2. WhispererSOLUTIONS FOR SOUND ATTENUATION AND EMISSION CONCERNS BY DANIEL P. DUFFY
Allimages:Miratech
NOISE
22 www.BusinessEnergy.net
A
nyone whose ears have
been on the receiving
end of a loud, pierc-
ing noise, knows that
sound pollution is a
serious problem. All
of us, at one time or another, have had
our hearing so assaulted. Sound attenu-
ation is a major concern. In the past few
decades there have been major strides
in the technology of noise abatement,
but not every advance has to be a great
leap forward. This is especially true in
the case of replacement components
that improve on existing systems. They
are employed in response to a toughen-
ing of noise pollution regulations or
changes in the facility’s local environ-
ment that make the upgrades necessary.
It’s these incremental improvements and
advances in acoustic customization and
retrofitting that add up to big improve-
ments in mitigating noise emissions.
What are primary sources of this
noise? For the distributed power indus-
try, gensets, turbines, and engines are
often as noisy as they are useful. Some of
these systems can generate noise levels
that are uncomfortable or even poten-
tially damaging to hearing. The solution
The Power
3. Business Energy June 2016 23
to this problem is the use of silencers
and sound attenuation enclosures. And
since ease of use can be as important as
functional utility, these noise abatement
systems are a valuable and necessary
part of any distributed energy system.
Noise is created by a generator
because there is no such thing as 100%
efficiency. To begin with, fuel is ignited
in an internal combustion engine, con-
verting chemical energy into mechanical
energy. If this conversion were 100%
efficient, all of the potential chemical
energy in the fuel would be converted
into the mechanical energy of driving
the pistons. But it does not; due to the
three laws of thermodynamics, some of
that energy will always be lost to heat—
even in the most efficient of engines.
This heat translates into an expanding
bubble of air, also known as “noise.”And
that is just the initial phase of fuel con-
sumption. Heat is created (and energy
wasted) due to friction during the recip-
rocal motion of the pistons. In addition
to heat, this friction creates vibration,
and with it noise. In short, due to the
laws of physics, there can be no such
thing as a silent engine or turbine.
So how much noise is too much?
The operator’s choice of a generator
will, in no small part, depend on its
noise level. This is not just for personal
comfort—maximum noise levels are
often mandated by local codes and ordi-
nances—violations of which can result
in law suits, fines, and expensive legal
fees). Even in remote areas, noise is a
concern. National parks and wilderness
areas have strict limits on noise gener-
ated during camping, ATV operation,
recreational vehicle use, and other out-
door events.
Measuring Noise Levels
For noise to be measured, rated, and
regulated, it needs to be quantified.
This is done with the decibel rating.
This is a logarithmic scale, not a linear
rating. So for each change in 10 deci-
bels (dB), noise is actually 10 times
greater. Ten decibels is, therefore, 10
times more than 1 decibel, 20 is 100
times more than 1, and 30 is 1,000
times more than 1, etc. And, in keeping
with its logarithmic nature, each addi-
tion of 3 decibels results in a doubling
of the sound’s acoustic energy.
So, the addition of
two separate sources of
noise operating at the
same decibel level will
result in a joint deci-
bel reading 3 decibels
higher than each indi-
vidual noise source. For
example, a machine that
generates 50 decibels is
10 times louder than one
operating at 40 decibels.
Decibels measure sound,
the manifestation of
pressure waves vibrating
and traveling through
the material medium of
air. (Though technically
sound can also travel
through water or solid
objects.) Table 1 com-
pares various decibel lev-
els with resultant human
pain levels and related
sources of sound.
Since the decibel scale is logarith-
mic, as a simple rule of thumb, the
addition of 3 dBA means a measured
doubling of the acoustical energy.
(Therefore, 80 dBA + 80 dBA = 83
dBA.) However, an addition of 10 dBA
represents a perceived doubling of the
sound pressure.
The decibel range of an operating
generator increases with its power out-
put. This is obvious since increases in
power output require larger and more
powerful engines. At the low end of the
decibel scale are portable, small (with
1,000-W inverters) generators whose
noise levels range from the quietest
models operating at about 45 decibels
(somewhere between a quiet room and
a rainstorm), to 75 decibels (between
city traffic and an operating vacuum
cleaner), for a 10,000-W generator
operating at 15 hp. To go from 45 to
65 decibels, a generator’s power needs
to increase from about the minimum
1,000 to 6,000 W. The increase from 65
to 75 decibels results from an additional
4,000 W of power.
Sound is one thing, but what people
hear is noise. Noise is sound as perceived
by the human ear. More specifically,
noise is an unwanted and undesirable
sound as perceived by the listener. It
is the potentially unpleasant nature of
sound that requires that adoption of
laws and regulations to minimize noise
as much as possible and to limit human
exposure to noise pollution.
As the human pain and discomfort
column on Table 1 indicates, sound and
noise levels can generate a great deal
of energy. The energy of sound and its
resulting pressure are measured on an
“A-weighted” scale, which takes into
account the elevation where the sound
occurs (from sea level to the top of a
mountain), and the density of the air.
The energy of a sound wave is shown
graphically by the height (amplitude) of
the wave itself. For example, larger com-
mercial generators can operate at 115
decibels (between a chain saw and pneu-
matic riveter) and produce 10,000,000
micro-Pascals of pressure, while the
sound waves from the smaller portable
generators described above operating at
about 50 decibels generate only 8,000
micro-Pascals.
The pitch of a sound is not the same
thing as its energy. Energy on the decibel
scale measures the overall loudness of
sound. Pitch describes how high or low
it sounds. Pitch is measured by frequency,
or the number of times a sound wave
occurs (as measured in cycles per second,
or Hertz). As a result of the physical
properties of air at sea level, sound trav-
els through air at a fixed speed. This is
y
xz
14007.410
12513.253
10949.096
9384.940
7820.783
6256.627
4692.470
3128.313
1564.157
0
Velocity [ft/min]
Two-dimensional model
showing flow velocities
Cut Plot 1:
contours
Flow
Trajectories 1
4. 24 www.BusinessEnergy.net
NOISE
equivalent to 1,126 feet per second,
otherwise referred to as Mach 1.
Given this fixed speed, a sound
wave’s frequency is the inverse of
its wavelength (the distance between
the peaks or troughs of adjacent
sequential waves). So a sound wave
with a frequency of 100 hertz would
have a wavelength of 112.6 feet (or,
1,126 ft. per second per 100 Hz =
112.6 ft.). The shorter the wave-
length, the higher the frequency, and
the lower the frequency, the longer
the wavelengths.
So, the decibel/energy and
hertz/pitch values objectively define
the physical characteristics of a
sound wave. But, what about the
subjective response of the human
ear? The range of human hearing
has been divided into octave bands,
which are further subdivided into
one-third octave bands. An octave
is defined as the frequency inter-
val between one musical pitch and
another with half or double its fre-
quency. In other words, a frequency
has an octave width when the its
upper band frequency is twice that
of its lower band frequency.
High pitch noise on the other
hand (sound with very high fre-
quency and very short wavelength
of 0.5 ft.) is defined as being at least
2,000 Hz. At this frequency, a lis-
tener experiences pain and discom-
fort and can suffer permanent hear-
ing loss from prolonged exposure.
Low pitch noise (sound with very
low frequency and very long wavelengths of up to 36 ft.) is
defined as occurring between 30 and 250 Hz.
The last physical factor affecting the impact of noise
pollution is distance. Because of divergence, sound natu-
rally attenuates over longer distances from the source of
the noise. This is a physical expression of the inverse square
law. The inverse square law states that the power intensity
(of sound, light, or whatever) varies inversely according
to the square of the distance from the source. Mathemati-
cally, this is expressed as A = 1/r^
2. Assuming no reflective
surfaces for sound waves to bounce off on and change direc-
tion, and a sound source that propagates in all directions,
the sound’s energy (as measured in decibels) will decrease
with the square of the relative distance from the source. So,
a listener to a sound source twice as far away will experience
a four times reduction in decibels, compared to the energy
at the source itself. As measured on the decibel scale, this is
a reduction of 6 decibels (remember a doubling or halving
occurs every 3 dB). Or more simply put, a sound source 10
times the distance will result in a decibel level of 100th the
energy at the source (10^
2 = 100).
Mechanical Means of Acoustical Attenuation
What are the goals of sound reduction? As defined by most
local codes and ordinances, the goal is to avoid noise distur-
bance in residential areas. In areas zoned for industrial activ-
ity, the goal is a more modest avoidance of potential hearing
loss. Typically, local governments set noise limits according
to location, hours of the day, zoning, permitting, and such.
These limits are set at property boundaries so that no noise
crosses these lines in exceedance of 60 decibels in residential
areas, 55 decibels in noise-sensitive zones, and 80 decibels in
commercial and industrial zones—or otherwise disturbs the
peace and quiet of a neighbor 50 feet away. A noise-sensitive
zone typically means any area designated by the planning
commission for the purpose of ensuring exceptional quiet
Table 1. Sound and Noise Comparison Chart
Decibel Level Human Hearing Example
0 Theoretical lower limit Silence
10 Normal breathing
20 Whisper at 6 ft.
30 Faint noise Silent library
40 Quiet household room
50 Moderate rainfall
60 Moderate to quiet Normal conversation
70 Busy traffic
80 Vacuum cleaner
90
Hearing damage at prolonged
exposure Shouted converstaion
100 Passing motorcycle at 6 ft.
110 Hearing damage at repeated exposure Chainsaw
120 Pneumatic riveter
130 Threshold of pain Jackhammer at source
140 Jet aircraft at 100 yd
150 Intolerable Fireworks at 3 ft.
160 12-gauge shotgun blast
170 Howitzer canon firing
180 Space rocket takeoff
190 Bomb or grenade at blast epicenter
200 Loudest possible sound (theoretical) Sound waves become shock waves at 194 dB.
Human ear drums rupture at 195 dB.
5. Business Energy June 2016 25
(parks, schools, and such).
But sufficient distance is not always
available for natural noise attenuation. This
is especially true in the tight confines of
urban and even suburban locations. And so,
the manufacturers of generators augment
their equipment with mechanical means of
acoustical attenuation. Each method has an
attenuation coefficient, which provides a rat-
ing for its sound reduction capability. Specifi-
cally, the attenuation coefficient measures the
energy loss of the sound wave as it propagates
through air or water. The attenuation coef-
ficient is also measured in decibels.
One method of achieving attenuation is
the installation of an enclosure around the
source of the sound with interior surfaces
capable of absorbing and annulling noise.
These surfaces absorb sound energy instead
of reflecting it back, concerting the energy into vibration and
waste heat. As there is no perfect sound reflecting surface,
there is no perfect absorbing surface—some of the sound gets
reflected back even with the best absorbers.
How much energy gets absorbed and how much gets
reflected is a function of several factors. The first factor is the
incident angle that the wave impacts on the surface. The sec-
ond factor is the roughness of the surface, with more porous
and pitted surfaces trapping and absorbing more sound. The
geometry and shape of the pitted surface can be designed to
maximize absorption. The third factor is the softness of the
surface. Dense and hard materials reflect sound, while low-
density, soft materials absorb it.
Manufacturers of gensets, engines, and turbines will
tend to equip their equipment with sound reduction enclo-
sures. But, often these enclosures are provided by third-party
manufacturers who specialize in the design, manufacture,
and installation of soundproof enclosures. Their capabilities
may range from only weather protection to thorough sound
elimination.
Major Suppliers of Sound Attenuation Equipment
Girtz Industries manufactures Z-GUARD, stationary equip-
ment enclosures for specialized applications with unique
post and panel designs for stationary sound attenuated
enclosures. These structures include several advanced fea-
tures (such as completely removable, fully welded roofing to
prevent water intrusion), which make for strong enclosures
resistant to extreme weather conditions and seismic activ-
ity. This inherent design strength is reinforced by the use
of carbon steel, aluminum, and stainless materials. Exterior
protection is provided by coatings up to marine-grade envi-
ronment level C5; per ISO 12944. Their modular design with
standardized dimensions allows for future add-ons with the
placement of these units side by side, limiting the need for a
large footprint. Sound attenuation levels can be customized
for client needs.
Harco Manufacturing exhaust silencers are built-in spark
arrestors to accomplish both sound attenuation, and provide
spark-arresting performance. Having patented the first low-
profile or “Hockey Puck” style engine exhaust silencers, Harco
provides a full line of residential-grade through super critical-
grade engine exhaust silencers, spark arrestors, spark arresting
silencers, and supporting products. A compact design makes
it suitable for situations in which space constraints are a major
concern. Harco was also the first manufacturer to develop a
low-profile, catalyst exhaust silencer, the SFH-F series, with
filters mounted inside the silencer itself.
Maxim Silencers is a supplier of industrial-grade silencers
for noise control in the oil and gas industry and the power
generation market. Maxim manufactures the QAC line
of catalytic silencers. It combines silencer technology with
catalytic housing, providing both noise and emission con-
trols. Their Maxim Quick Access is designed for natural gas
internal combustion engine emissions control. Its unified
construction saves space and ensures overall quality of con-
struction. It is designed to accommodate either three-way,
non-selective catalytic reduction (NSCR), which reduces
NOx
, CO, NMHC/VOCs, and formaldehyde on rich-burn
The attenuation
coefficient measures
the energy loss of
the sound wave as it
propagates through
air or water. The
attenuation coefficient
is also measured in
decibels.
Blower Silencer
6. 26 www.BusinessEnergy.net
engines, or a two-way oxidizing element,
which reduces CO, NMHC/VOCs, and
formaldehyde on lean burn/clean-burn
engines. Silencer configurations come in
either industrial or“hospital plus” grades.
MIRATECH, a global company,
is a leading provider of cost-effective,
reliable, and mission-critical emission
and acoustical solutions for natural gas
and diesel reciprocating engines used in
natural gas production, oil and gas drill-
ing, power generation, rail, marine, and
fluid pumping. Headquartered in Tulsa,
MIRATECH manufactures from three
locations in North America. Its approach
is customer-centric, utilizing advanced
engineering, industry knowledge, inno-
vative design solutions, and superior
product quality control. It produces a
wide and varied product line of indus-
trial silencers including emission-control
silencers, exhaust silencers, compres-
sor/blower silencers, vent silencers, and
vacuum pump silencers.
In addition to silencers, MIRATECH
has a comprehensive customer offer-
ing that includes catalysts, housings,
monitoring systems, and related services
that address and reduce engine exhaust
pollutants such as NOx
, CO, VOC, diesel
particulate, HAPs, and noise. MIRAT-
ECH specializes in the design, engineer-
ing, manufacturing, and delivery of total
acoustic or emission turnkey solutions
that fit unique customer needs.
MIRATECH’s capabilities extend to
project size, as well as product quality.
Their in-house engineering and project
management team are capable of provid-
ing highly customized solutions, even
for large-scale projects. Their teams are
practiced at the art of delivering total
acoustic and emission turnkey solutions.
In response to an ever-changing
technological and more stringent regula-
tory environment, MIRATECH is always
pushing for better solutions and prod-
ucts. In doing so, its engineers are adopt-
ing new techniques, shapes, and tools to
improve noise mitigation products.
Mehmood Ahmed, Director of
Acoustical Engineering for MIRATECH
explains one new approach:
Three-dimensional [3D] acoustical
modeling software that was used in
the automotive industry is making
its way into the industrial silencer
market. Since the analysis previously
needed comparatively large amounts
of computing power, it was generally
not practical to utilize it in relatively
larger industrial silencer applications
that required many points of analysis.
This is changing, and currently it is
relatively feasible to adopt 3D acous-
tical modeling for large silencers. As
an early adopter of such technology,
MIRATECH has been utilizing it
to help the engineers think outside
the box in applying new shapes and
elements to improve acoustical per-
formance. Even though this analysis
is still resource-intensive, it is helping
industrial silencer companies develop
better products. Not only has it
improved performance of their stan-
dard product range, but it also has led
to introducing higher grade silencers
that reduce noise to even lower levels
required for many of today’s critical
installations. Again, although the
3D acoustical modeling technique is
resource-intensive, our engineering
group has developed in-house noise
modeling codes that allow them to
quickly tune the silencer for low- and
mid-frequency bands. Having the
right tools at hand helps us effi-
ciently select from a wide variety of
acoustic elements for possible design
consideration.
MIRATECH is also a leader in the
application of cutting-edge technology
such as Computational Fluid Dynam-
ics (CFD) analysis to establish pressure
drop across the silencer. This differs
from the traditional approach of rely-
ing on empirical formulas, which can
be generic. This method allows design
engineers to push the boundaries to
improve flow distribution and reduce
turbulence, thus reducing flow gener-
ated noise that adversely affects the
overall performance of a silencer. Bet-
ter flow distribution allows for smaller
bodies or improves acoustical perfor-
mance. As a result, several different
shapes of silencers have been intro-
duced over the last decade to improve
space utilization, improve acoustics,
and reduce cost. Two of the leading
shapes are disk and oval silencers.
New designs are also being intro-
duced based on advanced data gather-
ing techniques. Again, Ahmed notes:
The advancement in handheld sound
meters and application software allow
users to capture and analyze a lot
more information than just overall
sound levels. In the recent years, it
has become feasible to easily measure
NOISE
Flex connectors
7. Business Energy June 2016 27
noise in narrow bands, which assists
in identifying dominant frequency
noise rather than looking at wide
frequency band measurements. For
example, this has allowed engineers
to utilize specific design elements such
as a tuned Helmholtz resonator to
cancel noise associated with engine
fundamental frequency.
A good example of MIRATECH’s
approach to the development of solu-
tions for difficult acoustic challenges
can be seen in a recent project for a
prominent US-based Vacuum Truck
Manufacturer. The specifications for this
project were tight, with demanding stan-
dards for acoustic performance, space,
weight, and cost. Previously, the typical
way to improve performance for indus-
trial silencers was to increase their size.
However, tight space constraints and
limited exterior dimensions prohibited
this approach.
Instead, 3D acoustical modeling
was used to design and refine the inter-
nal elements of the silencers to achieve
optimum results. Then, CFD analysis
was used to streamline the flow charac-
teristics inside the silencers to minimize
turbulence or flow restrictions, which
contribute to the silencer’s backpressure.
This approach resulted in reduced noise
levels as confirmed through acoustic test-
ing and exceeded the customer’s needs.
Robinson Custom Enclosures
specializes in unique noise reduction
applications. The company applies flex-
ible in-house manufacturing expertise to
provide customized solutions, whether
they are high-structural-integrity, or
semi-portable applications. The high-
strength steel containers range 20–40 feet
in length. They can provide weatherproof
enclosures and containers packaged
around a customer’s generator and acces-
sories, in addition to sound attenuation.
Universal AET manufactures both
the SU Series and U5 Series of absorptive
silencers. The SU series comes in three
models (SU5, SU4, and SU3). Made with
mild steel construction with a primer-
coated exterior, all models are designed
with an annular flow path with either
full or partly blocked line of sight. The
U5 Series Absorptive Silencer is designed
as a highly efficient straight-through
absorptive silencer. This makes it espe-
cially well-suited for inlet service on
small rotary positive or centrifugal blow-
ers, or the discharge of vacuum pumps.
It also features mild steel con-
struction with enamel paint exteriors.
Universal’s absorptive silencers with
discharge less than 15 psig are used for
sound attenuation of inlet and discharge
of high-speed, low-pressure centrifugal
compressors and blowers, industrial
fan inlets and discharges, high-pressure
centrifugal compressors inlets, gas tur-
bine inlets, dry vacuum pump discharge
units, certain low-pressure vents, high-
frequency noise sources, and the inlets of
turbocharged reciprocating engines.
Soundown has been in the business
of manufacturing noise control materi-
als for over 25 years. It has developed a
portfolio of products for the effective
treatment of packaged power units and
sound attenuated enclosures. The com-
pany’s products include acoustic insula-
tion and vibration-damping materials,
as well as a range of machinery-isolation
solutions. Their experience and diverse
product line allows them to meet speci-
fication and/or regulatory compliance
levels. J&A Enterprises, Sundown’s sister
company, also provides noise and vibra-
tion engineering support for new proj-
ects or existing applications.
Soundown offers a variety of acous-
tical foam products from the standard
polyether to high-temp polyimide foams.
These absorption materials are effective
at reducing the amount of reverberant
noise in an enclosure, which translates
into lower overall sound pressure levels.
The absorption treatments are particu-
larly effective when installed in conjunc-
tion with baffles to stop noise exiting the
sound shield through ventilation and
other unsealed penetrations.
Barrier composite insulation (com-
posites consisting of foam decoupler and
absorption layers sandwiched around
a TuffMass—mass loaded vinyl—bar-
rier) is also available from Soundown,
and these floating acoustic membranes
are available in variable thicknesses and
weights in response to requirements for
noise control, space, weight, and cost. In
doing so, they can specify a composite
that will most effectively treat airborne
noise radiating from machinery such as
engines, generators, pumps, etc.
Standard thicknesses range from
½ to 3 inches, and can be finished with
a range of facing options. PSA is also
available, as are precut kits. Soundown’s
TuffMass and damping materials have
a very low-gauge thickness (0.062 to
0.625 in.), which makes them ideal for
enclosures with low clearance between
the machinery and shell.
In addition to materials that muffle
noise, isolation mounts are key com-
ponents in reducing mechanical noise
from most pieces of equipment. This
vibration will transmit noise to booth
the sound shield and the equipment’s
structural foundation. Soundown’s
engineers study the dynamic character-
istics of machinery by using six degrees
of freedom analysis to ensure smooth
operation of the system.
Soundown also applies advanced
materials science to the problem of
noise abatement. Their Sylomer, Sylo-
dyn, and Sylomer HD are microcellular
urethanes designed with various stiff-
ness and damping properties that allow
them to be easily customized for specific
applications.
Unlike traditional rubber products,
Sylomer materials exhibit no surge fre-
quencies, show less creep and stiffening
over time, and require no profiling since
their volume compressible Sylomer
materials are commonly supplied as
pads or strips that can be placed directly
beneath machinery to provide vibration
isolation. Machines with high dynamic
forces and/or requiring lower natural
frequencies of the isolation system are
traditionally mounted on an additional
base or foundation for increased mass
and stability. Fully decoupling the foun-
dation with Sylomer materials signifi-
cantly improves the vibration isolation
and reduces forces that can be transmit-
ted to or from adjacent areas. BE
Daniel P. Duffy, P.E., writes on the topics
of energy and the environment.
BEFor related articles:
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