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3. d) Amphitheater Acoustical Evaluation Acoustical Design, Consulting, and Testing 2520 Central Ave NE, Suite 151 Minneapolis, MN 55418 p. (612) 567-3533 1 May 13, 2021 Prepared by: Bryan Knisley Central Acoustics bryank@centralacoustics.com c. (612) 987-3533 Overview: This document presents an environmental acoustic evaluation of a proposed amphitheater relative to neighbors in Scandia, Minnesota. Estimates of sound levels were determined with computer models of provided construction plans, current topography, proposed topographical changes, foliage, ground cover, and on-site statistical measurements of sound pressure levels. Computer modeling and measurements were performed to all applicable ISO 9613 standards. Results are presented with and without a bandshell as part of the proposed construction. Minnesota Noise Rules and Noise Descriptors: Minnesota noise standards are designed to be consistent with sleep, speech, annoyance, and hearing conservation requirements for receivers within areas grouped according to land use activities. The Minnesota standards are as follows: 7:00 AM to 10:00 PM 10:00 PM to 7:00 AM L10 L50 L10 L50 NAC-1 (Residential) 65 60 55 50 NAC-2 (Commercial) 70 65 70 65 NAC-3 (Industrial) 80 75 80 75 L10 means the sound level which is exceeded for 10 percent of the time for a one-hour period. L50 means the sound level which is exceeded 50 percent of the time for a one-hour period. Sound levels are expressed in dBA. Since the human ear does not perceive low or high frequencies as well as mid-frequencies, the A-weighted scale was developed. A dBA is a unit of sound level expressed in decibels and weighted for the purpose of approximating the human response to sound. The impact of the proposed amphitheater noise on a residential area is limited by the NAC-1 (Residential) values. Since it isn’t expected to operate at night past 10:00 PM, the limiting values would be L10 = 65 dBA and L50 = 60 dBA. Comfortable levels of speech are in the range of 60-65 dBA, so noise at or above this level would cause speech interference. Existing Ambient Noise Levels: Five measurement locations were selected for monitoring ambient noise levels at the proposed amphitheater and surrounding areas (see Figure 1). A 30-minute measurement was taken at each location between the hours of 2:30 and 6:00 pm on Wednesday, April 28 of 2021 to establish baseline ambient noise levels at a relatively busy time of day. Ambient noise levels were primarily from traffic on County Road 3 adjacent to the project location, but also included some local farming and mowing equipment operating on nearby properties. All measurements were taken with a calibrated Type 1 sound level meter and analyzer meeting all pertinent specifications of IEC 61672. Measurement results are as follows: 2 May 13, 2021 Test Location L10 L50 Noise Sources Noted During Test R 54.0 dBA 47.0 dBA Road Noise, Distant Lawn Mowing 1R 47.4 dBA 42.4 dBA Distant Lawn Mowing 2R 53.0 dBA 49 dBA Some Road Noise 3R 64.2 dBA 50.7 dBA Road Noise 4R 50.2 dBA 46.6 dBA Distant Lawn Mowing Fig 1. Existing Ambient Noise Level Test Locations Noise Levels from Proposed Amphitheater: The proposed amphitheater stage (Figure 2) faces northeast towards County Road 3, and has an earthen berm forming the seating area sloping up to approximately 9’ higher than the stage area. This berm provides some noise attenuation towards the north and northwest. If a bandshell is also constructed, it would provide attenuation to the south, southwest, and southeast. Although the sound levels of music performances at the proposed amphitheater will not be as high as those of large concert- level acts with high powered sound reinforcement equipment, much thought was given as to what would be considered reasonable for this project. After researching similarly sized venues and events, an average sound level of 90 dBA at the rear of the amphitheater seating was chosen and sound sources were placed on the stage area with sound power necessary to achieve these levels. Sources were assigned 103 dB per octave from 31 Hz to 8k Hz, with an overall equivalent sound power level of 110 dBA per source. Two of these sound sources are loudspeakers, placed on either side of the stage at a height of 5 meters. The third source is in the center of the stage at a height of 1 meter and represents a group of musicians with instruments and amplification. Although loudspeakers are directional and sound levels outside of their coverage pattern are lower, only non-directional sound sources were used in this model to be conservative when calculating the expected sound levels at nearby residences. 3 May 13, 2021 Computer modeling of the existing topography was modified to simulate the proposed amphitheater, berm, seating areas, buildings, and parking lots to predict noise levels at nearby residences. Existing foliage and ground coverage was also evaluated and entered to determine attenuation from the amphitheater. Figures 3 and 4 show 3D illustrations of the model. Fig. 3 Close up of 3D Noise Model Illustration without Bandshell Fig. 4 Close up of 3D Noise Model Illustration with Bandshell 4 May 13, 2021 Noise Predictions without Bandshell: Assuming that noise levels from music performances at the amphitheater are limited to an average of 90 dBA at the rear of the amphitheater seating, Figure 5 illustrates that average sound levels (Leq) at three nearby residences would range from 50 dBA to 60 dBA during amphitheater events without a bandshell. Residence 1 (W) 50 dBA Leq Residence 2 (SW and Nearest Residence) 60 dBA Leq Residence 3 (SE) 54 dBA Leq Fig. 5 Noise Level Contours without Bandshell These contours predict the average noise levels at Residence 2 (SW) to be essentially at the legal threshold for residential noise limits from 7am to 10pm. All music performance event sound levels would need to be carefully monitored and maintained to ensure the Minnesota noise standard is met for L50 = 60 dBA at Residence 2 (SW). 5 May 13, 2021 Noise Predictions with Bandshell: A simulation of a bandshell was entered into the model at a height of 6 meters. The bandshell consists of angled sides (following the provided stage) design and a roof over all three sound sources. The two speaker sources are at a height of 5 meters simulating a suspended installation of 1m from the underside of the roof. Assuming again that noise levels are limited to an average of 90 dBA at the rear of the amphitheater seating, Figure 6 illustrates the difference that a bandshell would make regarding sound levels at the three nearby residences. Average levels would now range from 36 dBA to 59 dBA during amphitheater events. Residence 1 (W) 52 dBA Leq Residence 2 (SW and Nearest Residence) 59 dBA Leq Residence 3 (SE) 36 dBA Leq Fig. 6 Noise Level Contours Stage with Bandshell The bandshell greatly reduces noise levels to the Residence 3 (SE) directly behind the amphitheater but does not significantly improve levels for Residence 2 (SW). All music performance events would still need to be carefully monitored and maintained to ensure the Minnesota noise standard is met for L50 = 60 dBA at this residence. 6 May 13, 2021 Noise Predictions with Bandshell and Amphitheater Rotated 30° Further experimentation with the model revealed that rotating the entire amphitheater, stage, and berm 30°clockwise in the plan view substantially reduced noise levels at the Residence 2 (SW). This rotation allows the western side of the bandshell to act as a barrier between the stage, speakers, and the residence to the west. Figure 7 illustrates that this rotation would reduce the estimated average sound levels at Residence 2(SW) to 48 dBA while maintaining the reduced levels at Residence 3 (SE). Residence 1 (W) 52 dBA Leq Residence 2 (SW and Nearest Residence) 48 dBA Leq Residence 3 (SE) 35 dBA Leq Fig. 7 Noise Level Contours Stage with Bandshell and Amphitheater Rotated 30° The combination of bandshell construction and amphitheater rotation in the model significantly reduces the average sound levels for both the nearest Residence 2 (SW) and Residence 3 (SE). Comparing the L50 site measurements taken near all three residences shows the calculated average sound levels from amphitheater events are not substantially different. Estimated Amphitheater Event Location Measured L50 Residence 1 (W) 52 dBA Leq 3R 50.7 Residence 2 (SW and Nearest Residence) 48 dBA Leq 2R 49 Residence 3 (SE) 35 dBA Leq 4R 47 7 May 13, 2021 Temperature Inversions: On a typical sunny afternoon, air is warmest near the ground and coolest at higher altitudes. This temperature gradient causes sound waves to bend upward, away from the ground and results in lower noise levels being heard at the listener’s position. At night, this temperature gradient will reverse, resulting in cooler temperatures near the ground. This condition is called a temperature inversion and will cause sound to bend downward toward the ground and results in louder noise levels at the listener position. As with wind gradients, temperature gradients can significantly influence sound propagation over long distances. Since nighttime events past 10:00 pm are not planned for the amphitheater, the likelihood of inversions during events is relatively low. Conclusions and Recommendations: 1. To ensure compliance with Minnesota noise rules, the amphitheater sound system output should be limited to an average of no more than 90 dBA Leq over 5-minute intervals measured at the rear of the amphitheater seating area. 2. All music performances and amplified events are to be finished before 10:00 pm. 3. Construction of an amphitheater bandshell and rotation of the entire amphitheater 30° clockwise (plan view) in the model greatly reduced expected noise levels at residences to the west and southeast. 4. Loudspeakers need to be installed within the bandshell. 5. Wind and temperature gradients can significantly affect sound propagation and noise levels can vary dramatically with weather.