ROOM ACOUSTICS DESIGN. Room Acoustics Design Intent Appropriate Articulation

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1 ROOM ACOUSTICS DESIGN (and continuing to look at OPR) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 1 Room Acoustics Design Intent Appropriate Articulation Articulation is a qualitative measure of the clarity of sound specifically speech (although the concept could also be applied to music) Articulation is analogous to visual acuity Articulation Index (AI) is a numerical rating of the quality of articulation AI is a great POE tool (it is easy to measure) not so great as a design tool (it is hard to predict accurately during design) AI is also used in speech privacy analyses (commonly associated with open office layouts) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 2 1

2 Articulation Index 0.7 (between good and very good) is a typical design criterion for AI; providing about 90% recognition of words and 98% recognition of sentences assuming language familiarity Architectural Acoustics: Egan Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 3 Room Acoustics Design Intent Appropriate Reverberation Time Reverberation time (RT) is a commonly-addressed room acoustics issue in many space types RT is defined as the time (in seconds) it takes for a sound pressure level impulse in a space to decrease (decay) by 60 db when the source of sound is turned off RT is a numerical indication of the acoustical liveliness/deadness of a space RT varies with frequency (because sound absorption varies with frequency) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 4 2

3 Live/Dead Spaces small space high absorption perception of reverberance is a function of both the scale of a space and its acoustical absorption big space low absorption Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 5 Live/Dead Space Examples enclosing planes designed to avoid standing waves Iive: reverberant chamber dead: anechoic room all six surfaces Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 6 3

4 Appropriate Reverberation Time Possible design intents (project-specific) Provide conventional reverberation time for speech Provide comfortable reverberation time for jazz quartets Provide adjustable reverberation time for symphonic music Potential design criteria A maximum RT of x.x seconds (+/- 10%) A minimum RT at 2000 Hz of x.x seconds See next few slides for suggested criteria Design tools Selection of appropriate interior materials and spatial volume Design validation As per the following information Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 7 Reverberation Time Criteria Optimum RT for speech (one recommendation) = 0.3 log (V / 10) where, V = room volume (m 3 ) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 8 4

5 Reverberation Time Criteria recommendations for maximum RT for speech Mechanical and Electrical Equipment for Buildings Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 9 Other RT Criteria for speech and for music Mechanical and Electrical Equipment for Buildings speech benefits from lower RT values which yield higher articulation Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 10 5

6 Other RT Criteria for various space types what s good depends upon space usage Mechanical and Electrical Equipment for Buildings RT is expressed quantitatively but user evaluations are qualitative Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 11 Visualizing Reverberation average space original sound impulse reflections SPL vs. time, if you hit a cow bell in an average space Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 12 6

7 Visualizing Reverberation anechoic space no reflections SPL vs. time if you hit a cow bell in a really, really absorptive space Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 13 Visualizing Reverberation reverberant space echo a strong, distinct reflection SPL vs. time if you hit a cow bell in a large, hard space (diagram simplified) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 14 7

8 Reverberation Time (RT) RT the time required for SPL to decay by 60 db when sound source is stopped 60 db Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 15 Estimating Reverberation Time RT = C V / (A + xv) where, RT = reverberation time (seconds) C = 0.16 in SI (0.049 in I-P) V = volume of the space m 3 (ft 3 ) A = total surface absorption (Sabins using appropriate units) x = absorption coefficient of air (a low value) Note: RT is frequency dependent because absorption is frequency dependent this equation shows that increasing room size increases RT; increasing absorption decreases RT Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 16 8

9 Total Surface Absorption (A) A = S 1 α 1 + S 2 α 2 + S 3 α 3 + where, A = total surface absorption (m 2 or ft 2 Sabins) S 1 = surface area of material #1 (m 2 or ft 2 ) α 1 = acoustical absorption coefficient of material #1 (dimensionless) S 2 and α 2 = material #2, and so on Note: A is frequency dependent (because α is) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 17 Sound Absorbers Sound absorbing materials generally fall into three broad categories: Porous absorbers Light, fluffy, with air pockets; spongy Absorb sound via friction of airflow within pores Panel absorbers Planes of flexible materials Absorb sound via friction in flexure of panel Cavity resonators Trap sound in a chamber (cavity) Absorb sound via multiple contained reflections Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 18 9

10 Sound Absorption Coefficients look at frequency patterns panel see next slide porous Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 19 Noise Reduction Coefficient (NRC) NRC is the average absorption coefficient of a material at four octave band center frequencies namely 250, 500, 1000, and 2000 Hz Used as a single-number absorption rating (providing yet more evidence of building designers love of singlenumber indices) NRC is terribly misnamed (it is not directly noise related) Use with caution (as always, averaging can lose valuable information) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 20 10

11 Examples of Porous Absorbers sound-absorbing batts (often with acousticallytransparent surface protection/cover) sound-absorbing boards acoustical tiles (which can also exhibit panel absorber properties) sound-absorbing movable partitions suspended sound-absorbing baffles Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 21 Porous Absorbers (continued) sound-absorbing banners space absorbers (three-dimensional absorbing objects, often suspended) sound-absorbing roof/floor decking (acoustical deck; metal or wood fiber-board) acoustical plaster thick fabrics (rich drapery, plush carpet) seating and occupants Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 22 11

12 Porous Absorbers: Absorption Patterns The absorption coefficient (α) for a porous material will be affected by: sound frequency (α generally increasing with frequency) basic composition and density of the material (α generally increasing with density within limits) thickness of the material (α generally increasing with thickness to a point of diminishing returns) spacing of material relative to backer/structure (an air space between material and structure will increase α) use of a protective membrane (an acoustically transparent cover will not greatly affect α) surface finish (access to air channels must be maintained beware of painting porous surfaces) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 23 Examples of Panel Absorbers gypsum board (drywall) walls and ceilings wood paneling acoustical tiles (supported at edges not glued to subsurface) large sheets of glass floating wood floors etc. panel must be installed so that it is free to move; like a drum head Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 24 12

13 Panel Absorbers: Absorption Patterns Absorption coefficient (α) for a panel is affected by: the frequency of sound (panels generally being useful below 500 Hertz) the surface mass of the material (mass and useful frequency are inversely related) the flexibility of the panel (bigger is better) the depth of air space behind the material (cavity depth and useful frequency are inversely related) the presence of a porous absorber material in the cavity behind the panel (such material increases absorption) Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 25 Cavity Resonators A specially manufactured product: performance data are given by manufacturer useful for exterior or hard-use (high impact or high humidity) locations available in unfilled and filled (with porous stuff) versions Architectural Acoustics: Egan Ball State Architecture ENVIRONMENTAL SYSTEMS 1 Grondzik 26 13