Telecom Industry Requires Very Low Noise Emissions
The telecom industry utilizes diesel-
fueled electric power generator sets
to provide reliable prime and standby
power for the equipment used at cell
towers. Some towers are often located
in close proximity to communities
and habitats.
By their nature, diesel generator sets
have the potential to produce excessive
and unwanted noise emissions that
can be disruptive to the communities
in which cell towers are located. And
while generator sets used in these
locations must not only comply with
regional noise regulations, directives
and standards, they are also subject to
telecom provider preferences for even
quieter operation.
The telecom industry also demands
generator sets that deliver maximum
Advanced noise reduction technology for the development of T-series Super Silent generator sets■ White PaperDr. Shashikant More, Group Leader — Acoustics (Mechanical Engineer) Global Applied Technology, Cummins Power Generation
Power topic #6054 | Technical information from Cummins Power Generation
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reliability and high performance in
a compact physical footprint. It’s a
common strategy to increase the
size of the generator set’s acoustic
enclosure and its overall footprint to
achieve desired noise reductions.
However, given the size constraints in
the telecom industry, generator set
manufacturers must significantly reduce
noise emissions without increasing the
enclosure’s size — all while keeping
units competitively priced.
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To achieve the targeted low noise levels, the T-series
generator sets were designed with specific component
and acoustic enclosure noise reduction strategies.
Each component — from the engine and radiator fan
to the exhaust muffler, air filter and alternators —
was designed and/or selected and packaged to
complement the noise reduction performance of the
acoustic enclosures. In addition to addressing noise
levels, all aspects relating to sound quality were
targeted through “Perception Based Engineering”
techniques.
The precision-grade, hemi-anechoic chamber at
Cummins’ state-of-the-art Acoustical Testing Center
provided advanced noise measurement capabilities.
Precision grade represents the highest rating possible
for hemi-anechoic chambers in the acoustic domain,
providing the greatest assurance of certainty in
determining noise levels.
All acoustic measurements were conducted in
accordance with ISO and ANSI standards and
recommendations, as well as the guidelines set forth
by European directives and/or Asian noise regulations.
Cummins Power Generation considered all these
telecom market requirements in the development of
its T-series Super Silent generator sets. Based on
advanced noise control technologies and innovative
mitigation techniques developed at our Acoustical
Testing Center (ATC) in Minneapolis, Minnesota, we
integrated acoustic optimization strategies at every
phase of product development.
Unique Acoustic Features of T-series Super Silent Generator Sets
From the advanced acoustic enclosure to component
design, every aspect of the T-Series generator sets
was designed and developed to meet the telecom
industry’s strict low noise requirement. Their quiet
operation can be attributed to the sound mitigation
techniques described in this paper.
■ Sound quality — pleasant sound qualities
were accomplished using “Perception Based
Engineering” principles. This guarantees speech
signal preservation while the generator set is in
operation, allowing normal conversations to take
place within close proximity. T-Series generator
sets feature a noise signature that does not
produce harsh or annoying tones.
■ Compact footprint — advanced acoustic tech-
nology was used to maintain a compact enclosure
and small overall generator set footprint, while
achieving a very high level of noise attenuation.
■ Relatively constant noise levels — from No
Load to Full Load conditions, noise levels remain
relatively constant throughout the full range of power
increments. Generator set design was focused on
achieving no acoustic distortion upon load changes.
Low noise level generator set technology development
Cummins applied noise modeling techniques
throughout the T-Series Super Silent generator set
design process. The design team placed particular
focus on component design and selection while
carefully employing innovative acoustic mitigation
techniques to the generator set enclosure. We
developed noise control strategies at varying frequency
ranges — from low, mid to high ranges — each
requiring a specific noise control scheme.
Figure 1 — Acoustic enclosure noise modeling techniques were
employed in the design of T-series Super Silent generator sets.
Figure 2 — Noise modeling validation compares experimental
and simulation results.
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Why precision acoustical testing matters
Most generator set manufacturers do not utilize
precision-grade acoustic measurement equipment
and facilities during product development. Those
who seek to exceed both regulatory compliance and
telecom industry noise emission preferences must
adhere to the highest standards of acoustic testing.
Noise levels at workspaces are regulated by federal
governments to protect workers’ safety. In addition,
communities are increasingly enacting ordinances
aimed at reducing sounds that are disruptive to
nearby residents.
For example, in North America, typically permitted
noise levels at the property line at daytime and
nighttime are around 60 dBA and 50 dBA, respectively.
However, in certain special zones, local authorities
have set even lower noise limits. That’s why it is very
important to measure noise levels using appropriate
and applicable national and international standards
in order to establish an accurate platform for
comparison.
These precisely measured noise levels are also
useful for understanding the noise characteristics
of generator sets so that noise control strategies,
such as acoustic enclosures and walls, can be
successfully implemented.
In order to bring a generator’s noise levels into
compliance with local ordinances and federal
regulations, facility design engineers must carefully
evaluate where to locate the generator and what noise
reduction strategies will be most effective. These
strategies may include the construction of sound-
attenuating enclosures or the addition of acoustic
insulation or barriers, exhaust silencers or isolation
mounts. By equipping power system designers
with precise acoustic data, they’re better able to
design a noise reduction system that reliably meets
local noise restrictions.
Understanding acoustical data measurements
When comparing generator noise and sound data,
it helps to understand some of the most common
units of measurement used for industrial products.
Sound levels are often expressed in terms of
Sound Pressure Level and Sound Power Level (see
Glossary, p. 06). Both are important measurements,
but they’re quite different.
A-weighted Sound Pressure Level (SPLA or LA)
is specific to the distance of the receiver from the
sound source. Sound Pressure Level decreases
as distance from the sound source increases, and
vice versa. Without the context of distance from the
source, Sound Pressure Level data is meaningless.
A-weighted Sound Power Level (SPWLA or LWA)
is a measure of acoustical energy produced by
a sound source, with no regard to its distance
from the point of observation. Sound Power Level
is calculated based on Sound Pressure Level
measured by using parallelepiped or hemispherical
array method per ISO or ANSI standards.
For telecom industry generator sets, noise levels
are also dependent on two major operating
parameters: engine speed and load. Since noise
levels increase as speed and load increase, it’s
important to know at what speeds and loads the
noise levels were measured.
Cummins Power Generation carefully considers
acoustic emissions in the product development life
cycle, from research and development, analysis-led
design, component selection and advanced
enclosure technologies.
We utilize our world-class Acoustical Testing
Center in Minneapolis throughout the product
development life cycle. This carefully engineered
and environmentally controlled facility provides
exceptionally reliable acoustic data through the
use of precision-grade acoustical equipment and
rigorous measurement techniques.
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A look at the Cummins Acoustical Testing Center (ATC)
The Cummins Power Generation Acoustical Testing
Center is among the largest in the diesel and power
generation industries, featuring a hemi-anechoic
chamber certified as “Precision Grade,” the maximum
accuracy level per ISO 3745:2009. Its vast size
is needed to accommodate large generator sets
and the many microphone positions required for
precision-grade testing, including those at 7 meters
from the face of the largest generator set Cummins
manufactures.
By using the microphone setup according to ISO
and ANSI standards, measurements are conducted
surrounding the generator set so that all unique
noise characteristics of its components are captured
and the directionality of the noise sources studied. This
precision-grade noise data increases the operator’s
confidence level and eventually helps in devising
noise control strategies quickly and economically.
The Cummins Acoustical Testing Center adheres
to the most stringent national and international
standards for acoustic noise measurements:
■ ISO 3744:2010
■ ISO 3745:2012
■ ISO 8528-10:1998
■ ISO 6798:1995
■ ISO 9614-1:1993
■ ISO 9614-2:1996
■ ISO 9614-3:2002
■ ANSI S1.13:2005
■ ANSI S12.18:1994
■ SAE J1074:2014
Figure 3 — T-series Super Silent generator set under developmental
testing in the Cummins Power Generation Acoustical Testing Center.
It features an exceptionally large hemi-anechoic chamber that is
105 feet (32 meters) long, 80 feet (24.5 meters) wide and 36.5 feet
(11 meters) high.
Details concerning the construction and acoustical
features of the Acoustical Testing Center are published
in the technical paper, Design and Performance of Acoustical Testing Center of Cummins Power Generation, authored by Shashikant More, Martin
Myers and Victor Clemente, as included in the
Proceedings of Inter-Noise 2012, New York, USA,
August 2012.
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5
Testing the Cummins T-Series Super Silent generator set
The T-series Super Silent generator sets are equipped
with 1.3, 2.5 and 3.3 liter diesel engines capable of
fulfilling different levels of electric power demands
in the telecom market. They were tested at a variety
of operating conditions and a range of acoustic
parameters to determine how different aspects of
normal operation would contribute to overall noise
levels. Testing focused on sounds that are most
impactful to the human ear and structures surrounding
the generator sets, and frequencies that have a higher
potential to perturb people in surrounding communities.
Cummins strived to achieve high precision and reliability
in its numbers, calculating average readings from an
array of 14 microphones located at 1 meter from the
surface of the enclosure surrounding the genset. This
covered the entire human audible frequency range,
from 20 to 20,000 Hz. Cummins conducted extensive
test configurations throughout the trial period.
By using advanced noise measurement techniques
developed at the Acoustical Testing Center, Cummins
was able to determine the noise contributions from
individual components of the generator set to overall
noise levels. This included the noise characteristics of
the engine, cooling system, exhaust system, air intake
system, fuel injection system and alternator. Acquiring
noise data in this manner helps ensure precise
information is incorporated in product specification
Figure 4 — Screen shot of acoustic measurement data set at
Cummins’ Acoustical Testing Center
sheets. It is also helpful for quickly devising noise
control strategies, whether it’s to meet regulation-
or directive-based noise limits or satisfy customer-
specific requirements.
Some highlights of the T-series Super Silent generator
set testing include:
■ Baseline noise data was acquired at 1 meter using
the parallelepiped method per ISO 3744:2010 with
14 microphone locations around the generator set
(engine and alternator ends, sides and top of the
generator set).
■ Baseline noise data was also acquired at 7 meters
using 8 microphones (8 locations spanning
360 degrees around the generator set, each
microphone placed at 45-degree increments)
based on Cummins’ internal noise measurement
standard derived from the U.S. Department of
Defense power generator set noise measurement
requirements. Noise data collection at 7 meters
from the surface of the generator set is important
because sound levels are generally stable at that
distance, which enables reliable extrapolation and
prediction of noise levels beyond 7 meters. Very
few hemi-anechoic chambers in the world are
large enough to conduct testing at that distance.
■ Cummins used 25 microphone locations to
measure noise levels at 1 and 7 meters from the
generator set, 1 meter from the line of exhaust,
and at operator location to achieve a very high
degree of reliability.
■ The average A-weighted Sound Pressure Levels
at 1 and 7 meters reported in the data sheets is an
average of 14 and 8 microphone locations.
■ The A-weighted Sound Power Level reported in
the data sheets was calculated using the average
A-weighted Sound Pressure Level measured by
using 14 microphones placed at 1 meter from the
generator set, per the parallelepiped method of
ISO 3744:2010.
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Several gigabytes (GB) of data were produced
during the testing of the T-series Super Silent
generator set, providing very high degrees of
accuracy and reliability in the data.
In addition, Cummins found that the T-series Super
Silent generator set’s noise frequency content is
very balanced and does not exhibit annoying noise
characteristics (e.g., harsh, unexpected or distinct
frequency components) in the audible human
frequency range.
These extensive testing configurations resulted
in highly detailed, three-dimensional (sound field)
data. This provides a valuable resource to end
users who seek to understand the T-Series Super
Silent generator set’s impact to their facility design.
Figure 5 — Top view of location of eight microphones positioned
7 meters from the T-series Super Silent generator set to achieve
360º baseline noise data
Figure 6 — A 3-D view of the sound field at 1 meter from the
surface of the generator set enclosure demonstrates the varying
acoustic emissions, from loudest locations (red dots) to the
quietest spots (black dots).
A glossary of acoustical terms
A-weighted Sound Power Level (SPWLA or LWA)
is a measure of the acoustical energy produced by
a sound source, no matter what its distance from the
point of observation.
A-weighted Sound Pressure Level (SPLA or LA)
is a relative measure of the impact of a sound wave
at a specific distance from the source.
Example: A generator set producing an
average Sound Pressure Level of “X” dBA
will be much quieter when heard from
50 feet away than from 5 feet away. Therefore,
the average Sound Pressure Level will be
lower at 50 feet than at 5 feet, although the
Sound Power Level has not changed.
Note: Both A-weighted Sound Power Level (SPWLA)
and A-weighted Sound Pressure Level (SPLA) are
represented by the same unit of measure — dBA —
but they are two different metrics.
Cycle The complete oscillation of pressure above
and below the atmospheric static pressure.
dBA A-weighted variation of the standard decibels
measurement. Since the human ear has different
sensitivity to different frequencies, noise levels are
often measured with an “A-weighted” filter that has a
frequency response similar to that of the human ear,
stated as dB(A) or dBA.
Free field A sound field in which there are no or
negligible sound reflections.
Hertz (Hz) Frequency of sound expressed by cycles
per second (see “Cycle”).
Noise Unwanted sound that is annoying or
uncomfortable.
Parallelepiped array A three-dimensional
arrangement of microphones at a certain distance
(most commonly at 1 meter) from an imaginary
parallelepiped that encloses the testing object.
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Recommendations for evaluating acoustical data
With the growing focus on generator set noise and
its impact on workers and nearby communities, noise
mitigation is an essential component of generator
set installation. Obtaining precise acoustical data
allows for the design of effective and cost-effective
noise control solutions that bring generator sets in
compliance with community and federal noise limits.
Comparing generator set acoustical data
isn’t as simple as it would seem. Not all testing
facilities meet the latest ISO, ANSI and SAE
standards. In addition, various manufacturers
may offer different types of data, which can make
comparing “like to like” numbers difficult.
Here are some questions to consider when comparing generator set acoustical data from different manufacturers:
■ Were the generators tested at the same
engine load/power node?
■ Were the generators tested at the same
engine speed?
■ Was the data collected with or without a
set-mounted radiator?
■ Are the units of measurement the same
(A-weighted Sound Pressure Level [SPLA] or
A-weighted Sound Power Level [SPWLA])?
■ Were Sound Pressure Level measurements taken
at the same distance?
■ Was the same standard or measurement method
used for the noise measurements?
■ How reliable is the data?
■ Were the measurements conducted at Precision-
or Engineering-Grade test facilities? Precision
Grade is more precise than Engineering Grade.
For additional information about the T-series Super
Silent generator set, visit power.cummins.com.
For technical support, please contact your local
Cummins Power Generation distributor.
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Cummins Power Generation 3850 Victoria Street North Shoreview, MN 55126 USA
Phone 763 574 5000 USA toll-free 877 769 7669 Fax 763 574 5298
Our energy working for you.™
www.power.cummins.com
© 2015 Cummins Power Generation Inc. All rights reserved. Cummins Power Generation and Cummins are registered trademarks of Cummins Inc.
“Our energy working for you” is a trademark of Cummins Power Generation.
GLPT-6054-EN (02/15) A4
Dr. Shashikant More is a group leader for
Acoustics in the Department of Applied
Technology at Cummins Power Generation
and leads the Applied Mechanics Functional
Excellence (AMFE) Acoustic Area sub-team
for Global Applied Technology. He received
his Ph.D. in mechanical engineering with
a specialization in acoustics from Purdue
University. Before joining Cummins, Dr. More
conducted doctoral research on aircraft noise
which was sponsored by the Federal Aviation
Administration, National Aeronautics and
About the author
Space Administration, and Transport Canada.
He is an active contributor to the field of power
generator noise research, including programs with
the National Academy of Engineering, National
Institute for Occupational Safety and Health and the
U.S. National Park Service. He is a member of the
Institute of Noise Control Engineering and a reviewer
of journal papers published in prestigious journals
such as International Journal of Environmental Research and Public Health, Basel, Switzerland;
Journal of the Acoustical Society of America, USA; and Noise Control Engineering Journal, USA.
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