Any method which shows possibilities of giving the captain and navigator soundings quickly and accurately is worth developing and using. In this paper an attempt is made to discuss some of the methods used for determining the depth of water by sound.
There are several distinct ways of using sound to obtain depths of water from a ship, namely:
- Stop-watch method.
- Angle of reflection method.
- Hayes sonic depth finder.
- The fathometer.
These methods will be taken up in this order.
Each of these methods, except the angle of reflection method, makes use of the fact that distance equals velocity times time. In the stop-watch method the only apparatus required is a stop-watch, a microphone or some other means for detecting sound in water, and a sound source. Let a sound impulse be made at the surface of the water. The sound will travel to the bottom and part of it will be reflected back to the surface where it is picked up by the microphone. The stop-watch is started when the impulse is made at the surface and is stopped when the echo from the bottom is picked up by the microphone. Thus, the time interval between initial impulse at the surface and the receipt of the echo by the microphone is determined. The velocity of sound in water is well known. It is about 4,800 feet per second, varying somewhat with the temperature and density of the water. For sake of simplicity 4,800 feet per second will be used throughout this paper. Knowing the elapsed time in seconds and the velocity, all that needs to be done to determine the depth of water is to multiply them together and divide by two.
This method is limited to some extent by the accuracy of the stop-watch, but to a much greater extent by the personal error entering into the starting and stopping of the watch. The average person stopping an ordinary stop-watch when he hears an echo from the bottom has an error of about one- fifth of a second. Suppose the depth is 2,400 feet or 400 fathoms. This error of one-fifth of a second in time would introduce an error of 480 feet or 80 fathoms in depth. This is an error of twenty per cent. No account has been taken of any existing error in starting the stop-watch when the initial impulse is made. Obviously this method without refinements is practically useless for accurate work but might be satisfactory for estimates, etc.
[Image: Tube, Microphone and Chronometer]
A German invention uses the stopwatch method in a manner slightly different from that described above. It makes use of a tube which runs down through the bottom of the ship from the pilot house, and a special microphone and chronometer arrangement. A pistol which expels an explosion cartridge is fired down the tube. The cartridge explodes when it strikes the water and makes a sound of good intensity. When the sound is made, it is picked up by the microphone and the chronometer is automatically started. The sound travels to the bottom and back to the ship where it is again picked up by the microphone and the chronometer is automatically stopped. Thus, the personal error in starting and stopping a watch is eliminated; the chronometer automatically registering the interval between the time the sound was made and the time of arrival of the echo at the ship. Knowing the velocity of sound in water and the time interval the depth is determined exactly as it was in the straight stop-watch method. This is much better than the straight stopwatch method, but still it is not very accurate.
With the angle of reflection method, use is made of one of the microphone lines of the electric M.V. compensator or of one side of the acoustic M.V. compensator. The compensator will not be described here, but it should be noted that this device is used to determine the relative angle as measured from the bow that a line drawn from the sound source to the geometric center of the microphone line makes with a line drawn through the microphones. Suppose that the compensator is adjusted so that it is “focused” for sound coming from a direction making a certain angle with the microphone line, and that this angle lies in a plane which is parallel to the surface of the water, as is normally the case when obtaining a bearing of a sound source. The compensator also is in “focus” for sounds coming from any direction so long as that direction makes the same angle with the base line as before. In other words, the compensator can be “focused” for any sound the source of which lies on the surface of a cone.
The propeller sounds from a ship will be reflected from the bottom to the microphone, as shown in larger figure below.
Now if the compensator be focused on this sound from the bottom, the angle from the bow to the direction of this sound from the bottom will be measured. Or what amounts to the same thing, the reciprocal of this angle, “a,” is determined. B is the base-line between the propellers, or the sound source, and the center of the microphone line. It is desired to determine the depth, D. Obviously, or D = .5B tan a.
A scale marked in fathoms of depth can be put on the compensator or, better, a table of compensator readings and depths in fathoms can be made. Some corrections that may be necessary are: inclination of the compensator line from the horizontal, depths of microphones and propellers below the surface, trim of the ship, shelving of the bottom. The toeing in of the compensator line toward the bow is usually neglected. This method has been and is used in practice. It gives good results up to a depth equal to about three times the length of the base line, B. Beyond this depth tan “a” changes so rapidly that a small error in setting the compensator results in such a large error in depth that the method ceases to be useful. It requires a trained listener for good results.
The Hayes sonic depth finder, invented by Dr. Hayes of the sound section of the naval research laboratory, operates according to the law that the distance a sound travels in a uniform medium in a given time equals the product of this time and the velocity of the sound in the medium. It is essentially a device whereby a continuous series of uniformly timed sound impulses may be sent out from a submarine oscillator. The operator hears these outgoing impulses in one ear while in the other ear he hears the echo of them as they are reflected back from the
bottom of the sea and picked up by the microphones. He is able to tell when each outgoing impulse exactly coincides with the returning echo of the previous impulse with a precision of about one one-hundredth of a second. The time interval between successive outgoing impulses may be varied continuously over a range of from one-tenth of a second to ten seconds, corresponding to the depths of water from approximately 40 fathoms to 4,000 fathoms.
When taking a sounding the operator adjusts the interval between successive impulses by trial until he observes unison between an outgoing signal and the echo of the previous outgoing signal. When this adjustment has been made, the time that it takes for the sound to travel to the bottom and return may be determined from a scale on the front of the apparatus. This time interval multiplied by the velocity of sound in water gives twice the required depth. It should be noted that this device does not measure the time required for a single impulse to go to the bottom and back. Such methods, as previously stated, under conditions found at sea, are capable of only the stop-watch precision of about one-fifth of a second which corresponds to a large error in depth. The Hayes sonic depth finder has a precision of four or five fathoms 01- better.
The diagram given below may clear up the elementary theory of this depth finder.
1a, 2a, 3a, etc., are outgoing impulses.
1a', 2a', 3a', etc., are the returning echoes for a depth of water “A.”
1b', 2b', 3b', etc., are the returning echoes for a depth of water “B.”
1c', 2c', 3c', etc., are the returning echoes for a depth of water “C.”
As shown in the figure, if the depth is varied, some depth such as C can be found for which the echo 1c' will coincide exactly with the second outgoing impulse 2a. As the time between impulses is accurately known it is easy to calculate the depth of water. Of course, in the actual depth finder the time intervals between the outgoing impulses are varied until the beginning, or “head,” of the echo of each outgoing impulse is brought into exact coincidence with the “head” of the next following outgoing impulse.
A technical description of this device will not be given, but it might be well to point out some of its features. The sonic depth finder is an accurately built piece of apparatus which is electrically and mechanically rugged. In the Navy type of installation the outgoing impulses are sent out by a skin type oscillator controlled by an automatic key. The speed of a rotary converter is accurately maintained constant at 1,800 revolutions per minute by means of a Leeds and Northrup tuning fork. By this means a horizontal table which is fifteen inches in diameter is driven at a constant speed of six revolutions per minute through a reduction gear. A small vertical wheel one and a half inches in diameter is driven by the horizontal table. This vertical wheel can be moved toward or away from the center of the table so that its speed can be varied continuously. The vertical wheel drives a shaft which rotates two cam-shaped discs. Either of these cams may be used. If the slow cam is used, one outgoing impulse is sent out for each revolution of the cam; if the fast cam is used, ten impulses are sent out for each revolution of the cam. A hand wheel rotates a horizontal, shaft which moves the vertical wheel in or out from the center of the table, and, therefore, controls the speed of rotation of the cams and the interval between outgoing impulses. A scale on the front of the instrument indicates the number of outgoing impulses sent out each second. Curves of depths in fathoms against scale readings in impulses per second can be plotted and used for quickly determining the depths from the scale readings.
The Hayes sonic depth finder seems to give excellent results in practice. At present it cannot be used to obtain depths much less than forty fathoms and this is a very serious drawback.
In brief, the fathometer is designed to give the navigator a visual and continuous record of depths which are less than one hundred fathoms. A constant speed motor drives a horizontal shaft, C, in the figure. This shaft rotates an arm A. Each time this arm sweeps past the contact B a sound impulse is sent out into the water. This sound travels to the bottom and is reflected back to the ship where it is picked up by a microphone. It is converted from sound energy into electrical energy and is filtered and amplified. Then it operates a spark coil. By this means a Geissler tube carried on arm A is lighted when the echo from the bottom reaches the ship. The arm A makes four revolutions per second so that there will be four flashes of the tube per second. A scale calibrated in fathoms is provided so that the depth can be read directly off at the point where the tube lights up. Actually the light dances around a mean point at any depth.
For depths greater than one hundred fathoms a clutch is engaged by means of a handle at the side of the fathometer. This slows the speed of rotation of arm A to one revolution in one and one half seconds. One and one half seconds of time corresponds to a depth of six hundred fathoms. A white light on arm A is lighted when the clutch is engaged. The operator wears ear phones and hears the outgoing impulse made when the arm A passes a fixed contact and also hears the returning echo from the bottom. He merely notes the location of the white light when the echo returns and reads the depth directly from a second scale (zero to six hundred fathoms). By this “White Light” method depths from one hundred to fourteen or fifteen hundred fathoms may be obtained.
The fathometer, as built by the Submarine Signal Company, is now a commercial product. It is limited to some extent by the fact that ships’ noises cover a large part of the audible sound spectrum. Some of these ships’ noises may be picked up by the fathometer and if they are of sufficient intensity may cause the tube to light up when it should not light and thus give an erroneous reading. The device, unlike the human ear, is unable to distinguish between ships’ noises and echoes, so long as the ships’ noises are of the proper frequency and intensity. Also, the fathometer has a rather delicate electrical circuit and this has handicapped it. However, with reasonably good personnel it gives good results and has possibilities of filling a great need in navigation.
No mention has been made of other uses of sound in navigation. Ranges of beacons can be obtained by observing radio and sound signals which are sent out simultaneously. The time elapsed between the transmission and the receipt of a radio signal is considered to be zero while the velocity of sound in both air and water is well known. Bearings on sound beacons can be obtained by compensators or other means. The location of icebergs and ships by sound offers a large field and needs further development.