Question

3. (25 points) A room is 75 ft (side to side) x 80 ft (front to back) x 30 ft (floor to ceiling). The front wall is made out of glass, the back wall is covered with curtains, both sidewalls are made of plasterboard, the ceiling is covered with acoustical board, and the floor is concrete. Table 8-3 Average Absorption Coefficients for Several Types of Building Materials at Octave Frequency Intervals Frequency (Hz) 4000 Material 2000 125 250 500 1000 0.03 0.0 0.02 Concrete, bricks 0.02 0.01 0.02 0.02 0.03 Glass 0.19 0.08 0.06 0.04 0.02 0.04 Plasterboard 0.20 0.10 0.08 0.15 0.09 Plywood Carpet Curtains 0.10 0.11 0.45 0.25 0.13 0.60 0.50 0.30 0.35 0.10 0.20 0.45 0.40 0.12 0.25 0.35 0.05 0.90 0.90 0.90 Acoustical board 0.25 0.45 0.80 a) What is the reverberation time of this room at 125 Hz? b) What is the reverberation time of this room at 1000 Hz? c) Would you describe this room as warm or brilliant? Why?

Answer 1

ANSWER The no8m ' 5ft length realth teght = 30-ft. (a) At 125 H2 Averge disorpti do-e Frent onll made ont o tGlays 61.0 Ayar) Curtins Back wall T) 50.0 (h) Plaster boord Sale 0 20 (Au) acensteal bownl: 0.2.5 W V flar CAnerete 11 :Tatl effective abtarbing wrea +arSa t aP Sde tar Se -Se + Seet 0-1975130) 0-05 35x10) +0-20x(80x30) 0.05 (orra3)o.0+ (7580) +001x(75K80) + 0.25x 3060 ft 75 tOR0) V=Velme ef # 180,000 reverboratio tme V . 0.0 Se Repofrouaen 0 049 x 180,000 17 3060 2.88 A

(b) At 1000 Hz z 0'35 aPl 004 Art tars 80.0 090 Com 70-0 (8ox3o) Here, Se 0-04 (a5K30) + 0:35x(75x30o) -0o8 (gor30)+ D 90K(7580)+ 002(75x yo) +0.08 6781.5 t ft3 V 180, (0-049 A/).V Carrespording heverbaration tie Se 0 049x 180,o0 O S.18t9 3 . the neverberatim time at The noom S L9NM as los freamerey hnds (125 H2)-2-88 the sreverberation time at high regmeney hods 7eater tham (tooo Hz)