US3550720A - Multiple wind screen noise attenuation system - Google Patents

Description

United States Patent Harold N. Ballard;

Miguel lzquierdo, El Paso, Tex.

Sept. 24, 1968 Dec. 29, 1970 The United States of America as represented by the Secretary of the Army. by mesne assignments Inventors Appl. No. Filed Patented Assignee MULTIPLE WIND SCREEN NOISE ATTENUATION SYSTEM 9 Claims, 8 Drawing Figs.

181/33. 181/05; l79/l78 Int. 11/04; H04m 1/19 Field 0fSearch.... l8l/33,31, 31.1.31A,0.5:179/178,l84,ll5.5(Late):55/276 References Cited UNITED STATES PATENTS 8/1936 Orem 8/1950 Anderson et al. 2,556,168 6/1951 Cragg et al.. 3,265,153 8/1966 Burroughs Primary Examiner-Stephen J. Tomsky Attorney-Kimmel, Crowell & Weaver ABSTRACT: A wind screen comprising a plurality of concentric cylindrical or hemispherical screens around a low frequency sound transducer for reducing or eliminating random noise developed by wind in the vicinity of the transducer to permit reception of low sonic and infrasonic waves is disclosed.

PATENTED UEEZQ 19m SHEET 1 or 3 l/V VE N T0196:

44/14/1454 Clean Eu F wamse MULTIPLE WIND SCREEN NOISE ATTENUATION SYSTEM BACKGROUND OF THE INVENTION This invention relates to acoustical sound attenuating devices for reducing the effect of wind on signals picked up by sound transducers. More particularly, this invention relates to an improved screen system for reducing or eliminating the effect of wind on signals picked up by microphones.

Screens or shields for microphones are well known in the prior art. Exemplary of devices of this type are those described by Rettinger in U.S. Pat. Nos. 2,325,424 and 2,346,394. These devices are effective in reducing the effect of air currents for microphones used in sound recording, radio broadcasts, and the like. At these frequency levels some improvement can be achieved using devices of this type. However, such devices have been found to generate, rather than reduce, undesirable random noise in the low sonic and infrasonic ranges of, for example, 0.5 to I Hz. Where a microphone is sensitive only at higher frequencies or has reduced sensitivity at low sonic frequencies, wind screens may be designed for microphones such that noise generated by the wind screens is in the very .low sonic or infrasonic ranges. For example, as described by Anderson et al. in U.S. Pat. No. 2,520,706, a wind screen which produces turbulent noise below 130 Hz. is still effective to reduce unwanted wind noise in the range of 130 to 330 Hertz, to which the microphone is sensitive.

Hemispherical wind screens separated by a packing material have been used to reduce the effect of wind on microphones used for radio broadcasting and public address purposes which operate in the range of 50 to I00 Hz.; see U.S. Pat. No. 2,623,957, Cragg et al. Devices of this type are not, however, capable of operating effectively in the infrasonic frequency range. It has been established that, at infrasonic frequencies, such a device will actually increase the turbulent noise level and is actually detrimental to performance.

Many screen systems have been proposed for absorbing sound, see, for example, Beranek, U.S. Pat. No. 2,595,047 and Orem, U.S. Pat. No. 2,050,581. Where it is the object to receive certain sounds, however, devices of this nature actually tend to defeat the purpose of the system. Such devices, as described in the aforementioned patents, have application to other systems, however.

While windscreen systems have been proposed for use in conjunction with standard microphones used in radio and television'broadcasts, public address systems, etc., heretofore no systems of this type have been found satisfactory in the very low sonic and infrasonic range. Indeed, most devices of this type have been found actually to be detrimental in this range. Transducers for detecting signals in the very low sonic and infrasonic range are useful in detecting distant explosions and the like which produce signals which are barely audible or inaudible to the normal human ear.

Various devices and systems have been proposed for improving the signal-to-wind-noise ratio of signals picked up by microphones which are constructed for use in the frequency range below I c.p.s. Such microphones are useful for the purpose of studying atmospheric pressure oscillations in this range. One such device, is described by Daniels, Noise Reducing Line Microphone for Frequencies Below 1 Cycle Per second," the Journal of the Acoustical Society of America, Volume 31, pages 529-31, (I959). Devices of this type are a very significant improvement over the devices theretofore proposed. It is, however, an object of this invention to provide a system for providing even greater signal-to-windnoiseratio improvements than are possible by the aforementioned line microphone design.

SUMMARY I transducer. The multiple wind screens are so positioned and arranged and are so constructed as to produce a series of attenuation steps wherein turbulent wind motion is converted to heat and dissipated according .to principles set forth hereinafter. It is, therefore, a principal object of this invention to provide an improved wind screen system for reducing the effect of turbulent noise generated by moving'fluid on transducer signals.

A more specific object of this invention is to provide a means for reducing or eliminating the effect of wind flow on signals received by a transducer in the low sonic and infrasonic ranges.

Still more specifically, it is an object of this invention to provide an improved wind screen system for increasing the signalto-noise ratio of transducers operating in the frequency range below 50 c.p.s.

An additional object of the invention is to provide a wind screen system having critical spacial dimensions.

Yet an additional object of the invention is to provide a wind screen system of critical configuration and construction.

A further object of the invention is to provide a wind screen system for use in conjunction with a transducer in receiving very low sonic and infrasonic signals for studying atmospheric oscillations in this frequency range.

Other objects of the invention will become apparent from the specification which follows and from the drawings to which reference is now made.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is an exploded view showing a preferred embodiment of the present invention.

FIG. 2 is a top plan view showing the concentric arrangement of the multiple wind screens constituting the noise attenuation system of this invention.

FIG. 3 is a partial side elevational view showing the concentric and vertical relationship of the multiple wind screens comprising this invention.

FIG. 4 shows an alternative construction of the present invention in which the wind screens are hemispherical in form and is shown in partial elevation.

FIG. 5 is a graph illustrating the random noise reduction achieved by the wind screen system of this invention.

FIGS. 6a, 6b, and 60 show a comparison of, respectively, a signal produced by the explosion ofa cannon using an unprotected transducer, a transducer protected by the system of this invention, and a transducer of the type useful in this frequency range described in the prior art.

Since the actual construction of wind screen systems of this invention are mechanically relatively simple in construction, the system of this invention will be described before discussing the theory and the development which lead to the ultimate design which was adopted. 7

Reference is made first to FIGS. I, 2, and 3. A preferred embodiment of this invention comprises a plurality of wind screens A,-B, C, D, and E, concentrically arranged about the transducer, normally a microphone, shown at M in FIGS. 2 and 3. Without contemplating any limitation'of the invention, in a specific embodiment of the invention the largest screen, A, may desirably be approximately I20 inches in diameter and slightly less, 114 inches for example, in overall height. This wind screen is constructed of a plurality of circular support rods 10-20 arranged vertically in successive horizontal planes for supporting the sidewalls of the housing. A domeshaped top is supported by a series of successively smaller circular support rings 24-28. The circular supports 10-20 are vertically supported by a plurality of longitudinal rods such as those shown at 30-32 while the dome-shaped cover support rings 24- 28 are supported by arcuately curved elongate support rods of the type shown at 34 and 36 which are, in turn, supported by rods of the type indicated at 30 and 32. The support rods may be either double, as shown at 30, for the purpose of constructing the wind screen in segments which may be joined at the site pr may be single as shown at 32.

The cagelike construction of each of the wind screens is generally the same as that described with respect to screen A, as will be apparentfromFlGS. 1 to 3. For example, screen B comprises circular supports of the type shown at 38, vertical supports of the type shown at 40, and arcuate top supports of the type shown at 42 although, since the size of the screen is smaller, circular supports in the dome-shaped top may be omitted if desired. Screen C is of similar construction, comprising circular support 44, vertical support 46, and arcuate top'support 48. Wind screens D and E are of similar construction comprising, respectively, circular support 50, vertical support 52, and arcuate support 54 in wind screen D and circular support 56, vertical support 58, and arcuate support 60 in wind screen E. Each of these wind screens, as shown, for example, in FIG. 2, is coaxially arranged with respect to the transducer, microphone M.

The support members in screen A may conveniently be constructed of metal tubing, such as .688 steel tubing welded together. The support structure of screens B, C, D, and E are conveniently made of .500 by .125 steel strap with the base ring support being made of 1.0 by .125 steel strap.

It will be apparent that the foregoing embodiment of the structural supports for the wind screen are merely exemplary of the type of constructions which may be used and any type of support which will form the desired configuration would be perfectly equivalent for purposes of this invention. For example, rigid, plastic, wood, fiberglass, and other types of construction could conveniently be used and could be equivalent for all relevant purposes.

The important feature of this invention is in the provision of metal or textile fabric covering the structures just described. In a typical embodiment, for example, .125 mesh hardware cloth forms a covering over the structure of screen A, as indicated at 62. 14 by 18 wire screen (14 wires per inch in one direction and 18 wires per inch in the other direction) may conveniently be used for the wall and domed top of screens B, C, and D. Fabric of the type conventionally used to cover speakers in sound systems is appropriately used to form the domed top and walls of wind screen E. For example, a fabric identified as Impala cloth manufactured by Columbia Mills Inc., Syracuse, N.Y., used for automobile seat covers and speaker system covers has been found to be satisfactory for the present invention.

Because of the mechanical simplicity of the construction and configuration of the wind screen system, the foregoing description is adequate to permit one skilled in the art to construct such systems based upon the considerations to be described hereinafter. The distinguishing feature, mechanically, ofthe invention is the provision ofa plurality of cylindrical wind screen systems, preferably having a domed top, arranged coaxially on a planar surface about a microphone or other transducer arranged on such surface.

The configuration of FIG. 4 is similar in all essential respects to the configuration just described except that the successive wind screens, three of which are shown, are hemispherical in configuration and are concentrically arranged about the microphone M. In this construction three, for example, concentric hemispherical screens A, B',.and C are constructed using arcuate and circular support members and screen coverings of the type described. Given the configuration, any skilled mechanic can easily construct the individual wind screens and assemble them as illustrated in FIG. 4. In a specific embodiment of this configuration which was found to be successful, the respective dimensions of screens A, B, and C were 106 cm. diameter, 84 cm. diameter, and 62 cm. diameter. Screens A' and B were formed ofa support made of ls-inch copper tubing and galvanized wire covered with 014 mesh-brass screen while screen C was constructed similarly insofar asthe frame was concerned, but was covered with conventional acoustic cloth designed to pass frequencies to 10,000 c.p.s.

Quite apparently, the embodiment of FIG. 4 is smaller in dimension than the embodiment of FIGS. 1 to 3 wherein screen A is, in the specific embodiment described, 120 inches in diameter, screen B is 46 inches in diameter with a height of 52 inches, screen C is 34 inches in diameter with a height of 38.5 inches, screen D is 28 inches in diameter with a height of 31.5 inches, and screen E is 18 inches in diameter with a height of 22.25 inches. The significance of these diameters will be considered in greater detail hereinafter.

Clearly, the mechanical construction'of these devices is relatively simple from the foregoing disclosu re; but it was far from apparent at the outset of this research-that wind screen systems would be successful. IndeedQthe prior' art suggested that in the frequency ranges of greatest interest windscreens were not only not successful 'in reducing random noise generated from air flow, but that" suchsystems actually increased random noise and lowered 'the signal-to-noise ratio.

Based upon certain theoretical considerations and experiments, however, it was determined that, using proper design techniques, a wind screen system of the'type described he rein could be developed to give quite an expected improvement in signal-tomoise ratios.

DISCUSSION Acoustic signals in the atmosphere which may be detected by a microphone may be classified either as coherent pressure changes or extraneous random pressure fluctuations. The in discriminate reception of both types of acoustic signals by a microphone interferes with the reception of the coherent signal from which intelligence is derived or through the analysis of which atmospheric studies and studies of pressure producing phenomena may be made. The random pressure fluctuations are generally classified as acoustic noise.

The frequency of the acoustic pressure fluctuations may range from a fraction of a cycle per second to several thousand cycles per second. If the frequency lies below 20 c.p.s. (Hertz), then-the signal is said to be in the infrasonic range. If the frequency of the signal is above 20,000 Hz., the acoustic signal is said to be in the ultrasonic range.

One means of excluding acoustic noise is to limit the range of acoustic frequencies to which the microphone is sensitive. This is done in many types of microphones. If the acoustic noise lies within the same frequency range of the acoustic signal to be detected and from which intelligence is to be derived for studies to be made, then some other means must be sought to eliminate the acoustic noise or to reduce the level of the noise significantly below the level of the signal to be detected.

This latter condition exists with regard to microphones which are utilized in connection with the multiple wind screen system of this invention. These microphones were designed to operate at the earth's surface in an outside environment. The frequency range of these microphones is from 0.] to Hz.

A detailed study of the characteristics of the acoustic signals recorded in this frequency range by these microphones was undertaken. From these studies it was determined that there were several sources of acoustic noise and that the primary source of such noisewas associated with wind flow. It was further established'that two types of wind noise in the frequency range of 0.1 to 100 Hz. existed. The first type of noise was associated with pressure variations produced by the inte raction of the air stream, the wind, with the surface terrain inthe vicinity of the transducer. The other type of noise was generated by interaction of the air stream with the transducer itself, that is, the interaction of the air stream with the boundaries of the recording microphone proper. This latter source of noise was determined to be the more predominant of these sources of noise.

From these studies and observations, it was determined that the acoustic microphone noise'in the 0.1 to 100 Hz. frequency range, with the microphone in an outside environment, could be greatly reduced by inhibiting the flow of air over the boun daries of the acoustic sensing system. It was not at all obvious, however, how this could be accomplished without excluding or badly distorting the desired signal which was within this frequency range. A review of the literature related to the reduction of microphone wind noise was made and it was found that various types of baffles or screens had been utilized to prevent the flow of air over the acoustic sensing device in public address system microphones and in radio broadcast system microphones. In all of these devices, however, in inhibiting the air flow over the microphone, the protective devices actually generated acoustic pressure variations which were within the desired frequency range, 0.1 to 100 Hz.

Various rather acoustically complicated multiport microphones had previously been designed and used to reduce infrasonic acoustic noise, such as that described by Daniels, A Noise Reducing Line for Frequencies Below 1 Cycle Per Second" Journal of the Acoustical Society of America, Volume 31, pp. 529-31, (1959). A line, similar to that of Daniels, was used in connection with the wind screen reduction study. A comparison of the noise reduction capability of this type of complex multiport system is compared later herein with the experimental wind screen noise reducing device of this invention.

Three points should be fully understood in understanding the operational principle of the Multiple Wind Screen Systems described herein for the reduction of infrasonic wind noise.

First, a clear distinction must be made between those physical phenomena which are classified as acoustic and those phenomena which are classified as hydrodynamic. Acoustic pressure variations are relatively small pressure changes which are superposed on the ambient pressure existing within the medium, the atmosphere in the present case, and which are transmitted through the medium molecule to molecule by elastic forces brought into existence by the pressure fluctuation. There is no net movement of the particles of the medium, but rather simply random motion of the molecules of the medium.

Contrasted with this, pressure changes which are imposed on the medium and which cause a net movement of the particles of the medium from one point to another point are said to produce hydrodynamic flow. For example, when a person purses his lips and blows strongly another person a short distance away will very shortly thereafter, depending on the distance between the persons and the speed of sound in the medium, hear the sound generated by the interaction of the air stream with the first person's lips. Sometime after this event, the heater will detect the flow of air past him as caused by the pressure difference produced by the first persons blowing.

Secondly, viscosity forces act within a fluid, the air in this case, to dissipate hydrodynamic energy. It has been experimentally established that adjacent layers of the fluid, air, when moving at different velocities relative to each other generate frictional (viscous) forces between the adjacent relatively moving fluid layers. The greater this velocity difference, the greater is the viscous force. The mathematical relationship is stated by the following equation:

where F, is proportional to the velocity gradient, V represents the velocity gradient and where n is the coefficient of viscosity for the medium under consideration.

The velocity gradient is the change in the fluid velocity which takes place in a given distance. Thus, if each particle of moving fluid is moving at the same velocity, there is no dissipation of energy through fluid friction. An example of this would be the case of a steady wind. If this flow pattern can be modified, however, so that large velocity differences exist between the various particles of the fluid, then large amounts of flow energy will be converted to heat energy with little flow energy being converted to acoustic energy.

Third, it has been established experimentally that when fluid flow becomes turbulent, fluid eddies exist within the fluid with adjacent fluid layers moving at different velocities. The

smaller the eddy size, the larger is the velocity difference between adjacent fluid layers, i.e. the velocity gradient is larger. These large velocity gradients are associated with small eddies which, as will be seen from the equation above, generate large fluid frictional forces which, in turn. reduce the fluid velocity by reason of the conversion of the kinetic energy of flow to heat energy with a resultant increase in temperature of the fluid, but without the generation of acoustic energy.

Fourth, it has been determined experimentally that the eddy size in a turbulent flow is approximately the same size as the dimension, the characteristic length, of the region in which the flow takes place.

Fifth, a flow becomes turbulent when the Reynolds number, defined by the following equation, exceeds a particular value:

where p is the fluid density, v is the flow speed, and 1 is the characteristic length of the flow region; 1;, of course, is the fluid viscosity.

Based upon the preceding considerations, it seemed feasible to at least inhibit, and possibly to prevent, the flow of air over a given acoustic detection system by interposing a fine wire or cloth mesh in the air stream to break up the existing flow pattern into a flow pattern consisting of very small turbulent eddies which would dissipate the wind energy in the form of heat. The number of screens necessary to reduce the wind speed to zero, or to a tolerable figure, is determined primarily by the initial wind speed.

The correctness of these determinations was proved by an experiment conducted in a wind tunnel with a sensor in the form of a hot-wire anemometer. Three concentric hemispheric screens which were covered with an acoustic cloth conventionally used in the covering of loudspeaker enclosures, as previously described, were utilized. The acoustic cloth was designed to pass frequencies up to 10,000 Hz. The experiment was conducted by recording the signal generated by variations in the temperature of the anemometer and by sequentially interposing the several screens. The results of this experiment are shown in FIG. 5.

As shown in FIG. 5, the record indicates that at time t to laminar air flow at a speed of 30 miles per hour existed in the wind tunnel. At time t the smallest of the three screens, screen C, was placed over the anemometer with the anemometer at the center of the screen base. At time t the second screen, screen B, was placed over the anemometer in a position concentric with the first screen. At time t;, the largest of the screens, screen A, was similarly placed over the first and the second screens.

A study of FIG. 5 indicates that when the smallest screen was placed over the anemometer the air speed dropped from 30 miles per hour to approximately 15 miles per hour and that the flow pattern became quite turbulent. At the time the second screen was placed in position, the flow speed was reduced to an average of 3 miles per hour with some larger scale turbulence remaining. When the third screen was placed in position, the flow speed was reduced to essentially zero with some small fluctuations not exceeding 0.3 miles per hour remaining.

While these wind tunnel tests verified that the wind screen systems effectively reduced the wind speed, these tests in no way indicate to what degree an acoustic signal would be attenuated by the screen system. Tests were, therefore, carried out in an anechoic chamber in the frequency range of between 5 and 5,000 c.p.s. Acoustic pressure levels were determined with and without the three wind screens over the recording microphone. From these tests it was determined that there was no detectable attenuation of the acoustic signal due to the screen system in this frequency range. Thus, the principle could be extended to higher frequencies than 0.1 to Hz. if desired. The desired acoustic signal will be attenuated, however, when the wave length of the sound approaches the distance between two successive wires in the screen mesh.

Field tests were then conducted to determine the effectiveness of the screen system in reducing the microphones wind noise under actual wind conditions and to determine the screen geometry and configuration which would be most effective. Two screen systems were constructed, one in the form of concentric cylinders, the other in the form of concentric hemispheres Both systems initially consisted of a three screen configuration with the two outermost screens formed by a 14 copper wire mesh while the innermost screen was formed from the acoustic cloth previously described. The wind screens, an open microphone, and a noise reducing line (similar to that described by Daniel) were set up in adjacent locations. The acoustic transducers were capacitor microphones having the same frequency response and sensitivity. The frequency response of these microphones is given in Table I.

TABLE I.FREQUENCY RESPONSE OF CAPACITOR MICROPHONES Sensitivity Frequency (Hertz): (volts/dyne/cmfl) 0. 3 0. 25

2. 0 0. 40 Above 2. 0 0. 40

The pressure variations, as recorded by these sensors, are shown in FIGS. 6a, 6b, and 60, under conditions in which the average wind speed was 12 miles per hour with gusts up to 12 miles per hour as determined by the anemometer.

The signal appearing in these FIGS. was produced by the firing of a small cannon placed a distance of approximately 200 yards from the recording systems.

A comparison of these records indicates that the wind noise as recorded by the microphone placed within the screen was very much reduced in comparison with the wind noise recorded by the microphone associated with the noise-reducing line and by the open microphone. The firing of the cannon was distinguishable from the acoustic noise generated by the wind flow in the unprotected microphone, FIG. 6a, but little information could be gained therefro'm. in contrast, the cannon firing is very pronounced in the signal generated by the wind screen protected microphone of this invention, FIG. 6b. There was noticeable improvement using the noise-reducing line microphone system, FIG. 6c, but the signal-to-noise ratio of the wind screen system protected microphone, 6b, is very greatly improved as compared with either the unprotected microphone system or the noise-reducing line system.

Since the wind noise generated by the flow of air over the recording device is a function of wind speed, the ratio of wind noise generated over the open microphone to the wind noise generated in the screened systems increased with the wind speed until at a wind speed of miles per hour the noise generated in the screen system was reduced by a factor of 30' db relative to the open microphone wind noise.

From the several tests which were conducted, it was found that the screens in the form of hemispheres were somewhat more effective than the cylindrically shaped screens in reducing the wind noise. While the dimensions of the screens are not extremely critical, the spacing between the successive screens is important in producing maximum wind noise reduction for a given number of screens.

The important point, however, is that screens of successively smaller mesh be used in a series such that turbulent eddies are created to generate heat without the wind screen system interacting with the screen (or cloth) to create acoustic frequencies in the range of the signal to be detected and that these same screens pass, undistorted, the signals to be detected. The number of screens necessary and their gradation of mesh size will be determined by the maximum expected wind velocity and the frequency range of interest. The screen configuration and mesh size described hereinbefore were suitable for frequencies of from 0.1 to Hz. and wind speeds up to 30 miles per hour.

While specific embodiments have been described, it is not our intention to limit the invention thereto, since these are simply exemplary of constructions which have been tested and have proved out the theory underlying this invention. Furthermore, without intending to place absolute limits on the variations available, the following design criteria have been determined. Variations within and, under certain circumstances beyond, these criteria may be made based upon the foregoing teachings depending upon the frequency range of interest and the expected wind velocities.

The smallest of the concentric screens is preferably in the range of 15 to 24 inches in diameter and when a cylindrical design is used of approximately the same height, plus from 2 to 6 inches for dome configuration at the top. The diameter of this screen, may, however, be in the range of from 2 inches to about 30 inches. The diameter of the largest of the screen configurations is preferably from about 96 to about 144 inches, for example, inches, but may be from 24 inches to inches. The fabric should be designed to pass at least 100 c.p.s. and preferably as high as 10,000 c.p.s.

The foraminous covering, the screen, of the largest of the wind screens is preferably a wire screen (though a fabric could be used) having between about 10 and about 18 wires per inch, for example, 14 mesh, and may be symmetrical or asymmetrical, though symmetrical screens are preferred. Screens of 8 mesh to about 18 mesh may, however, be used. In any event, the foraminous material having larger openings than the openings of the foraminous cover of the smallest screen should be used.

Three wind screens are preferred for this invention, although some improvement may be noted using only two such screens. Considering this invention to consist of at least three concentric screens, the intermediate screen preferably is covered with a foraminous material having an intermediate opening size, although it is possible to use foraminous covers on the intermediate screen as are used on the outermost screen with good results.

The intermediate screen should be more nearly the diameter of the smaller screen than of the largest screen.

Where a plurality of intermediate screens are used, these should be nearer the diameter of the smallest screen than of the largest screen and should be covered with a foraminous material having openings larger than the openings in the foraminous cover of the smallest screen and preferably having openings smaller than the openings in the foraminous cover of the largest screen, although the latter limitation is not essential for satisfactory operation.

As previously indicated, the sizes, the diameters, of the respective screens is dependent in large measure upon the expected wind speed. Table II shows suggested values for these respective diameters.

TABLE II.DESI GN CONSIDSIIEZIEA'TIONS F0 R WIND SC BEEN Diameters of respective screens Smallest, Intermediate, Largest, inches ches inches While certain of the allowable dimensions may overlap, it is to be understood that each wind screen is larger than the wind screens inside thereof. The minimum distance between the smallest screen and the next adjacent screen is about 2 inches, the minimum distance between any two other adjacent screen is about 3 inches, and the minimum distance between the outer screen and the next adjacent inner screen is about 4 inches. The ratio of the diameters of the largest screen and the smallest screen is preferably in the range of from about 3 to about 4 but may be from about 3 to about 24. The ratio of the diameter of the smallest of the intermediate .screens to the diameter of the smallest screen should be about 1.6 to about 3.

With the foregoing considerations n mind, variations from theembodiment described may be made without departing from thespirit and scope of the invention as defined in the following claims:

We claim: r

l. A system for reducing infrasonic turbulence generated noise for use with an acoustic sensor which, in combination,

' comprises:

a low frequency acoustic sensor; a

1 an outer wind screen formed of a foraminous material from between 8 and 18 size mesh; and a at least one inner wind screen comprising a substantially smaller enclosure formed of forarninous material of smaller size mesh than the outer wind screen; said inner wind screen being constructed and disposed to be positioned around said acoustic sensor. and said outer wind screen being constructed and disposed around the inner wind screen concentric therewith and spaced a minimumdistance of 4 inches-from the next adjacent innerscreen.

2. The system of claim 1 wherein the wind screens are cylindrical in configuration and comprise a top enclosure member of foraminous material corresponding to the material of each respective cylinders.

respective wind screen. 3. The system of claim 2 wherein each top enclosure is formed in an upwardly eonvexed dome configuration.

4. The system of claim 2 wherein the height of the respective cylinders is approximately equal to the diameter of said 5. The system of claim 1 wherein theclosures are hemispherical in configuration.

6. A system for reducing infrasonic turbulence generated noise for use with an acoustic sensor which.- in combination.

comprises: a low frequency acoustic sensor, at least three enclosures of like configuration being of successively larger dimension, being formed of foraminousmaterial having. from the smallest to the largest enclosure, foraminous openings successively relatively larger than the foraminous openings of the next interior adjacent enclosure wherein the material of the largest or outermost enclosure is within the range of 8 to l8 mesh while the material of the innermost enclosure is a good acoustical cloth, said enclosures coaxially positioned around the acoustic sensor.

7. The system of claim 6 wherein the ratio of the diameter of the largest enclosure to the smallest enclosure is between about 3.1 and about 24:1 and the ratio of the diameter of the intermediate enclosure to the smallest closure is between about 1.621 and about 3:1.

8. The system of claim 7 wherein the distance between the smallest enclosure and the innermost intermediate enclosure is about 2 inches and the distance between thelargest enclosure and the outermost intermediate enclosure is about 4 inches.

9. The system of claim 8 wherein the smallest enclosure has a diameter of between about 2 and about 30 inches, the intermediate enclosure has a diameter of between about 6 and about 60 inches and the largest enclosure has a diameter of between about 12 and about l 44 inches.

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