level of the Kmax at a given distance are displayed in figure 4a. Figure 4b displays the average maximum noise level plus 2 standard deviations. Statistically, this accounts for 97.5 percent of all maximum noise events in the sample (the upper tail of the normal distribution curve). Both figures display a trendline curve generated using power regression (least squares fitting) to more clearly illustrate the relationship between helicopter sound and distance.

Reviewing the average values (figure 4a), the data shows that the Kmax is below the target 92 dBA level at almost all distances. The more rigorous criterion in figure 4b presents the more conservative prediction that the Kmax should be below the 92 dBA level at distances greater than about 100 feet.

Figure 4a. Average sound of Kmax. The solid black line represents a power regression of the data. Click on image for larger view.

Figure 4b. Average sound of Kmax plus (+) two standard deviations. The solid black line represents a power regression of the data. Click on image for larger view.

Vertol 107

The two sets of sound measurements taken of Columbia's Vertol 107 (Cedar House timber sale) are shown in appendix 3. We took a set of measurements at an upper and lower log landing on 18 April and additional measurements at the lower landing on 19 April (see figure 3b in Methodology).

Figure 5a displays the average maximum readings of the noise level of the Vertol at a given distance. Figure 5b displays the average noise level plus 2 standard deviations. As was done for data on the Kmax, both figures display a curve generated using power regression (least squares fitting) to more clearly illustrate the relationship between helicopter sound and distance.

A review of the simple averages (figure 5a) indicates the 92 dBA level occurs at about 225 feet. The more rigorous criterion in figure 5b presents the more conservative prediction that the Vertol should be below the 92 dBA level at a distance of 450 feet or greater.

Figure 5a. Average sound of Vertol 107. The solid black line represents a power regression of the data. Click on image for larger view.

Figure 5b. Average sound of Vertol 107 plus (+) two standard deviations. The solid black line represents a power regression of the data. Click on image for larger view.

Note: The regression curves generated in figures 4 and 5 are consistent with the theoretical relationship of the decrease in sound pressure level as distance increases, that is, sound pressure level decreases by 6 dBAs with every doubling of distance (Harris 1998; Piercy and Daigle 1998). In this study, doubling of distance resulted in a reduction of 4 to 6 dBAs. These results are as expected for the relatively short distances at which we measured. Since our results do not account for atmospheric absorption, sound shadows, etc., using these results to extrapolate to greater distances would result in overestimating the predicted sound level.

Consistency of results with measurements taken by others.

Provided below are sound measurements of helicopters (Kmax, Boeing Vertol 234, and Mil Mi-8) taken by other investigators which gives the reader some additional information about the distance from these helicopters to the 80 to 90 dBA contour. The results reported by them are, in general, consistent with the results of this study.

Kmax

According to FAA (2001) (i.e., Advisory Circular 36-1H, Appendix 11), the Kaman Kmax with no load is listed as having a noise level of 82.5 dBA for a flyover at 492 feet (150 meters) above ground measuring instruments. This is roughly equivalent to 92 dBA at 165 feet (actual vertical distance, not horizontal from the helicopter). (This estimate is based on an extrapolation using geometric spreading calculations.)

(Note: FAA's regulations require that aircraft noise be quantified as part of Airworthiness Certification [14 CFR 36]. Subpart H deals specifically with helicopters. The intent of this requirement is to note progress in the control and abatement of aircraft noise that continues to be made. AC 36-1H provides the noise level data for aircraft certified under this authority. This regulation, however, only applies to helicopters designed for carrying passengers. Helicopters designed for "agricultural aircraft operations", for firefighting, or for caring external loads are not required to quantify their noise levels [14 CFR 36.1(a)(4)].)

During timber sales, resource specialists with the BLM and Forest Service have monitored noise generated by the Kmax to ensure noise levels are at acceptable levels for sensitive resources. Typically only one or a few measurements are taken, but the results recorded during that monitoring are within the range of those noted in this study. Chatt and Menten (2007) with BLMs Coos Bay District recorded sound data on the Kmax during an April 2006 timber sale as peak decibel levels (instead of the maximum slow dBA levels recorded in this study). They reported levels of 83 and 87 dBA (peak) at 250 to 325 feet.

Measurements taken of the Kmax during logging operations on the Olympic National Forest in 2006 recorded maximum sound levels of 118 dBA at 50 feet from the helicopter (at the service landing); 86 to 89 dBAs at distances of 490 feet; and 84 to 86 dBAs at distances greater than 500 feet (Piper 2007).

Oregon’s Occupational Safety and Health Division (2006) performed routine noise exposure monitoring of a Kmax during a helicopter logging operation by Superior Helicopters. They reported maximum readings of 90.6 to 93.7 dBA at a log landing at a distance of 150 feet.

Boeing Vertol 234

No specific sound measurement data on the Boeing Vertol 107 II (which was measured in this study), or its military version (the CH-46) could be located. However, in 1984 the FAA conducted a detailed sound measurement test of the larger Boeing Vertol (BV)-234 (military version - CH-47) (see FAA 1984). (Note: The BV-234 has twice the gross weight and over twice the lifting capacity of the BV-107). The FAA (1984) reports the sound level of the BV-234 for the flyover test (at 492 feet or 150 meters) as 87 to 89 dBA. This is approximately equivalent to 92 dBA at about 310 feet (vertical).

Mil Mi-8

Finally, Messinger and Tajnos (2006) performed a detailed sound analysis of the Mil Mi-8 helicopter during helicopter logging operations, including modeling the 70, 80, and 90 dBA sound contours. This helicopter appears to be intermediate in size between the Kmax and Vertol 107 (see table 1 of Messingerova and Tajnos 2006). A model of the 90 dBA noise level contour around the Mil Mi-8 helicopter during yarding activities is about 164 feet (50 meters) from the source.