Acoustical Analysis: Design Objective The primary acoustical goal of space planning is to place noisy equipment as far as possible from noise sensitive areas. New House did everything possible to achieve this goal. The main mechanical room is located in the south-east corner of the basement, below grade. Surrounded by a stairwell, storage rooms, and vestibules, this is an ideal location. The only area of interest concerning possible noise and vibration transfer is one floor above. The dining room, which also doubles as a reading room during off hours, is located right above the mechanical room. The location of the dining room was of no concern early in the design phase because it was not known that this area was to be used as an additional space for studying. Typical Noise Criteria (NC) values for dining rooms range from 40-45. Values for reading rooms are much lower at NC-35. Using the Noise Criteria Method, it will be determined whether or not the relative loudness of the mechanical room will have any effect on the sound pressure levels in the room above. The NC rating will be used because of its simplicity and I am only looking for defined limits of octave band spectra that will meet occupant satisfaction. Analysis Using Trane Acoustical Program (TAP), an average sound power level (L w ) was developed for the mechanical room. The room is constructed from 8 cmu blocks with an 8 concrete slab ceiling. From the absorption characteristics, a room constant (R T s) was found. The following table provides characteristics of the mechanical room. l W h LWs αc αw αf Scαc Swαw Sfαf ΣSiαi αsab RTs 125 Hz 15 7 3.25 103.01.1.01 1.05 14.30 1.05 16.40.05 17.2 250 Hz 15 7 3.25 98.01.05.01 1.05 7.15 1.05 9.25.03 9.5 500 Hz 15 7 3.25 96.02.06.02 2.10 8.58 2.10 12.78.04 13.3 1000 Hz 15 7 3.25 98.02.07.02 2.10 10.01 2.10 14.21.04 14.8 2000 Hz 15 7 3.25 95.02.09.02 2.10 12.87 2.10 17.07.05 17.9 4000 Hz 15 7 3.25 91.02.08.02 2.10 11.44 2.10 15.64.04 16.4 Table 8.1: Mechanical Room Absorption characteristics (SI units) 20
The room constant (R T r) for the Dining Room was then developed. The room absorption coefficients are based on a wood parquet floor, acoustical tile ceiling, and gypsum board walls. Glass windows were also accounted for. l W h αc αw αwgls αf Scαc Swαw Sfαf ΣSiαi αsab RTr 125 Hz 15 7 3.25.76.55.35.04 79.80 75.40 4.20 159.40.45 290.6 250 Hz 15 7 3.25.93.14.25.04 97.65 21.81 4.20 123.66.35 190.3 500 Hz 15 7 3.25.83.08.18.07 87.15 13.07 7.35 107.57.30 154.7 1000 Hz 15 7 3.25.99.04.12.06 103.95 7.02 6.30 117.27.33 175.6 2000 Hz 15 7 3.25.99.12.07.06 103.95 16.35 6.30 126.60.36 197.4 4000 Hz 15 7 3.25.94.11.04.07 98.70 14.59 7.35 120.64.34 183.3 Table 8.2: Dining Room Absorption characteristics (SI units) Typical study rooms have an NC rating of 35. After converting the mechanical room sound power levels to sound pressure levels and using the typical Sound Transmission Class (STC) and Transmission Loss (TL) values of mechanical equipment rooms found in ASHRAE s 2003 HVAC Applications Handbook, it was found that the sound pressure levels reaching the dining room had an NC rating of 50. Additional absorptive material must be added to the mechanical room. LPs TL TL-(5) Safety Factor NR Lpr Actual LPr Desired 125 Hz 97 35 30 34 62 54 250 Hz 94 41 36 39 56 46 500 Hz 91 44 39 41 50 40 1000 Hz 92 50 45 47 45 36 2000 Hz 88 57 52 55 34 35 4000 Hz 85 64 59 61 23 34 Table 8.3: Dining Room SPL vs. Desired SPL 21
Conclusion The mechanical room is comprised of highly reflective surfaces. After some initial research and trial and error, it was determined that a NC rating of 35 could be achieved if a 2 inch thick, porous soundabsorbing material (mtg. B) is added to the ceiling. This material will help absorb excessive low frequency sound associated with a rumbling sound. Supporting calculations may be found in Appendix K. In this case, the sound quality of the dining room was not of great concern. By adding materials only to the mechanical room and not disturbing the original design of the dining room, this may be a feasible plan of action. 22
ACOUSTICS - EQUATIONS APPENDIX K 60
ACOUSTICAL ANALYSIS EQUATIONS R T = ΣS i α I / [1-α sab ] α sab = ΣS i α I / ΣS i L P mech = L W mech 10Log[R T mech] + 6 NR = TL sf 10Log[S w / R T receiver] L P receiver = L P mech NR NC-50, Original Design NC-35, Reading Rooms Table 8.4: Typical NC Curve, L P Rating