US3699508A

US3699508A - Means for increasing the acoustic power output of underwater transducers

Info

Publication number: US3699508A
Application number: US61199A
Authority: US
Inventors: Frank Massa
Original assignee: Dynamics Corp of America
Current assignee: MASSA DONALD P COHASSET; Dynamics Corp of America; Massa Products Corp
Priority date: 1970-08-05
Filing date: 1970-08-05
Publication date: 1972-10-17
Anticipated expiration: 1989-10-17

Abstract

An underwater transducer is force limited and mounted in a baffle structure which presents less than a 100 percent rho-c loading. By so reducing the radiation resistance of the acoustic impedance of the transmission medium, the acoustic output from the transducer can be increased.

Description

United States Patent Massa [451 Oct. 17,1972

i541 MEANS FOR INCREASING THE ACOUSTIC POWER OUTPUT OF UNDERWATER TRANSDUCERS [72] inventor:

[73] Assignee: Massa Division, Dynamics Corporation of America, Hingham, Mass.

[22] Filed: Aug. 5, 1970 [21] Appl. No.: 61,199

Frank Massa, Cohasset, Mass.

[52] US. Cl. ..340/8, BIO/9.1, 340/9 [51] Int. Cl. "601v 1/00 [58] Field of Search ..340/8i4; 3 10/91 [5 6] References Cited UNITED STATES PATENTS Massa, Jr. ..340/1 2 R 2,906,991 9/1959 Camp ..340/9 X 3,492,634 1/1970 Massa ..340/9 3,019,661 2/1962 Welkowtiz ..340/8 R 2,923,782 2/1960 Armstrong et al. ..340/8 R Primary Examiner-Benjamin A. Borchelt Assistant Examiner-W. J. Tudor Attorney-Louis Bernat [57] ABSTRACT An underwater transducer is force limited and mounted in a baffle structure which presents less than a 100 percent rho-c loading. By so reducing the radiation resistance of the acoustic impedance of the transmission medium, the acoustic output from the transducer can be increased.

12 Claims, 2 Drawing Figures MEANS FOR INCREASING THE ACOUSTIC POWER OUTPUT OF UNDERWATER TRANSDUCERS This invention relates to means for improving and increasing the acoustic power output from underwater transducers, and more particularly to an array of spherical transducers.

The following description uses the term rhoc which is well known in acoustic engineering. This term refers to the characteristic acoustic impedance of the medium (such as ocean water, for example) into which the transducer is driving. The description also refers to a spherical shaped transducer which behaves as a dipole source of sound. This is different from a pulsating sphere which acts as a point source.

Generally, it has been assumed that the optimum perfonnance for an underwater transducer is realized when a full 100 percent rhoc loading is achieved for the operating environment of the transducer. At the lower audio frequencies, this full loading condition is often achieved by assembling a large number of transducer elements into an array configuration. The array is usually configured so that the radiation resistance acting upon the transducers approaches the desired 100 percent rhoc loading.

Another general assumption has been that a transducer increases its ability to generate acoustic power responsive to an increase of the radiation resistance. For the usual amplitude limited transducer, the power output does increase as the acoustic loading on the transducer is increased, and this is in accordance with the generally accepted belief. However, the general assumption is not necessarily true for all other types of transducer. For example, l have found that it is possible to reduce the radiation resistance below the 100 percent rhoc loading factor and, under certain circumstances, to actually increase the acoustic output from the transducer. The circumstances under which this may be achieved includes the use of a transducer in which the electromechanical drive system is force limited. This is in contrast with a more conventional type of underwater transducer in which the vibrating system is amplitude limited.

Accordingly, an'object of this invention is to increase the acoustic power output from an underwater transducer. Here, an object is to combine a force limited transducer with a baffle structure which presents less than a 100 percent rhoc loading on the radiating surface of the transducer.

Another object of this invention is to increase acoustic power output of a transducer array. Here, an object is to mount a number of inertial type electromagnetic transducers in an array configuration which results in less than 100 percent rhoc loading on the transducers. Still another object of this invention is to increase the acoustic power radiation from an inertial type electromagnetic transducer by coupling the transducer to an underwater horn which presents less than a 100% acoustic resistance loading to the vibrating surface of that transducer.

Yet another object of this invention is to increase the acoustic power generating capability of an array of inertially driven, electromagnetic, dipole transducers. In this connection, an object is to assemble a group of such transducers into an array configuration in which open spaces are provided in the array between adjacent transducers. Here, an object is to use the open spaces to reduce the resulting radiation resistance upon the radiating surfaces of the transducers.

The structure for accomplishing these and other objects is set forth with particularity in the appended claims.

However, for a better understanding of the invention itself, together with further features, objects, and advantages thereof, reference is made to the accompanying description and drawings, in which:

FIG. 1 is a schematic cross sectional view of an underwater horn coupled to a force limited transducer, the horn being designed to present less than a lOO percent rhoc loading to the transducer; and

FIG. 2 is a schematic view of an array of force limited inertial type transducers, arranged in a configuration that reduces the eflective acoustic loading on the array, thereby increasing the acoustic power output of the transducers.

In FIG. 1, a spherical transducer 11 is shown as being mounted into a small opening at the end of an underwater horn 12. The underwater horn may be constructed of concrete loaded with scrap steel for incre asing the density of the horn structure. Details of a suitable underwater horn design are given in U.S. Pat. No. 3,360,771, issued Dec. 26, 1967, and assigned to the assignee of this invention. The transducer 1 l is an inertial, electromagnetic, force limited, spherical transducer which may be similar to the structure described in U.S. Pat. No. 3,319,220, issued May 9, 1967, for example, also assigned to the assignee of this invention.

The transducer is mounted in the small or rear end of the horn by means of the brackets 13 and the bolts 14. The brackets 13 are preferably molded into the peripheral edge of a rubber belt 15 which equatorially surrounds the transducer sphere 11. This belt serves to resiliently hold the transducer in a fixed position, while enabling the vibrations of the spherical transducer to reach surrounding medium, when electrical power is supplied thereto via a suitable cable (not shown).

The acoustic resistance at the throat D of an exponentially shaped hom is proportional to l/A,,, where A,, is the area of the horn at the diameter D, shown in FIG. 1. If loaded with a percent rhoc loading factor, theacoustic resistance on the radiating surface of the vibrating sphere is proportional to llA where A, is the projected area of the sphere. Thus, if A is made greater than A,, the acoustic loading on the transducer is less than 100 percent rhoc. This is assumed to be the case illustrated in FIG. 1.

in greater detail, the diameter D of the horn in FIG. 1 is small as compared to the wave length of the sound being radiated. More particularly, an exponential horn has a throat diameter which must be small compared to the wave length if the structure is to behave as a horn. Here, the concem is to make the throat diameter D larger than the diameter of the transducer attached thereto. in this way, I achieve a reduced acoustic load on the vibrating sphere and thereby increase the acoustic power radiated, provided the transducer is a force limited vibrating structure.

Therefore, the diameter of the spherical transducer element is small compared to the wave length of sound being generated. If this were not so, there would be no point in having a horn. The acoustic radiation resistance on the surface of a vibrating structure approaches 100 percent rhoc, for the medium, when the size of the vibrating structure is large in comparison to the wave length of the radiated sound.

If the throat diameter of the exponential horn were made equal to the diameter of the vibrating sphere, the acoustic impedance presented to the transducer would be equivalent to 100 percent rhoc acoustic loading. on the other hand, if the diameter D were made less than the diameter of the transducer, the acoustic radiation impedance presented to the transducer would be greater than 100 percent rhoc loading. This latter case is typical of conventional types of horn design, and it is particularly suitable if a horn is driven by an amplitude limited transducer. In other words, if the transducer were generating a constant amplitude, the acoustic power would increase if the throat diameter D of the horn were made less than the diameter of the transducer. This is exactly the opposite of what is here achieved by the use of a force limited transducer, as described herein.

The acoustic power is increased for a force limited transducer coupled to a horn. This increase in power occurs since the acoustic power output of a force limited transducer is proportional to F IR where F is the electromagnetic force generated in the electromagnetic drive system of the transducer, and where R,, is the acoustic radiation resistance load. The acoustic radiation resistance load R is made lower as the area of the horn at the diameter D (FIG. 1) is made larger. The result is an increased level of radiated acoustic power. The electromagnetic, force limited transducer delivers higher power with decreasing radiation resistance loading since the amplitude of vibration of the transducer increases to satisfy the fundamental equations of motion. The practical limit upon this increase in output is imposed by the design of the vibrating system in the transducer and by the maximum permissible displacement of the structure.

The same conclusions cannot be applied to an amplitude limited transducer such as a piezoelectric transducer, for example, because, at a constant amplitude of the transducer, the acoustic power output decreases as R, decreases.

FIG. 2 illustrates another arrangement comprising a plurality of transducers 11A, each of which is the same as transducer 11 in FIG. 1. The transducers are here used to give a reduced acoustic loading for the transducers. The individual transducers are mounted with the same elastic belt suspension system, as illustrated at 15 in FIG. 1. Each transducer 11A is mounted within a circular rigid sleeve member 16, a plurality of which are attached within a frame structure 17 by means of weldin g as at 18.

The openings 19 between the adjoining sleeve members 16 extend through the array assembly. These openings may be either left open or partially blocked by filling the interstices with rigid structural barriers (not shown). If the openings 19 remain in the array, they reduce the effective radiation resistance loading on the transducers. Thus, an increased acoustic power is achieved from the electromagnetic transducers, for

the reasons described above in connection with FIG. 1. It should be understood that modifications and variations may be effected without departing from the spirit and scope of the novel concepts of this invention. Therefore, the appended claims are to be construed to cover all equivalent structures.

I claim:

1. A structure for radiating sound underwater, said structure comprising at least one force limited transducer, means for increasing the acoustic power radiated from said transducer, said means including a baffle structure associated with said transducer, said baffle structure having a throat diameter relative to the diameter of the transducer which presents less than percent rhoc resistance loading to the transducer.

2. The invention in claim 1 characterized in that said transducer is driven by a force limited system, of the inertial type.

3. The invention in claim 1 and means for electromagnetically driving said transducer.

4. The invention in claim 1 characterized in that said transducer is driven electromagnetically, and further characterized in that said baffle structure is a tapered horn.

5. The invention in claim 4 wherein said tapered horn is made of cement loaded with metallic parts.

6. The invention in claim 4 further characterized in that said tapered horn has said throat area which is larger than the working area of said transducer.

7. The invention in claim 1 wherein said baffle structure includes a frame, a plurality of tubular sleeves mounted within said frame, and a plurality of transducers mounted within individually associated ones of said tubular sleeves.

8. The invention in claim 7 further characterized in that said transducers are electromagnetic devices.

9. The invention in claim 8 further characterized in that said transducers are of the inertial type.

10. The invention in claim 7 further characterized in that there are unfilled openings between said tubular sleeves whereby the radiation resistance on the transducers is effectively reduced.

11. The invention in claim 10 further characterized in that said transducers are spherical electromagnetic devices, each having a mounting structure comprising an equatorially positioned resilient belt, said belt being compressed between the wall of said sphere and said tubular sleeve.

12. The invention in claim 11 still further characterized in that said transducers are of the inertial type.

Claims

1. A structure for radiating sound underwater, said structure comprising at least one force limited transducer, means for increasing the acoustic power radiated from said transducer, said means including a baffle structure associated with said transducer, said baffle structure having a throat diameter relative to the diameter of the transducer which presents less than 100 percent rhoc resistance loading to the transducer.