Power topic #6054 Advanced noise reduction technology · PDF fileAdvanced noise reduction...

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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5

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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