DATE: July 12, 2004 SUBJECT: Gaudet School Chiller Noise Survey Results

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1 799 MIDDLESEX TNPK. BILLERICA MA PHONE: FAX: noise-control.com JOB MEMO TO: John McKenney COMPANY: McKenney Mechanical Contractors, Inc. FROM: Jesse Spence DATE: July 12, 2004 SUBJECT: Gaudet School Chiller Noise Survey Results Background NCE has performed a noise survey of the McQuay Air Conditioning GeneSys Air Cooled Rotary Screw Chiller system which is located on the roof of the Gaudet Middle School in Middletown, RI. The survey was performed by one NCE engineer on July 7, 2004 from 6:45 AM to 9:30 AM. The weather was clear, 75 degrees with an occasional light wind. The purpose of the survey was to assess the noise created by the chiller system as it impacts the neighboring residential community. NCE has been made aware of complaints that have been issued regarding the noise from the chiller since it has started to be used approximately one month ago. The chiller system uses two screw compressors which operate as needed. In addition there are several fans located at the top of the unit for cooling the system. The chiller system currently operates between 7:00 AM and 5:00 PM, however NCE has been informed that it may run earlier or later in the day at some point in the future. A photograph of the chiller is given in Picture 1. During the survey NCE performed airborne noise measurements at the property line of the Gaudet Middle School which borders several residential homes located on Turner Rd. and Columbia Rd. Measurements were taken while the chiller system was both on and off. During the survey it was noted that different compressors were running at different times, and occasionally both compressors ran at the same time. An effort was made to note the operating condition of the chiller system. There was no control over which compressor was running during the survey. Airborne noise measurements were also made around the chiller system on the rooftop. Only one compressor was operating during these measurements. All measurements were made with a Type I sound level meter, as specified by ANSI Standard The measurement system was field calibrated to 94 db at 1000 Hz at the beginning of the survey. Page 1

2 Results Figure 1 shows the maximum airborne noise levels measured along the entire school property line. The levels are presented in third octave bands. The maximum levels were measured near the house located at the corner of Turner Road and the school property line (south-east corner of the school property). This house has an unobstructed view of the chiller system, meaning there is a direct noise path. Only one compressor was running during this measurement; however similar levels were measured at the same position when both compressors were running. The levels in Figure 1 are plotted against the measured background levels at the same position which were taken with the chiller system turned off. From Figure 1 it is seen that the maximum db(a) level measured at the school property line is 50 db(a). This is largely due to increased levels (over the background) between 800 and 2000 Hz. Narrowband measurements (3.125 Hz bandwidth) made at the same position show that these levels are due to several tones, the largest of which is at 1785 Hz. Figure 2 shows the measured narrowband levels at this position. Nominal airborne noise levels of the chiller system measured at a distance of 30 feet were provided to NCE by McKenney Mechanical Contractors Inc (MMC) [1]. These levels were compared to measurements of the Gaudet chiller system measured at 30 feet. An example of this comparison is presented in Figure 3. While the data is not an exact match, it does indicate that the unit is operating normally. It should be noted that only one compressor was operating during these measurements at the Gaudet School. However it was observed that both compressors produce approximately the same noise levels. Measurements made close to the compressor system show that the dominant noise levels are originating from the compressors. The fans located on top of the unit do not produce significant noise levels relative to the compressors. Noise Ordinance The Town of Middletown, RI Noise Ordinance section states that sound levels can not be in excess of those listed in the Normal Limit column of Table 1 at the property line of a residential receiver [2]. However, paragraph C of this section states For any source of sound which emits a pure tone 1, the maximum sound level limits shall be reduced by 5 db(a). This reduced limit is shown in the Tonal Limit column of Table 1. Due to the tonal nature of the sound created by the compressor system (as shown in Figure 2), the tonal limits would apply in this case. Comparison of these limits to the measured data at the school property line (which is effectively the property line of the residential receiver at the position of maximum measured 1 Section of the Middletown Noise Ordinance states that a pure tone is defined as any sound which can be distinctly heard as a single pitch or set of single pitches. It is clear from the data presented in Figure 2 that the tested chiller system creates a pure tone as defined by this ordinance. Page 2

3 noise) shows that the current generated levels of 50 db(a) are 10 db below the daytime limit. However, since the unit may be run during nighttime hours it is recommended that the nighttime limit of 50 db(a) be set as the noise goal at the property line. In this case, the measured sound level is equal to the Middletown limit as shown in Table 1. Technically the data measured by NCE is within compliance of the Middletown nighttime tonal noise limit. Table 1 Noise Limits set by the Town of Middletown, RI Time of Day Normal Limit Tonal Limit 7 AM to 10 PM (day) 65 db(a) 60 db(a) 10 PM to 7 AM (night) 55 db(a) 50 db(a) Recommendations The maximum measured sound level at the Gaudet School property line was 50 db(a), which is equal to the recommended limit based on nighttime noise limits set by the Town of Middletown, RI. It is conceivable that under certain conditions (i.e. different wind speed and direction) this level could be higher or lower by a few db. Because of this it is recommended that noise treatments be installed to help lower the noise levels at the property line. NCE was informed of a 2.5 lb/ft 2 sound blanket which is available for treatment of each individual compressor. The AGS Sound Cover Material sound blanket is made from a single layer of 2 PSF loaded vinyl barrier which is sandwiched between two layers of quilted aluminum cloth-faced fiberglass. The transmission loss of this barrier was provided to NCE by MMC, and is given in Table 2. Table 2 Sound Blanket Transmission Loss (from vendor) Octave Band Frequency, Hz Transmission Loss, db This treatment should provide significant sound reduction, especially since the tones created by the compressors are in the 250 to 2000 Hz range (as seen in Figure 2). NCE recommends that this treatment be used to abate the noise from the chiller system. Note that care should be taken to make sure each compressor and compressor motor are completely covered by the sound blanket. Gaps in the treatment should be minimized if not avoidable. Page 3

4 References 1. McQuay Air Conditioning, GeneSys Air Cooled Rotary Screw Compressor Product Manual, PM AGS Town of Middleton RI, Town Code Book/Title XIII: General Offenses/CHAPTER 130: GENERAL OFFENSES, Section Page 4

5 Figure 1 Third Octave Band Measurement Maximum Measured Airborne Noise Levels at Property Line Compressor System On Background SPL, db re 20 upa Third Octave Band Frequency, Hz Figure 2 Narrowband Measurement Maximum Measured Airborne Noise Levels at Property Line db(a) SPL, db re 20 upa Frequency, Hz Page 5

6 Figure 3 Measured Source Levels Feet from Unit Gaudet Unit McQuay Nominal Data 65.0 SPL, db re 20 upa db(a) Octave Band Frequency, Hz Page 6

7 Page 7 Picture 1 Compressor System