Page 449 echo sounding 516 



516. Description and Function of Parts 



Every echo-sounding instrument is composed of three principal parts, which per- 

 form individual functions, the combination of which results in a depth measurement. 

 The three parts are: the acoustic transmitting unit or sound producing part, and the 

 source of energy to operate this unit; the acoustic receiving unit and the echo ampHfier; 

 and the registering device, including a motor whose speed is controlled, an index and 

 depth scale, and the necessary keying circuits. Under the following headings are given 

 descriptions and functions of each of the principal parts, and a brief summary of the 

 different kinds of equipment used to perform these functions. 



5161. Acoustic Transmitting Units 



The function of the transmitting unit is to generate acoustic waves in the water 

 with sufficient intensity and duration so that an echo can, be received from the bottom, 

 of sufficient strength to be amplified to register the depth. Acoustic waves are ordi- 

 narily generated in the water by the motion, or vibration, of a surface or diaphragm, 

 which alternately condenses and rarefies the water next to it, thereby starting a train 

 of acoustic waves that is propagated away at the velocity of sound. The transmitting 

 miit may have its emitting surface in direct contact with the sea water or the entire 

 unit may be enclosed in a water-ffiled tank, the acoustic waves passing through the 

 ship's plates. The transmitting unit is frequently housed in an internal water-ffiled 

 tank on a wooden-hulled vessel but, because the wooden hull interferes with the trans- 

 mission of sound, a hole must be cut in it, so that the water in this tank is separated 

 from the outside water by only a metal plate. On small vessels and launches the 

 transmitting unit of a portable instrument is sometimes housed in a streamlined casing, 

 known as ajish, which is secured in a submerged position alongside or over the bow of 

 the vessel. 



Electric energy is normally used to operate the transmitting unit, but mechanical 

 energy is employed in some cases. 



Transmitting units may be divided into four classes: (1) electromagnetic; (2) mag- 

 netostrictive ; (3) piezoelectric; and (4) mechanical, that is, the hammer or striker type, 



A. ELECTROMAGNETIC UNIT 



The electromagnetic type of acoustic transmitter has a diaphragm, or emitting surface, actuated 

 by the alternate attraction and repulsion of an electromagnet. The magnetic force may act directly 

 on the diaphragm or be coupled to it through a system of levers. The diaphragm is sometimes tuned 

 and in such case, when the frequency of the driving force corresponds to the natural frequency of the 

 diaphragm, the amplitude of vibration of the latter is at a maximum and hence the maximum energy 

 is transmitted to the water. Electric energy to actuate the electromagnetic transmitting unit is 

 normally derived from some source of alternating current, such as a rotary generator. It is common 

 practice to use an alternating current of half the frequency of the sound to be transmitted in the 

 water. The frequency of vibration of the diaphragm of the transmitting unit will be doubled if there 

 is no constant polarizing force to prevent the diaphragm from following each alternation of the elec- 

 tric current, irrespective of its direction. Electromagnetic transmitting units are generally used at 

 sonic frequencies because of their greater efficiency at these frequencies. 



B. MAGNETOSTRICTIVE UNIT 



An increasing number of acoustic transmitting units utilize the magnetostrictive properties of 

 certain metals and alloys in the production of supersonic signals. Certain metals change their linear 

 dimensions when placed in an electromagnetic field. This property or phenomenon is called magneto- 



