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The Wisconsin Engineer UNIVERSITY OF WISCONSIN VOL. XXV, NO. 3                           MADISON, WIS. FEBRUARY, 1921 SUBMARINE DETECTION BY MULTIPLE UNIT HYDROPHONES By MAX MASON Research Professor of Mathcnatical Physics

Oryalioatio11 of Research o01 Submarine Detection Research on methods of submarine detection was car- ried on during the war under the control of a special board of the Navy. The board was organized with Ad- miral A. W. Grant as chairman, Admiral S. S. Robison being appointed chairman soon afterwards. Commander C. S. McDowell was secretary. The other naval member of the board was Commander M. A. Libbey, while H. J. W. Fay, F. B. Jewett, R. A. Millikan, and W. R. Whitney were named as advisary members. The research work was centered at two stations, one at Nahant, near Boston, the other at New London, Conn. The Nahant group was formed by the Submarine Sig- nal Company, the Western Electric Company and the General Electric Co., the personnel being composed of men picked from the research staffs of these organiza- tions. The New London group was the outgrowth of a committee on submarine detection organized, with the approval of the Navy, by the National Research Council through its chairman G. E. Hale and the chairman and vice-chairman of the Division of Physical Sciences, R. A. Millikan and C. E. Mendenhall. Of the members of this committee H. A. Bumstead, E. F. Nichols. and H. A. Wilson experimented at Yale University during the sum- mer of 19I7, coming to New London for tests, while Ernest Merritt, G. W. Pierce and the writer were sta- tioned at New London. Facilities for experimentation and tests with submarines were furnished by the Navy at the Submarine Base at New London. In September the special board formed the Naval Experiment Station at New London. Under the able leadership of Commander C. S. McDowell facilities and personal were at once collected, and experimentation and tests proceeded with great rapidity. Four main research groups were formed under the direction of P. WV. Bridgman, Ernest Merritt, G. W. Peirce, and the writer, respectively. A school for the training of listen- ers was organized, and later a hydrophone school for officers. Production, installation, and maintenance were cared for by appropriate organizations. and close corre- lation maintained between these different organizations, as well as between the research groups at New London and at Nahant. Through liaison officers and interchange of weekly reports research activity in this field in the allied countries was made a matter of common know- ledge among the workers. Hundreds of problems were studied by the different experimenters. The work under the direction of the writer was con- cerned with the development of submarine detectors in which a large number of individual sound receivers were arranged to focus the sound coming from any particular direction, so as to eliminate disturbing noises coming fromn other directions, and to enable the operator to determine with accuracy the direction from which the sound came. A brief account of some of the steps in development and a description of the final form of the instruments which operate on this principle is given in the following pages. This work was made possible in its initial stages, be- fore the establishment of the Naval Experiment Sta- tion, by the support of the University of Wisconsin through its War Research Fund, a support secured by the efforts of the chairman of its Research Committee. Professor C. S. Slichter. During the summer of 1917, Professors E. M. Terry and J. R. Roebuck devoted their time to the work and contributed in large measure to the developments. At their return to the University, from New London in the fall, Professor Roebuck started work on an instrument for sound analysis to be used in connection with the design of submarine detectors. He joined the group in New London in the winter, remaining there, while the laboratory work on sound analysis was continued by Dr. Frank Gray, first at Wisconsin and latter in New Lon- doll. Professor Terry again joined the group at New London in the summer of 1918. Professor W. L. Dab- ney of the University contributed greatly to the success of the station by his aid in organizing and administer- ing the work of the shops. In addition to those members of the faculty of the University of Wisconsin, the writer had throughout the work the able assistance of 'Messrs. L. B. Slichter and D. L. Hay, graduates of the class of 1917. Their aid was invaluable and to them should be credited much that was accomplished both in plan and execution. The writer was continually aided by the council of his associates at the Naval Experiment Sec- tion, who while engaged primarily along different lines, gave many valuable suggestions. The interest of Lieut. Commander S. C. Houlghton. R. N. V. R.. British liai- son officer on submarine detection, was of especial val- tie -his enthusiasm stimulated effort, and his unstinted cooperation hastened production. The problem of determining all the elements of a suc- cessful acoustical and mechanical design was one of

The WISCONSIN ENGINEER great complexity. The progress made in the short time was due to the enthusiasm and spirit of co-operation of all who were concerned in the work, a spirit maintained uniformly throughout the personnel of the Navy Experi- ment Station by the leadership of Commander McDowell. During the summer of 1918 the writer was engaged with the destroyers and chasers of the United States Navy in European waters, installations being made in

FIG. 1. A DIAGRAMMATIC ILLUSTRATION OF THE HYDRO- PHONE. It shows the Process by Which the Differences in Time of Arrival of a Sound at Successive Receivers are Compensated by Altering the Length of the Paths from the Receivers to the Ear.

English dockyards under the direction of Ensigns L. B. Slichter and Edward Rice, Jr.  The work was done under the direct control of Captain R. H. Leigh, head of the anti-submarine division of the Navy in European waters, whose cordial and complete co-operation stimu- lated all to maximum effort.

General Principles, The First M-V Tube and Compensa- tor

Early in the nineteenth century Calladon and Sturm experimented with sound under water in Lake Geneva. The clearness with which sound produced by metallic objects under water, -the clanking of anchors for exam- ple -was propagated to large distances led Calladon to remark that this effect might play an important role in future naval warfare. It is interesting that this predic- tion was made long before the days of the submarine. A submarine produces sounds under water by its machin- ery,-main motors, rudder motors, pumps, fans and pro- peller-and the problem is to detect and locate a subma- rine through this noise. Sound under water is not transmitted directly from water to air. but is reflected back, the reflecting power being very high. A sound under water may however readily be heard by simple receiving devices. A good receiver is made by a short piece of soft rubber tube closed on one end, the other end being slipped over a metal tube so as to leave an inch or two of the rubber beyond the metal. The metal tube leads above the sur- face of the water and to the listener's ears through stethoscope ear-pieces. The compressional wave in the water produces a compression of the rubber tip which sends a sound wave through the air of the tube. With a single receiver of this kind sounds would be heard. but no idea of the location of their source would be obtained. The direction from which the sound comes may be determined by the use of a pair of receivers. Let these be fixed at the ends of a horizontal arm which may be rotated under water by a vertical column attached to the center of the arm, the vertical column carrying the sound from the two receivers through two tubes to the listener's ears separately. The listener may deter- mine the direction of the source of sound by rotating the arm about the vertical column until the arm is at right angles to the direction of the sound wave. In this posi- tion the sound pulses reach the listener's ears simultan- eously and it seems to him that the source of sound lies directly ahead of him. By reading the amount of rota- tion of the arm on a dial when the sound is thus "cen- tered binaurally" the direction of the source of sound may be determined. The receivers thus act as a kind of extension of the listener's ears into the water, and the same physiological effect is used to determine direc- tion as is used ordinarily for sound in air. An instru- ment based on this principle, known as the "C" tube, was developed at Nahant and extensively installed. The main difficulty in submarine detection by sound lies, however, in the fact that, under normal circum- stances, the detecting apparatus is mounted in the neigh- borhood of many sound sources, and the submarine must be heard and identified in the presence of breaking waves, wave slaps against the listening ship, noises originating within the listening ship, and sounds from other ships in the neighborhood. These disturbing noises are many times greater than the sound of the submarine. The dif- ficulty from this cause is especially great when the attempt is made to listen from a ship which is under way. It is clear that under these circumstances sensitivity of the sound receiving apparatus is a matter of secondary importance. It becomes necessary to devise an instru- ment which will amplify sound coming from a definite. direction, without correspondingly magnifying the in- tensity of sounds from other directions. Acoustical instruments, such as sound lenses and the spherical Walzer plates developed by the French navy, may be built by direct analogy to optical focusing instruments. Such instruments must have a great area to show a marked advantage over the single sound receiver on ac- count of the length of wave for sounds in water, and they require large open spaces behind them for their

(5 lll oollllooi~ooolliiiiio FIG. 2. THE FIRST "M-V" TUBE INSTRUMENT The sound Impulse is received at R and passes to the collecting cone through a path whose length can be adjusted by means of the lever, L.

76 Volume 25, No. 5

The WISCONSIN ENGINEER

operation. In the acoustical case, however, it is possible to connect the members of a group of receivers by tubes to a central collection point and to obtain a focus at this point for sound waves incident at any angle, by properly adjusting or "compensating" the lengths of the connect- ing tubes. The use of this principle was proposed by the writer at a meeting of the Submarine Committee of the National Research Council and representatives of the U. S. Navy on July 3, 1917. An instrument was at once designed and constructed in the physical laboratory of the University of Wisconsin and tested with success- ful results at Madison on July 17, and at New London on July 30. A description of this first instrument may be of inter- est, for, though crude in construction and difficult to manage, it operated sufficiently well to justify the prin- ciples involved, and lead ultimately to a successful and easily operated device. The accompanying figure (Fig. 1) is diagrammatic only. It illustrates the process by which the differences in times of arrival of sound pulse, coming from any direction, at successive receivers of a line are "compensated" by altering the lengths of trans- mission paths from the receivers to the ears, so that the effects of the different receivers are superimposed on the ears, while sounds coming from other directions are not brought together in phase. Let A, B,. . H indicate a line of equally spaced receivers mounted, for example, on the wall of a ship. Suppose the paths A1M, B1M, etc.. are all equal, and that the paths AA,, BB,, etc., are com- posed of trombone slides, the line AlH1 being capable of rotation about its center. The paths AA,, BB,, CC,, etc., then differ successively by the same amount, - d sin b - dependent on the angle b. If sound comes from the direction of the arrow x, making an angle a with AH, the instrument will collect the sounds from the separate receivers in phase at M, if                 PB    AA, -BIB, Vw       Va where Vw and Va are the velocities of sound in water and in air respectively, or if          d cos a   d tan b             Vw        Va or, Va          tan b    - cos a= .23 cos a.                   Vw It is clear that the same angle b will compensate for the symmetrical directions x and y of a sound wave. In practice a pair of lines mounted parallel to the keel on the port and starboard sides can be used, and the am- biguity removed either by comparing intensities on the two sides, or by connecting all of the port line to one ear and all of the starboard line to the other. A multiple unit device of this type,-maximum or binaural or both being obtained by varying tube lengths (variable compensation)-was called an "AI-V" tube. The first actual instrument which was based on this principle consisted of two ten foot rows of 30 receivers each, (Fig. 2). Tubes S, one-half inch in diameter, ex- tended upward from the receivers of each row to a com- mon level, and into inverted "U" tubes which slid over the upright tubes. Fixed vertical tubes led the sound from the sliding "U" tubes and entered the base of a cone, C. The ear piece was connected to the top of the cone. The sliding "U" tubes were driven by a single lever, L, as is shown in Figure 2. Although crude in construc- tion, the instrument showed a fair focus. There were two collecting cones for each row, the halves of a row being led to the ears separately, so that, as the lever was adjusted, the maximum of sound was obtained simul- taneously with the binaural center. The illustration on the cover shows this instrument being tested from a raft on Lake Mendota. The trombone slides are seen to be of unequal lengths, their line rising toward the center. The lengths were determined so that the sound paths from all receivers to the collecting cone were equal when the driving lever was horizontal. In operation, if the tube lengths were adjusted by the lever L for an angle different from that of the sound wave, the sound was heard dimly and confused in character; since one half of the line received the sound as a whole in advance of the other half, corresponding sounds were heard in the ears, but one ear,-the right for example-received the sound before the other. Then it appeared to the listener as if this confused sound were coming from his right. As the lever was adjusted to compensate more nearly, the sound became louder and more distinct, and the source of the sound seemed to move towards a position directly in front of the listener, reaching this position at its maximum of loudness and clearness. Direction could thus be determined to within five degrees. The apparatus was installed in a yacht and much information gained from its use, although its range was but slight and the difficulty in operation very great. More encouraging results were, however, soon obtained when, through detailed study, improve- ment was made in the type of receivers, methods of con- nection, and in the compensator.

EDITORS NOTE-This is the first of a series of four arti- cles by Professor Mason on submarine detection.

Flooring for Highway Bridges A method for providing a satisfactory floor for old steel highway bridges that are too light for a concrete floor, is described by MARTIN W. TORKELSON, Wis. '04, Bridge Engineer for the Wisconsin Highway Commis- sion, in the Engineering News-Record for Dec. 30, 1920. He states that the high costs that have prevailed recently have made it necessary to retain bridges that would otherwise be replaced. A floor built up of laminated units, i. e., planks laid on edge. and covered with a bituminous surfacing was introduced prior to 1917 and has been giving satisfaction. Either tar or asphalt may be used with screened gravel or stone chips as an absorbent. The surface is rolled to a thickness varying from 1/2-inch at the edges to 2 inches at the center. 77 February, 1921