511 HYDROGRAPHIC MANUAL PaGE 438 



vibrating surface in contact with the water. This is commonly known as an oscillator, 

 or transmitting unit. The surface, or diaphragm, may be made to vibrate by either 

 electric or mechanical means. The alternate condensation and rarefaction of the 

 water produced by the vibrating surface are transmitted to the adjacent layers of water, 

 and the sound wave travels away from its source at a velocity of approximately 4,900 

 feet per second. The transmitting unit has an effect in water similar to that of a radio 

 loudspeaker in air. 



Echo-sounding instruments employ a receiving device, whose function is similar to 

 that of a telephone transmitter, to detect the echo. This device is referred to as the 

 receiving unit, or sometimes a hydrophone. The receiving unit is immersed in the water 

 in a favorable position to receive the echo, and it converts the sound energy into electric 

 energy. As this electric energy is very small it must be amplified by means of a vacuum- 

 tube amplifier. 



The accuracy of echo soundings is directly dependent on a knowledge of the velocity 

 of sound and on the precision with which the travel time of the sound can be measured. 

 Numerous devices of varying precision are utilized to measure the travel time. They 

 constitute timing parts that operate in conjunction with the registering device of the 

 instrument. 



Although echo soundings are primarily measurements of the time required for the 

 sound to travel from the surface of the water to the bottom and return, the time units 

 must be converted into depth units to be of practical use. That part of the echo-sound- 

 ing equipment that measures the time interval in terms of depth is known as the register- 

 ing device. It may record the echo graphically or indicate it visually. The function of 

 the registering device is to control the timing of the original sound with reference to a 

 depth scale and to register the receipt of the echo on the same scale, the depth indicated 

 corresponding to the elapsed time. 



All types of echo-sounding equipment contain parts, in one form or another, which 

 perform these separate functions, all arranged to operate collectively. To illustrate the 

 relations between the various parts and the contribution of each to the whole operation, 

 a complete sounding cycle is described. A timing mechanism in the instrument, at 

 a precise instant, actuates the transmitting unit to emit a sound of short duration in the 

 water. This instant is usually near the zero of the depth scale. The sound travels 

 downward, is reflected from the ocean bottom, and returns as an echo to be detected 

 by the receiving unit, or hydrophone. The electric energy generated in the receiving 

 unit by the echo is increased sufficiently by the amplifier to register on the depth scale 

 at a point corresponding to the echo-travel time. This process is continuous, the cycle 

 of transmitted sounds and received echoes being repeated in rapid succession. 



The range of w^ater depths that can be measured by echo sounding is practically 

 unlimited. Soundings have been taken in depths from a few inches below a trans- 

 mitter to the greatest ocean depths (about 6,000 fathoms) — one echo-sounding instru- 

 ment is usually incapable of this, as instruments that are designed for accurate soundings 

 in shoal water are usually not suitable for soundings in deep water, and vice versa. 



The ease and rapidity with which water depths can be measured by echo sounding, 

 for use in navigation or hydrographic surveying, is remarkable as compared with other 

 means; and the results obtained with a properly adjusted instrument, if adequately 

 corrected, are far more accurate than those obtained by the older methods. Echo 

 soundings are not subject to the inherent uncontrollable errors and difficulties associated 

 with the leadhne, wire, and other depth-measuring devices. The deepest echo soundings 



