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SENIOR AND STAFF CONSULTANTSANDREW C. CARBALLEIRA

EMILY L. CROSS, P. ENGWILLIAM J. ELLIOTAARON M. FARBOJOHN T. FOULKESBRION G. KONING

MICHAEL D. MAYNARD, CTSCHRISTOPHER A. STORCH

MARKETING MANAGERPATRICIA A. CASASANTO

ASSOCIATED CONSULTANTSNICHOLAS BROWSE, SMPTESTEWART RANDALL, CTS-D

MARTIN CALVERLEY, CTS

SENIOR PRINCIPALS WILLIAM J. CAVANAUGH, FASA, Emeritus GREGORY C. TOCCI, PE, FASA, PRESIDENT

PRINCIPALS DOUGLAS H. BELL LINCOLN B. BERRY TIMOTHY J. FOULKES, FASA, INCE, Bd. Cert. MATTHEW J. MOORE, CTS

ADMINISTRATOR DONNA L. RAFUS

CAVANAUGH TOCCI ASSOCIATES, INCORPORATED 327 F BOSTON POST ROAD, SUDBURY, MA 01776-3027 • TEL: (978) 443-7871 • FAX: (978) 443-7873 • E-MAIL: [email protected]

October 19, 2007 Patrick Reffett, Director of Community Development Town of Natick 13 East Central Street Natick, MA 01760 Subject: Helipad Sound Study – Measurements and Analysis Dear Patrick, We have conducted a study of noise created by the Boston Scientific helicopter. This study was commissioned by the Natick Planning Board for the purpose of evaluating sound from current helicopter operations and reviewing possible noise mitigation measures. The study has included the following measurements and analyses:

• One week continuous monitoring of ambient sound at three locations. • Sound measurements of a typical arrival and departure operation from five different

vantage points. • Calculation of helicopter sound exposure at existing and future residential locations. • Estimates of indoor sound at residences during helicopter operations. • Prediction of helicopter sound exposure if the helipad is moved to an alternate location on

the Boston Scientific campus. Continuous Sound Monitors The audibility and annoyance of a sound source is related to the amplitude of the sound compared with other existing ambient sounds. In order to understand the existing ambient environment, we installed continuous sound monitors at three locations for one week. Figure 1 is an area map showing middle Lake Cochituate and the surrounding area. The Massachusetts Turnpike is at the top of the photo and Route 9 at the bottom. The location of the helicopter landing pad and the three sound monitor locations are shown on the map. The first sound monitor was north of the helipad at Cochituate State Park. The second monitor was 2000 ft. east of the helipad in a residential back yard at 18 Crest Road. The third monitor location was at Mary Bunker Park, 3000 ft. southeast of the helipad.

MEMBER FIRM, NATIONAL COUNCIL OF ACOUSTICAL CONSULTANTS

Patrick Reffett, Town of Natick Helipad Sound Study

Cochituate State Park (1300 ft)

18 Crest Road(2200 ft)

Rail trail (120 ft)

Landing pad

Mary Bunker Park (3000 ft)

Figure 1: Area map showing sound measurement locations

Table 1 is a summary of the sound level statistics from each monitor. The full strip charts are attached in the Appendix. The monitors sampled the instantaneous sound level four times per second. For each thirty minute interval, the monitor calculates sound levels statistics. The most important of these statistics is the L90, also called the background sound level. This is the sound level exceeded during 90% of the interval. During the daytime, the background sound was the continuous traffic on the Massachusetts Turnpike. This background was punctuated by louder sounds from trucks, aircraft, birds, and other sources.

Patrick Reffett, Town of Natick Helipad Sound Study TABLE 1: Analysis of Ambient Sound Statistics (Natick Helipad Sound Study) Background (L90) Average (Leq) Statistical Max (L01) Location Day Night Day Night Day Night Cochituate State Park 55 45 58 47 65 55 18 Crest Road 50 43 55 48 70 50 Mary Bunker Park 52 47 55 50 63 53 The second important statistic is the Leq or the energy average sound level. The third statistic is the L1, the loudest 1% of the interval. For a one hour sample, the L1 represents the sound exceeded during 36 seconds of the hour. The L1is typically determined by some loud repetitive sound, such as loud trucks on a nearby roadway. The time history charts (see appendix) show the normally expected variation with the day/night cycle. The quietest hours occur between midnight and 6 am. The quietest background levels were 43 dBA. Helicopter Sound Measurements Our measurements of helicopter sound were conducted on Thursday, September 20, 2007, around noon. The weather was 80o, clear and sunny with no significant wind. The helicopter made its standard arrival approach and landed on the pad. It then idled for a few seconds and powered down. Five minutes later, the helicopter powered up, went through a warm up cycle of two to three minutes, and departed using its standard departure track. This is typical of a normal operation. During this observation, we had observers with sound meters at five different locations.

• 120 feet in front of the helipad • On the rail trail, adjacent to the helipad • Cochituate State Park • 18 Crest Road • Mary Bunker Park

These locations are shown in Figure 1. For each location, there is a time history graph showing the sound pressure level versus time. The following table summarizes the measured sound levels at each location.

Patrick Reffett, Town of Natick Helipad Sound Study TABLE 2: Helicopter Sound Measurements – September 20, 2007 Background Helicopter Duration within

Location Distance Sound Sound ___ 10dB of Max__ 1. Helipad 120 ft. 43 dBA 96 dBA 30 sec 2. Rail Trail 120 ft. 45 dBA 94 dBA 20 sec (adjacent to Helipad) 3. Cochituate State Park 1350 ft. 50 dBA 84 dBA 30 sec 4. 18 Crest Road 2200 ft. 44 dBA 60 dBA 30 sec 5. Mary Bunker Park 3000 ft. 47 dBA 68 dBA 30 sec

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Helicopter Sound Levels Measured on Landing Pad at 120 feetBoston Scientific - Natick, MA (September 20, 2007)

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Helicopter Sound Levels Measured at Rail Trail near Landing PadBoston Scientific - Natick, MA (September 20, 2007)

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Helicopter Sound Levels Measured at Cochituate State ParkBoston Scientific - Natick, MA (September 20, 2007)

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Helicopter Sound Levels Measured at 18 Crest RoadBoston Scientific - Natick, MA (September 20, 2007)

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Helicopter Sound Levels Measured on Beach at Mary Bunker ParkBoston Scientific - Natick, MA (September 20, 2007)


Patrick Reffett, Town of Natick Helipad Sound Study The helicopter sound level is a function of distance from the helicopter. The location at 18 Crest Road is shielded by trees, so helicopter sound at this location is 8 decibels quieter than Mary Bunker Park, even though 18 Crest Road is closer to the helipad. The location at Mary Bunker Park had a clear line-of-sight across the lake, with the only obstruction being trees on the far side of the lake and the helipad sound wall. Helicopter Sound to Residential Receptors In addition to the measurement locations used for the helicopter flyover test, there are other residential locations of interest. The closest houses on the Evergreen Road Peninsula are only 1400 feet from the helipad, and have a more direct view across the lake. There is also a series of houses on Lake Shore Road, 3000 feet from the helipad. There are also several new multi-family buildings under construction in the area. These include the new Cloverleaf Apartments, currently in construction; the proposed Chrysler Road Apartments; and the Nouvelle Natick Condominiums, which are currently in construction. For each of these locations, I have shown the distance to the helipad, and the calculated helicopter sound reaching the property. I have calculated sound for the helicopter in flight and also for the helicopter on the pad. This analysis is summarized in Table 3. TABLE 3: Helicopter Sound Calculated at Residential Receptors Near Boston Scientific Distance Helicopter Sound Location to Pad in Flight on Pad 18 Crest Road 2000 ft 60 dBA 45 dBA (measured Sept 07) Closest houses on Evergreen Rd. Peninsula 1400 ft 70 dBA 55dBA Lake Shore Road 3000 ft 68 dBA 55 dBA Nouvelle Natick Condos 2200 ft 60 dBA 40 dBA (shielded) (in construction) Cloverleaf Apartments 600 ft 85 dBA 61 dBA (shielded) (in construction) Chrysler Road Apartments 800 ft 83 dBA 59 dBA (shielded) (proposed)

Patrick Reffett, Town of Natick Helipad Sound Study Based on our measurements, the helicopter is ten decibels quieter when idling on the pad. For residential locations to the west and south, the existing warehouse building provides a sound barrier with an additional 10 decibels of sound reduction when the helicopter is on the pad. The in-flight sound level represents the loudest part of an arrival or departure operation. This lasts 20 – 30 seconds depending on the location. The “on pad” sound level is longer, and may last 4-5 minutes. As expected, the most significant helicopter noise is at the Cloverleaf Apartments and the proposed Chrysler Road Apartments, due to their proximity to the helipad. Fortunately, these new buildings will be modern, commercial grade construction with steel frames and concrete floor slabs. They will provide better isolation of all exterior sound, and steel/concrete structures will not resonate with low frequency sound the way wood frame houses do. The new construction will be sealed more tightly and the windows will be more resistant to sound transmission. Table 4 shows a comparison of exterior versus interior sound levels for these residential locations. This analysis is based on the in-flight sound levels, since the “on pad” sound level is barely significant indoors. For this analysis, I used a 20 decibel reduction for a standard wood frame house, and 30 decibel reduction for a new multi-family structure with a concrete and steel floor structure. These values are conservative based on our experience measuring and analyzing many different buildings. TABLE 4: Helicopter Sound Calculated at Residential Receptors Near Boston Scientific Maximum Distance Helicopter Sound Duration Location to Pad Exterior Interior of Sound 18 Crest Road 2000 ft 60 dBA 40 dBA 30 sec (measured Sept 07) Closest house on Peninsula 1400 ft 70 dBA 50 dBA 30 sec Lake Shore Road 3000 ft 68 dBA 48 dBA 40 sec Nouvelle Natick Condos 2200 ft 60 dBA 30 dBA 20 sec (construction) Cloverleaf Apartments 600 ft 90 dBA 60 dBA 60 sec (construction) Chrysler Road Apartments 800 ft 85 dBA 55 dBA 60 sec (proposed) Table 4 also shows the duration of the maximum sound, which is in the range of 30-60 seconds for each arrival or departure.

Patrick Reffett, Town of Natick Helipad Sound Study Analysis of Low Frequency Sound The Boston Scientific helicopter is larger than the typical news or police helicopter and creates significant low frequency sound. Most of this study is based on A weighted sound, since dBA is the predominant metric for environmental noise studies. A weighting is shown to correlate with human perception of loudness. Unweighted sound levels are dominated by low frequency sound, and humans are less sensitive to low frequencies. Figure 7 shows a time history for both the A weight sound and the unweighted sound during the 10 minute interval including the arrival and departure for the helicopter operation of September 20, measured at Mary Bunker Park.

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Helicopter Sound Levels Measured on Beach at Mary Bunker ParkBoston Scientific - Natick, MA (September 20, 2007)

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The unweighted helicopter sound was louder than the A weighted sound, but this is also true of the continuous background sound. For the unweighted sound the helicopter maximum was 77dB vs. a background of 64 dB. The A weighted helicopter maximum was 68 dBA vs. background of 47 dBA. This is not to say that the low frequency sound is not significant to the annoyance experienced by residents, but the magnitude of the helicopter noise above background sound is greater when using the dBA scale than for the unweighted sound.

Patrick Reffett, Town of Natick Helipad Sound Study Analysis of Alternate Helipad Locations Given the level of concern regarding the helicopter noise, the question of alternate helipad locations (on the Boston Scientific property) needs to be asked. I have analyzed the distance and shielding geometry for two alternate helipad locations, at grade in what is now a parking lot near Rt. 9, and on the roof of the office building (formerly the Carling Building). These locations are not necessarily feasible from a flight safety standpoint, but serve to illustrate how helicopter noise exposure might change as a function of helipad location. Table 5 shows predicted helicopter sound levels at four important residential locations in the immediate vicinity of Boston Scientific. In addition to Evergreen Road, Lakeshore Road, and the new Cloverleaf apartments, the neighborhood directly across Rt. 9 from the Boston Scientific campus has been included. The table shows distance to the helipad and estimated helicopter sound (both in-flight and on-pad) for the existing helipad and the two hypothetical alternate helipad locations described above. TABLE 5: Analysis of Noise with Alternate Helipad Locations Distance/Estimated Sound (in flight/on-pad) Helipad Location Receptor Helipad in South Helipad on Roof Location Existing Pad Parking Lot of Carling Building Evergreen Road 1400 ft. 2500 ft. 2700 ft. Peninsula 70dBA/55dBA 65dBA/50dBA 68dBA/55dBA Lakeshore Road 3000 ft. 1500 ft. 2000 ft. 68dBA/55dBA 64dBA/54dBA 72dBA/58dBA Cloverleaf 600 ft. 1400 ft. 2000 ft. Apartments 85dBA/61dBA 75dBA/55dBA 75dBA/55dBA Archer Drive 2200 ft. 800 ft. 350 ft. Robinhood Road 60dBA/45dBA 83dBA/59dBA 90dBA/75dBA


Figure 8: Area Map Showing Alternate Helipad Locations and Residential Areas

Patrick Reffett, Town of Natick Helipad Sound Study The residential areas on Evergreen Road and Lakeshore Road could expect slight reductions in helicopter noise for the south parking lot helipad. For a helipad on the roof of the Carling Building, the Evergreen Road neighborhood would experience a slight decrease and the Lakeshore Road neighborhood a slight increase in noise compared to existing. For the Cloverleaf apartments, the table shows a 10 decibel reduction in airborne helicopter noise and a 6 decibel reduction on-pad noise. The Robinhood Road neighborhood would experience much higher levels of helicopter sound for both the alternate helipad locations. There are 10-20 houses within 800 feet of these helipad locations and another 50 or so houses within 1500 feet. As mentioned before, wood frame houses are less effective isolating exterior noise than commercial construction (such as the Cloverleaf Apartment building), so the problem is even more significant when interior sound is considered. Sound Mitigation Measures The sound from the helicopter operation is significant, especially for closer receptors and for those with a clear view across the lake. The maximum helicopter sound at Mary Bunker Park was 20 decibels above the background sound. Fortunately the duration of the sound is very limited, less than 60 seconds per operation, and the operations are relatively infrequent. The current permit limits the number of helicopter operations, and it is my understanding there are weeks with no operations. Sound mitigation measures generally fall into three categories – source, path, and receiver. In this case, I see no opportunity for further noise control of the source. It would also be impractical to modify the receiver, since this would involve soundproofing private homes. Attenuation of the sound path is not practical for an airborne sound source. Boston Scientific has already built a significant sound barrier wall (200 ft. long by 17 ft. tall) along the Rail Trail to mitigate sound propagating to the East while the helicopter is on the pad, and our measurements show the effectiveness of this barrier. The only additional mitigation measure I see is to carefully consider the operating schedule to avoid creating this noise during times when residents are most sensitive. Please contact me if you need additional information. Sincerely, CAVANAUGH TOCCI ASSOCIATES, INC.

Timothy J. Foulkes tjf/pac/07283


Town of Natick Helipad Sound Study

October 2007

Appendices

1) Sound Monitor Graph – Cochituate State Park

2) Sound Monitor Graph – 18 Crest Road

3) Sound Monitor Graph – Mary Bunker Park

4) Table A1 – Analysis of Rail Trail Sound Barrier

5) (reference) dBA Sound Levels

6) (reference) Environmental Sound statistics


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September 20, 2007 - September 26, 2007

Thursday Friday Saturday Sunday Monday Tuesday WednesdaySeptember 20 September 21 September 22 September 23 September 24 September 25 September 26

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Patrick Reffett, Town of Natick Helipad Sound Study TABLE A1: HELICOPTER SOUND, LANDING PAD VS. RAIL TRAIL Operating Mode At Pad Rail Trail Difference Helicopter idle on pad 80 dBA 70 dBA 10 dBA Helicopter arrival (100 ft. altitude) 96 dBA 94 dBA 2 dBA Helicopter departure (100 ft. altitude) 96 dBA 90 dBA 6 dBA

c tocci associates, incorporatedutil.wickedlocal.com/s/pdf/helipad.pdf · is an area map showing...

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