U.S. patent number 3,920,095 [Application Number 05/438,736] was granted by the patent office on 1975-11-18 for free flow sound attenuating device and method of using.
This patent grant is currently assigned to Brunswick Corporation. Invention is credited to Raymond C. Clark.
United States Patent |
3,920,095 |
Clark |
November 18, 1975 |
Free flow sound attenuating device and method of using
Abstract
A sound attenuating device characterized by several novel
structures which include (1) a foraminous conduit surrounded by (2)
a housing having in series after the inlet end (3) a low frequency
attenuator and (4) a high frequency attenuator, the conduit acting
in operative association with the low frequency attenuator to
enhance the effectiveness of the high frequency attenuator by
providing a lower resistance to the high frequency sound entering
the high frequency attenuator.
Inventors: |
Clark; Raymond C. (Lake Forest,
IL) |
Assignee: |
Brunswick Corporation (Skokie,
IL)
|
Family
ID: |
23741811 |
Appl.
No.: |
05/438,736 |
Filed: |
February 1, 1974 |
Current U.S.
Class: |
181/248 |
Current CPC
Class: |
F16L
55/033 (20130101); F16L 55/02 (20130101) |
Current International
Class: |
F16L
55/033 (20060101); F16L 55/02 (20060101); F01N
001/04 () |
Field of
Search: |
;181/42,46,48,59,47,41,33G,50 ;23/288F |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Tomsky; Stephen J.
Assistant Examiner: Gonzales; John F.
Attorney, Agent or Firm: Heimovics; John G. Olexa; D. S.
Epstein; S. L.
Claims
What I claim is:
1. A device for attenuating sound in a fluid flowing therefrom, the
fluid having a known acoustical impedance measured in rayls,
comprising a housing surrounding a conduit having an inlet and an
outlet, the conduit having in series after the inlet:
first means for attenuating low frequency sound entering the
device; and
second means for attenuating high frequency sound entering the
device,
the second means having a foraminous duct in series with said
conduit, the duct having a preselected acoustical resistance
measured in rayls substantially the same value in rayls as the
impedance of the flowing fluid, the first means attenuating the low
frequency sound to a level sufficiently low to enable the
foraminous duct to effectively function at its preselected
acoustical resistance.
2. A device as recited in claim 1 wherein the device is a free flow
device.
3. A device as recited in claim 2 wherein the fluid is a flowing
gas.
4. The device as recited in claim 1 wherein the first means is a
Helmholtz resonator.
5. The device as recited in claim 4 wherein the first means is
tuned to 250 Hz.
6. The device as recited in claim 1 wherein the first means is a
baffle system.
7. The device as recited in claim 6 wherein the baffle system is a
multi-dimensional baffle system.
8. The device as recited in claim 1 wherein the first means is a
retroverted system.
9. The device as recited in claim 1 wherein the first means is an
expansion chamber.
10. The device as recited in claim 1 wherein the second means is a
quarter-wave standing wave cavity.
11. The device as recited in claim 10 wherein the depth a of the
standing wave cavity is an odd multiple of one-quarter wavelengths
of the sound pressure level.
12. The device as recited in claim 11 wherein the second means is
tuned above 1000 Hz.
13. The device as recited in claim 1 wherein the foraminous duct
has an effective impedance in the range 3-30 cgs rayls.
14. The device as recited in claim 1 wherein the foraminous duct
has an effective impedance in the range 31-100 cgs rayls.
15. The device as recited in claim 1 wherein the foraminous duct
has an effective impedance in the range 101-400 cgs rayls.
16. The device as recited in claim 1 wherein the foraminous duct is
a laminated screen structure.
17. The device as recited in claim 1 wherein the foraminous duct is
a perforated tube.
18. The device of claim 17 wherein the perforated tube is made of
metal with metal fiber in the openings.
19. The device as recited in claim 1 wherein the foraminous duct is
a metal fiber web structure.
20. The device as recited in claim 19 wherein the metal fiber web
structure has the inside surface thereof covered with a layer of
fibers having a protective coating.
21. The device as recited in claim 20 wherein the fibers are made
of metal.
22. The device as recited in claim 20 wherein the fibers are of
organic material.
23. The device as recited in claim 20 wherein the fibers are of
ceramic material..
24. The device as recited in claim 20 wherein the protective
coating is an oxidation catalyst.
25. The device as recited in claim 1 further including third means,
located in series after the second means, for attenuating middle
frequency sound entering the device.
26. The device as recited in claim 25 wherein the third means is a
Helmholtz resonator.
27. The device as recited in claim 25 wherein the third means is
tuned within the range 500 Hz to 1000 Hz.
28. The device as recited in claim 1 further including in series
after the second means,
a plurality of quarter-wave standing wave cavities surrounding a
portion of the periphery of the foraminous duct;
at least one Helmholtz resonator surrounding the plurality of
standing wave cavities and communicating with the foraminous duct
by ports located in the conduit at those portions not communicating
with the standing wave cavities.
29. The device as recited in claim 1 wherein the foraminous duct
comprises two foraminous tubes parallel to each other.
30. The device of claim 1 wherein the second means is a laminar
absorber.
31. A method for attenuating the level of sound in a fluid flowing
through a muffler, the fluid having a known acoustical impedance
measured in rayls, comprising the steps of:
a. providing a high frequency sound attenuator for absorbing
frequencies over 1000 Hz, the attenuator having an acoustical
resistance in rayls matched to the acoustical impedance of a fluid
entering the muffler; and
b. providing a low frequency sound attenuator to absorb sound
having a frequency of under 1000 Hz to a level sufficiently low to
enable the high frequency attenuator to effectively function at its
preselected acoustical resistance.
32. The method as recited in claim 31 wherein the low frequency
attenuator is a Helmholz resonator.
33. The method of claim 30 wherein the high frequency attenuator is
a foraminous tube.
34. The method of claim 33 wherein the foraminous tube is a mesh of
metal fibers.
35. The method of claim 33 wherein the tube is a woven metal fiber
tube.
36. The method of claim 35 wherein the woven metal fiber tube has
the inside surface thereof covered with a layer of metal fibers
coated with an oxidation catalyst.
37. The method as recited in claim 30 wherein the high frequency
attenuator includes a quarter wave standing wave cavity.
38. The method of claim 37 wherein the depth a of the standing wave
cavity is an odd multiple of onequarter wavelengths of the sound
pressure level.
Description
BACKGROUND OF THE INVENTION
The present invention is directed to a sound and noise attenuating
device for use with flowing gas systems. In particular the
invention is directed to a free flow sound attenuating device
wherein the sound attenuated encompasses both low and high
frequency sound and to a sound attenuating device for use
particularly with flowing gases of single phase. By single phase it
is meant flow comprising substantially 100 percent gaseous state
with little or none of the flow in a liquid state.
By low frequency it is meant frequencies 500 Hz or less, middle
frequency being defined as between 500 Hz and 1000 Hz, and high
frequency any frequency above 1000 Hz. The device may be used for
intake and exhaust gas flow systems such as mufflers and resonators
for internal combustion engines. By a low frequency attenuator it
is generally meant those known in the art such as a Helmholtz type
resonator, a single or multi-dimensional baffle system, retroverted
systems, or expansion chambers. By high frequency attenuator it is
meant those known in the art as attenuating frequencies above 1000
Hz, one example being the use of quarter-wave standing wave;
cavities which are closed with acoustically absorptive foraminous
material. It is noted that best results are obtained of the low
frequency attenuator used is of the Helmholtz type and the high
frequency attenuator is of the quarter-wave standing wave type.
By foraminous conduit it is meant any conduit having porosity such
that the flow resistance across the conduit wall is properly
selected relative to the acoustical impedance of the gas and sound
pressure level entering the conduit. The flow resistance may be
uniform or non-uniform along the conduit length, a uniform
distribution being preferable because of its lower manufacturing
cost.
SUMMARY OF THE PRIOR ART
Free flow or straight through sound attenuating devices are known
in the art, and by free flow it is meant those devices where the
flowing gas passage is direct and open with minimal back pressure
developed as compared to devices where the flowing gas must pass
through a multi-directional baffle system, retroverted systems, or
through expansion chambers. Prior art devices have used resonators
of the Helmholtz type separately or in combination with baffled
systems for attenuating sound in exhaust gas flows for internal
combustion engines, blower duct systems, fuel burning systems, etc.
The art also discloses using a single Helmholtz resonator for
tuning out a specific frequency or multiple resonators for tuning
out a multiple of frequencies, such that sound accompanying such
gas flows have sound removed by the action of certain frequencies
resonating in the Helmholtz chamber. Whether single or multiple
resonators are used the sound attenuation is dependent on the
frequency characteristics of the single resonator or the sum of the
frequency characteristics for multiple resonators.
For large gas flows and sound pressure levels, the prior art sound
attenuating devices have necessitated very large units in absolute
terms, increasing the problem of the design, handling and
installation of these units.
An additional problem with respect to free flow systems is that
unless large in size the efficiency of the attenuator is not
capable of attenuating sound to an acceptable level to the general
public or for industrial applications.
SUMMARY OF THE INVENTION
It is a general object of the present invention to provide a sound
attenuating device that has greater sound attenuating
characteristics than the sum of the sound attenuation of the
individual sound absorbers if used separately. In other words, a
synergistic effect is realized by the structural combination of the
present invention.
Furthermore, recognizing that prior art sound attenuators could be
efficient when subjected to gas flows having a high sound pressure
level, they did so at the expense of being very large in size,
weight and complexity. The present invention has as one object and
advantage, the provision of a sound attenuation device that because
of the operative association of the foraminous conduit with the low
frequency attenuator, the size, weight and complexity found with
prior art structures are eliminated and a very simple free flow
device can be defined. The device of the present invention
accomplishes this by permitting several attenuating functions to
take place in close proximity resulting in a synergistic sound
attenuation characteristic.
Moreover, with prior art sound attenuators utilizing baffling,
retroverted flow patterns, resonators and expansion chambers, a
great number of production sequences were required in their
manufacture. Accordingly, another object of this invention is to
provide a sound attenuator which is simple in structure, easier to
manufacture and easier to maintain than devices heretofore known in
the art. In addition the device of the present invention is lower
in cost and more durable in operation than prior art devices having
equivalent sound pressure level and attenuation
characteristics.
Furthermore, although the prior art teaches free flow devices, the
structure of the present invention provides an improvement over
them in its utilization of a central foraminous tube which may have
an internal layer of metal fibers having a special coating. The
coating applied will depend on the function it is to serve one
example being a material in an oxidized or unoxidized state
resistant to corrosive substances that may be present in the
flowing gas. The foraminous tube operating in conjunction with a
low frequency resonator also permits greater decibel attenuation in
the high frequency resonator, heretobefore not known in the art or
expected.
It is another object of the invention to provide for a more compact
rugged constructed attenuator that has improved erosive, corrosive
and temperature resistant characteristics.
Additional objects and advantages of the invention will become
apparent to those skilled in the art from the following discussion
of the several illustrative embodiments thereof, which will be
described in connection with the attached drawings, in which:
FIG. 1 shows a schematic of the basic structure of the sound
attenuating device in accordance with this invention;
FIG. 2 is a section of FIG. 1 taken along lines 1--1;
FIG. 3 is a graph detailing the improvement in sound attenuation by
the unit shown in FIG. 1;
FIG. 4 shows a first alternate embodiment to that shown in FIG.
1;
FIG. 5 is a graph detailing the sound attenuation by the attenuator
shown in FIG. 4;
FIG. 6 shows a second alternate embodiment to that in FIG. 1;
FIG. 7 shows a section of the structure in FIG. 6 taken along lines
1--1;
FIG. 8 shows a third alternate embodiment of the sound attenuation
device of the present invention;
FIG. 9 shows an embodiment of the central tube used in the present
invention, sectioned along its centerline;
FIG. 10 shows a second embodiment of the central tube of the
present invention, sectioned along its centerline;
FIG. 11 shows a third embodiment of the central tube of the present
invention, sectioned along its centerline;
FIG. 12 shows a fourth embodiment of the central tube of the
present invention, sectioned along its centerline;
FIG. 13 is a cross sectional view of another embodiment of this
invention;
FIG. 14 is a cross sectional view of another embodiment of this
invention;
FIG. 15 is a cross sectional view of another embodiment of this
invention; and
FIG. 16 is a cross sectional view of another embodiment of this
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In general terms the basic features of the preferred embodiment of
this invention comprise (1) a foraminous conduit surrounded by (2)
a housing having in series after the inlet end (3) a low frequency
attenuator operatively associated with the conduit and (4) a high
frequency attenuator operatively associated with the conduit end
positioned after the low frequency attenuator to enhance the
effectiveness of the high frequency attenuator by providing lower
resistance to the high frequency sound entering the high frequency
attenuator due to the absence of low frequency sound entering the
high frequency attenuator.
FIG. 1 shows a schematic of a section of a sound attenuator
according to the present invention. The attenuator 8 has a central
conduit 10 having an inlet end 12 and outlet end 13 which outlet
and inlet end can be connected to conventional intake or exhaust
pipes according to the particular application. The attenuator 8 has
a high frequency attenuator 8A and a low frequency Hz attenuator
portion 8B. Alternatively, the low frequency attenuator may be a
baffle system 8C, a multi-dimensional baffle system 8D, a
retroverted system 8E, and an expansion chamber system 8F, shown in
FIGS. 13-16 respectively, and used as desired. The low frequency
attenuators of FIGS. 13-16 do not prevent the high frequency
portions from being flowed through. The central conduit 10 is
generally described as a foraminous tube, meaning a tube having a
preselected flow resistance such that the pressure drop across the
wall of the conduit is selected to match the impedance of the
flowing fluid. That is, it is necessary that the conduit not be
totally impervious to a flow of gas. The choice of material for the
conduit may be made of a number of materials some examples being,
laminated screen structure, perforated tube, metal fiber web,
knitted fiber or wire material, compacted fiber, foamed metals or
non-metals, glass fiber, plastic fiber, porous ceramics or
combinations thereof. However, it is noted that the choice of
material for the conduit will be related to its particular
application. As an example, for a flow of gas at a high temperature
a metal fiber web will work best whereas for gases at a low
temperature a plastic may be used. In addition, for gas flows
containing surges of pressure level increases in flow, a material
capable of withstanding a great pressure drop across the conduit
wall will be necessary. Whatever the situation it is obvious that
one skilled in the art can determine the material most suitable for
the particular gas flow by application of known engineering
principles.
A number of different conduit strucutres may be used depending on
the physical characteristics of the gas flow, four structures are
shown as example in FIGS. 9, 10, 11 and 12.
FIG. 9 shows a tubular section 48 made of perforate plate having a
plurality of holes 49. These holes may all have s similar or
different diameters.
FIG. 10 shows a tubular metal structure section 50 made of metal
fiber web or mesh structure, which web and mesh structures may be
produced by processes known in the art as described in U.S. Pat.
No. 3,505,038; 3,127,668 and 3,469,297 each of which is
incorporated by reference. The mesh and web structures can be made
of metal fiber made in accordance with the descriptions given in
U.S. Pat. No. 3,394,213; 3,505,038; 3,505,039; 3,698,863; 3,379,000
and 3,277,550 each of which is also herein incorporated by
reference. It is also possible to provide chopped fibers useful in
the tubular structure in accordance with U.S. Pat. No. 3,504,516.
The above listed patents being owned by the assignee of the present
invention.
In addition, it is also possible to use coated metal fibers in
accordance with U.S. Pat. No. 3,698,863 and 3,505,038. Other ways
of producing the fibers and fiber webs for the tube of this
invention are known in the prior art and such other references are
not excluded by the citing of the above references which are given
as example only.
It is noted that the tubular structure made of fiber web or mesh as
described above is desirable to other structures, since it has been
found that this structure provides the greatest amount of
frictional losses for optimum sound absorption.
FIG. 11 shows a tube made of a screen-like material 51 which may be
used either separately or in combination with the tube shown in
FIG. 9, the controlling factor being whether the outer shell 11
(See FIG. 1) of the structure has sufficient rigidity to provide
support for the screen-like tube. An equivalent form for the
screen-like material would be a highly perforated tube or mat
structure tubular in shape and having an external diameter
approximately equal to the internal diameter of the tube.
FIG. 12 shows a preferred embodiment of the present invention
comprising a conduit 10, wherein a conduit 52 (in accordance with
the conduit shown in FIG. 9) has ports 53 of similar diameter shown
large for purposes of illustration. It also has metal, organic or
ceramic fibers 54 provided on the internal surface thereof, the
fibers having a protective coating of oxidation-catalyst such as
nickel, platinum, aluminum oxide, copper oxide, etc.
Referring to FIG. 1 and FIG. 2 whatever the form of the central
conduit used, it is surrounded by and secured to an inner wall 21
closing cavities 19, with an outer shell 11 enclosing this inner
wall except for the inlet and outlet ends projecting a short
distance for easy attachment to the device with which it will be
used. It is noted that the foraminous portion of conduit 10 is
foraminous only for that portion of the conduit associated with
cavities 19, this being indicated between points c and b. The
remaining portions of conduit 10 being non-porous to fluid flow
except for ports 15. The outer shell 11 is of a conventional
nature, one example being sheet metal.
The flowing gas having concomitant sound to be attenuated enters
the sound attenuator 8 at the inlet end 12. The gas comes into
contact with low frequency Helmholtz resonator ports 15 which use
chamber 17 for resonating. Although it was defined earlier in the
specification what is generally accepted as definition for low,
middle, and high frequency, the cut-off range for low, medium and
high frequency is relative and will depend upon the application.
The gas stream then passes by annular high frequency, quarter
wavelength depth tuned cavities 19 which communicate with the
central tube through the tube's foraminous wall 20. The alpha
character a indicates the depth of the quarter wavelength
cavity.
FIG. 3 is a graph plotting attenuation of sound power in db verses
frequency of the sound with the area under the curves representing
the amount of sound absorbed. If for example the low frequency
resonator was tuned, by appropriate adjustment of chamber volume 17
and port diameter 15 well known in the art, to 250 Hz the response
curve would be that designated by lines L. A high frequency
resonator line duct will have the typical curve shown as line H,
usually tuned somewhere above 1000 Hz. From theory and practice in
the art it would be expected and predicted that if a sound absorber
containing two attenuating means, one tuned to low frequencies and
one tuned to high frequencies were to be used in one structure the
resulting curve would follow line L for frequencies just above 250
Hz and then follow line A, a transition phase, and continue along
line H in normal fashion for the high frequencies. What was found
with the device of the present invention, however, was a
synergistic effect. For low frequencies the curve looks like that
expected in the art, line L, but for higher frequencies the
response follows curve I attenuating the sound in a gas flow
approximately 10 db higher than expected. By definition in the art
a decibel increase in sound power of 3 decibels is equivalent to
doubling the sound power level. Therefore, a reduction of 10
decibels (db) in sound power level is equivalent to reducing the
sound power level to one-tenth of the original level. This one
improvement of the invention and it is significant in that none of
the prior art structures have disclosed or suggested this
possibility. The improvement is believed to be the result of the
elimination of low frequency sound from interacting with the
foraminous tube and thereby enhancing the effectiveness of the tube
at high frequencies. Specifically, the wall of the foraminous
conduit provides a certain resistance to the flow of high frequency
sound to resonant cavities 19. This resistance is increased if low
frequency sound is present along with the high frequency sound.
There is a further increase in resistance at the wall of the tube
if the low frequencies are at high sound pressure levels. Maximum
attenuation will occur in thin-walled foraminous tubes backed by
quarter wavelength cavities, when the tube wall resistance is
effectively equal to the characteristic impedance of the fluid
flowing through the tube. The acoustical impedance of fluid gases
can range between 3 - 400 cgs, rayls, the following given as
example only with impedance values for other gases available in
standard reference books:
air at 6 psia and 1000.degree. F - 10 rayls.
hydrogen at 14.7 psia and 0.degree. C - 11.4 rayls,
air at 14.7 psia and 0.degree. C - 42.86 rayls,
air at 55 psia and 800.degree. F - 100 rayls,
air at 115 psia and 400.degree. F - 250 rayls,
air at 100 psia and 100.degree. F - 300 rayls,
Freon -- 22 at 76 psia and 150.degree. F - 380 rayls (the condition
of Freon inside of a sealed refrigeration unit)
The matching resistance of the foraminous portion of conduit 10 is
also measured in units of rayl being defined as the change in sound
pressure level drop (measured in dynes/cm.sup.2) across the conduit
wall divided by the velocity of fluid flow (measured in cm/sec). A
unit of rayl is then dynes-sec/cm.sup.3.
It has been shown that if the effective resistance of the conduit
wall is too low, the quarter-wave standing wave will be established
but maximum sound absorption will not occur. If the conduit wall
resistance is too high the quarter-wave standing wave will not be
strongly established and sound absorption will again be low. The
prior art literature indicates that a high sound pressure level
high frequency sound present will cause the effective acoustical
impedance (resistance) of the material used to increase; "high"
pressure level for high frequency sound being defined as above 130
db, with particular concern in the 140-160 db range. Any high
frequency sound pressure level below 130 db being defined as low.
It has been found by the inventor, that the effective resistance of
the material used also increases if low frequency sound at sound
pressure levels of 110-120 db are present. The result is that with
a broad spectrum of sound the low frequencies present reduce the
ability of the foraminous material to absorb energy at the high
frequencies since the material will exhibit a higher effective
resistance. This higher resistance is great enough to seriously
reduce the effect of the high frequency quarter-wave standing wave
cavities and correspondingly reduce absorption. What is believed to
be happening in the present invention is as follows. It was found
that resistance of the foraminous tube 10 changes with the
frequency and sound pressure level of sound passing therethrough.
If low frequencies are present the wall of the tube in effect acts
as high resistance. However, if low frequencies are absent the
resistance of the tube wall decreases. The sound attenuator of the
present invention utilizes this effect for synergistic results. At
the inlet end the low frequencies in the flowing gas communicate
through ports 15 to the low frequency resonator chamber 17, low
frequencies resonate in chamber 17 with the result that the low
frequencies are substantially attenuated. The high frequencies in
the flowing gas then encounters the high frequency resonators 19.
Since the low frequencies are now absent, the high frequencies pass
through a low resistance wall, establish standing waves and are
attenuated. The high frequencies can then be attenuated with
greater efficiency in the high frequency resonators which is the
improvement shown as curve I in FIG. 3. If the low frequencies were
present with the high when encountering chambers 19, the low
frequencies would cause the tube wall to exhibit a high resistance.
The high resistance of the wall however will prevent the high
frequencies from entering chambers 19 efficiently and thereby
result in lower attenuation, which is the curve H in FIG. 3. There
is no suggestion or disclosure in the prior art that an interaction
exists between the different frequency ranges such that the
presence of low frequencies in a flowing gas will cause a high
frequency resonator of the type described to be less capable and
efficient in attenuating the high frequencies. This effect has not
heretobefore been recognized or utilized.
Another embodiment of the attenuator shown in FIG. 1 is that shown
in FIG. 4 which is identical to the attenuator in FIG. 1, but which
has in addition a middle frequency Helmholtz attenuator indicated
by chamber 18. Tube 10 communicates with chamber 18 by ports 16 to
attenuate the middle frequency generally designated in the art as
between 500 and 800 Hz. The response of this attenuator is shown in
FIG. 5 where line H-1 would be expected according to the teachings
of the prior art and where line I-1 is the improvement according to
the structure of the present invention.
In the above description, frequencies of 0 to 500, 500-1000 Hz and
1000 Hz and above were used in defining the low, medium and high
frequency ranges respectively. The particular frequencies generated
by a sound source, will vary with style, size, etc. of the
application. Thus, the frequencies necessary and useful in the
above structures will vary. The maximum efficiency of the
attenuator of the present invention will thus depend upon the exact
frequency characteristics of the sound source. Utilizing this data
the frequencies to be used for tuning the low, medium and high
resonators can be calculated by standard formulae as described in
"The Theory of Sound," by John William Strut, Baron Rayleigh,
published in 1894 and republished by Dover Publications, Inc. in
1945.
It has been noted earlier that for large gas flows and sound
pressure levels, the prior art attenuating devices have
necessitated very large units in absolute terms. As example, a
centrifugal compressor moving a fluid comprising air at a
temperature of 300.degree. F, pressure of 15 psi, and flow velocity
of 145 to 290 ft/sec. required a prior art muffler (attenuator) 18
inches in diameter by 86 inches long to reduce the sound level
below 90 db, whereas with the present invention for air at the
given temperature, pressure and flow velocity a unit only 141/2
inches in diameter by 291/2 inches long is required. A volumetric
reduction of 449% over the prior art device.
As another example, for a compressor application for a given gas
flow at a given temperature, pressure and flow velocity, a selected
attenuation in db required a Burgess BEO-10 unit 30 inches in
diameter by 136 inches long whereas a structure by the present
invention achieved equivalent results with a unit 22 inches in
diameter by 32 inches long. A volumetric reduction of 790% over the
prior art.
The effect of the Helmholtz resonators used in this invention is
maximized by knowing the frequencies of the system to be treated.
Variations in gas flow volume and sound pressure level will
obviously necessitate increases or decreases in the size and design
of the attenuator. However, adjustments for these parameters are
well known in the art.
FIGS. 6 and 7 show another alternate embodiment of the present
invention wherein a shell 36 surrounds and is secured to a
foraminous tube 31 having an inlet 32 and outlet 38. FIG. 7 more
clearly shows the low frequency resonators chamber 41 communicating
with the gas flow by ports 37. The high frequency standing wave
cavities 42 communicate with the gas flow by the porous nature of
the tube. In this embodiment the gases enter at inlet 32 and come
in contact with a low frequency resonator chamber 41 through ports
37. The gas stream flows by annular high frequency cavities 42
through the tube's foraminous wall 43, and then continues to flow
and contact simultaneously a series of both low frequency
resonators and high frequency standing wave cavities. Use of this
structure having a plurality of structures as shown in FIG. 1 is
required where the sound pressure level of the gas is very high. In
other words, every sound attenuating structure has a certain
efficiency in attenuating the sound accompanying gases flowing
therethrough. The magnitude of sound accompanying the gas flow is
directly related to the sound pressure level. If the sound pressure
level is very high, an attenuator, say for example having a given
amount of efficiency, may not attenuate a sufficient amount of
sound to bring it to an acceptable level. Accordingly, it will be
necessary to flow the gas through a second unit to further
attenuate the sound. The structure in FIG. 6 is for this purpose,
indicating an attenuator having a plurality of single attenuators
similar to that shown in FIG. 1 for applications where large sound
pressure levels are encountered.
FIG. 8 shows a third embodiment of the present invention having a
double, parallel ducted structure having a shell 23 provided with
an inlet 24 and outlet 25. There are two foraminous tubes 27
communicating with a low frequency resonator 30 through ports 28
and with high frequency standing wave cavities 32 through the
porosity of the tubes walls. It is noted that the foraminous
portion of tubes 27 extend for only those portions of tubes 27
associated with cavities 32. The remaining portions of tubes 27
being non-pervious to fluid flow except for ports 28 and 29. In
addition, and similar to FIG. 4, a middle frequency resonator 31 is
provided and communicates with tube 27 through ports 29. The
resonators, depending on the system on which it will be used, will
be tuned in accordance with the above discussion regarding
frequency design characteristics and the mesh or web structure
discussed previously. This embodiment is useful for higher volumes
of gas flow.
It is also noted that the depth a of the high frequency resonators
(see FIG. 1) is measured from the outer wall E of the foraminous
material. These resonators being preferably tuned by making a an
odd multiple of one-quarter wave lengths of the high frequency
sound. This will cause the sound pressure level at the tube wall to
be substantially at zero pressure with the sound velocity being at
its maximum.
While the foregoing description sets forth the principles of the
invention in connection with specific resonators known in the art,
the terms and expressions employed are terms of description in the
art, and not of limitation, with no intention in using such terms
to exclude any equivalent of the structures described. It is also
understood that the description is made only by way of example and
not as a limitation of the scope of the invention as set forth in
the general aspects thereof and in the accompanying claims.
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