Reflectance Profile Method for the Acoustical Design of Buildings

Providence, RI 2016 June 13-15 NOISE-CON 2016 Reflectance Profile Method for the Acoustical Design of Buildings Paul Battaglia, AIA, ASA, INCE Jin Young Song, AIA, LEED Department of Architecture 140 Hayes Hall University at Buffalo Buffalo NY 14214 plb@buffalo.edu ABSTRACT The acoustic character of interior enclosed spaces is typically described by Reverberation Time (RT-60) and presented as a single value based on the Noise Reduction Coefficient for the total number of sabins in a spatial volume. When a series of reverberation times are calculated at frequency band centers, well-accepted terminology can be applied for describing the space for conditions where reverberation is missing, prevalent, or excessive. Therefore, reverberation time provides a spatial description but little about the material properties of room surfaces that create the condition. This paper presents a simple methodology for calculating the Reflectance Profile of a space. Examples of typical spaces such as hotel rooms and elementary school classrooms are included to demonstrate its utility as a design tool. The Glass House, a proposed music hall entirely enclosed in glass, is also included to indicate how the Reflectance Profile can provide an easy, understandable means of identifying corrective measures that should be employed during design to avoid frequency imbalance. It has led to the study of triangular non-damping reflective glass panels for use in the Glass House. 1 INTRODUCTION Reverberation time provides a means for presenting a single number value to describe the natural decay of sound introduced into an enclosed space. The equation for reverberation time (RT60) is extremely simple. Reverberation time is proportional to the cubic volume of the space divided by the sum of the surface areas of various materials times their sound absorption rates with a constant related to the amount of decay in decibels. For SI units and 60 dB of decay: T = 0.16 V/ΣSα In presenting the reverberation time any information about the actual makeup of the materials used to absorb the sound is lost. As a result it is difficult to see how to easily modify these combined values to improve conditions in the room. Sounds heard in a space have been changed by the surfaces of materials comprising the enclosure by reflecting various frequencies in different amounts to contribute to the reverberant qualities. It is fair to say that architects are more interested in the reflections of the sounds than the loss of reflections through absorption since the reflections are what are heard. This is true even Reflectance Profile Method Battaglia & Song when the intent is to achieve sound privacy through absorption since some reflections always remain to lend some character to the space. 2 REFLECTANCES AND REVERBERATION Rooms can be described with commonly-used adjectives associated with reverberation at various frequencies and whether the reflected sound frequencies are missing, prevalent, or excessive1 as shown in Table 1. The terms associated with missing and with excessive reverberation generally Table 1: Glossary of terms to describe sound indicate an unpleasant condition; the terms for prevalent reverberation, such as warm, bright, airy, intimate, mellow and delicate, indicate a relatively pleasant condition. Reverberation is essentially a combination of the myriad reflections in the room. That is why the reflection coefficient ) rather than the absorption coefficient () is used in these analyses. 3 MATERIAL REFLECTANCES Materials will absorb, reflect, and transmit sound energy to relative degrees, and the total amount of energy incident on the surface will be equal to the sum of the absorbed (, reflected () and transmitted (τ) energy incident on the surfaces (Law of the Conservation of Energy) and is defined: α+ρ+τ=1 For commonly used architectural materials the amount of sound energy transmitted through a material is negligible compared to the amount absorbed or reflected. For example, a typical gypsum board partition of STC-38 has a transmission coefficient  equal to 0.0063, which is quite insignificant. For that reason the reflectance coefficient  is effectively 100% less the absorption coefficient . The Material Reflectances Chart shown in Table 2 was constructed for the reflectance coefficients of common interior materials for rooms based upon a chart of absorption coefficients compiled by David Egan2 and several manufacturer’s catalogs. NOISE-CON 2016, Providence, Rhode Island, 13-15 June, 2016 2 Reflectance Profile Method Battaglia & Song Table 2: Material Reflectances Chart 4 REFLECTANCE PROFILE METHOD The reflectance coefficients at common centerband frequencies of the materials found in a room can be multiplied by their surface areas, summed up, and divided by the total surface area in order to get an average reflectance at each centerband frequency: avg = S/S This weighted average across the centerband frequencies presents the Reflectance Profile. To simplify use of this method a Microsoft Excel template as shown in Table 3 can be developed and copied for every room to be analyzed, placing the formulae in the proper boxes. Insert the name of the material, its surface area, and reflectances from Figure 2 and the template will calculate the profile. NOISE-CON 2016, Providence, Rhode Island, 13-15 June, 2016 3 Reflectance Profile Method Battaglia & Song Table 3: Template for Reflectance Profile Method When the weighted average is calculated it presents a profile of reflectances in the room. The terms presented in Figure 1 can then be used to describe the subjective effect of these reflectances at various frequencies by applying limits to the amounts. Somewhat arbitrarily, we consider a weighted reflectance over 90% as excessive; above 80% as prevalent; between 65% and 80% as preferred; and less than 65% as missing. The preferred range indicates a balance of frequency reinforcement and dampening in a space. 5 TYPICAL ROOM EXAMPLES Three-dimensional models of rooms can be developed to easily collect data on surface areas of the materials. Google Sketchup3 models of a typical classroom, a hotel room, and a large open plan office floor illustrate how the Reflectance Profile Method can be used. A typical c. 1920 elementary school classroom, 7.62 x 9.72 x 3.66 meters high (25’x 32’x12’), has a vinyl tile floor, plaster walls, concrete deck ceiling, and large windows (Fig. 1). The Reflectance Profile for this classroom is tallied in Table 4 and the results indicate all weighted reflectances are above 93%, or excessive, so this room can be termed chesty, boomy, nasal and shrill all at the same time. Table 5 shows a contemporary version of the same classroom with a suspended acoustic panel ceiling in the space at 2.75 meters (9’) and insulated gypsum board Figure 1: Model of Typical Classroom walls; it shows a different Reflectance Profile with all reflectances close to or within the preferred range. A renovation scheme for the 1920’s plaster-walled classroom is tallied in Table 6 and includes a perforated wood back wall and sound-absorbing foam panels on the exposed concrete deck ceiling. This scheme has reflectances at all frequencies in the preferred range except at 125 Hz. Table 4: Reflectance Profile for typical 1920's classroom CLASSROOM: Exposed hard surfaces Vinyl composition tile floor Plaster walls Glass Concrete ceiling, painted Total ( Σ S, Σ Sρ ) Weighted Average Area 74 111 16 74 275 NOISE-CON 2016, Providence, Rhode Island, 13-15 June, 2016 125 0.98 0.86 0.82 0.99 256 0.93 250 0.97 0.90 0.94 0.99 260 0.95 500 0.97 0.94 0.96 0.98 264 0.96 1000 0.97 0.95 0.97 0.98 265 0.96 2000 0.97 0.96 0.98 0.98 267 0.97 4000 0.98 0.97 0.98 0.98 268 0.98 4 Reflectance Profile Method Battaglia & Song Table 5: Reflectance Profile for a contemporary classroom of similar area CLASSROOM: Insulated GWB, Lay-in Panel Ceiling Vinyl composition tile Gypsum board, insulated cavity Glass Acoustical panel ceiling Total ( Σ S, Σ Sρ ) Weighted Average Area 74 82 13 74 243 125 0.98 0.45 0.82 0.62 195 0.80 250 0.97 0.86 0.94 0.62 200 0.82 500 0.97 0.92 0.96 0.49 196 0.81 1000 0.97 0.96 0.97 0.23 180 0.74 2000 0.97 0.88 0.98 0.11 165 0.68 4000 0.98 0.89 0.98 0.05 162 0.67 125 0.98 0.86 0.50 0.82 0.99 0.78 243 0.89 250 0.97 0.90 0.25 0.94 0.99 0.41 216 0.79 500 0.97 0.94 0.15 0.96 0.98 -0.07 197 0.72 1000 0.97 0.95 0.15 0.97 0.98 -0.31 188 0.69 2000 0.97 0.96 0.00 0.98 0.98 -0.46 179 0.66 4000 0.98 0.97 0.10 0.98 0.98 -0.49 182 0.67 Table 6: Reflectance Profile for remodeled 1920's classroom CLASSROOM: Perforated wood and melamine foam Vinyl composition tile floor Plaster walls Micro-perforated wood on fiberglass (Decustic) Glass Concrete ceiling, painted Panels, 1.5" melamine foam Type D mounting (STC) Total ( Σ S, Σ Sρ ) Weighted Average Area 74 83 28 13 34 40 272 The typical hotel room has a sleeping area 3.66 x 5.50 x 2.44 meters high (12’x18’x8’) with a 1.50 x 2.44 meter (5’x8’) vestibule area (Fig. 2). Typical finishes include carpeted floor, gypsum board walls and ceiling, window wall with drapes, and two queen-size beds with bedding. The Reflectance Profile for this room shown in Table 7 indicates that all reflectances are close to or within the preferred range. Figure 2: Typical Hotel Room Table 7: Reflectance Profile for typical hotel room HOTEL ROOM: Typical Carpet, glue down Gypsum board, insulated cavity Bedding Glass Drapes Total (ΣS, ΣSρ) Weighted Average Area 189 797 131 214 214 1,545 NOISE-CON 2016, Providence, Rhode Island, 13-15 June, 2016 125 0.98 0.45 0.81 0.82 0.86 1,040 0.67 250 0.94 0.86 0.63 0.94 0.65 1,286 0.83 500 0.86 0.92 0.44 0.96 0.45 1,255 0.81 1000 0.63 0.96 0.33 0.97 0.28 1,195 0.77 2000 0.40 0.88 0.39 0.98 0.30 1,102 0.71 4000 0.35 0.89 0.41 0.98 0.35 1,111 0.72 5 Reflectance Profile Method A typical open plan office floor plate of 1,500 square meters (about 16,000 sf) with carpet, gypsum board, glass exterior windows, and acoustic panel ceiling suspended at 2.75 meters (9’) above the floor (Fig. 3). This space has a Reflectance Profile shown in Table 8 where the midrange frequencies are missing resulting in a space that sounds dull. Removing the suspended ceiling and adhering foam panels to a portion of the exposed metal deck at 3.50 meters (11-6’) above the floor restores these missing reflectances as shown in Table 9. With prevalent levels of low midrange and bass the open floor now feels warm and speech sounds are much more natural. Battaglia & Song Figure 3: Typical Large Open Plan Office Floor Table 8: Reflectance Profile for large open plan office with acoustical panel ceiling LARGE OPEN PLAN: Lay-In Panels Carpet, glue-down Gypsum board, single layer, insulated cavity Glass, heavy Lay-in acoustical panel (Armstrong Fine Fissured) Total ( Σ S, Σ Sρ ) Weighted Average ( Σ Sρ /Σ S) Area 1,500 392 234 1,500 3,626 125 0.98 0.45 0.82 0.62 2,768 0.76 250 0.94 0.86 0.94 0.62 2,897 0.80 500 0.86 0.92 0.96 0.49 2,610 0.72 1000 0.63 0.96 0.97 0.23 1,893 0.52 2000 0.40 0.88 0.98 0.11 1,339 0.37 4000 0.35 0.89 0.98 0.05 1,178 0.32 Table 9: Reflectance Profile for large open plan office with exposed deck and sound absorbing ceiling panels LARGE OPEN PLAN: Exposed with Foam Panels Carpet, glue-down Gypsum board, single layer, insulated cavity Glass, heavy Concrete or steel deck Panels, 1.5" melamine foam Type A mounting (STC) Total (SS, SSr) Weighted Average (SSr/SS) Area 1,500 518 234 1,000 500 3,752 NOISE-CON 2016, Providence, Rhode Island, 13-15 June, 2016 125 0.98 0.45 0.82 0.99 0.86 3,315 0.88 250 0.94 0.86 0.94 0.99 0.60 3,365 0.90 500 0.86 0.92 0.96 0.98 0.28 3,111 0.83 1000 0.63 0.96 0.97 0.98 0.11 2,704 0.72 2000 0.40 0.88 0.98 0.98 0.01 2,270 0.61 4000 0.35 0.89 0.98 0.98 -0.01 2,190 0.58 6 Reflectance Profile Method Battaglia & Song Figure 4: Glass House view from seating Figure 5: Glass House view from stage left Figure 6: Model of the Glass House 6 THE GLASS HOUSE The Reflectance Profile Method can be applied to more complex spaces. The Glass House was conceived with a stage enclosure and roof entirely made of glass in order to engage the city on the exterior of the building as illustrated in Figures 4 and 5. When we first considered the acoustical conditions we were creating we were very concerned about the excessive reverberation and reflectances that were probable. NOISE-CON 2016, Providence, Rhode Island, 13-15 June, 2016 7 Reflectance Profile Method Battaglia & Song The Sketchup model includes a sloped glass canopy that mimics the fragmented glass reflectors (Fig. 6) so that data collection and analysis with the Reflectance Profile can be easily completed. These glass reflectors are proposed to provide good diffusion and distribution of strong, earlyarriving reflections to every seat. Their support configuration avoids creating regular, rectilinear patterns that could cause an imbalance in frequency reflection by creating resonant panels. The Reflectance Profile in Table 10 indicates an extraordinary balance assisted by the fabriccovered upholstered seating and the use of micro-perforated wood veneer panels at balcony fasciae and behind seating areas. All reflectances at every frequency are within the preferred range. Table 10: Reflectance Profile of the Glass House performing arts center AUDITORIUM: GLASS HOUSE Glass, heavy Perforated wood panelling, insulated cavity Wood strips Carpet, glue-down Fabric covered audience seating Gypsum board Total ( Σ S, Σ Sρ ) Weighted Average ( Σ Sρ /Σ S) Area 3,888 1,371 344 142 4,892 1,032 11,669 125 0.82 0.60 0.85 0.98 0.81 0.71 9,138 0.78 250 0.94 0.10 0.89 0.94 0.63 0.90 8,242 0.71 500 0.96 0.20 0.90 0.86 0.47 0.95 7,718 0.66 1000 0.97 0.50 0.93 0.63 0.41 0.96 7,863 0.67 2000 0.98 0.60 0.94 0.40 0.39 0.93 7,881 0.68 4000 0.98 0.70 0.93 0.35 0.41 0.91 8,084 0.69 7 SUMMARY The Reflectance Profile Method can give the architect an indication of the amount of sound reflection in a room at common centerband frequencies. It is easily determined whether reflections are excessive, prevalent, missing, or within a preferred range for each frequency band. It also indicates when there is an unbalanced set of reflections leading to a clear description of the sound qualities of the room. Refinement is needed. The limits that determine the descriptions are not absolute, and quite arbitrarily assigned in these examples. Furthermore, the proper shape for a plotted profile curve is yet to be determined. For acoustical comfort it might resemble the NC-40 plot with higher values in the lower frequency. For music a flat shape might be preferred. For speech, reinforcement with reflections in the 160-2,500 Hz range with lower values above and below that range might be preferred. 1 There are various glossaries on-line with no reference to the sources. A good comprehensive list can be found at http://www.head-fi.org/t/220770/describing-sound-a-glossary 2 Egan, M. David, Architectural Acoustics, J. Ross Publishing, Plantation, Florida, 2007. 3 Sketchup by Google is available online at no cost. The software has a tool to calculate the area of selected surfaces, and with care in modeling can calculate the total surface area of a selected material. NOISE-CON 2016, Providence, Rhode Island, 13-15 June, 2016 8