HARRIS MILLER MILLER & HANSON INC.
15 New England Executive Park
Burlington, MA 01803
Tel. (781) 229-0707
Fax (781)229-7939
Email for Christopher Menge: cmenge@hmmh.com

TECHNICAL MEMORANDUM

To: Larry Lee, City of Bloomington, Minnesota
From: Christopher Menge
Subject: Proposed Black Dog Amphitheater - Comments on EAW
HMMH No. 296810
Date: June 12,2000
 

This memo incorporates our comments on the revised noise and meteorological analysis presented in the EAW. We have reviewed the plans for the `Proposed Black Dog amphitheater in Burnsville, MN, dated Apr. 13, 2000, the EAW dated May 8, 2000, the attached "Acoustical Analysis" report prepared by Paoletti Associates, Inc., dated Apr. 19, 2000 and the attached "Wind and temperature structure at the proposed Wild Amphitheater Site," report by Bruce F. Watson, dated March 28, 2000. We have also revised our independent sound level calculations at the seven Bloomington homes on the bluffs based on the proposed revisions to the Arnphitheater development.

Comments on EAW

Our primary concern relates to what we see as a significant potential for regular violations of the MPCA noise limits, as opposed to the "small potential for noise violations" and "noise problems under a rare combination of factors" as stated on pp 92 and 93 of the EAW. Our conclusion that violations could be the rule rather than the exception is based on information provided in the Paoletti and Watson reports included in the EAW. Specifically, the Paoletti report predicts L10 levels of 66 dBA at the nearest Bloomington bluffs homes under downwind/temperature inversion conditions. This is a very significant 11 dB higher than the MPCA nighttime limit. This level is predicted for the worst-case condition, which is for "extremely loud" concert events (105 dBA at the mixing table'), which are predicted to be 20% of the concerts, and for "full-size" venue, where the attendance is near full capacity. Note that this analysis accounts for only sound from the sound system, and not from the audience, which is predicted to reach 60 to 65 dBA at the homes on the bluffs.

Our files show a range of L10 levels measured at the sound table/mix console during popular music concerts in outdoor amphitheaters ranges from about 85 dBA to 110 dBA, and it depends primarily on the particular band performing. The average of our data is in the mid 90s, with 105 dBA toward the high end.

For "calm and neutral" atmospheric conditions, L10 levels are predicted to be 57 dBA, still 2 dB higher than the MPCA nighttime noise limits. The Paoletti report predicts that calm and neutral conditions will occur 66% of the time during concerts, with downwind conditions only 22% of the time. However, in our opinion, the Watson meteorological report contradicts this statement. Table 2 in the Watson report shows that during concert times (summer evenings), winds will - have a component from the South (sum from ESE to WSW) between 0 and 10 knots approximately 73% of the time, and between 4 and 10 knots about 39% of the time. Since we must be concerned with the entire Bloomington bluffs, we must consider all winds with a southerly component. Further, the Watson report indicates that in-valley inversions are likely to occur very frequently on summer evenings. We believe, based on our experience and on the comments of the bluffs residents, that inversions will enhance sound propagation from the valley to the bluffs, such that sound propagation similar to downwind conditions, or perhaps more enhanced than downwind conditions, will occur.

The new location of the theater is about 775 ft south of the original location, according to the plans submitted. Due to the increased distance alone, this would result in sound levels 1 to 2 dB lower at homes on the bluffs. Our evaluation of the lines of sight to the lawn seating area from the homes on the bluffs reveals that unobstructed lines of sight will exist to between 30% and 60% of the lawn, depending on the location of the homes. According to the Paoletti report, crowd noise levels are likely to be in the range of 60 to 65 dBA at the nearest homes, accounting for distance and a capacity crowd. Downwind and/or temperature inversion atmospheric conditions may increase these levels above 70 dBA, such that crowd noise could be up to 20 or 25 dBA higher than the existing background sound levels. This condition would be very intrusive.

With the potential for L10 sound levels over 65 dBA on the bluffs during loud concerts, we are concerned about the feasibility of a sound-level reduction plan in achieving an 11 dB reduction. Short of eliminating the loud bands altogether, we believe that several noise-abatement approaches will be needed to achieve such a significant reduction. A reduction of 10 dB can be difficult to achieve; it represents a 90% reduction in sound energy, which would be perceived as a halving in loudness.

The Paoletti report indicates that most of the sound-system noise that would radiate to the community would emanate from the distributed lawn loudspeakers, rather than from inside the pavilion. (This is confirmed by our analysis.) Note 8 of Tables 1-3 in the Paoletti report caught our attention; it states: "Lawn speakers are operating only for the full-size venue condition." We believe that restriction would impose difficult constraints on the operators of the amphitheater, and that there would be pressure to operate the lawn speakers during most concerts. Lawn tickets are usually priced significantly lower than those inside the pavilion, so we would expect lawn tickets to be sold for most if not all concerts. In addition, the sound on the lawn will be substantially improved (made clearer as well as louder) by using the lawn speakers, making the experience much more satisfying for the patrons. Therefore, we think the analysis should assume that the lawn speakers are operating during all concerts, to be realistic. Given that, the sound levels shown in Table 1 should be increased accordingly for the mid-size and minimum- size venues.

The noise mitigation proposals suggest a noise monitoring system will operate during all concerts on amphitheater property and that a community noise monitoring program will be implemented. These are both very good mitigation proposals. However, the amphitheater operators will need an effective enforcement procedure, when agreed-upon limits are exceeded. As we have seen from the numbers presented in the Paoletti and Watson reports, sound levels will regularly exceed the MPCA limits during loud concerts. The residents and the City of Bloomington will need protection and/or recourse in the cases where noise limit violations do occur.

Analysis Approach

Without any details in the EAW on the proposed sound system, our analysis focused on the expected music sound levels to be radiated from the side openings of the amphitheater. Our reading of the drawings shows that approximately the front 85 ft along each side of the amphitheater will be a full height wall, acting as barrier to sound that would otherwise radiate to the outside. However, the back sections of the sides (also approx. 85 ft long) remain open to the outside. It is these openings on both sides of the theater that represent the most significant areas to radiate sound generated inside the amphitheater to the closest homes in Bloomington. Our revised analysis accounted for the lowered roof at the building sides, which reduced the radiating areas by approximately one-half.

Our analysis considered the shielding effects of both the proposed concession and rest room buildings and the berm to the north of the amphitheater. We accounted for the baffler effect in cases where one or both of the structures interrupt the lines of sight to the homes. In our analysis, we examined the sound propagation to the following 7 properties (from west to east): 2833 Overlook Circle, 2501 W. 112th St., 2305 Sherwood Circle, 2257 Overlook Circle, 11024 Glen Wilding, 11010 Humbolt, and 10769 Hopkins Circle (see Figure 1). Neutral atmosphere sound propagation was based on normal spherical divergence with atmospheric absorption at 702 F and 50% RH. No ground-effect attenuation was assumed, due to the high sound propagation path. For enhanced propagation atmospheric conditions, we assumed that much of the building/berm shielding attenuation would be lost due the downwardly-curved paths that sound travels under downwind or temperature inversion conditions. We also checked this assumption with the USArmy Construction Engineering Research Laboratory's SoundProp model, which was developed specifically for long-distance sound propagation; the results were very similar. Our predicted sound levels are lower than those shown in the Paoletti report, which supports our conclusion that the lawn loudspeakers will be the controlling factor with respect to the Bloomington residents.

Final Remarks

Sound levels and audibility of events at many Bloomington homes overlooking the proposed amphitheater will be strongly dependent on a number of factors including:

 The final design of the amphitheater and its sound system
 Shielding elements between the theater and the homes (buildings, berm)
 The size and enthusiasm of the audience
 The particular band performing
 The atmospheric conditions during each concert
 Agreed-upon maximum sound levels from a noise monitoring system, and
           enforcement of those limits

Violations of the MPCA noise limits of up to 11 dB are expected to occur on a regular basis. We are concerned that an 11 dB reduction will be difficult to achieve without changing substantially the nature of the amphitheater's construction or operation.

Finally, several Bloomington residents along the Minnesota River bluffs have mentioned that they and guests frequently hear conversational (not shouting) voices in their backyards as if the people are very close when, in fact, these voices are from people on barges on the river over 1000 ft away. These situations imply that especially enhanced sound propagation conditions (such as channeling) may occur regularly between the river and the bluffs. We believe that a test measurement program should be undertaken during several summer evenings to determine quantitatively the significance of this effect. This would assist in determining the validity of assumptions made about the effects of atmospheric conditions on sound propagation in this area with unique topography. This will be the only way to determine the true summertime sound propagation conditions that occur between the valley and bluffs, and their potential effect on sound system and crowd noise.
 

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 Click Here for Comments By Bloomington Amphitheater Coalition's Consultants