THE SONIC DEPTH FINDER.

BY HARVEY C. HAYES, PH.D.

(Read April 26, 1924.)

PREVIOUS PAPERS.

This paper is the third of a series dealing with developments that have resulted from research work in the field of submarine acoustics carried out by the U. S. Navy during the past five years. The first paper, entitled "Detection of Submarines," described a superior type of submarine sound detector called the MV Hydrophone which had proved effective during the war and predicted that this device would eventually prove to be a distinct aid and safeguard to navigation. The second paper, entitled "The U. S. Navy MV Type of Hydrophone as an Aid and Safeguard to Navigation," 2 described the various ways in which the MV Hydrophone could serve the navigator, as determined on various ships of the Navy, and gave the results of some practical tests.

PRESENT PAPER.

The present paper may be regarded as an extension of the second, inasmuch as it has to do with developments that are of prime importance to the navigator and which were predicted in that paper. It is also closely related to the first paper since it deals with an installation of submarine sound apparatus for ships that includes the MV Hydrophone. In order to avoid repetition the author will, therefore, only describe very briefly those parts of the sonic depthfinding apparatus that have been fully described in the two previous

papers.

GENERAL DESCRIPTION.

The sonic depth-finding apparatus comprises three distinct units, viz. a submarine sound transmitter, a submarine sound receiver 1Proceedings of the American Philosophical Society, Vol. XIX., No. 1,

Proceedings of the American Philosophical Society, Vol. LIX., No. 5,

and the so-called "Sonic Depth Finder," which is a connecting link between the transmitter and receiver and which serves to accurately measure the time interval between the sending of a submarine sound signal and the return of its echo from the sea-bottom or any other submarine sound-reflecting surface. This time interval, together with the velocity of sound in sea water, serves to determine the distance to the reflecting surface. And if the sound receiver is a type that can determine the direction of the echo, as is the MV Hydrophone, then the reflecting surface can be definitely located and its slope determined because the direction of the echo is perpendicular to the reflecting surface. Such an installation of sound apparatus can, therefore, furnish complete data for making a survey of the sea bottom. The three units comprising the installation will be described in turn.

The Submarine Sound Transmitter.

The submarine sound transmitter consists essentially of a rigid. steel diaphragm about eighteen inches in diameter one side of which is in contact with the water and to the other side of which is attached electrical means for exerting alternate thrusts and pulls of sufficient magnitude to cause the diaphragm to vibrate against the highly incompressible water and of sufficient rapidity to give an audible sound. The driving force is usually produced by powerful electromagnets through the coils of which is passed an alternating current. The force so exerted on the diaphragm may reach several tons. Such a transmitter, which is commonly called a submarine sound oscillator, can be used for sending code signals if it is provided with a suitable transmitting key for making and breaking the A.C. circuit.

The pitch of the sound generated is dependent on and numerically equal to the frequency of the A.C. current. The coefficient of reflection of the sea bottom is doubtless a function of the pitch of the sound but very little is known of this function. As a result the pitch of 1100 which is used has been chosen from considerations other than the coefficient of reflection. This comparatively high pitch is employed for two reasons. First, it permits the use of a high-pass filter that greatly weakens the low-pitched local noises but allows the signals to pass. Secondly, high-pitched transmitters are better adapted for sending short signals in rapid succession. It

may be stated that signals having any pitch between 550 and 1100 have been found to reflect well from the sea-bottom.

The Submarine Sound Receiver.

The submarine sound receiver may be a very simple and inexpensive device provided it is not required to determine the direction of the sound. It will be shown later that good sounding data can be taken with such a receiver, but for securing data of the highest accuracy it becomes necessary to determine the direction of the echo. The MV Hydrophone, developed by the U. S. Navy during the war has proved to be superior to other types of direction finding sound receivers for use on ships for the reason that it is simple and rugged, permits of easy and rapid manipulation, responds with almost equal sensitivity to sound waves having a wide range of frequency, and through the use of several receptors intensifies the response to sound waves from any one direction while at the same time the response to sound from all other directions is weakened. As a result much of the interference from the numerous local noises such as are caused by propellers, auxiliary machinery and slapping of waves is eliminated, thus enabling the operator to hear comparatively weak signals and echoes which otherwise would not be audible.

The MV Hydrophone employs a double telephone head set, one receiver of which connects with one half of the submarine sound receptors and the other of which connects with the other half of these receptors. Direction of sound is determined through the operation of the binaural sense and is accomplished by so varying the time of energy transit from each submarine sound receptor to its respective telephone receiver that the responses from all of these receptors reach the two phones at the same time and in phase. Under such conditions the sound will appear to the listener to be binaurally centered, i.e., it will give him the impression that the source is located neither toward his right nor his left. The time of electrical transit of the several receptor circuits is varied by means of an "electrical compensator" through which all the receptor circuits pass before reaching the telephones, and the amount of retardation that must be introduced into these circuits to bring their currents into consonance at the phones determines the direction of the sound.

The electrical compensator is so ingeniously designed that the variations of current in the several receptor circuits, caused by the action of sound waves actuating the receptors from any definite direction, can be brought into consonance at the telephones by turning a single hand-wheel. This hand-wheel operates a rotary scale that is calibrated to give the direction of the sound with respect to a straight line passing through the sound receptors.

Two lines of sound receptors are employed, one for receiving sounds off the port and one for the reception of sounds off the starboard side of the ship. They occupy symmetrical positions on opposite sides of the hull, are placed as far forward and as far below the water-line as conditions will permit, and are directed parallel with the ship's keel. Only half of the sound receptors are used at any one time and the compensator is provided with a multipole switch whereby all the receptors on either side of the ship or the forward or rear half of the receptors on both sides of the ship can be connected through the compensator. When either of the first two combinations is used the compensator gives the direction of the sound with respect to the ship's keel and when either of the last two combinations is employed the direction of the sound is given with respect to a line running athwart ship and perpendicular to the keel.

The Sonic Depth Finder.

The Sonic Depth Finder is an automatic key used to make and break the A.C. circuit of the transmitter. It is designed to close this circuit for an instant at the end of equal time intervals and contains. means for continuously varying this interval through all values included between about ten seconds and one tenth of a second. It consists of a phonograph-like disk rotated at constant speed by means of a tuning-fork-speed-regulated motor. A small wheel having its axis parallel with a radius of the disk is driven by friction contact with the surface of the disk and carries means for moving it radially in and out along its axis. The axis of this wheel carries two cams, one having a single saw-tooth-shaped depression and the other carries ten such depressions uniformly spaced about its circumference. Each cam operates a pair of contact points, and by means of a twoway switch either pair can be connected in series with the coil of a

relay. The relay operates a pair of heavy contacts connected in series with the transmitter circuit.

In practice the disk is driven at the uniform rate of one revolution in ten seconds and the ratio of the diameter of the disk to the friction wheel is ten. Such an arrangement permits the speed of the friction wheel to be varied from one revolution in about ten seconds to one revolution per second, and by making use of the two cam wheels the time interval between oscillator signals can be given any value between ten seconds and one tenth of a second. It is obvious that this period can at all times be computed from the radius of the circle that the friction wheel scribes on the disk. This radius can be determined to within a very small fraction of an inch by means of a micrometer arrangement used for sliding the friction wheel along its axis.

In order that the operator may use the same pair of head phones for determining the direction of sounds picked up by the MV Hydrophone and also for making depth determinations, the telephone terminals of the compensator are connected by cable to the Sonic Depth Finder where, by means of a two-way multipole switch, both phones can be connected to the compensator or one phone can be connected to the compensator and the other to the secondary coil of a variocoupler the primary of which is energized by an inductive take-off from the A.C. circuit of the sound transmitter. These two positions of the selector switch may be called the (A) and (B) position respectively. The (A) position connects the two phones directly to the compensator. The apparatus is then adjusted for determining the direction of any sound picked up by the sound receptors and among others the echoes of the oscillator signals. The (B) position connects one phone inductively to the transmitter circuit and the other to the compensator. Under such conditions the operator hears each oscillator signal in the inductively connected phone at the instant it is transmitted and he hears the echo of each signal in the other phone which connects with the MV Hydrophone receptors. The vario-coupler serves to bring the signals in the two phones to approximately the same intensity.