Fluidic element noise and vibration control constructs and methods

Fluidic constructs, including grouped stacks of fluidic elements, that provide countersound to control sound in a noisy environment, prevent radiation of sound from vibrating surfaces, reduce sound-induced vibration of surfaces, and absorb sound that might otherwise impact on surfaces, are provided. These constructs may have a wide range of geometries for specific applications, but generally include a face plate on one side, and a back plate on the other side. Supply ports on the back plate provide a supply of fluid that flows through the construct, while undergoing acoustic modulation through fluidic amplifiers. The face plate includes input ports that sense sound waves to be controlled, and transmits this sound to influence the acoustic modulation of the supplied fluid. The construct produces an amplified output, having sound out of phase with the sound sensed by the input ports, at output ports on the face plate in a sufficient volume to substantially neutralize incoming sound waves, or reduce sound radiation from an object. Any sound produced by the construct of the invention that is substantially in phase with sound to be neutralized is dumped at a sufficient distance from the produced countersound to minimize interference.

FIELD OF THE INVENTION 
The invention relates to the field of noise reduction, and provides 
constructs that comprise fluidic elements for controlling the impedance of 
the construct to attenuate sound waves over a broad range of frequencies. 
BACKGROUND OF THE INVENTION 
Several techniques have been developed for noise reduction. These include, 
for instance, the use of passive mufflers, such as those found on the 
exhaust systems of automobiles. Other techniques include the use of 
noise-reducing enclosures around the noise-creating device and 
sound-absorbing materials to reduce the reverberation of sound in the 
environment. In addition, active techniques using the generation of 
"counternoise" to neutralize the noise have also been demonstrated 
successfully. For example, a system of electrically powered microphones 
for detecting noise, linked to electrically powered speakers for 
generating a counternoise, has been used successfully in the cabin of 
propeller-driven aircraft. The electrical microphone-speaker system 
requires a plurality of these devices distributed along the walls of the 
cabin, and is limited to reducing noise within a narrow bandwidth. Thus, 
the system is well adapted for attenuating the periodic sound pressure 
generated by a rotating impeller, but is not well suited for reducing the 
broad sound wave band generated by a jet engine or the aerodynamic 
boundary layer of a flying aircraft. 
There exists a need for a device that is able to attenuate sound waves, 
across a broad frequency band, that is reliable and cost-effective. 
Preferably, the device should not require significant input of 
maintenance, and should be able to operate effectively for long periods of 
time without continuous monitoring. Furthermore, the device should 
desirably be energy efficient, either not using power, or using very 
little power. Moreover, the device should be space-efficient, and not 
bulky, so that it can be readily used in a variety of applications where 
space limitations are important. Finally, the device should also be light 
weight to allow use in weight-sensitive applications, such as aircraft 
cabins. 
SUMMARY OF THE INVENTION 
The invention provides constructs of controlled, typically low, sound 
impedance that effectively reduce broad frequency band noise in an 
environment. These constructs may be fabricated in a variety of shapes, 
including planar shapes suitable for use as wall coverings, and 
cylindrical shapes suitable for use in mufflers, and other noise reduction 
applications. The constructs are of light weight, and are relatively thin, 
so that they are space efficient. Moreover, the constructs do not require 
an input of electrical, or another power other than an input of a suitably 
pressurized fluid, gaseous or liquid. 
The constructs of the invention comprise an array made up of a plurality of 
grouped stacks of sheets having cut out fluidic elements thereon. Each of 
the stacks of sheets of fluidic elements includes at least one sheet, and 
preferably many sheets, having fluidic amplifiers. These fluidic 
amplifiers may be cascaded so that each of the stacks is able to amplify 
significantly the acoustic pressure of the fluid in contact with the 
stack. The fluidic construct also has at least one control port (or 
"microphone") in a face plate of the construct that faces the environment 
in which sound must be controlled. Input received in this control port 
modulates the fluid flow through the construct from the supply port to 
produce sound destructively out of phase with the sound in the 
environment. The amplified and out-of-phase sound ("countersound") 
generated is expelled from the construct through at least one output port 
("speaker") and controls or reduces incident sound waves. At the same 
time, an unwanted portion of the amplified sound pressure is dumped, via 
at least one dump port of the array of fluidic elements, to a sufficiently 
remote location so that it does not generate significant interference with 
the attenuation of the sound. 
Due to the travel time of the air supply through the fluidic element 
construct to the output port, instabilities in the fluidic circuit of the 
construct could occur at high frequencies. To counteract this possibility, 
acoustic low pass filters, in the form of orifices and volumes, are 
included in the construct to filter out the high frequencies. 
In a preferred embodiment, the "sheets of fluidic elements" are each 
fabricated from relatively thin sheets of material about 0.1 mm to about 
0.5 mm thick. A range of materials are useful, including metal foil, 
plastic sheeting, etc. Each of these sheets preferably has a plurality of 
fluidic elements cut out of the sheet. A multiplicity of such sheets 
having fluidic amplifiers, alternating with sheets having transfer 
elements, are grouped together into a first "stack" of elements. The 
transfer element on one sheet controls the flow or transmission of fluid 
between fluidic elements on sheets on either side of the one sheet. A 
plurality of these stacks of fluidic and transfer elements are then 
grouped together to form "an array" of stacks. Depending upon the geometry 
of this array, it comprises the noise control "wall paper", or cylindrical 
roll muffler embodiments of the invention, described in more detail below. 
While constructs of the invention may be customized to particular 
applications and therefore come in a range of geometries, each suited to a 
particular application, in one embodiment described herein, the noise 
control construct of the invention is in the form of a "sound absorbing 
wall paper" that includes substantially planar fluidic elements, such as a 
series of sheets, arranged in a predetermined sequence to achieve the 
desired attenuation of noise. This noise-reducing "wall paper" may be used 
in a variety of applications, including the lining of the side walls of 
cabins of aircraft and other vehicles, use in theaters, recording studios, 
and opera halls to tailor acoustics, in certain manufacturing environments 
that generate high levels of noise that pose a hazard to health, and the 
like. 
In another embodiment of the invention, the noise control construct is in 
substantially cylindrical form, with the thin sheets of fluidic elements 
are rolled up together like a roll of sheets of parchment. This type of 
construct is used as a muffler for sound in the fluid that is passing 
through the axial bore of the construct. In another version of the muffler 
embodiment, the cylindrical roll of sheets of fluidic elements is axially 
aligned with a cylindrical passive muffler to form a combination muffler 
that is highly effective for noise attenuation. In a further embodiment, 
the fluidic element constructs are interspersed with passive elements, 
either in a planar or a cylindrical arrangement. In this latter type of 
combined construct, the passive elements serve to increase the acoustical 
stability of the construct and increase its frequency range of 
attenuation. 
The fluidic element noise control constructs of the invention may be 
fabricated in a variety of thicknesses, the thinner constructs being 
preferred. However, when used in the "wall paper" embodiment, the 
thickness of the construct is generally expected to be in the range from 
about 1.0 to about 5.0 mm. Sound waves having a frequency in the range 
from about 0 to about 400 Hz can be attenuated with such a construct. 
While it is desirable for most applications to minimize thickness and size 
of the fluidic elements, currently feasible technology appears to limit 
the thickness of the "wallpaper" to this 1.0-5.0 mm range. However, if 
thinner and smaller fluidic elements are feasible, then the constructs may 
attenuate sound waves having a frequency in the range from about 0 to 
about 2,000 Hz.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The invention provides constructs that actively control sound impedance. 
The constructs are composed of stacks of laminated sheets that are 
arranged in the form of an array. Preferably, each sheet in the array 
contains either a fluidic element or a transfer between fluidic elements. 
Some of the fluidic elements are fluidic amplifiers, and these amplifiers 
are preferably cascaded in series. The input to the series of amplifiers 
is either from the side exposed to the noisy environment, and so excited 
by sound waves, or the side where sound radiation from an object should be 
controlled. The construct also receives a supply of fluid that is 
modulated by the input to produce a volume of "countersound" or sound out 
of phase with the sound to be controlled. The effect is to actively 
control the acoustic impedance such that an exciting sound wave is 
absorbed, or sound radiation from a vibrating object (such as an aircraft 
cabin wall) which the construct is shielding, is minimized. 
It is a unique aspect of the invention that it uses fluidics as the medium 
for providing the cancellation of sound waves (noise) and thereby allows 
the use of potentially the entire surfaces of walls and other objects, as 
"speakers" for canceling these noise vibrations using input from 
"microphones" in surfaces exposed to the noise. 
The definitions that follow are not intended to override the usual 
understanding of the meanings of these terms in the art but to clarify the 
terms to facilitate understanding of the invention. In the specification 
and claims, the tern "substantially planar" is intended to include 
constructs that have a large radius of curvature, such as wall coverings 
for the side walls of an aircraft which has a cylindrical fuselage. The 
tern "sheet," as used in the specification and claims, means a sheet 
fabricated from a material suitable for use in making fluidic elements and 
transfer elements, such as organic polymer (plastic), metal foil, and the 
like. Preferably, the sheets used to produce the fluidic constructs of the 
invention are as thin as possible for least mass. Typically, sheets are in 
the thickness range from about 0.2 to about 0.5 millimeters, although they 
may be as thin as 0.05 millimeters, and thickness may range upward, 
depending upon the specific application. A "fluidic element" is a 
precisely shaped cut-out section of a sheet that has at least an input 
point to receive fluid and an output point from which fluid is discharged. 
While the sizes of the cut-out fluidic elements will vary depending upon 
the specific application of the fluidic construct, the elements may 
typically be in the size range of about 5 mm to 50 mm square. A 
multiplicity of small cut-out elements in each sheet of an array makes up 
a "wallpaper" type of construct. A "fluidic amplifier" is a fluidic 
element that amplifies acoustic pressure of a supplied fluid. A "transfer 
element" is also in a generic sense a fluidic element, but it generally 
does not amplify and it is interposed, usually on a sheet between a first 
and second sheet, to control fluid communication from a fluidic element on 
the first sheet to a fluidic element on the second sheet. The term "stack" 
relating to fluidic elements means the repeating unit of a group of sheets 
containing fluidic elements stacked one atop the other, usually with 
transfer elements interposed between to control fluid flow. The term 
"array of stacks" or "array of stacks of fluidic elements" means a series 
of stacks of fluidic elements grouped together and in fluid communication. 
Typically, stacks are compiled into a fluidic construct for noise 
reduction, in accordance with the invention. Generally, an array of 
fluidic elements will include several stacks, each of which has at least 
one, and preferably several, fluidic amplifiers. A "vent" is an area in an 
element of a sheet, such as part of the body of a fluidic amplifier, where 
pressure is kept at ambient levels. A "face plate" is the top sheet of an 
acoustic fluidic array, where the "microphone" (input or control port) and 
the "loudspeaker" (output port) openings are located. A "back plate" is 
the rear sheet of an acoustic fluidic array, where the dump ports (or dump 
openings) are located. 
It is one of the objectives of the invention to create a desirable acoustic 
impedance. For a wall absorber in a room, this may be a resistive 
impedance in the range 1-2 .rho.c (where .rho. is the air density, and c 
is the speed of sound), and for a muffler an impedance proportional to 
(1.8-1.5j) .omega. over some frequency range in order to effectively 
suppress the least attenuated mode (where j=.sqroot.-1, and 
.omega.=circular frequency). For a vibrating wall, the optimal impedance 
would be zero in order to completely suppress radiation. It is desired to 
create an impedance in the range 0.5-1.0 .rho.c over the frequency range 
where excessive noise exists, or to create a very low impedance, of the 
order of 0.1 .rho.c, at some discrete frequency or frequencies. 
The general concept may better be understood with reference to a specific 
example. Thus, consider a wall lining that consists of an array of fluidic 
and transfer elements. The fluidic elements are arranged so that the 
control port of the first amplifier stage ("microphone"), and the output 
port of the last amplifier stage ("speaker"), are both exposed to an 
incident wave. The ports are arranged in such a way that a positive 
pressure at the control port causes a negative pressure (or 
"counternoise") at the output port, thus counteracting the incident wave. 
Due to the time delay of the response of the output port, the counternoise 
may only arrive in time for frequencies that are lower than a limiting 
frequency, defined below, while for frequencies higher than the limiting 
frequency, the damping in the circuit must be sufficiently large to 
prevent self-excited oscillations. The limiting frequency, f, is set by 
the accumulated time delay, d, through the fluidic circuit (i.e., from 
control port to output port). At this frequency, the time delay 
corresponds to a phase shift of about 60.degree. to about 90.degree., 
i.e., .pi./3&lt;fd&lt;.pi./2. At a frequency where fd equals .pi., the gain 
around the circuit, closed over the microphone and loudspeaker openings, 
must be less than 1.0 in order to avoid the occurrence of self-excited 
oscillations. This requirement is fulfilled by insertion of acoustic 
filters, in the form of resistive orifices or capillaries, and volumes, in 
the circuit. However, these filters further reduce the upper frequency 
range at which the circuit is effective. 
The invention may be better understood with reference to the attached 
drawings, not to scale, that represent certain embodiments of the 
invention. Clearly, the invention is not limited to the embodiments 
illustrated, but encompasses all of the technology that is disclosed and 
claimed herein, as well as variations and modifications that may become 
apparent to one of skill in the art who has read this disclosure. 
FIG. 1 is a schematic representation of an example of a fluidic amplifier 
10. Clearly, other designs are also useful. In the amplifier shown, there 
is a supply port 12 at one end for carrying a fluid through a throat 11 
into the amplifier body 14. The amplifier body 14 flares outward from the 
end of throat 11 to an opposite end of the body that includes two output 
ports 16a and 16b. Output ports 16a and 16b are separated by a V-shaped 
splitter 15 at the output end of amplifier body 14, with the apex of the 
vee oriented directly opposite, and in line with, a line of center L (in 
this case L is also a line of symmetry of the amplifier 10) of supply port 
12. Thus, fluid entering supply port 12, and moving through a throat 11 
into the amplifier body 14 in a straight line as shown by arrows, would be 
split in half by the vee so that one-half would enter each of the output 
ports 16a and 16b. In order to control the division of the fluid pressure 
between output ports 16a and 16b, the amplifier illustrated has a pair of 
opposed control ports 18a and 18b, disposed at right angles to fluid 
moving in a jet 13 through the body 14 of the amplifier from the supply 
port 12 to the output ports 16a, 16b. Thus, by varying the pressure of 
control fluid entering through ports 18a and 18b, the flow of fluid 
through amplifier body 14 may be deflected to control the amount of fluid 
entering output ports 16a and 16b. As the control port (18a, 18b) pressure 
is controlled by an acoustic signal, the output port (16a, 16b) pressures 
will reflect this pressure signal, with a time delay and a pressure gain. 
Additionally, the exemplified amplifier 10 shown has two pairs of opposing 
vents 17a, 17b and 19a, 19b, located on either side of the amplifier body 
14, that are at substantially ambient pressure. 
In order to simplify the analysis of a series of fluidic amplifiers, 
mathematical relationships have been developed. Moreover, in order to 
simplify the illustration of fluidic amplifiers conventional illustrations 
have also been developed. For example, FIG. 2 illustrates an example of a 
proportional fluidic amplifier 20 in a simple fluidic circuit, in 
accordance with the invention. Air supplied to the fluidic amplifier 20 
enters at supply port 22 and its acoustic modulation is controlled by 
fluid entering on opposite sides of the fluidic amplifier 20 through 
control ports 24a and 24b so that the output acoustic pressure appears 
amplified and reversed at output ports 26a and 26b. If this amplifier were 
the first stage of a multi-stage amplifier, it would be followed by 
another amplifier stage, with the two output ports 26a and 26b connected 
to the control ports of the next stage. If this is the last amplifier 
stage, then, in accordance with the invention, the output of the port with 
sound waves in phase with the first stage control port pressure is dumped 
at a sufficient distance from the fluidic circuit to prevent substantial 
interference with its function of controlling the acoustic impedance. The 
output of the other output port, out of phase with the sound waves at the 
first stage control port, is exposed to the environment where noise must 
be reduced. This output port is effectively the "speaker" that produces 
"counternoise," i.e., the out of phase sound. 
The acoustic pressure to be amplified is applied to volume 28 which acts 
like a capacitor. The volume is connected to control port 24a via 
resistive orifice 30. The combination of volume 28 and orifice 30 acts as 
a low pass filter 35, i.e., at low frequencies volume 28 is pumped up and 
its pressure is transmitted to control port 24a, while at high frequencies 
volume 28 is emptied after a pressurization, before the pressure has time 
to be transmitted to control port 24a. In addition, the figure shows the 
vent 36 as a clashed circle connected by 32 and resistive orifice 34 to 
the environment. Resistance 34 is large enough to substantially prevent 
transmission of sound pressure to the vent 36. 
While FIG. 1 has illustrated an apparent single fluidic element cut out of 
a sheet, more typically multiples of such fluidic elements will be cut out 
of a sheet. FIG. 3 illustrates an example of a sheet 100 having multiple 
cut-out fluidic elements 10, in this case fluidic amplifiers. As pointed 
out above, each individual cut-out fluidic element 10 may have the 
dimensions from about 5 mm to about 50 mm square. Consequently, a fluidic 
construct "wallpaper" for use in reducing or controlling the sound in an 
aircraft cabin would contain stacks of sheets that together have literally 
millions of cut-out fluidic elements. The back plate of the fluidic 
element construct would be equipped with supply tubes (not shown) attached 
to supply ports of its cut-out fluidic elements to supply the necessary 
fluid for operating the fluidic construct. The back plate would also be 
equipped with tubes to collect the fluid output from the dump ports. 
FIG. 4 is a schematic simplified representation of an exploded view of a 
stack 50 consisting of a plurality of sheets of fluidic elements that may 
be grouped together to form a controlled impedance construct, in 
accordance with the invention. Typically, a plurality of stacks of sheets 
of fluidic elements are grouped side-by-side to form an array in order to 
produce a useful fluidic construct. For simplicity, each of the planar 
sheets 40, 41, 42, 43, 44, 45, and 46 of the stack 50 has a single cut-out 
fluidic element, 40a, 41a, 42a, 43a, 44a, 45a, and 46a, respectively, 
although in practice each sheet will contain many such cut-out elements, 
as discussed above, with reference to FIG. 3. For simplicity, each sheet 
in FIG. 4 will be referred to as a "fluidic element" since the sheets have 
one fluidic element each Also as shown, the stack 50 of planar fluidic 
elements 40-46 includes a supply port 40b in the first element 40 of the 
stack 50, known as the "back plate." In the event that the planar stack of 
elements makes up, for example, a section of an acoustic wallpaper for an 
aircraft cabin, then the air supply for port 40b may be from the air 
conditioning system of the aircraft. Otherwise, another convenient source 
may be used. The fluid supply flows into the supply port 40b of the 
fluidic element 40 and thence into the supply port of fluidic element 41 
where it is divided into two outputs: 41c and 41d. The proportion of flow 
to each of those output ports 41c and 41d is determined by the pressure at 
control port 41e of amplifier 41a. Control port 41e is connected to 
"microphone" port m of face plate 46 of the fluidic stack via fluidic 
elements 45, 44, 43, and 42. The two output ports of element 41, 41c and 
41d, are in fluid connection with the control ports 43c and 43d of the 
next amplifier stage 43a, on sheet 43, via the transfer sheet 42 (i.e., 
through portals 42c and 42d, respectively). Note that fluidic amplifier43 
is supplied at port 43bb through portals 42bb, 41bb, and 40bb, which in 
turn is connected to the same supply of fluid as portal 40b. The output 
ports 43e and 43f of amplifier 43a are in turn in fluid connection with 
the control ports 45e and 45f of the final amplifier stage 45 via the 
transfer 44 (i.e., ports 44e and 44f, respectively). One output 45g (the 
"speaker") of the final amplifier 45a is connected to the environment via 
orifice p of face plate 46, while the other output 45h is dumped 
sequentially via orifices 44j, 43j, 42j, and 41j to dump port 40j of back 
plate element 40. As will be appreciated, the output of any stack may be 
successively amplified through a plurality of fluidic amplifiers before 
being output into the environment. The output of 45g, with its amplified 
and inverted (or "out-of-phase") acoustic pressure, then encounters the 
incoming sound wave, illustrated as 55, to attenuate that sound wave. It 
should be noted that the pressure at "microphone" port m is the residual 
pressure of the incoming sound wave 55 after being counteracted by the 
efflux from the loudspeaker port p. 
Typically, the function of the first few amplification stages is to amplify 
the pressure, while the function of the last amplification stage is to 
increase the fluid flow. For this purpose, the last stage might consist of 
one or more amplifiers in parallel. The aim of the last stage is to match 
the volume velocity of the incoming sound wave. 
In order to better understand this design requirement, an example will be 
given. Assume that a sound wave with 85 dB amplitude is normally incident 
on the fluidic construct. The peak particle velocity of that wave is then 
0.0027 meters per second. Assume further that the steady flow through the 
last amplifier stage can be modulated with acoustic pulsations at .+-.30%. 
Then, this steady flow would have to be 0.009 meters per second. If the 
repeating-unit stack area is 0.0001 sq. meters, and the two amplifiers are 
used in parallel, then the volume flow through each of the last two 
amplifier stages is 9.times.10.sup.-7 m.sup.3 /sec. 
In order for the last amplifier stage (45 in FIG. 4) to produce this amount 
of flow, the preceding amplification stages have to amplify the residual 
sound pressure by a factor of about 10 to about 1,000, and most typically 
a factor in the range about 50 to about 500. Each amplification stage 
increases the sound pressure by a factor of about 4 to about 25, depending 
on local feedback in the amplifier, as will be discussed below. The 
thickness of the fluidic element construct may typically vary between 1 mm 
and 5 mm, but other thicknesses may also be useful in specific 
applications. The number of sheets making up the construct will typically 
vary between 10 and about 50. The unit stack of the construct would be an 
approximately square area, with a side of 3 mm to 100 mm, or most 
typically, from about 5 mm to about 50 mm. The smaller the side of the 
unit area, the greater the high frequency limit of performance of the 
construct. A construct with parameters like these would be able to 
attenuate sound waves in the frequency range about 0.1 Hz to about 2,000 
Hz, and most typically in the range about 1 Hz to about 400 Hz. 
FIG. 5 is a schematic representation of a plurality of series of cascaded 
fluidic elements, such as those illustrated in FIG. 2. As shown, each of 
the fluidic amplifiers 20x, 20y, and 20z have an input supply of fluid 22, 
two control ports, and two output ports. Following the diagram from left 
to right, the outputs 20b and 20c from the first amplifier 20x are 
amplified in the second amplifier 20y, and its outputs 20d and 20e are in 
turn further amplified in the third amplifier 20z. Clearly, many more than 
three amplifiers may be cascaded, depending upon the specific application. 
As before, the acoustically amplified output 26a (or "speaker") from the 
third (last) amplifier 20z is exposed to the environment where noise must 
be reduced, for example the interior of an aircraft. The environment is 
also connected to one control port of amplifier stage 20x (equivalent to 
the microphone port of FIG. 4). The other output 26b is directed away from 
the zone of interaction between the amplifier output and the environment, 
and is preferably dumped at a distance from the interaction zone to 
minimize interference with the output from 26a. As is evident to those 
versed in the art of designing fluidic amplifier circuits, elements of 
resistance and volume (shown as 28) may have to be added at various points 
in the circuit in order to achieve pressure biases necessary for all 
amplifier stages to operate within the linear range. 
FIG. 6 is a schematic illustration of a further embodiment of the noise 
reduction constructs of the invention. This construct represents a muffler 
75, in which the array of fluidic stacks is arranged in a cylindrical 
rather than an essentially planar shape. As shown, the construct includes 
a tubular body 70 surrounded by a cylindrically coiled array of fluidic 
element stacks 72, located around the mid-section of the tube 70. As 
before, the fluidic elements include a plurality of cascaded amplifiers 
for amplifying the acoustic pressure at the construct surface within tube 
70. Pressurized fluid is supplied to the construct through tube 74. This 
supplied fluid is modulated acoustically by the pressure in tube 70, and 
the resulting countersound again emerges into tube 70, to cause sound 
attenuation. The unwanted sound from the final amplifier output port is 
dumped into tube 76, which leads that sound, and the accompanying steady 
flow, back into the central tube 70. Tube 74 may join tube 70 either 
upstream of the fluidic array or downstream (shown in broken lines), as 
shown in FIG. 6. Alternatively, the unwanted sound may be dumped in tube 
78 to a remote location. 
The fluidic arrays may consist of a planar array which has been bent into a 
cylindrical shape, or may consist of stacks formed by continuous sheets of 
fluidic elements wound around a central tube 70. The fluidic elements of 
the stack arrays, cylindrical or essentially planer, may be complemented 
by purely passive sound-absorbing elements in order to effect the 
stability of the active fluidic circuit and to increase the frequency 
range of attenuation beyond the frequency range of the fluidic array by 
itself. An example of such a design will be shown among the examples 
discussed below. 
The invention also provides methods of attenuating sound waves in an 
environment, methods of reducing sound radiation from a vibrating object 
into an environment surrounding the object, methods of reducing 
sound-induced vibration of an object in a noisy environment, and methods 
of absorbing sound waves that would otherwise be incident on an object. 
The latter methods of absorbing sound include the steps of interposing a 
fluidic construct of the invention between the sound waves and the object 
to be protected from sound waves. Pressurized fluid is continuously 
supplied to supply ports of the fluidic construct. Simultaneously, sound 
pressure of sound waves to be absorbed is continually sensed at input 
ports of the construct. Thus, the sensed sound pressure is continuously 
modulated to generate sound waves that are out of phase with the sensed 
sound waves, i.e., countersound waves. The fluidic construct continuously 
outputs a sufficient quantum of fluid having countersound waves in the 
vicinity of the object being protected from sound waves in the 
environment, to substantially reduce the sound pressure in the environment 
and thereby the pressure of these sound waves on the object. 
In order to reduce sound radiation from a vibrating object, a similar 
procedure is followed, except that the continuous countersound output from 
the fluidic construct of the invention is in the vicinity of the vibrating 
object and essentially cancels out the sound radiation from the vibrating 
object. Thus, there is a substantial reduction of noise transmission from 
the vibrating object into its surrounding environment. Likewise, 
sound-induced vibration of an object may be reduced by continuously 
outputting a sufficient volume of amplified fluid from output ports of a 
fluidic construct according to the invention, in a location adjacent to 
the surfaces of the object that would otherwise be exposed to the noisy 
environment. This reduction in sound in the environment able to impact 
upon the object causes significant reduction in sound-induced vibration 
excitation of the object. 
Thus, the invention provides not only fluidic constructs in a wide range of 
geometries suitable for specific applications to reduce noise, but also to 
reduce sound-induced vibration of objects, radiation of sound from objects 
into an environment, and for absorbing sound waves that might otherwise 
impact on an object. In addition, the fluidic constructs of the invention 
offer, for the first time, the capability of controlling broad wave band 
sound over a wide range of frequencies, ranging from about 0 to about 
2,000 Hz. The control of such broad band sound, or noise, is generally 
regarded as not feasible with the use of electronic microphone and speaker 
systems, which would require literally thousands of such devices. 
The following examples illustrate specific embodiments of the invention, as 
described above and claimed herebelow. These examples are for illustrative 
purposes, and to facilitate understanding of the invention, and do not 
limit the scope of the invention. 
EXAMPLES 
The individual components of a fluidic amplifier circuit may be modeled 
with groups of standard components that are used in conjunction with the 
EASY5 (Engineering Analysis System 5) software that is provided by The 
Boeing Company of Seattle, Washington. A simulation using this software 
yielded the following observations and results which may provide useful 
guidelines to design low-impedance constructs of the invention for 
specific applications. Clearly, however, the invention is not limited to, 
or by, the following simulation examples. The examples illustrate 
conventional transfer function analysis of the open loop (for stability) 
and the closed loop) (for performance). 
The first application is a trim panel, such as may be used in a jet 
aircraft, that has low radiation efficiency. The panel is designed to have 
an impedance of the order, or smaller than, the characteristic impedance 
.rho.c of the medium into which it radiates. If the panel impedance is 1 
.rho.c, then the noise from a vibrating panel will be from about 6 to 
about 10 decibels lower than that from a hard panel, depending upon 
whether the radiation is primarily in the form of plane waves normal to 
the panel, or in a diffuse field in all directions from the panel. 
The second application is a duct muffler, for example, an auxiliary power 
unit exhaust, or an air-conditioning duct. In general, in jet aircraft 
low-frequency air conditioner noise is generated in the forced, turbulent 
mixing of compressed air from the engines outside air, and recirculated 
cabin air. The amount of attenuation cannot be directly calculated by the 
use of the EASY5 software, but the impedance output from this program can 
be used to predict performance using existing duct-acoustic programs. 
The basic amplifier model selected is shown in FIG. 7, although other 
models may also be useful in certain applications. A summing amplifier 85 
was selected in order to allow an additional feedback path within the 
stage to boost the gain, as discussed below. Pressure amplification 
through gains 84a and 84b respectively were assumed to be a factor of 
four, from the first control port 80a and a factor of three from the 
second control port 80b. Corresponding time delays 86a and 86b were 
assumed to be 0.07, and 0.06 milliseconds, respectively. The time delays 
were modeled with an 8th order Pade approximation, i.e., the ratio of two 
8-order polynomials in the s-plane with unit magnitude. This provides a 
good linear approximation of the phase over the entire frequency range of 
interest (0 to 1,000 Hz). The Outputs were summed in 88 for output 89. 
There are also input and output impedances, as well as small volumes at 
each port, that introduce phase lags, to consider. These were modeled as 
first order low pass filters 82a, b, with unit gain in the pass band, and 
a variable time constant. The filters were combined into a single filter 
at the control port. While there is a minimum time constant set by the 
volumes and the impedances, a larger constant may be selected if filtering 
for circuit stability is desired, by adding to the resistance by use of 
smaller orifices or adding to the volumes. 
A final stage amplifier, as modeled, is shown in FIG. 8, with the EASY 5 
symbol 95 shown above the connection of the circuit elements. Here a 
pressure-amplification factor is not appropriate due to the small output 
load impedance. As can be seen, in this case an amplifier with a single 
control port pair was selected, since pressure feedback was not practical. 
The signal from the control port 90 is filtered through input filter 92, 
amplified in gain 94, and time delayed in delay 96 to produce an output to 
output port 98. 
A connected five-stage system is shown in FIG. 9A. This circuit is 
appropriate for analysis of an aircraft interior trim systems performance. 
The sound from the primary source 100 is mixed at the microphone port 102 
with the counternoise from the counternoise output of the circuit via the 
feedback 110, through the acoustic space at the trim surface. The residual 
noise is fed through the four pressure amplification stages 104 (of type 
shown in FIG. 7), and then to the flow amplification stage 106 (of type 
shown in FIG. 8) to emerge into the environment, symbolized with the 
radiation impedance 108, which has been assumed to be 1 .rho.c. It has 
been assumed that the output load impedance on amplifier 106 is 
negligible. The signal from this output is delayed by the propagation time 
from the loudspeaker port to the microphone port, which are assumed to be 
0.01 meters apart. The open loop gain is measured from the 
summing-junction 102 output 103 to the top input 109 of the same summing 
junction; and the closed loop performance is measured from the left input 
101 of the summing junction to its output 103. 
The open loop gain is shown in FIG. 9B. The component parameters have been 
adjusted such that there is 10 dB gain margin where the phase around the 
loop is 180.degree.. The phase margin at zero loop gain is 90.degree.. The 
corresponding closed loop performance is shown in FIG. 9C. The component 
parameters assumed to achieve this performance are as follows: for each 
pressure amplification stage in assembly 104, an amplification by a factor 
of 4, time delay 0.07 ms, and low pass corner frequency of 10,000 Hz. For 
the flow-amplification stage 106 in FIG. 9A, a transfer admittance of 
3.2.times.10.sup.-8 cubic meters per second per newtons per meter square, 
time delay 0.07 milliseconds, and a low pass corner frequency of 80 Hz 
have been assumed. Somewhat better performance in the attenuation band 
could be obtained with smaller margins, but then the out-of-band 
amplification would be greater. 
A method for reducing the number of fluidic amplifier elements in the stack 
circuit is explained below. Such a design lead to a thinner stack and may 
therefore reduce the bulk, weight and cost of the construct. By adding a 
positive feedback loop around each pressure amplifier the gain may be 
stably boosted, as long as the loop gain is less than 1. In FIG. 10 the 
amplifier 112 has a gain of F.sub.1 from input 111a to output 114 and a 
gain F.sub.2 from input 111b to output 114, without feedback impedances 
Z.sub.1 (116) and Z.sub.2 (118) connected. With Z.sub.1 and Z.sub.2 
(typically resistive orifices) connected, part of pressure P.sub.2 at 
output port 114 is sensed at input port 111b. This part is .beta.=Z.sub.1 
/(Z.sub.1 +Z.sub.2). The pressure P.sub.2 at output 114 will therefore be 
a sum of the pressure P.sub.1 at input port 111a amplified by gain F.sub.1 
and the fed back pressure at input port 111b, amplified by gain F.sub.2. 
Therefore, P.sub.2 =F.sub.1 P.sub.1 +.beta.F.sub.2 P.sub.2 or P.sub.2 
=(F.sub.1 /(1-.beta.F.sub.2))P.sub.1. Without feedback, the relation would 
be P.sub.2 =F.sub.1 P.sub.1. With the arrangement shown in FIG. 10 gain is 
thus boosted by a factor of 1/(1-.beta.F.sub.2). The time delay associated 
with the travel distances and the capacitances associated with the volumes 
of the feedback loop must be considered in calculating Z.sub.1 and 
Z.sub.2, but as long as .beta.F.sub.2 is not equal to 1, the circuit is 
stable. 
In the EASY5 modeling of the feedback, illustrated in FIG. 10, it was 
assumed that the feedback is made to the second control port pair 80b in 
FIG. 7 which has a smaller gain 3. Z.sub.1 is the second control port 
input impedance, and Z.sub.2 is an appropriate orifice resistance. 
It should be appreciated that variations in the performance of the fluidic 
circuits can be accomplished by appropriate filtering at the amplifier 
inputs. If band pass filtering is used, instead of low pass filtering, the 
frequency region of useful performance can be extended upward, at the 
expense of some low-frequency performance drop. The realization of such 
filters using resistive and volumetric elements are apparent to those 
versed in the art of acoustic filtering. 
FIG. 11A illustrates schematically a pressure-amplification stage with 
feedback boost. Essentially, FIG. 11A is a combination of the circuit 
shown in FIG. 7 and the circuit of FIG. 10, with an associated delay in 
the feedback loop. The benefits of such a system include a thinner 
construct due to fewer fluidic elements in the stack but they are bought 
at a reduce high-frequency performance of the circuit. 
FIG. 11B is a graphical representation of the output of the circuit of FIG. 
11A. FIG. 11B clearly shows that the gain from first control port 140 to 
output port 142 is greater (20 dB) than it would be without the feedback 
via second control port 144, in which case it would be a factor of 4 (12 
dB) of gain block 146. Due to the time delays in the circuit, the gain 
boost persists only up to a few hundred Hz. 
FIG. 12A illustrates a simplified schematic of a muffler lining where 
active 120 and passive 122 lining elements have been combined, and its 
corresponding acoustic performance is shown in FIG. 12B. The passive 
lining 122 has a two-fold purpose. Firstly, it provides damping of the 
feedback from the active lining microphone ports to its loudspeaker ports. 
Secondly, it provides attenuation at frequencies above the attenuation 
band of the active lining. 
The active lining elements 120 shown in FIG. 12A occupy about one-half of 
the total lining surface and face the sound waves 128 to be controlled. 
The active lining elements 120 consist of stacks of fluidic elements 
substantially with the configuration shown in FIG. 9, except that only two 
pressure amplification stages are used. Each of these stages has the 
configuration shown in FIG. 11A. In addition, the face plate 125 of the 
stack is covered with a resistive sheet of impedance 4 .rho.c. It is 
understood that this resistance is averaged over the whole stack area, 
i.e., if the loudspeaker ports occupy five percent of the total stack 
area, then the resistance in front of the loudspeaker ports is 5% of 4 pc. 
The passive part 122 of the lining consists of a resistive face of sheet 
126 of impedance 1 .rho.c, over an array of cavities 124 of depth d of 
about one inch, that space the passive and active elements from the 
muffler housing 130. Note that the cavities occupy the space under the 1 
.rho.c base sheet 126, as well as the space under the active lining 
elements 120, which have been assumed to be 0.25 inches deep. 
The performance graph FIG. 12B gives an estimate of the attenuation of the 
configuration of FIG. 12A per unit length, equal to one diameter of the 
duct in an air conditioning muffler. The muffler was assumed to have a 
cross section with internal diameter of 11 inches. 
While the preferred embodiments of the invention have been illustrated and 
described, it will be appreciated that various changes can be made therein 
without departing from the spirit and scope of the invention as described 
above and claimed hereafter.