As a result of the exhaustive experiments on under-water sound conducted during the war, although primarily relating to the detection of submarines, many useful peace-time applications have developed. Among these are numbered the various means of utilizing underwater sound for determining ocean depths. Sounding methods may be divided into two classes: those that depend on determining the direction of a reflected sound from the bottom; and those that depend on measuring the time of transit of a sound pulse from a ship to the bottom and back.
In this article it is intended to review the work conducted during the past few years at the University of California at Los Angeles, with the cooperation of the Bureau of Engineering, on an instrument utilizing the second principle, i.e., utilizing the time of transit of a sound pulse from the ship to the bottom and back.
Due to the exhaustive work of such investigators as Behm in Germany, Langevin in France, and Dr. H. C. Hayes of the U. S. Naval Research Laboratory, instruments had been developed by 1923 that made it possible to chart large areas in a small fraction of the time previously required. These instruments were not, however, direct reading. An investigation was therefore undertaken at the University of California at Los Angeles early in 1923 to determine the practicability of developing a continuously recording depth sounder.
Permission was obtained in 1924 from the Bureau of Engineering and the commander in chief of the Battle Fleet to obtain oscillographic records of the echoes received from a standard Navy oscillator over various depths and over different bottom conditions. These tests were carried out aboard the U.S.S. Maryland during a full-power run in November, 1924. It was found from these tests that although the intensity of the reflected sound varied within wide limits, in most cases the echo was well distinguished from the background of ship’s noise. These results were encouraging for they showed that it would be possible to amplify the returning echoes.
It was felt that a depth recorder to be of the greatest use should embody the following features: it should be capable of giving continuous visual indications automatically, i.e., it should be direct reading and as independent as possible of the personality of the operator; it should be as rugged and free from adjustments as possible. In a word, it should be “seagoing,” and capable of being maintained by the ever-changing personnel of a ship.
An experimental instrument embodying these features was then built and tried out aboard the U.S.S. Maryland in January, 1925. This instrument is known as the “graphic acoustic sounder.” It is an instrument designed to use the standard Navy installation of oscillator and microphones to give a continuous graphic record of the depth and at the same time provide a visual indication. It depends for its operation upon a measurement of the time that elapses between the arrival of a pulse of sound direct from the oscillator and of the same pulse as an echo from the bottom. It is assumed that for practical purposes the velocity of sound is constant and that the speed of the ship is slow in comparison to this velocity. The time lag of the echo may be computed as follows: if
d = the distance from oscillator to microphone center in fathoms.
h = the depth of water below oscillator microphone line in fathoms.
V = the velocity of sound in sea water in fathoms per second.
t = the time lag of the echo over the direct sound in seconds.
The Constant-Speed Motor Governor
Since the device depends for its operation on the pointer moving with constant angular velocity, an electrical governor has been provided which maintains the speed within narrow limits. Under normal fluctuations of the supply voltage the speed is maintained within 1 per cent of the correct value and even with a drop of 50 per cent in the line voltage the speed does not drop more than 1^2 per cent. The operation of the governor is as follows: in Fig. 3 A is the armature and F the shunt field of a no-volt D.C. motor. The contact, D, and the friction ring, B, are attached to the same support. D is arranged so that it may vibrate between the contacts C and E. When the motor is not running, contacts C and D are held together by a light spring not shown in the figure. A resistance /?j in series with the armature is shunted by contacts C and D. A resistance R2 in series with the shunt field is shunted by contacts D and E. When the motor is started the governor balls are driven out radially by the centrifugal force, causing the friction disc, H, to approach the friction ring, B. If in proper adjustment the disc, H, will bear on the ring, B, with sufficient force to cause B to rotate slightly and separate the contacts C and D. This action includes resistance R1 in the armature circuit, which decreases the armature current and consequently the torque, and tends to decrease the speed. Should the speed, however, further increase, the contact D is driven over until it closes with contact E. This action cuts out the resistance R2 from the shunt field circuit, thus strengthening the field and further tending to slow the motor. When operating normally contact D vibrates between C and E and maintains a uniform speed. An adjustment nut K is provided to regulate the speed at which the motor is to operate. A reed tachometer is provided as a ready check on the motor speed. The reeds are actuated by a small bar magnet fixed to the motor shaft. The center reed is tuned to exactly 30 vibrations per second, the right one 1 per cent above this value, and the left one 1 per cent below. By this arrangement it is possible to check the speed at a glance prior to obtaining a sounding.
Performance of the Graphic Acoustic Sounder
An exhaustive test of the experimental instrument was authorized by the Bureau of Engineering during the Australian cruise of the fleet in 1925. The instrument was installed aboard the U.S.S. Maryland during passage from Honolulu, T.H., to Sydney, Australia. The first graphic record taken upon leaving Sydney Harbor en route to Auckland, New Zealand, is reproduced in Fig. 4. The nearest charted soundings have been inserted as circles. At 1200, 1215, and 1345 the ship passed directly over charted depths at which times it is seen that the agreement is good. At 1418, the depth having exceeded 100 fathoms, the pointer speed was changed from p = 1 to p = 10. Fig. 5 shows a continuation of the record to this new scale. By 1630 the depth had reached 1,000 fathoms, at which time the scale was again changed to p = 50. Records similar to these were obtained upon entering and leaving Auckland, New Zealand; upon passing over submerged peaks in the vicinity of Samoa; and upon entering and leaving Honolulu Harbor, T. H.
At the conclusion of the Australian cruise in 1925, this instrument was transferred to the U.S.S. Eagle 35. Lieutenant Commander A. N. Offley, at that time captain of Eagle 35, continued the experimental work with this instrument. In an article in the United States Naval Institute Proceedings for March, 1929, under the title of “Modern Fog Navigation,” Lieutenant Commander Offley gives in detail his experiences.
The Indicating Acoustic Sounder
As a result of the experimental data obtained from the graphic acoustic sounder, and the criticisms of naval personnel, an indicating acoustic sounder was designed. It was felt that the graphic feature would find its greatest use for charting purposes, and that for purely navigational use an indicating instrument would suffice. An instrument of this type was built aboard the U.S.S. Holland in December, 1926. It has been in constant use since then.
In this instrument two speeds are provided. The pointer can rotate clockwise once a second, or in the reverse direction once in ten seconds. This arrangement permits depths up to 400 fathoms to be read from one scale and up to 4,000 fathoms on the other. The shallow limit of both sounders is fixed by the minimum duration of sound that can be produced by the particular oscillator used. The least depth readable is reached when the train of sparks due to the echo begins to overlap with that due to the direct sound. Fig. 6 illustrates this point: a is an oscillogram of the current flowing through the oscillator when the circuit is closed for a very short interval; b is a record of the direct sound from the oscillator as received by the microphones. It should be noticed that the sound persists in this type of oscillator for a considerable time. It is possible, however, by using the directional property of the compensator, to eliminate some of the interference. With a 540-cycle oscillator the shallow limit is twenty fathoms, while with a 1,050-cycle oscillator it is five fathoms. The shallow limit is thus fixed by the damping of the oscillator. The maximum depth is limited by the intensity of the oscillator signal, the type of bottom, and the amplification used. With a 540-cycle oscillator and three stages of amplification it is possible to obtain visual indications or graphic records as deep as 2500 fathoms.
In order to test these instruments over an extended period of time and under actual operating conditions at sea, four have been built by the Washington Navy Yard. They are to be placed on various types of naval vessels. The results of the tests should definitely establish the practicability of these instruments.