1、550 2010 ASHRAEABSTRACTAn investigation was conducted into HVAC-related airflowrates and noise levels in five classrooms in a mechanically-ventilated building at the University of British Columbia(UBC), the relationship between them, and how to control thenoise. Sound-pressure levels were measured i
2、n terms of total,A-weighted level, Noise Criteria (NC) and Room Criteria (RC)ratings, and the results compared with established accepta-bility criteria. Airflow velocities were measured at each ven-tilation outlet in each classroom and corresponding volumeairflow rates determined. The volume airflow
3、 data were com-pared with UBC design standards and specifications in previ-ous balancing records. Comparison of the measured volumeairflow rates and noise revealed a direct relationship betweenthe two factors. Classrooms meeting the minimum airflowrequirements tended to be excessively noisy. Classro
4、oms withacceptable noise levels tended to have inadequate volume air-flow rates. The main source of noise was excessive turbulenceover dampers and diffusers, intensified by poor damper place-ment and high airflow velocities. To make noise levels accept-able, three of the classrooms could have their
5、ventilationsystems balanced to lower volume airflow rates to the appli-cable minimum standard. In addition, face dampers could bereplaced by volume extractors; branch takeoffs could belengthened and their ducts enlarged. Two of the classroomscould have their volume airflow rates increased to comply
6、withthe ventilation standards. The results suggest that achievingboth acceptable ventilation and noise quality in mechanically-ventilated buildings can be a challenge; they also confirm thatenvironmental factors are not independent, and must be opti-mized from a multi-disciplinary perspective if hig
7、h-qualityenvironments are to be achieved for building occupants.INTRODUCTIONA study of the classrooms in a university building wasundertaken, with the goal of investigating the acceptability of,and the relationship between, airflow rates and noise levelsfrom the heating, ventilation and air-conditio
8、ning (HVAC)system, and how the noise can be controlled. The HVAC noisewas evaluated according to three different criteria for ratingbackground-noise levels. Measured volume airflow rates werecompared to their specifications, and related to the back-ground-noise levels. This paper reports the tests d
9、one and theresults. The objective was not to perform a detailed, exhaustiveinvestigation of system performance. It was to provide directevidence of the relationship between ventilation performanceand noise, and to discuss how HVAC noise can be controlled.This paper is directed at ventilation enginee
10、rs who may notalways be aware of the acoustical consequences of their work,not to acoustical engineers.There is no one way to rate the acceptability of classroomnoise. Existing and proposed standards use a variety of back-ground-noise rating methods to quantify suitable classroom-noise levels. The c
11、lassroom noise in the study building wasevaluated according to three methods. An ANSI standard usesan A-weighted sound-pressure-level rating (dBA), and limitsclassroom noise to below 35 dBA for classrooms with vol-umes less than or equal to 20,000 ft3(566 m3), and 40 dBA forthose of greater volumes
12、(ANSI 2002). A second standardunder consideration uses the Noise Criteria (NC) method ofevaluating noise, and recommends a limit of NC 30 (Lilly2000). The Room Criteria (RC) rating is recommended byASHRAE when assessing HVAC noise, and limits classroomnoise to RC 35 (N), the “N” indicating that the
13、noise spectrummust be of neutral quality (broadband noise) (ASHRAE 2007).Relationship between HVAC Airflow Rates and Noise Levels, and Noise Control in a Mechanically-Ventilated University BuildingMurray Hodgson, PhD, CEngMurray Hodgson is Professor of Acoustics in the School of Environmental Health
14、 and the Department of Mechanical Engineering, Universityof British Columbia, Vancouver, BC, Canada.OR-10-058 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additi
15、onal reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. ASHRAE Transactions 551Of these criteria, the Room Criteria method is the only onewhich evaluates sound quality as well as level.The building investigated in th
16、is study was chosenbecause previous work had shown it to contain classroomswith high HVAC noise Hodgson 2002. It had five classrooms(here called Rooms A-E) on the second and fourth floors. Itwas built in 1962. The three unrenovated classrooms and theirventilation systems had negligible sound absorpt
17、ion. How-ever, classrooms A and C on the second floor were renovatedin 1999 and 2001, respectively. Renovations included upgrad-ing wall and ceiling sound absorption, and upgrading theHVAC systems by increasing the number of branch ducts sup-plying air to the classrooms and adding duct liner inside
18、them,to provide sound absorption and control internal noise. All fiveclassrooms were located on the west side of the buildingsmain hallways three on the second floor and two on thefourth floor. A fan located in the first-floor mechanical roomsupplied the west side of the building. A second fan, posi
19、-tioned in the penthouse, supplied all other rooms. Plan layoutdrawings of the classrooms and HVAC systems of the secondand fourth floors are shown in Figure 1.EXPERIMENTATIONMeasurement ProceduresIn each of Rooms A-E of the building volume airflowrates were measured across each ventilation outlet.
20、HVACnoise levels were also measured, using a sound-level meter.Measurements were made at four to nine equally-spaced posi-tions throughout the seating areas of the rooms, and energy-average, 31.5- to 8000-Hz octave-band levels calculated; fromthese, total A-weighted levels were calculated. As per th
21、erequirements of the ANSI standard (ANSI 2002), measure-ments were made with windows closed, and computers andaudio-visual equipment turned off.The airflow velocity across each classrooms HVAC-ductoutlets was measured using a hot-wire anemometer. The datawere collected over four equally-spaced areas
22、 over eachdiffuser face, and the results averaged to obtain the effectiveairflow rate. These were converted into volume airflows bymultiplying the measured velocity by the outlets effectivecross-sectional area. The effective cross-sectional area was thefraction of the outlets cross-sectional area no
23、t blocked bydiffuser vanes, and was taken, since values for the actual diffus-ers were not available, from manufacturers data for similardiffuser grills, to be 0.78 of the total cross-sectional area.Noise-Level ResultsFigure 2 shows the classroom-average HVAC noiselevels. It shows that noise levels
24、were highest at low frequen-cies; in the two renovated classroomsRooms A and Cnoise levels decreased with increasing frequency, whereas thethree unrenovated classrooms displayed peak noise levels inthe 63-Hz octave band. Room E had the highest levels in the31.5 to 125 Hz octave bands, while Room B h
25、ad the highestlevels in the 250 to 4000 Hz bands. In general, the two reno-vated classrooms, A and C, had the lowest levels of back-ground noise. Though Room E had the highest noise levels atlow frequencies, Room B had the highest total A-weightedlevel (46 dBA), since it had the highest levels at hi
26、gherfrequencies.The three noise-rating methods were applied to all fiveclassrooms; the results are shown in Figure 3. For Rooms Aand C the acceptable noise-level limit was 40 dBA, whereasfor Rooms B, E, and D it was 35 dBA due to their smallervolumes (ANSI 2002). With regards to the NC criterion (Li
27、lly2000), the limit of NC 30 applied to all five classrooms, as didthe limit of RC 35 (N) for the RC rating method (ASHRAE2007; ASHRAE 2009). These limit values are displayed inFigure 3.Figure 1 Plan layout drawings of the classrooms andHVAC systems of the second (top) and fourth(bottom) floors of t
28、he study building, beforerenovation of classrooms A and C.Figure 2 Measured octave-band and total A-weightedHVAC noise levels. 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal
29、 use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. 552 ASHRAE TransactionsWhen rated by the total, A-weighted level (ANSI 2002),Room A was slightly below the 40 dBA limit (with a value of39.5 dB
30、A), and Room C marginally exceeded it (at 40.6 dBA).Rooms B, E, and D had unacceptably high noise levels relativeto the recommended limit of 35 dBA, with values of 46, 45 and43 dBA, respectively. With regards to the NC criterion (Lilly 2000), all fiveclassrooms had NC levels exceeding the NC 30 limi
31、t. RoomsB and E had ratings of NC 40, Room D had a rating of NC 37,and both Rooms A and C had ratings of NC 33.When evaluated according to the RC criterion (ASHRAE2007), only Rooms B and E exceeded the specified RC 35limit. Considering the quality of the noise spectrum, all of theclassrooms except f
32、or Rooms A and B had the desirableneutral quality. The noise spectra of both Rooms A and Brated “H (hiss)”. Although Room A was below the specifiedRC 35 level, it did not have a “neutral” noise spectrum.However, in this case the noise quality was determined to bemarginally acceptable, given that its
33、 high-frequency compo-nent was likely not perceptible to occupants (ASHRAE 2007).Note that the values obtained from these measurements are anapproximation to the actual RC ratings, as the proper proce-dure requires the measurement of sound-pressure levels in the16-Hz octave band, which exceeded the
34、capabilities of thesound-level meter used.All three methods of evaluating the classroom noiseresulted in the conclusion that Room B was the most problem-atic, followed by Room D, then Rooms E, C, and finally A. Thetwo renovated classrooms, Rooms A and C, were either below,or slightly above, their ac
35、ceptability limits for all three criteria.The decibel attenuations required in each octave band (inparticular, to meet the NC 30 criterion) were determined. RoomB had the highest noise levels, and thus required the greatestattenuation. Attenuation was needed mainly in the 250 to2000 Hz frequency ran
36、ge, with maximum values of 9.6 and9.3 dB at 500 and 1000 Hz, respectively. Room E had thesecond highest noise levels; it required attenuation in the 125to 2000 Hz octave bands, with the maximum attenuationrequired (7.6 dB) at 125 Hz. Room D was slightly less noisythan Room E, but showed a similar pa
37、ttern of required atten-uations. Its maximum required attenuation was 5.2 dB, againin the 125 Hz octave band. Rooms A and C required the leastattenuation to meet the NC 30 standard; the required reduc-tions were in the range 500 to 2000 Hz, and were a maximumof 2.9 dB at 1000 Hz.Airflow ResultsThe a
38、verage volume airflow rate across each diffuser wascalculated from the airflow-velocity measurements for all fiveclassrooms. Then, for each room, the total volume airflow ratefrom the HVAC system was found by summing the individualoutlet values. These results are summarized in Figure 4. RoomB had th
39、e highest volume airflow rate, with an average totalvalue of 2500 ft3/min (1180 L/s), even though it contained thesmallest number of outlets (three). Room E had the next highestat 2443 ft3/min (1153 L/s), followed by Room D at 2357 ft3/min (1112 L/s), Room C at 2000 ft3/min (944 L/s) and, finally,Ro
40、om A at 1722 ft3/min (813 L/s). The calculated volumeairflow rates for each classroom were compared with thecorresponding values specified by the most recent balancingreport or the renovation specifications. These specified valuesare also shown in Figure 4. The noisiest classroomsmostnoticeably Room
41、 Bhad measured volume airflow rates wellabove those specified. Conversely, the renovated classrooms(Rooms A and C) had volume airflow rates below their spec-ifications.Figure 3 Comparison of measured noise levels in fiveclassrooms with three acceptability criteria.Figure 4 Comparison of measured tot
42、al volume airflowrates in five classrooms with two acceptabilitycriteria. 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or
43、transmission in either print or digital form is not permitted without ASHRAEs prior written permission. ASHRAE Transactions 553The University of British Columbia requires a minimumof 8 to 10 air changes per hour (ACH) for classrooms (Lis2002). The volume airflow rate corresponding to the absolutemin
44、imum of 8 ACH was determined for each classroom and isalso shown in Figure 4. According to Figure 4, Rooms B, E,and D could have their volume airflow rates reduced belowtheir current specifications, and still meet minimum require-ments. Conversely, renovated classrooms A and C currentlyhave volume a
45、irflow rates below the regulated minimumvalues.It is difficult to determine how much of the apparent noisereductions in Rooms A and C were the result of their renova-tions, and how much was due to their extremely low volumeairflow rates. There are two other rooms supplied by the west-zone fan on the
46、 second and fourth floors; one houses graduate-student offices, the other is a seminar room. The former roomhad also recently been renovated. Even though neither was aclassroom, the airflows into these rooms were measured forcomparison. As before, the renovated room had volumeairflow rates which wer
47、e below its specifications, while theother room had volume airflow rates above those indicated inthe balancing report. It seems that there was a lack of properbalancing after renovations were completed.The speed of the supply fan was also measured, to deter-mine whether it was operating as intended.
48、 When the fan wasoriginally selected in 1962, it was to operate at 425 rpm.However, during balancing in 1987 for energy-conservationpurposes, the fans speed was lowered to 405 rpm. The currentoperating speed of the fan was measured as 413 rpm, anapproximately 2% increase since 1987, which is negligi
49、ble.Assuming the fan was originally selected to operate at or nearpeak efficiency, it should still be operating in this range; thusit would not be expected to be generating excessive noise(ASHRAE 2007). Furthermore, the airflow output of the fanincreased from 27,300 ft3/min (12 880 L/s) in 1987 to approx-imately 27,890 ft3/min (13 160 L/s; an increase of 2.2%); thisdoes not account for the magnitude of the increased airflow inthe non-renovated rooms. The problem was likely not the fan,but rather the reduced airflows in Rooms A and C (and thegraduate-student area) which c
