US3517390A

US3517390A - High power acoustic radiator

Info

Publication number: US3517390A
Application number: US709388A
Authority: US
Inventors: Layne Whitehead
Original assignee: Individual
Current assignee: Individual
Priority date: 1968-02-29
Filing date: 1968-02-29
Publication date: 1970-06-23
Anticipated expiration: 1987-06-23

Description

Juhe 23, 1970 WHITEHEAD 3,517,390

HIGH POWER ACOUSTIC RADIATOR Original Filed Oct. 14, 1965 2 Sheets-Sheet 1 go m 34 52 DRIVER 20 i l m l DRIVER DFQ/VER I FIG. I B

DR/VER 4g DRIVER Layhe Whitehead INVENTOR BY Michael F. Breston ATTORNEY DR/ VE R FIG. 4

June 23, 1970 WHITEHEAD "3,517,390

HIGH POWER ACOUSTIC RADIATOR Original Filed Oct. 14, 1965 z Sheets-Sheet 2 DRIVER LOAD F G. 6 f0 FREQ. I

Layne Whitehead F G. 5

IN VE N TOR BY Michael PBreston ATTORNEY United States Patent 3,517,390 HIGH POWER ACOUSTIC RADIATOR Layne Whitehead, 7220 Selma St., Houston, Tex. 77025 Continuation of application Ser. No. 496,746, Oct. 14, 1965. This application Feb. 29, 1968, Ser. No. 709,388 Int. Cl. G08b 3/00; Gk 9/00 US. Cl. 340-384 17 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a new and improved acoustic radiator such as a fog radiator for allowing an acoustic source to generate a relatively large amount of acoustic power into a gas medium. During each cycle, a portion of the acoustic energy from the source is stored in the radiator to establish standing pressure waves therein; the remainder of the energy is radiated to the gas medium. The geometry of the radiator is selected so that the vibrating gases confined therein present an optimum load to the acoustic source thereby allowing it to generate optimum power into the gas medium. The radiator can be made in sections having diverse cross-sectional areas.

This application is a continuation of applicants copending application Ser. No. 496,746, now abandoned, entitled Acoustical Impedance Matching and Array Forming.

BACKGROUND OF THE INVENTION While the teaching of this invention may find application in various fields, it will be described as being particularly useful to the field of fog warning devices such as are required by the US. Coast Guard for offshore platforms. Warning devices for emitting audible long range signals to warn ships of dangerous obstacles or to signal them are well known. In the recent past, offshore oil production and oceanographic work have greatly expanded. The need for dependable, eflicient and long-lasting acoustic radiators is ever growing.

It was generally accepted by workers with fog warning devices that the classic design theory of horns provided optimum design parameters. Although various geometric horn configurations were built, the classic design principles of horns were strictly adhered to. These principles were based on the following factors: When a diaphragm is set in motion in free space the air particles in front of it are given a certain velocity. The pressure of the diaphragm against the volume of air in front of it is accompanied by a reaction pressure against the diaphragm. This reaction pressure is proportional to the air particle-velocity, and must obviously be small compared with the forces due to the inertia and stiffness of the diaphragm itself. The diaphragm therefore works into a very small load, and its motion is almost entirely determined by its own stiffness and mass. Consequently the useful work done by the diaphragm on the air will in general be very small.

The paramount function of the horn is to properly load the diaphragm. An acoustic generator without a load is like an engine without a load or like a radio transmitter without an antenna.

In addition to providing a load to the diaphragm the horn is designed to meet three important requirements: (1) A given applied force acting on the diaphragm must cause the air at the throat of the horn to have a nearly uniform air volume velocity at the operating frequency; (2) the area of the mouth of the horn must be such that little sound energy is reflected, otherwise air-column resonances will occur; and (3) the law of increase of area of cross section with length and the rate of area increase must be such as to avoid discontinuities, that is, standing 3,517,390 Patented June 23, 1970 waves. This will provide a constant ratio of pressure to air volume velocity at the throat at the operating frequency. That ratio is known as the throat impedance.

If the preceding requirements are met and if the throat area is small compared with the diaphragm area, the air in the throat is given a proportionately high particle velocity. As the pressure generated is proportional to this velocity, the reaction pressure on the diaphragm is correspondingly high and the work done on the air correspondingly great. It appears that the smaller the area of the throat, the more eflicient the horn becomes. If the diameter of the throat is less than a certain value, however, the energy used up in overcoming viscous resistance becomes intolerably high. Also, the smaller the throat the longer the horn must be, and this is an additional disadvantage.

Another limiting factor is found in the requirement on the area of the mouth of the horn. This area must be large enough to give negligible reflection at the operating frequency. Since a horn is designed to eliminate reflection waves from its open end, its mouth is made as wide as possible; the wider the mouth the less reflection there is from the open end. In sum, properly designed horns do not allow standing waves within their confining walls.

To serve as a fog warning device it is desirable that the horn, for a given quantity of input energy, radiate a maximum amount of audio energy in the horizontal direction. This can be best accomplished by using an array of horns. Since the designer was already committed to a small throat and a large mouth he could not escape from using a relatively long horn which was frequently folded. An array of folded horns constitutes a heavy and bulky structure. One such structure is shown applicants Pat. 3,153,783.

SUMMARY OF THE INVENTION The paramount function of the acoustic radiator of this invention is much like that of the horn that is to load the diaphragm.

This radiator, however, is based on entirely different design considerations. The area of each mouth of the radiator is selected so that appreciable sound energy is reflected back from the mouth to establish air-column resonances, that is, standing waves within the confining walls of the radiator which can be made in sections.

The law of increase of area of cross section with length is particularly adapted to benefit from sharp changes in the cross sectional areas between consecutive sections forming the radiator. Thus, the radiator of this invention, instead of avoiding, suppressing, and minimizing standing waves or air column resonances within its confining walls, makes good use of them. The air columns within the radiator are subjected to forced vibrations. If the frequencies of the vibrating diaphragm coincide with the natural frequencies of the radiator having a proper acoustic length, the amplitude of the reaction pressure against the diaphragm may become very large indeed. During part of the vibration cycle of the diaphragm, the radiator is drawing energy from the diaphragm. An appreciable portion of that energy is stored in the air column confined between the walls of the radiator in the form of standing waves. Some Waves escape from the open end of the radiator to do work on the surrounding air. The radiator therefore makes use of backward-and-forward traveling waves to establish a vibrating sound field within the in terior of the radiator. The outward-and-backward traveling waves add in magnitude to produce standing waves which allow power to flow toward the open end of the radiator. While the sections forming the radiator of this invention can assume various configurations, in practice it is convenient to Work with regularly-shaped sections having circular or rectangular cross-sectional areas. Cylindrical sections in particular lend themselves better to analytical treatment.

Analytically and experimentally, the teachings of this invention allow the building of simple and complex acoustic radiators. The building blocks for these radiators are sections in which air-columns resonate (called resonators).

The teachings of the present invention avoid the undesirable design limitations previously discussed in connection with fog-horns and allow the design of eflicient, relatively-inexpensive, compact radiators which are particularly useful as fog warning devices for offshore platforms.

It is a primary object of the present invention to provide an improved acoustic radiator capable of efficient operation with conventional signal generating equipment.

It is another object of the present invention to provide a improved acoustic radiator, acting as an impedance transformer in conjunction with a conventional acoustic generator to provide efficiet and optimum operation.

Another object of the present invention is to provide such an acoustic radiator which can be built in sections to form arrays.

A further object of the present invention is to provide such an acoustic radiator which is relatively light, occupies a minimum of volume, and is relatively simple to manufacture.

The novel features which are believed to be characteristic of the invention, both as to its construction and method of operation, together with further objects and advantages thereof will be better understood from the following description considered in connection with the accompanying drawings in which presently preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and descrip tion only, and are not intended as a definition of the limits of the invention.

BRIEF DESCRIPTION OF THE DRAWING In the drawings:

FIG. 1A shows in cross-section a driver coupled to a single resonant coupler;

FIG. 1B shows an array of three couplers.

FIG. 2 shows schematically a driver feeding four single-step resonant couplers.

FIG. 3 shows another arrangement of a driver feeding three single-step resonant couplers;

FIG. 4 shows schematically a driver feeding three couplers each of increasing cross-sectional area;

FIG. 5 shows various response curves;

FIG. 6 shows a broadside array of multi-step resonant couplers; and

FIG. 7 shows another arrangement of a broadside array of multi-step resonant couplers.

Although the present invention is described hereinafter in connection with fog warning devices and particularly with respect to the generation of cyclic signals of narrowband frequency it is to be understood that the present invention is equally applicable to the generation of acoustic signals for other purposes.

Referring to FIGS. 1A and 1B there is shown a fog radiator generally designated as 10 having a support structure 12 for supporting three

resonators

14, 16 and 18, driven by untuned electro-

magnetic drivers

20, 22 and 24, respectively. Each driver has a permanent magnetic structure 25, a coil 26 and a diaphragm 28. Drivers of this type are commercially available and are used primarily for public address systems. An alternating-current electric signal is applied. to the

input terminals

30, 32 of each coil 26. The signal may have a square shape. Preferably it is sinusoidal. Each resonator consists of a section of cylindrical pipe having a radius R and a length L. The drivers and resonators are suitably secured to ribs 13 of the structure 12 as by welding. The length L is selected so that the fundamental resonant frequency of the air column confined within the Walls of the tube resonates at a fundamental frequency corresponding to the frequency 4 f of the signal applied to coil 26. A standing wave is then established within each of resonators 14, 1'6 and 18 having a wavelength \=c/f, where c is the velocity of sound in air. The length L is chosen so that the effective acoustic length of each tube is one quarter wavelength or an odd multiple thereof, i.e.

where n is an integer. Each driver-tube combination can be thought of as a coupled system consisting of a driver and a vibrating air column. The column is the predominant element in this combination and optimum results are achieved when the frequency of the vibrating diaphragm coincides with the frequency of the fundamental mode of vibration of the air column. As is known to those skilled in the art, the effective acoustical length of each pipe is not equal to L because of what is known as the end-effect. For a cylindrical pipe the end correction is about 0.6R. The magnitude of the end correction for other than cylindrical pipes depends on the degree of openness to the air. Thus for a tube the actual value of L is One can think of the maintenance of air vibrations in each tube as follows: As the diaphragm vibrates back and forth it produces sound waves which travel into the tube. The intial vibration of the diaphragm causes a compression to enter into the pipe. The compression travels up to the open end of the pipe. A small fraction of the energy in the compression is radiated into the open air; the remainder is reflected back toward the diaphragm. When the reflected wave reaches the diaphragm, it has traveled one-half Wavelength since the effective acoustical length of the tube is one-quarter wavelength. At this instant, the diaphragm reverses its direction of travel, hence the reflected wave is in phase with the wave produced by the diaphragm. Since the forward traveling wave is in phase with the backward traveling wave, the two waves add and produce what is known as a standing Wave in the tube. The standing wave has a maximum air volume velocity and a minimum pressure at the open end or mouth of the tube, and a minimum volume velocity and a maximum pressure at the throat of the tube. This maximum pressure at the throat exerts a great reaction pressure against the diaphragm which allows the diaphragm to pump a correspondingly great quantity of energy into the tube. In other words, as the driver feeds power into the tube against a maximum reaction pressure, it sees a much higher load impedance than it would see if the driver worked directly into the air. It is known that for maximum efliciency the driver should work into a specific load. It is possible to provide this specific load by suitably selecting the dimensions of the tube. It may also be helpful to think of the tube as an impedance transformer transforming the low impedance at the mouth of the tube into a. high impedance at the throat of the tube.

While

resonators

14', 16 and 18 have been described in connection with a cylindrical tube having a circular cross section for the sake of simplicity, it will be understood by those skilled in the art that the resonators may have a. rectangular cross section or any other cross sectional area. It is desired to maintain standing waves only along the longitudinal axis, hence the diameter of the tube should be substantially less than a wavelength corresponding to the fundamental frequency of the driver.

For a fog radiator it is desirable to have an array of sound sources arranged to make the sound intensity greater in a given direction usually horizontal. In FIG.

1B resonators

14, 16 and 18 are arranged in what is known as a broadside or linear array. The spacings between the open ends of the resonators should be properly adjusted for the desired directivity pattern. For a full discussion of directivity patterns and desirable spacings reference is made to a text book on Acoustics by Leo L. Beranek, published by McGraw-Hill, 1954.

In FIG. 1B, the drivers are mounted on top of the resonators and are covered with a cover 34 to protect them from rain and snow. It is desirable to drive all the drivers with electric signals having the same frequency and phase. If there is a phase difference between any two electric signals, undesirable pressure cancellations may take place in the open air. All the drivers may be driven from the same electric source. If necessary, phase shift networks may be used in the circuits to compensate for phase differences.

When viewed from the driver, each resonator in FIG.

1B represents a single-step resonator, i.e., the driver sees only one impedance changing load sometimes referred to as coupler. In FIG. 2 is shown a radiator 40 having four

singlestep resonators

42, 43, 44 and 45 connected in parallel to a single driver 46. The single-

step resonators

42, 43 may each have an effective acoustic length of A or /1 wavelength at the drivers operating frequency. Resonators 414, 445 may each be /4)\ or 7/4 long. The ends of

resonators

44, 45 are curved to fit the requirements of array forming. The space separations between the open ends of the resonators 42-45 should be suitably selected for the desired directivity pattern. Since the theory of operation of odd-multiple quarter wavelength couplers is the same as that of single quarter wavelength couplers, reference is made to the prior description.

In FIG. 3 is shown a different arrangement of a radiator 50 having three single-

step resonators

52, 53 and 54 connected in parallel and driven by a single driver 56 similar to that shown in FIG. 2. Each resonator has a length equivalent to an odd multiple quarter wavelength at the operating frequency of the driver. The open ends of the resonators are bent to keep water and snow out as Well as to space them to form a linear array.

So far the arrays were shown as being formed of singlestep resonators. Such arrays are susceptible of having the effective acoustic length of each coupler or resonator change with changes in environmental conditions, primarily temperature. When the air temperature changes, the sound velocity c in the air also changes and hence A changes, thereby introducing a phase shift between the forward and backward traveling waves. That phase shift can deleteriously affect the load of the air column precented to the diaphragm. One way to overcome the consequences of changes in the wavelength x is to use a driver, the frequency of which is automatically regulated with changes in temperature. For example, the oscillator generating the electric signal for the driver may have a frequency-determining component (a resistor or capacitor) therein, having a value which changes with temperature by an amount sufficient to offset the change in the wavelength with temperature, and hence in the acoustic length of the resonator. Another method is to use a feedback loop including the driver or a microphone in the coupler to keep the operating frequency at an optimum value.

In sum, without compensating means, single-step couplers have a very narrow frequency pass-band. In practice, this means that due to a change in temperature, the acoustic length of the radiator may become ineffective to properly load the diaphragm and, therefore, to accomplish useful work on the outside air. Of course, if compensating or feed-back means are provided than any change in the effective acoustic length of the radiator will automatically be compensated by a corresponding change in the frequency of the operating electric signal, and no net change in the radiated acoustic power will be noted. In accordance with one aspect of this invention, there is provided a radiator including a cascade of suitably connected resonators which allows; (1) to broaden the frequency pass-band of the radiator, and (2) to achieve a greater impedance transformation ratio.

In FIG. 4 is shown a cascade or series assembly 65 of three quarter-

wavelength sections

61, 62 and 63 driven by a driver 60. For an understanding of the operation of a cascaded radiator, it may be helpful to remember that coupler 63 constitutes the load for coupler 62, and the latter forms the load for coupler 61. The interface between the first-step coupler 61 and the second-step coupler 62 marks a substantial change (step) from the cross sectional area of the first coupler to the second coupler. A step is also provided between the second and

third couplers

62 and 63. The ratio between adjacent couplers areas is selected to give an impedance mismatch between the couplers. Depending upon the amount of impedance mismatch, reflections will occur at the interfaces, thereby creating a sound field to wherein forward-and-backward traveling waves add to 'form standing waves in each coupler. In general, a coupler presents a resistive load and a reactive load to an adjacent coupler. At resonance, a quarter-wavelength coupler presents primarily a resistive load: the reactive load substantially vanishes. If the coupler is shorter than a quarter wavelength, then it presents a negative reactive load; if it is longer than a quarterweavelength it presents a positive reactive load. These reactive loads are vector quantities. In a quarter-wavelength coupler the positive and negative reactive loads are equal in magnitude, and hence their resultant is zero. Each coupler may be represented by an equivalent electrical circuit using resistive, capacitive, and inductive ele ments. Thus, for example, a quarter-wavelength coupler may be represented by a series or parallel tuned circuit where the inductive impedance is equal to the capacitive impedance. Whereas in an electrical resonant circuit there is a cyclic transfer of energy between the inductive and capacitive loads, in a quarter-Wavelength resonator there is a cyclic transfer between the kinetic and potential energles stored in the air column confined between the walls of the coupler. In this respect, the air column is similar to a pendulum, a tuning fork, or a stretched elastic string. Couplers shorter or greater than a quarter-wavelength may be represented by equivalent resistive and capacitive or inductive loads. On the other hand, the radiated power from the open end of the radiator constitutes a resistive load designated hereinafter as R In the cascaded radiators just as in the single step radiators, it 1s desired to transform R into a much greater load nR where n is the transformation ratio of the radiator.

The value of the transformation ratio n of an cascaded radiator assembly depends on the transformation ratio of each coupler, in turn, depends on its pressure standing Wave ratio which can be calculated from a knowledge of the geometric parameters, or it can be experimentally measured. The pressure within a coupler can be easily measured by inserting a small hollow probe tube into the coupler. The ratio between the maximum pressure and the minimum pressure represents the pressure standing wave ratio. If the pressure is uniform througout a coupler, the standing pressure wave ratio is one: no standing gvlalves exist and the coupler acts as a conductor of sound In sum, the load of each coupler depends on its pres sure standing wave ratio, which in turn depends on its cross-sectional area. The total transformation ratio n of the radiator assembly can therefore be analytically or experimentally determined from a knowledge of the individual couplers standing wave ratios. For example, the standing wave ratio in a cylindrical coupler opening into the air can be approximately calculated as the ratio of the area of the coupler to the area of a circle whose diameter is twice the wavelength, at the operating frequency a divided by 1r.

Because acoustical phenomena are continuous, slight deviations from ideal design parameters produce only slight deviations from desired results. This allows multiple step resonators to be built from standard size pipes.

It will be appreciated that the reactive impedance of a coupler depends on the deviation of the Wavelength from the design wavelength. As previously mentioned, a change in temperature results in a corresponding change in the wavelength. Each coupler when loaded with a reactive load inverts its load about its characteristic impedance at its input end. Since the coupler also develops a reactive component due to the change in wavelength, the inverted component is partially cancelled by the developed reactive component. The greater the number of couplers, the easier it is to balance out the reactive components due to a change in the wavelength of the radiated sound waves, i.e., to maintain the total transformation ratio n of the radiator constant. Consequently, a multi-step radiator allows a relatively Wider frequency pass-band.

Mathematically, it can be shown that for a desired particular type frequency pass-band and transformation ratio, the ratios between the cross-sectional areas of the couplers can be made proportional to the coefficients of certain mathematical functions. For example, the areas of the couplers can be proportional to the coeflicients of a binomial expansion or of the well-known Tchebycheif polynomial. In the latter case, the cascade of quarterwavelength resonators follows a Tchebycheff-type frequency response. Because the computations become somewhat tedious they can be conveniently programmed on a digital computer. The information derived can be tabulated to serve for the design of other cascaded step couplers.

In FIG. are shown three frequency response curves 70, 71 and 72. Frequency is plotted on the X axis and the acoustic response (normalized) on the Y axis. Curve 70 represents the response of a singlestep coupler. Curve 71 represents the response of a cascaded step coupler having a flat frequency response, and curve 72 represents the response of a cascaded step coupler having a rippled response or a Tchebycheff-type response. Curve 70 has a relatively narrow frequency response, that is, on either side of an operating center frequency i the response falls off sharply. Curve 71 represents a wider frequency response, and within the pass-band the response is uniformly flat. Curve 72 represents a rippled response within the pass-band. The amount of ripple that can be tolerated depends on the particular application. By suitably adjusting the size of the steps in a cascaded radiator, the amount of ripple can be controlled.

In FIG. 6 is shown a broadside array of couplers which are arranged in a plurality of assemblies acoustically connected in series-parallel combinations. A driver 80 feeds into a coupler 81. Coupler 81 feeds

couplers

82 and 83 connected in parallel. Since the load of a coupler is determined by its size or cross-sectional area rather than by its shape, the division of a main tube into two parallel branch tubes, each equal in size to the main tube, is equivalent to doubling the area of the main tube. As previously mentioned, if a tube adjoins a tube of greater or smaller area, then a greater or smaller step results. Coupler 82 is connected in series with a larger area coupler 84 which, in turn, is divided into

parallel couplers

85 and 86. Coupler 85 is connected in series with an end coupler 87, and coupler 86 feeds into an end coupler 88. Similarly, coupler 83 feeds into a larger size coupler 89 which, in turn, is divided into

couplers

90 and 91. Coupler 90 feeds into an end coupler 92 and coupler 91 feeds into an end coupler 93.

In a broadside array as shown in FIG. 6, the desired spacings between the

end couplers

87, 88, 92 and 93, to achieve a horizontal or other directivity pattern, may not allow the couplers to be all one-quarter wavelength long. If one coupler is made shorter or longer than one-quarter wavelength, then to offset its resulting reactive component another coupler in the array is made either shorter or longer than one-quarter wavelength.

On the other hand, if it is necessary to make one or more couplers, for sound conduction purposes, several one-quarter-wavelength long, the pressure standing wave ratio in these couplers should be lower than What would be used in a quarter wavelength coupler, so as to develop approximately the same reactive component as that oif a quarter wavelength coupler when operated of resonance. For example, if coupler is shorter than onequarter wavelength, then coupler 84 may be made shorter than one-quarter wavelength to compensate for the reactive component in coupler 85. Also, if

couplers

82 and 83 are three-quarter wavelength long, then the standing wave ratio in these couplers should be approximately onethird the optimum standing wave ratio for one-quarter wavelength coupler. The pressure standing wave ratio in a coupler may be made lower or higher by suitably selecting the area of the coupler. If standing waves are not desired in a particular coupler, then its size or area is selected to achieve that purpose. For example, if it were desired to have no standing waves in

couplers

82 and 83 then the ratio of the cross-sectional areas of

couplers

82 and 84 should be equal to the pressure standing wave ratio in coupler 84. It will be appreciated that the pres sure standing wave ratio in coupler 84 depends only on the load that coupler 84 is presented with, that is, on

couplers

85, 86, 87 and 88. The ratio between the area of a coupler and the area of the load it is equivalent to is equal to the pressure standing wave ratio in the coupler. If, for example, a tube having a four square inch crosssectional area is found to have a pressure standing wave ratio of four-to-one, then it will load the acoustic source the same as an infinite pipe of one square inch crosssectional area. Thus, the loading effect of that coupler is the same as that of an infinite pipe of one square inch area. The same holds whether the source of acoustic energy is a driver or a coupler.

The arrangement shown in FIG. 6 allows a relatively broad band-pass which could be of the maximally flat type or of the rippled type, The reason Why a relatively broad band-pass is desired is to allow for variations in atmospheric conditions, which aflect the wavelength of the sound waves propagated in the air, and for variations in the operating frequency of the acoustic energy source. The manner in which the areas of the couplers, i.e., the steps, are selected determines the type of frequency passband obtained for a given number of couplers.

In FIG. 7 is shown another series-parallel arrangement of a multi-step radiator which forms a broadside array.

A driver 101 feeds into a coupler 102. The output of coupler 102 is fed into coupler 103. The output from coupler 103 is fed into

couplers

104 and 105. Coupler 104 feeds into

end couplers

106 and 107. Coupler feeds into

end couplers

108 and 109. To preserve the crosssectional areas of the radiator,

closed tubes

110, 111, and 112 are provided, as shown. The flow of sound within the radiator from the driver is indicated by the arrows.

In FIG. 7 all tubes are concentric. The area of coupler 106 can be obtained from a knowledge of the inside diameter of tube 106 and the outside diameter of tube 110. The other areas can be similarly calculated or measured. The area changes and lengths in FIG. 7 are similar to the ones shown in FIG. 6 yet the physical configurations are different. Other physical arrangements will readily suggest themselves to men skilled in the art for the purpose of obtaining broadside arrays in accordance with this invention.

A reflection plate 115 is used to form an image of the end coupler 106, thereby making the effective acoustic length of the array longer than its actual length. A similar reflection plate 116 is used to form an image of the end coupler 109. Another function of

reflection plates

115 and 116 is to allow the

end couplers

106 and 109 to see the same load as their symmetrically opposed

couplers

107 and 108, respectively. In other words, coupler 107 sees coupler 108 and coupler 106 sees its reflected image. The same holds for

couplers

108 and 109. The use of the

reflection plates

115, 116 allows greater directivity or beaming, both because the couplers are 9 evenly matched and because the effective length of the radiator is longer than the actual physical length. Reflection plates may also be used in the other embodiments, if desired.

Drivers of the electromagnetic type have both mass and compliance in their moving parts, thereby leading to resonant effects. When such drivers are operated off resonance, the reactive component of the current flowing through the voice coil resistance increases heat losses and reduces the operating efficiency. The radiators of this invention lend themselves to reducing the deleterious effects caused when the frequency of the signal driving the driver shifts. Thus, the radiator is arranged to have a rcactance, as seen by the driver, equal to but of the opposite sign than the rcactance of the driver, when operated off the desired operating frequency. For example, the first step in FIG. 6 between

couplers

81 and 82, 83 is chosen to be a two-to-one step which is a larger step than would normally be indicated for the first step of the radiator. The relatively larger first step provides the reactive component required to cancel or offset the reactive component of the driver 80. Hence, by proper selection of the areas of the tubes both an optimum load and driver rcactance cancellation may be achieved. In this manner, the optimum driver efficiency within the pass-band is obtained.

As an example of a type of radiator shown in FIG. 6, which is driven by a 360 cycle signal, the following dimensions in inches are given for the sake of illustration only:

Tubes Length Diameter 87, 93 (each) While the invention has been described in connection with particular type couplers, it is not limited thereto and other embodiments will readily suggest themselves to those skilled in the art. Also, while electromagnetic drivers were discussed other type drivers can be equally employed.

What I claim is:

1. In combination,

a signal generator,

a sound-emitting structure for emitting powerful audible acoustic signals at a predetermined frequency into a fluid medium when said structure is coupled to the output of said signal generator, said structure comprising:

at least two distinct housings acoustically arranged in series, said housings having substantially different cross-sectional areas thereby establishing a sound reflecting interface between said areas,

each housing defining at least one sound conducting chamber,

each chamber having a determined length dimension in the direction of sound travel to cause each chamber to act as a resonator at said predetermined frequency when said structure is coupled to said signal generator,

said length dimension being related to (2nl))\/4 wherein n is an integer and A is the wave length of said acoustic signals, and

each chamber upon becoming a resonator having pressure standing waves established therein to allow said generator to transmit into said medium said powerful acoustic signals.

2. The combination as defined in claim 1 wherein said length dimension is substantially equal to (2m1))\/4.

3. The combination as defined in claim 1 wherein said signal generator is electrically driven.

4. The combination as defined in claim 1 wherein said fluid medium is air.

5. The combination as defined in claim 1 and further including:

at least one sound reflector positioned adjacent to said structure to allow said acoustic signals to become reflected from said reflector.

6. In combination,

a signal generator,

at least two assemblies acoustically connected in parallel,

each assembly including at least two distinct housings acoustically arranged in series, said housings having substantially different cross-sectional areas thereby establishing a sound reflecting interface between said areas,

each housing defining at least one sound conducting chamber,

said length dimension being related to (2n-1))\/4- wherein n is an integer and A is the wave length of said acoustic signals, and

7. The combination as defined in claim 6 wherein said length dimension is substantially equal to (2n-1))\/4.

8. The combination as defined in claim 6 wherein said signal generator is electrically driven.

9. The combination as defined in claim 8 wherein said fluid medium is air.

10. The combination as defined in claim 9 wherein the chambers in said structure which transmit said acoustic signals into said medium are suitably displaced from each other to allow said acoustic signals to become transmitted in a directional pattern.

11. The combination as defined in claim 10 and further including:

12. In combination,

a signal generator,

a plurality of assemblies acoustically connected in series-parallel combinations,

each housing defining at least one sound conducting chamber,

said length dimension being related to (2n1))\/ 4 1 1 wherein n is an integer and A is the wave length of said acoustic signals, and each chamber upon becoming a resonator having pressure standing waves established therein to allow said generator to transmit into said medium said powerful acoustic signals. 13. The combination as defined in claim 12 wherein said length dimension is substantially equal to 14. The combination as defined in claim 12 wherein said signal generator is electrically driven.

15. The combination as defined in claim 12 wherein said fluid medium is air.

16. The combination as defined in claim 15 wherein the chambers in said structure which transmit said acoustic signals into said medium are suitably displaced from each other to allow said acoustic signals to become transmitted in a directional pattern.

17. The combination as defined in claim 16 and further including:

References Cited UNITED STATES PATENTS 1,221,859 4/1917 Honold 340-384 1,733,718 10/1929 Blondel 181-26 1,761,568 6/1930 Kersten 18127 XR 2,087,052 7/1937 Spens Steuart 181,-.5 2,225,312 12/1940 Mason 18l.5 2,598,994 6/1952 Gougeon 340-3 88 2,720,934 10/1955 Schenkel l81.5 2,790,164 4/1957 Oberg 34038 8 3,046,544 7/1962 Auer et a1. 340-388 3,138,795 6/1964 Wallace et a1. 340-384 3,153,783 10/1964 Whitehead 340-384 3,214,753 10/1965 Dodge 340-384 FOREIGN PATENTS 6404318 10/ 1964 Netherlands.

STEPHEN J. TOMSKY, Primary Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3 ,517 ,390 June 23 1970 Layne Whitehead It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2 line 32 after "shown" insert in Column 3 line 15, "a should read an line 17, "efficiet" should read efficient line 41, "couplers." should read couplers line 43, "couplers." should read couplers; Column 4, line 29, "intial" should read initial Column 5, line 27, after "multiple" insert a comma; line 66, "than" should read then Column 6, line 15, cancel "to"; line 46, "an" should read a line 48, after "each" insert individual coupler. The transformation ratio of each line 69, "a" should read a comma. Column 8 line 4 "of" should read off line 59 after "6" insert a comma.

Signed and sealed this 16th day of February 1971.

[SEAL] Attest:

EDWARD M.FLETCHER,JR. WILLIAM E SCHUYLER, JR. Attesting Officer Commissioner of Patents