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Engineering Report May 1, 2019 John Shirley Anderson Shirley Architects 95 Commercial St SE #5 Salem, OR 97301 [email protected] Re: South Salem High School Addition – Acoustical Report REPORT: ABD Engineering & Design, Inc. (ABD) has reviewed the SD drawing set and our notes from meeting with the music faculty. Based on this information, we have prepared the following recommendations for the Auditorium and Music Suite. Once you have a chance to review our comments, some further refinement may be necessary to meet your architectural requirements. Nothing in these documents should be construed as modifying the requirements of any applicable building code. Where conflicts exist, the most restrictive condition should be satisfied. Room Acoustics Criteria Reverberation Time Reverberation time (RT) is the length of time that it takes for a sound to drop in level by 60 decibels (dB). Speech intelligibility is related to the room acoustics and the sound reinforcement system; it is the ability to hear and understand what is being said. Excessively long reverberation time can seriously degrade speech intelligibility. Reverberation times that are too short also detract from the musicality of performance spaces. The speech and music demands of the Auditorium and Music Rooms dictate the reverberation time criteria to be selected as indicated in Table 1. ABD Engineering & Design ◼ Architectural Acoustics ◼ AV Design ◼ Noise and Vibration 321 SW 4th Avenue, Suite 700, Portland, OR 97204 124 Fulton Street East, Second Floor, Grand Rapids, MI 49503 Phone (866) 272-9778 ◼ www.abdengineering.com SSHS Addition – Acoustical Report May 1, 2019 Page 2 Table 1: Reverberation Time Targets Target Room Type Reverberation Time (s) Auditorium 1.5 Choir Room 0.8 – 0.9 Orchestra 0.7 – 0.8 Band Room 0.6 – 0.7 Ensemble Room 0.6 – 0.7 Black Box 0.8 Ensemble/Practice Rooms < 0.5 Speech Intelligibility Speech intelligibility is defined as how well a listener can understand a speaker; it is the ability to hear and understand what is being said. This is often described as a percentage of how many words or phrases can be understood. An important factor to determining the speech intelligibility is the signal-to-noise ratio (SNR) which is the difference between the source level (speaker) and the noise level (background noise). When considering speech intelligibility, the SNR in the speech frequency range of 250 Hz to 4000 Hz is most important. Additional noise within this range can reduce the SNR and lower speech intelligibility. Excessively long reverberation time can seriously degrade speech intelligibility. Background Noise (NC) The background noise of a space is represented by a single number criterion called the NC or Noise Criterion. The NC level is determined by comparing the measured sound levels within the room with charts that compensate for human sensitivities to noise and reduce the sound levels measured at various frequencies to a single number. NC levels are most often used in the design process to indicate the level of the background noise for each room. High background noise is often the result of mechanical ventilation noise or noise infiltration from other areas. Low background noise is a key factor in providing good listening spaces. Mechanical noise guidelines will be submitted under a separate report. Diffusion Diffusion is the breaking up and scattering of sound waves. A flutter echo results from the bouncing of sound between hard parallel surfaces. Normally, a flutter echo is masked by the reverberance of the room or reduced by putting sound absorbing or diffusing elements at strategic locations to break up the parallelism. ABD Engineering & Design ◼ Architectural Acoustics ◼ AV Design ◼ Noise and Vibration 321 SW 4th Avenue, Suite 700, Portland, OR 97204 124 Fulton Street East, Second Floor, Grand Rapids, MI 49503 Phone (866) 272-9778 ◼ www.abdengineering.com SSHS Addition – Acoustical Report May 1, 2019 Page 3 Room Acoustics Recommendations Auditorium Our recommendations are based on the SD drawings and notes from our meetings with you. With this information, we have prepared a baseline acoustical model assuming carpeted aisles, concrete floors under padded theater seating, a standard non-acoustical metal deck, and painted masonry walls. We note that further refinement will be necessary as the design of the space progresses. We recommend: • We have assumed that the stage will have a full complement of curtains. If so, the stage area should not be overly reverberant. However, if this is not the case, we recommend spraying the deck of the stage house with 3” K13 by International Cellulous Corporation. • We recommend that the stage be made rigid to prevent "drumming" of the surface due to the airspace below. Typically there are two construction methods stages, our recommendations are based on each type of construction: EITHER o Concrete Floor Slab: We recommend that 2” thick rigid coated ductliner be applied to the underside to absorb sound that is generated from “stomping” on the platform. OR o Wooden Platform on Joists: We recommend that the platform should be made rigid with joists spaced closer together (12"), and with two layers of 3/4" plywood as a subfloor. We also recommend that 2” thick rigid coated ductliner insulation or 3” batt insulation can be placed between the joists to absorb sound that is generated in the space. • We recommend that front surfaces of the cheek walls be integrated with curved reflectors to evenly redirect sound from the stage to the audience area. The reflectors should have a minimum of 1” of facing material, consisting of (2) layers of ½” gypsum wallboard. The reflectors can be curved with a radius between 8’-12’. In addition, the cavities should be lined with a minimum 1” thick fiberglass batt insulation and fully enclosed at the top and bottom. We will forward the AutoCad drawings of the recommended curved sections in a separate file in the DD phase of the project. • We note that the rear wall directs acoustical energy back to the stage. Plan for approximately 1500 square feet of 4” thick acoustical wall panel on the rear walls. Alternately, a combination of wood diffusive panels directly behind seating from 4’-0” AFF to 8’-0” (minimum) AFF on the rear wall with absorptive panels above could be an option. • Treat the rear 1/3of the structural deck with a spray-on acoustical treatment such as K-13 by International Cellulous (http://www.spray-on.com/products/k13/). We recommend a minimum spray depth of 4”. ABD Engineering & Design ◼ Architectural Acoustics ◼ AV Design ◼ Noise and Vibration 321 SW 4th Avenue, Suite 700, Portland, OR 97204 124 Fulton Street East, Second Floor, Grand Rapids, MI 49503 Phone (866) 272-9778 ◼ www.abdengineering.com SSHS Addition – Acoustical Report May 1, 2019 Page 4 • Use a tight loop, glue down carpet on the open floor aisle areas. This provides some sound absorption, and more importantly, muffles footfalls and foot shuffling sounds. Its use enables patrons to enter or leave unobtrusively during a performance. The floor area below the seats should be acoustically “hard” such as sealed concrete or vinyl tile. • The auditorium seating should have upholstered seats and backs with perforated seat pans. These seats are designed to approximate the sound absorption of a seated person, which will help to avoid undesirable changes in RT with audience size. We recommend that acoustical data be requested from the manufacturer prior to approval to bid. • In order to provide ideal early reflections and reinforce sounds generated on stage, we recommend approximately 4-5 overhead ceiling reflectors in the house and one at the proscenium opening. The one at the proscenium opening may be larger than the house reflectors and ideally, as close to the proscenium opening as possible. These reflectors can be field fabricated with two layers of 1/2” gypsum wallboard on stud framing. They should be a minimum of 8 feet wide and should extend the entire width of the seating area. All reflectors should be installed complete with 3” batt insulation rolled out on the backside of the reflector. We will forward the AutoCad drawings indicating the size and placement of these reflectors in a separate file in the next phase of this project. • In addition to the house ceiling reflectors, we recommend choosing a good quality stage music shell as currently planned by PLA. The combination of a stage shell with overhead acoustical reflectors are important to enhance the natural acoustics of the auditorium. In addition we recommend that stage risers be purchased from the manufacturer in order to ease coordination. All of the following manufacturers can be considered: o Wenger Corporation models “Diva” or “Forte” (www.wengercorp.com) o StageRight model “Opus 2” (www.stageright.com) o Secoa model “Maestro” (www.secoa.com) • The face of the balcony should have a curved or angled facing or be completely absorptive to minimize harmful reflections back to the stage. • Although variable acoustics were not discussed as a desired option for the Auditorium, variable acoustics could be beneficial if the space is to be used for speaking events. In order to accommodate a speaking event, we recommend planning for variable acoustical treatment on the rear side walls. Choose one of the following options: o 25 ounce velour or similar fabric curtains. These curtains should be open when used for musical performances and closed when used for lectures. The curtains should be offset at ABD Engineering & Design ◼ Architectural Acoustics ◼ AV Design ◼ Noise and Vibration 321 SW 4th Avenue, Suite 700, Portland, OR 97204 124 Fulton Street East, Second Floor, Grand Rapids, MI 49503 Phone (866) 272-9778 ◼ www.abdengineering.com SSHS Addition – Acoustical Report May 1, 2019 Page 5 least 6” from the wall and have 100% fullness meaning that the amount of fabric needed is twice the amount of the area being covered.
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