May 11, 2016
Mr. Chris Cocallas
University Architect and Director Capital Planning and Construction
Colorado School of Mines
1801 19th St.
Golden, Colorado 80401
Re:
GRL and GRLA Building Noise Study
Wave #1434
Dear Chris,
We completed the noise assessment for rooftop equipment on the General Research Laboratory
(GRL) and General Research Laboratory-Annex (GRLA) buildings at the Colorado School of
Mines (CSM). I measured sound levels on the roof near each noise producing piece of equipment
and on the ground in the vicinity of nearby residences. This report is an update of our original
report dated April 8, 2016. We have re-measured sound levels at night after modifications have
been made to fans on both the GRL and GRLA rooftops.
The study shows that the most significant noise sources were initially the large General Exhaust
fans and motors on the GRLA building and the induced flow Lab Exhaust fans on the GRL
building. The GRL Lab Exhaust fans were re-sheaved on March 25, 2016, to slow the fan speed,
and this significantly reduced noise from these fans. The Variable Frequency Drives serving the
GRLA exhaust fans were adjusted and the annoying “chirp” or “whine” was reduced and shifted
to a higher frequency. The annoying “chirp” is no longer audible at most locations on the ground.
The GRL Lab Exhaust fan noise is reduced below the background sound and other equipment.
The GRLA General Exhaust fans are still audible to the north, but the smaller GRLA Lab
Exhaust fans are louder than the larger fans at locations northeast of the building.
This report is intended to summarize the measurements and mitigation that have been done so
far, and to comment on additional noise mitigation measures that can be considered.
Mr. Chris Cocallas
May 11, 2016
Page 2
TestProcedures
To minimize the influence of background noise from traffic on Highway 93 and local streets, I
measured sound levels in the area on Friday March 25, between 12:30 a.m. and 3:30 a.m. I
measured sound levels at or near five locations that you had previously measured at to allow
comparison with those measurements. The same testing procedure was repeated on April 22,
2016, at the same time of day. The measurement locations are shown on Figure 1.
Figure 1: Measurement locations
Mr. Chris Cocallas
May 11, 2016
Page 3
The following test equipment was used.
SoundLevelMeterM1(Locations1,4,and8)
 Larson Davis Model 831 sound level meter S/N 0002878, Type 1 per ANSI S1.4
 PCB preamp PRM831, S/N 021453
 PCB ½ʺ microphone Model 377B02, S/N 130873
SoundLevelMeterM2(Location7)
 Larson Davis Model 831 sound level meter S/N 0001119, Type 1 per ANSI S1.4
 PCB preamp PRM831, S/N 026106
 PCB ½ʺ microphone Model 377B02, S/N 138652
SoundLevelMeterM3(Location2)
 Larson Davis Model 831 sound level meter S/N 0004081, Type 1 per ANSI S1.4
 PCB preamp PRM831, S/N 036934
 PCB ½ʺ microphone Model 377B02, S/N 153301
Larson Davis Electronic Calibrator CAL200, S/N 2905
The system calibration was checked in the field before and after each series of measurements.
The calibration of all equipment has been certified by the manufacturer.
Meter M1 was moved between Locations 1, 4, and 7 during each equipment scenario. The data
shown in Table 1 are two to three minute averages during each scenario.
Meters M2 and M3 ran continuously at their respective locations from approximately 12:45 a.m.
to 3:30 a.m. on Friday March 25, 2016, and Friday, April 22, 2016. The data shown in Table 1
are five minute averages during each scenario.
I measured sound levels under a variety of operation conditions to help determine the individual
contributions of each piece of equipment. Mike Willey of CSM turned equipment on and off to
create the different equipment scenarios. Please note that the GRLA General and Lab Exhaust
systems, and the GRL Lab Exhaust systems each include two fans, but only one runs at a time.
Only one fan from each system was running during our tests.
I also measured sound levels close to the equipment on the rooftops of the GRL and GRLA
buildings (with Meter M1). We used this data in our analysis to help judge the most significant
noise sources, and to evaluate mitigation. These measurements were performed on March 10, 22,
25, 29, and April 6, 2016.
Mr. Chris Cocallas
May 11, 2016
Page 4
The equipment scenarios for the early morning March 25 and April 22 measurements are
identified as follows.
All off)
Scenario 3a)
Scenario 3b)
Scenario 3c)
Scenario 4d)
Scenario 4f)
All on)
all rooftop equipment on GRL and GRLA turned off. This condition is used to determine
the background noise levels
GRLA small Lab exhaust fan & MAU on, GRL rooftop equipment off
GRLA Gen Exh fan & MAU on, GRL rooftop equipment off
GRLA Gen Exh fan, small Lab Exhaust fan & MAU on, GRL rooftop equipment off
GRL Gen Lab Exh Fans on, MAU on, Lab Exhaust fan off, GRLA rooftop equipment off
GRL Gen Lab Exh Fans on, MAU on, Lab Exhaust fan on, GRLA rooftop equipment off
all rooftop equipment on GRL and GRLA was on.
TestResults–March25,2016
Table 1: Sound pressure levels (in dBA) under various equipment scenarios, 3/25/16
March 25, 2016 Scenario All off, background 3a 3b 3c 4d 4f All on 1 36 34 37 37 37 39 39 Location No. 2 4 7 39 37 38 39 38 39 44 37 38 44 38 38 41 40 42 43 43 45 47/47* 45 47/44* 8 36 41 40 43 39 46 47 * Mar 25/Mar 29
I measured during the early morning on March 25 except where noted otherwise.
The GRL Lab Exhaust fans were re-sheaved after the March 25 measurements. I re-measured on
March 29 during the day at Locations 2 and 7. That data is also shown in Table 1.
The sound levels in Table 1 are L90 sound levels in A-weighted decibels (dBA). The L90 is the
sound level that is exceeded 90% of the time. Since the sound from fans and mechanical
equipment is relatively steady, the L90 can be used to show the sound level of the equipment
while minimizing the effect of intermittent cars passing on Highway 93 or local streets, aircraft
flyovers, and other intermittent noises. This gives us the best indication of sound levels from the
mechanical equipment.
The L90 was also used for the background sound levels (all equipment off), so sound levels
shown represent the quieter periods during lulls in traffic.
Mr. Chris Cocallas
May 11, 2016
Page 5
InitialResults
Table 1 data shows that the GRLA small Lab Exhaust fan raised the sound level at Location 8,
but was insignificant at other locations.
The larger GRLA General Exhaust fan sound was significant at Locations 2 and 8. The “whine”
associated with the motor was audible at Locations 1, 4, and 7, but the sound level was not
significantly raised by the fan. Locations 4 and 7 are screened from direct line-of-sight of the
General Exhaust fans.
The GRL General Exhaust fans and MAU were measureable at all locations except Location 1,
but only 2 to 4 dBA above the background sound level.
The GRL Lab Exhaust fan was much more significant and clearly audible at Locations 4, 7, and
8 before it was re-sheaved.
It is worth noting that sound that appeared to be from the Coors plant(s) to the east was distinctly
audible at Location 4 and is part of the background noise.
ResultsafterGRLLabExhaustFanswerere‐sheaved
The GRL Lab Exhaust fans were re-sheaved after the March 25 measurements to slow the fans
down. This significantly reduced the sound produced by the fans. This was shown by the
measurements on the roof and also by a measurement made during the day on March 29 at
Location 8.
ResultsafteradjustmentoftheGRLAGeneralExhaustFanVFDs
The GRLA General Exhaust fans exhibited a loud “whine” or “chirp” that was audible at all of
the measurement locations except Location 7. Since the March 29 measurements, the VFDs
causing the audible motor whine were adjusted. I visited CSM again during the day on April 6 to
observe the improvement. The “whine” was not audible on the ground at any of the measurement
locations. I re-measured on the roof close to the equipment. The single-number dBA level is not
a good way to judge the severity of the “whine”, so I measured sound levels across the frequency
spectrum in two different ways. You can think of them as two levels of “resolution.”
First, I measured very close to the GRLA General Exhaust fan motor that was producing the
strong “whine” using Narrow Band analysis. This measurement breaks the frequency spectrum
from 0 to 5,000 Hz into constant bandwidth bands that are about 1.6 Hz wide. This gives a very
accurate indication of the frequencies with the strongest sound energy and of the frequency of
any tones. The result of the “before” measurement is shown in Figure 2.
Mr. Chris Cocallas
May 11, 2016
Page 6
Figure 2: Narrow band measurement of GRLA General Exhaust fan motor before VFD
adjustments
Figure 2 shows the decibel level (y-axis) versus frequency (x-axis). The series of many sharp
spikes, or tones, create an unusual “chirp” or “whine” at about 2,200 Hz and higher frequencies.
Figure 3 shows a measurement at the same location after the VFD adjustment.
Figure 3: Narrow band measurement of GRLA General Exhaust fan motor after VFD
adjustments
Mr. Chris Cocallas
May 11, 2016
Page 7
The difference between Figures 2 and 3 is dramatic. You can see the reduction in amplitude and
number of spikes, and also that heavy blue areas (indicating more sound energy) shift to higher
frequencies. The most distinct spikes now start at about 4,000 Hz. Higher frequencies are
attenuated at a higher rate as sound travels through the atmosphere. The plots in Figures 2 and 3
are interesting, but it is somewhat difficult to see how this affects the sound levels that we hear
or measure.
Figure 4 shows the sound levels measured at the same times and locations, but presented in onethird octave bands. One-third octave bands show the sound levels with less frequency resolution,
but are standardized so the actual sound levels are more meaningful.
Figure 4: 1/3-octave band measurements of GRLA General Exhaust fan motor before and after
VFD adjustments
The one-third octave band data show that the peak sound energy was reduced in amplitude and
moved from the 3150 Hz to 5,000 Hz band. I could still hear the much reduced “whine” when
standing near the fan, but I could no longer hear it on the ground. Having said this, my visit on
April 6 was during the day when ambient noise was higher than at night. After my work on the
roof, the wind picked up and made measurements on the ground meaningless. The sound levels
should be evaluated again at night when ambient sound is low.
Mr. Chris Cocallas
May 11, 2016
Page 8
ReductionoftheGRLLabExhaustFanNoise
I took similar measurements next to the GRL Lab fan discharge stack. Figure 5 shows the onethird octave data before and after the fan was re-sheaved to slow its speed.
Re-sheaving the fan reduced the sound level across the frequency spectrum. The overall sound
level was reduced by about 18 dBA. The resulting reductions at Location 7 was about 3 dBA
since other sources contribute as well. Further reduction should not be necessary for the GRL
Lab fans.
Mr. Chris Cocallas
May 11, 2016
Page 9
TestResults–April22,2016
The nighttime round of testing was repeated on April 22, 2016.
The results are shown in Table 2.
Table 2: Sound pressure levels (in dBA) under various equipment scenarios, 4/22/16
4/22/16 testing
Location No.
Scenario
1
2
4
7
8
2) All off,
41*
41*
43*
38*
41*
background
3a
42*
44*
45*
38*
46*
3b
41*
45*
42*
41*
43*
3c
41*
45
42*
39*
46*
4e
41*
40
41*
41
41
4f
40*
42
43
44
42
1) All on
41*
46
42*
45
46
* Background sound from the wind at times, traffic, and other sources was louder than
equipment at GRL and GRL-A. There was a strong hum audible much of the time from the
direction of Coors or Golden.
The intent of this round of measurements was to get a full set of data after improvements have
been made to equipment on GRL and GRLA.
Unfortunately the background sound was louder on April 22 and makes direct comparison with
the March 25 data (Table 1) difficult.
In Table 2, the * indicates when the sound of wind, traffic, or equipment in the direction of
Golden or Coors was more significant than sound from the GRL and GRLA equipment. The
sound levels from GRL and GRLA equipment could not be determined (even though it may have
been audible in some cases) in the presence of the background sound. We only know that the fan
noise is no louder than the measured sound level, and in many cases is less than the measured
level.
The GRL Lab Exhaust fans are no longer the loudest source on the GRL roof. Sound from the
condensing units and/or make-up air unit are louder than the fans.
The GRLA General Exhaust fans are more noticeable at Location 2. The GRLA Lab Exhaust
fans are more noticeable at Location 8.
Mr. Chris Cocallas
May 11, 2016
Page 10
NoiseLevelCriteriaandAssessment
Colorado Revised Statute 25-12-103 sets a limit of 50 dBA at night at a location 25′ beyond a
residential property line. The Table 1 and Table 2 data show that the GRL and GRLA rooftop
equipment did not exceed the state limit even before re-sheaving the GRL Lab Exhaust fans.
Nonetheless, sound from the equipment on both buildings was clearly audible above background
sound in the area on March 25, and the “whine” of the GRLA General Exhaust fan motor was
distinctly audible and drew specific complaints from some neighbors. Since then, the “whine”
has been mitigated and is no longer noticeable.
The April 22, 2016 measurements continue to show that the GRL and GRLA rooftop equipment
does not exceed the state limit.
PotentialforAdditionalMitigation
Two of the most significant noise sources have been mitigated. The remaining GRL and GRLA
noise sources contributing to noise levels in the area include the GRL Make-up Air unit and
condensing units, and the GRLA General and Lab Exhaust fans (fan noise as opposed to motor
noise). The cooling tower on the GRLA may be contribute to a lesser extent.
ExhaustFanStacksilencers
The GRLA General Exhaust and smaller Lab Exhaust fans produce noise, but the noise does not
appear to be as annoying as the motor “whine” was. It is possible to reduce the General Exhaust
and Lab Exhaust fan noise with stack silencers.
Some manufacturers of these induced flow fans make Circular Dissipative silencers that are
essentially a collar around the duct above the fan. Unfortunately this type of silencer is not
available as an “off the shelf” item from Twin City Fans, the manufacturer of the GRLA General
Exhaust and Lab Exhaust fans.
We have received preliminary designs for inline stack silencers from Vibro-Acoustics. The
silencers would be inserted between the fan discharge and the induced flow stack. They would
raise the General Exhaust stack height by approximately 72ʺ, and the Lab Exhaust stacks by
approximately 36ʺ. If you wish to pursue the stack silencers, a structural engineer will need to
determine if additional structural support, guy wires, etc., are necessary to support the increased
stack height. Fan noise could be reduced by 5 to 10 dBA. Straight or elbow silencers may also be
necessary the bypass damper intakes on each set of fans.
Keep in mind that as the neighborhood sound level is reduced closer to the background sound
level, further reduction will be less noticeable.
Mr. Chris Cocallas
May 11, 2016
Page 11
TallScreenWall
We evaluated the possibility of a screen wall around the GRLA rooftop equipment. A screen has
to completely block the line-of-sight to reduce noise. Screens are more effective at mid and high
frequencies than at low, and are most effective when placed close to the noise source. I
understand that the GRLA building structure cannot accommodate the additional weight and
wind load of a screen.
A screen would have to be located at the edge of the roof along the northwest and northeast
sides. It would have to be supported from the ground on a new structure. It would have to be
approximately 50 to 55 feet tall (from ground level) to be effective. In theory a screen could
reduce GRLA equipment noise in the neighborhood from 10 to 15 dBA, but this much reduction
would not be realized because noise from other sources would become more significant. For
example, even if the fan noise is reduced by 10 to 15 dBA, the noise level at Location 2 would
only be reduced by about 3 dBA (from 44 to 41 dBA per Table 1) due to the background sound
level. This is a “just noticeable” change.
For your reference, a 2 to 3 dBA change is “just noticeable” by most people. A 5 to 6 dBA
change is clearly noticeable, and a 10 dBA change is perceived as “half as loud.”
I have only evaluated the screen wall conceptually. At this point I have not tried to evaluate its
feasibility, or if and how it would work architecturally and structurally. If feasible, it is sure to be
quite expensive for the relatively small improvement to be gained.
Conclusions
Sound levels from GRL and GRLA rooftop equipment comply with State of Colorado limits at
residential property lines. The previous source of noise complaints was most likely the GRL Lab
Exhaust fans and the GRLA General Exhaust fans and motors.
Sound from the GRL Lab Exhaust fans has been greatly reduced by re-sheaving and slowing the
fans. The “whine” of the GRLA General Exhaust fan motors has been reduced or eliminated.
Sound from the GRLA General Exhaust and Lab Exhaust fans is still contributing to noise levels
that are likely above the nighttime background sound at some locations in the neighborhood.
There are times, even at night, when the background sound is too loud measure the fan noise.
The GRL condensing units and MAU may also be contributing to noise in the area.
Additional mitigation measures could include silencers on the exhaust fans or a tall screen wall
around two sides of the GRLA building.
Mr. Chris Cocallas
May 11, 2016
Page 12
I am available to meet or discuss this further by phone if you wish. Please let me know if you
would like to proceed with design or investigation of additional noise mitigation.
Sincerely,
Jeff
Kwolkoski
Digitally signed by Jeff Kwolkoski
DN: cn=Jeff Kwolkoski, o=Wave
Engineering, Inc., ou,
email=jeffk@WaveEngineering.c
o, c=US
Date: 2016.05.11 10:09:28 -06'00'
Jeff Kwolkoski, P.E., INCE Bd. Cert.
President
cc:
Mike Bowker, CSM