Multi-Disciplinary Senior Design Conference
Kate Gleason College of Engineering
Rochester Institute of Technology
Rochester, New York 14623
Project Number: 11221
NOISE REDUCTION FOR INTERNAL COMBUSTION ENGINES
Project Leader
Christopher S. Morehouse
Lead Engineer
Caroline Bills
Resonator Lead
Julie Maier
Faculty Guide
Ed Hanzlik
ABSTRACT
The primary goal of this project is the reduction of
noise emitted from the exhaust of an internal
combustion engine while minimizing engine
performance degradation. The main resulting design
options after an exploratory search are absorption style
mufflers, wave canceling resonators, and electronic
active noise cancelation. The ANC options were
explored and developed with partner team 11227. In
order to determine an optimal solution, test rigs where
designed and built to test various factors for each
design. A resonator demonstration module was built
to facilitate exploratory studies into the behavior of the
wave canceling resonators. The two types of
resonators demonstrated are Helmholtz and Concentric
Tube.
Three different geometries of absorption
mufflers were built and tested using three different
packing materials: fiberglass, mineral wool, and steel
wool.
These materials were tested at different
densities. Sound intensity was measured using an Aweighting sound pressure scale at various locations in
the sound field. The testing yielded noticeable results
with both absorption and resonator devices and
suggests an optimal system comprising of both
systems.
NOMENCLATURE
ANC
Active Noise Cancelation
BS
Briggs & Stratton
c
Wave Speed
C
ci
dB
dBA
DSP
EGT
fe
fh
Hz
ICE
λ
N
nc
RPM
SAE
ANC Lead
Ted Zachwieja
Number of engine cylinders
Cubic inch
Decibels
A-weighted Decibels
Digital Signal Processor
Exhaust Gas Temperature
Engine Firing Frequency (Hz)
Helmholtz frequency (Hz)
Hertz (1/seconds)
Internal Combustion Engine
Wave length
Engine speed in RPM
Rotations per cycle
Revolutions per Minute
Society of Automotive Engineers
1. PROJECT BACKGROUND
1.1 Introduction
There is currently an increased focus on the reduction
of noise emitted from an internal combustion engine’s
exhaust. For consumer vehicles it is largely an issue of
comfort, convenience, and legal regulation. However,
for performance competitions, such as the SAE
Formula student racecar competition, it is an issue of
meeting competition requirements while limiting
performance degradation.
The original scope of the project was to create a noise
dissipation system to be installed on the SAE Formula
car. The logistics of coordination and implementation
were determined to be too eager for a single design
Copyright © 2011 Rochester Institute of Technology
Proceedings of the Multi-Disciplinary Senior Design Conference
project. Therefore, a Briggs and Stratton lawnmower
engine was used as the noise source for testing the
noise reduction methods.
1.2 Basic Sound Physics & Measuring
Sound is a series of transverse pressure waves
traveling through a medium.
Figure 1.1 [1]
demonstrates the motion of a sound wave through
successive compression and expansion of the
molecules as condensation and rarefaction sections,
respectively. The pitch and loudness of sound is due to
the frequency and pressure differential of the wave
respectively. The human ear can typically perceive
sounds from 20-20,000 Hz.
FIGURE 1.1 SOUND WAVE MOTION [1]
Sound intensity is measured on the logarithmic decibel
scale that describes the ratio of pressure between the
condensation and rarefaction of a sound wave. A 1 dB
difference in sound intensity is considered the
threshold of human perception. For this reason,
differences in sound intensity are rarely given in units
less than 1 dB.
Sound readings are rarely given in pure dB. This is
due to the fact that human hearing is more sensitive to
certain frequency ranges then others, therefore a 60 dB
sound source at 500 Hz may be perceived as quieter
than a 60 dB sound source at 10,000 Hz. For this
reason most sound pressure readings are given in dBA.
This is a weighted sound pressure scale that fits to a
curve designed to account for frequency perceptions of
the human ear. The SAE Formula specifications for
sound pressure are taken at 110 dBA at an RPM
derived based on piston speed, thus all readings in this
project were recorded using the dBA scale.
It is important to note that due to the logarithmic scale
of sound pressure readings, apparent small changes in
sound pressure may be significant. A 3 dB increase or
decrease represents a doubling or halving of the sound
pressure ratio, respectively.
1.3 Engine Noise Sources
There are three primary sources of noise from an ICE.
Engine body noise is emitted from the vibration of the
engine itself as well as the operation of valves, gears,
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cooling systems, alternator, etc. There is also the
noise generated by the engine’s air intake system.
This noise is mostly muffled by the air intake filter
system, but for engines with a turbo charging system
or complicated vacuum system this noise can be
distinct and complex. Lastly there is noise emitted
from the exhaust system, which is generally the
greatest noise source. For the purpose of this project,
this last source of noise was assumed to be the major
noise contributor and was the focus of noise reduction.
Exhaust noise can be broken up into two components,
deterministic and stochastic.
The deterministic
component comes from pressure pulses in the exhaust
stream that are produced when the high pressure
combustion gasses of the engine first contact the
relatively low pressure gasses of the exhaust stream at
the opening of the exhaust valves. In a four-stroke
engine, this occurs once every two rotations of the
crankshaft per cylinder. Thus a four-cylinder fourstroke engine produces two pressure pulses per
revolution. The time rate of these pulses is what is
called the engine firing frequency. The engine firing
frequency is a crucial factor in attenuating noise
coming from the engine’s exhaust.
The stochastic sound component is due to turbulent
airflow in the exhaust stream and is a broadband
source. Compared to the deterministic component of
exhaust noise, the stochastic component is not a
significant contributor for most exhaust setups.
Methods of reduction to the deterministic components
that would add significantly to the stochastic
component also inherently decrease engine
performance, and thus where not implemented in this
project. For this reason the stochastic component of
exhaust noise was not a considered in this project.
1.4 Current Industry Noise Reduction
In industry the most common forms of noise reduction
for ICEs are resonance silencers and absorption
mufflers. Absorption mufflers function by allowing
sound waves to pass through an absorption material
that removes part of the acoustical energy from the
sound pressure wave. The absorption material
transfers the acoustical energy to mechanical energy,
which is dampened by the properties of the material
and transferred to heat.
Resonating chambers
implement changes in geometry to reflect sound waves
in such a way so they may self-cancel. These can have
high attenuation for specific frequencies, but as they
must be designed to target a specific frequency, their
effect is limited over a narrow range.
The majority of vehicles commonly seen on the road
do not use a single resonator or absorption type
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Proceedings of the Multi-Disciplinary Senior Design Conference
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muffler.
Instead, most cars use a combination of
several resonating chambers and absorption material.
While this achieves a relatively high level of
attenuation, the effect on backpressure is not
negligible.
1.5 Project Engine
The original intent of the project was to design
systems to fit directly to the SAE Formula Engine, a
600 cc (36.61 ci) 4-cylinder 4-stroke 86 hp engine.
However, due to logistics issues between the SAE
team and 11221, an alternate engine was used.
The engine that has been the focus of this project has
been a Briggs & Stratton Model 127802-1913-D1
engine. This is a 12 ci 1-cylinder 4-stroke 5.5 hp
engine.
The performances differences of these two
engines are not insignificant. Due to the operational
RPM range differences between the two the
frequencies generated by the BS engine were
significantly lower than those of the SAE engine. This
would have a significant impact on the practicality of
resonator designs, which will be discussed later.
2. PROCESS
2.1 Customer Needs
The original focus of this project was to developed
noise reduction systems to be applied to the formula
SAE car. For this reason the customer’s needs focused
heavily on SAE standards and maintaining engine
performance. Another need was to do the preliminary
work for our partner team 11227 to define their design
space for an ANC system with ICE applications. Our
primary customer needs are tabulated below.
FIGURE XX. CUSTOMER NEEDS
2.2 Engineer Specifications
The needs above where translated into key
specifications that our designs where intended to meet.
These are tabulated and ranked below.
FIGURE XX. ENGINEERING SPECIFICATIONS
2.3 Concept Selection
Several concepts where considered to meet the needs
and specifications defined in this project. Ultimately
the requirement for mechanical systems to have low
performance impact led to a rejection of any method
that did not allow for free flow of exhaust gasses.
This constrained our choices to designs that did not
require exhaust gasses to be redirected through
complicated geometry. It was decided that the
mechanical solutions should focus on a single pass
concentric tube resonator, side branch Helmholtz
resonator, and non-restrictive absorption mufflers.
2.4 Benchmarking
As there was no prior work done in this field by a
previous MSD group at RIT, significant research and
benchmarking was necessary to understand properly
the design space.
Our basic assumptions on the characteristics of engine
noise were investigated with a series of test runs on
both the Briggs & Stratton and Formula engines. The
goals of these tests were to confirm that the
deterministic exhaust noise source is the major
contributor to engine noise and to quantify the degree
to which it exceeds other noise sources.
One of the preliminary tests involved recording the
Formula SAE team’s 598 cc 4-cylinder 4-stroke
Honda engine running through a range of RPMs and
performing a frequency response analysis. A zero
response microphone was placed at a 45 degree angle
0.5 m from the exhaust outlet, as specific for the sound
check in the Formula SAE regulations [XX]. A
waveform was recorded using Audacity at a sample
rate of 96,000 Hz. Audacity is a program for
recording and playing sound recordings and tones.
Tachometer readings were recorded directly from the
Engine Control Unit and the data was correlated to the
waveform recording.
Copyright © 20011 Rochester Institute of Technology
Proceedings of the Multi-Disciplinary Senior Design Conference
For the Formula SAE engine, at an RPM of 7650, the
engine firing frequency is 255 Hz. The generic
equation for an ICE’s firing frequency is listed below.
(1)
Analyzing the waveform of the engine recording at the
time correlating with a 7650 RPM tachometer reading
is expressed in the figure below.
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Casing Shape
In order to further understand how length and packing
material thickness can affect sound attenuation,
standard, short, and thin designs were considered. The
standard design was meant to be a baseline with which
to compare to other tests. The standard configuration
consists of a metal outer tube and a perforated inner
tube connected by two end caps, as seen in Figure
XX.
The short configuration was designed to be the same
packing material thickness but half the length of the
standard design. The thin option was chosen to be the
same length but half the packing material thickness of
the standard design. In these designs, the inner
perforated tube is straight, indicating that there is a
straight shot for the sound waves to travel through.
Creating a wave in the inner tube would, theoretically,
force the sound waves to bounce off the perforated
tube before exiting the end of the muffler.
FIGURE XX. FREQUENCY RESPONSE OF
FORMULA ENGINE
From these tests, it was confirmed that the highest
noise amplitude does indeed occur at the engine firing
frequency. The frequency of the next highest noise
amplitude is a multiple of fe. From the above figure, it
can be seen that a significant noise reduction (up to 8
dBA) could be obtained by targeting fe.
The same test was conducted on the BS engine and
resulted in a similar trend. When equation (1) was
applied to the BS engine for its operating RPM of
3800, the resulting frequency was 31.7 Hz.
The wavelength of these firing frequencies can be
calculated using the below formula.
(2)
The wavelength for the Formula SAE engine at its
tested RPM is 1.33 m. However, the BS engine
produced a wavelength of 10.73 m at its firing
frequency. This would become a crucial obstacle
when considering the potential resonator designs.
3. PROJECT DESIGNS
3.1 Absorption Mufflers
3.1.1 Preliminary Design
There are three critical parameters of absorption
muffler design: casing shape, packing material, and
packing density.
The fourth option, “sine wave”, was designed to have
an inner tube which would bend slightly to mimic the
curve of a sine wave just enough so the sound pressure
wave would not have a straight shot through the
muffler. The more the sound waves bounce off of the
perforated inner tube and absorb into the packing
material, the more the sound will be attenuated.
The “cavity” muffler is designed to hold the sound
waves in the muffler longer. It opens up so that
halfway through the muffler the inner tube is three
times larger than the inlet diameter. Then, the diameter
is reduced toward the exit so that the exit diameter is
equal to the inlet. This should enable the sound waves
to bounce around inside the inner tube before exiting
the muffler. However, because of the decrease in
diameter there should be increased back pressure
within the muffler. There should also be slightly
higher back pressure for the ‘sin wave’ option because
of the modified inner tube shape.
Packing Material and Density
Packing material for a muffler must be able to
withstand the exhaust gas temperatures and should be
porous, even when packed to high densities. The
baseline material was chosen to be the fiberglass
strands the RIT Formula team uses, at their ‘optimal
density’ and just above and below it. The second
packing material chosen was a spun mineral wool,
similar to, but more dense, than common household
insulation. This material has very small fibers which
create small openings for sound waves to get trapped
and attenuated and is designed to withstand high
temperatures. Foams are commonly used in sound
attenuation, however few can handle the exhaust
Project P11221
Proceedings of the Multi-Disciplinary Senior Design Conference
temperatures. Thus metal foams were researched. The
conclusion was that steel wool would provide similar
characteristics to a foam. Three grades, high, medium,
and low, were chosen, all at a less compact density
than the other tests to accommodate the thicker fibers.
3.1.2 Engineering Model
The standard, thin, ‘cavity’, and ‘sin wave’ options
each have a length of twelve inches. This was shorter
than the RIT Formula SAE team and accommodates
the smaller inner tube diameter. The inner tube
diameter was chosen to be one inch, the diameter
exiting the engine used for testing. Packing material
thickness was chosen to be equal to the team’s, at 1
11
/16 inches. The bend in the inner tube for the ‘sin
wave’ option was chosen to be enter the muffler at the
centerline of the muffler end cap, then ½ inch over the
centerline at the 3-inch mark, then back down to ½
inch below the centerline at the 9-inch mark, then back
up to the centerline at the exit. For simplicity, the outer
casing was designed as a straight tube with the
thinnest packing section at the RIT FSAE thickness, as
shown in Figure 2. The ‘cavity’ option started with a
one-inch inner tube diameter for one inch, then
increased steadily for 3.5 inches to a 3-inch inner
diameter, which continued for 3 inches. The inner tube
then decreased steadily for 3.5 inches to a 1-inch
diameter which continued for one inch to the exit. The
outer casing was design as a straight tube with the
thinnest packing section at the RIT FSAE thickness, as
shown in Figure 3. Standard packing density
(medium), equal to the RIT FSAE optimal density,
was 200 g/L. The high and low values were 225 and
175 g/L, respectively. These values were used for both
the fiberglass and the mineral wool packing materials.
The density chosen for the three grades of steel wool,
0000, 1, and 4, was 136.1 g/L. It was assumed that the
ambient air temperature, rotational speed of the
engine, and weather were constant and consistent
between and during tests.
22 gage galvanized sheet steel was cut, rolled, ground,
and TIG welded to form the outer casings for the five
mufflers. Tabs with ¼-inch holes were welded on for
attachment to the flange. 22 gage perforated
galvanized steel was cut, rolled, and TIG welded to
form the inner tubes. End caps were cut, ground, and
welded to one side of the inner tubes and outer
casings. A flange was manufactured to connect the
exhaust from the engine to the mufflers. One side
consisted of a __-inch thick piece of steel with two
holes for screws to attach to the engine. This was
welded to a 4-inch long, 1-inch inner diameter piece of
steel tubing. The tube was welded to an end plate. The
end plate was made of __-inch thick steel with a 1inch hole at the center for the exhaust gasses and slits
Page 5
for the muffler casing tabs were cut in the plate, as can
be seen in Figure 4. The muffler tabs fit into the slits
and tapered pins were slipped in to secure the muffler
to the end plate, which was screwed into the engine.
3.1.3 Experimental Setup and Procedure
A 5 hp Briggs and Stratton engine was fixed to a
rolling cart to run tests with. A flange was
manufactured to allow easy attachment to the engine
with tapered pins. Each muffler was packed with the
desired material, weighed at certain points to ensure
consistent density throughout. Any parameters which
could affect the results were recorded throughout, e.g.
ambient temperature, weather, and engine rotational
speed. The muffler was mounted to the engine, then
the engine was started and allowed to reach a
somewhat steady rotational speed. A sound map of 24
points around the engine (along a horizontal plane the
height of the end of the muffler) was recorded using a
sound pressure level meter on A-weighting. Then a 30
second sound recording was taken using a hyper
cardioid microphone at the FSAE distance and height.
3.2 Resonators
3.2.1 Preliminary Design
The layout of a basic Concentric Tube Resonator is
shown in Figure XX. As the sound waves travel
through the inner pipe, a portion of the through-flow
enters the cavity formed by the outer pipe through the
holes in the inner pipe. Within this sealed cavity the
sound waves continue to travel in the same direction
as the main stream until encountering the plunger end.
At this point, the sound waves reflect back and, if
properly tuned, will cancel, or significantly reduce, the
incoming sound waves to the cavity. Tuning of the
resonator is achieved by making the length of the
cavity equal to certain fractions of the predominant
frequency’s wavelength. Making the cavity length ½
the wavelength should achieve the maximum amount
of attenuation, with ¼, 1/8, 1/16, etc of the wavelength
achieving lowering levels of attenuation. Based on
discussion with an industry expert, the optimal open
area for the inner pipe is around 5%.
FIGURE XX. BASIC LAYOUT OF CONCENTRIC
TUBE RESONATOR
Copyright © 20011 Rochester Institute of Technology
Proceedings of the Multi-Disciplinary Senior Design Conference
Page 6
The Helmholtz Resonator, as seen in Figure XX,
is a type of side branch resonator, situated
perpendicular to the main sound stream.
FIGURE XX. BASIC LAYOUT OF HELMHOLTZ
RESONATOR
As the sound waves pass by the resonator, a portion of
the stream enters the cavity through a hole in the
straight pipe. Based on the neck cross-sectional area
and length and the cavity volume and length, the
sound exits the cavity at a certain frequency. When the
resonator is tuned so that the sound exiting the cavity
is at the same frequency as the main stream, a portion
of the main sound stream is cancelled. The simple
equation for calculating the frequency the resonator
“produces” is shown below.
(3) [2]
FIGURE XX. HELMHOLTZ FREQUENCY
EQUATION [2]
3.2.2 Engineering Model
Based on the theory, the cavity length of the
concentric tube should be about ½ the wavelength of
the firing frequency of the engine. The engine firing
frequency of the Formula car engine at 10,000 RPM is
about 333 Hz, which results in a ½ wavelength of
about 20.1 inches (50.1 cm). However, the Briggs &
Stratton engine has a firing frequency of about 25 Hz
for normal operation at about 3000 RPM, which
results in a ½ wavelength of about 22.3 ft (6.81 m). It
was therefore deemed infeasible to construct a
Concentric Tube Resonator for attachment to the
Briggs & Stratton Engine. So, a demonstration module
was instead created utilizing a frequency generator to
produce a pure tone.
The design of the Concentric Tube Resonator
consisted of a variable length cavity and an
interchangeable outer pipe. A solid model of the two
configurations is shown in Figure XX. This set-up
allows for the exploration of diameter effects on sound
attenuation as well as cavity length.
FIGURE XX. SOLID MODEL OF LARGE AND
SMALL OUTER PIPE OPTIONS
The diameter of the inner pipe was constrained to be
the same as that on the lawnmower engine (1 in) for a
closer comparison. Therefore, the plunger rods were
chosen to be the next larger size pipe (1.125 in). The
value of the small outer pipe diameter was decided
upon based on available pipe sizes and having a large
enough cavity volume. The large outer pipe diameter
was calculated based on the configuration having
twice the cavity volume as the small outer pipe
configuration. The 90 holes in the inner pipe had a
diameter of 0.25 in. This resulted in about 6 % open
area. The length of outer pipe was determined based
on the length needed for the SAE engine, which came
out to be about 24 in.
The resonator was constructed out of PVC, as it would
not be exposed to high pressures or temperatures or
gases other than air. It was fabricated using a lathe and
drill press. Figure XX is a picture of the final product.
FIGURE XX. FINISHED CONCENTRIC TUBE
RESONATOR
Using the simple equation for frequency, the
Helmholtz Resonator was sized to be able to apply to
the Formula car as well as standard pipe sizes. The
figure below is the solid model of the resonator.
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Proceedings of the Multi-Disciplinary Senior Design Conference
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the sound level for different frequencies. The
frequencies tested consisted of four readings at five Hz
intervals both above and below the predicted
frequency based on the simple equation. The three
lengths tested were 5 cm, 10 cm, and 15 cm.
FIGURE XX. SOLID MODEL OF HELMHOLTZ
RESONATOR
The resonator was constructed out of PVC pipe and
machined using a lathe and drill press. Figure XX
shows a picture of the completed product.
The Concentric Tube Resonator was tested using both
of the tests. The first test was performed the same as
with the Helmholtz, with the three lengths of 30 cm,
40 cm, and 50 cm. For the second test, three
frequencies (150 Hz, 350 Hz, and 550 Hz) were tested
and the locations of maximum and minimum
attenuation were recorded. Each of these tests was
conducted on both the large and small configurations.
A third test was conducted on all of the resonators
using the waveforms of both the Formula and Briggs
& Stratton engines. The purpose of this test was to
determine what the resonator would look for each
engine and what the attenuation would be.
Backpressure testing was also conducted on all of the
resonators.
FIGURE XX. FINISHED HELMHOLTZ
RESONATOR
3.2.3 Experimental Setup and Procedure
The test set-up consists of four components:
resonators, laptop, speakers, and sound level meter. A
typical test set-up with the Concentric Tube Resonator
is shown in the below figure. The speaker is located
beneath muffling medium, which in this case consists
of polyester batting and a jacket. The speaker is also
placed inside of a box filled with polyester fill. The
muffling of the speaker is important because the
likelihood of it contributing as a secondary sound
source is minimized. The program Audacity is being
used at the tone generator through the laptop to ensure
accuracy of the source frequency.
FIGURE XX. TYPICAL TEST SET-UP
There were two different tests to access the
performance of the resonators: fixed length or fixed
frequency. The Helmholtz Resonator was tested using
the fixed length method. This entailed setting the
resonator cavity to a certain length and then recording
3.3 Active Noise Cancellation
3.3.1 Overview
Active noise control is the creation of a pressure wave
that is 180 degrees out of phase of the targeted sound
wave that is to be attenuated. This particular wave is
considered an anti-phase wave and ideally it will
attenuate the sound 100 percent.
Active noise control is usually only modeled for either
1-D or 3-D waves. The more complex method is to
attenuate sound for 3-D waves. The goal of the
project is to attenuate the targeted noise from an
internal combustion engine as much as possible. Since
the project now focuses on a lawnmower engine, with
minimal regard to weight and size requirements, it is
possible to apply an ANC system to attenuate the
sound. However, a requirement of the project is to
make recommendations for application to the Formula
SAE car. This would be a theoretical design of the
system taking into account the weight and size
considerations of the SAE car.
In order to completely attenuate a sound wave the antiphase wave needs to match the targeted wave’s
intensity or audio power level. This can be achieved
using active control in one of two ways: attenuate the
sound at a particular location, such as the use of
headphones, or co-locate the active control with the
source of the noise generation. Co-locating the active
control with the source of the noise generation is the
method chosen for the system. This method will
require the co-located speaker(s) to have the ability of
Copyright © 20011 Rochester Institute of Technology
Proceedings of the Multi-Disciplinary Senior Design Conference
matching or nearly matching the intensity of the noise
generated.
Active noise cancellation loses its efficiency as the
frequency increases because of its unpredictable
nature. High frequency is considered at 1000 hertz or
higher. These high frequency sound waves have a
tendency to reinforce and/or cancel themselves
unpredictably. This nature hinders the ability to
attenuate noise at these frequencies. Frequencies of
less than 1000 hertz are where active noise
cancellation can be best utilized to attenuate unwanted
noise. The Briggs and Stratton lawnmower engine
runs at low frequencies (about 30 hertz), so this should
not be an issue for the application of ANC on this
particular motor. The Formula SAE car engine runs at
a maximum speed of 12,000 rpm, which correlates to a
frequency of 400 Hz. Therefore, ANC is still
applicable to the SAE Formula car.
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This section should describe your final product or
process, whether it met specs (results of testing), and
how you evaluated its success. Most conference
papers include enough information for your work to be
reproducible.
5.
CONCLUSIONS
RECOMMENDATIONS
AND
Optimal geometry for Absorption
Optimal Resonator
Hybrid systems
SAE Team recommendations
Follow on work recommendations
4. RESULTS AND DISCUSSION
Absorption Sound- Results
-Geometry comparison
-Material comparison
-Optimum solution
REFERENCES
[XX] Bell, Lewis H. Industrial noise control:
Fundamentals and Applications. New York : M.
Dekker, 1982. Print.
Resonator Sound-Results
-Geometry comparison – attenuation at target
frequency
-Geometry comparison – attenuation off target
frequency
Absorption
comparison
&
Resonator
Flow-Results
and
ACKNOWLEDGMENTS
Be sure to acknowledge your sponsor and
customer as well as other individuals who have
significantly helped your team throughout the project.
Acknowledgments may be made to individuals or
institutions.
Project P11221