Page 451 echo sounding 5162 



electric field and, conversely, will create an electric field when subjected to mechanical stress. It is 

 this change of dimension, as in the magnetostrictive units, that is utilized for the production of 

 acoustic waves. 



Most echo-sounding transmitting units making use of the piezoelectric effect, one type of which 

 is illustrated in figure 98, consist of a mosaic assembly of quartz crystals A, which are clamped between 

 two metal plates B and C. The thickness of the assembly of crystals and plates determines the period 

 at which the system will be resonant. One of these plates B is in direct contact with the water and 

 forms the emitting surface. The energizing force to operate this unit is the electric field between the 

 two metal plates. Electric energy to excite the piezoelectric oscillator may be from an alternating- 

 current generator, but ordinarily excitation is produced by the high-frequency oscillations resulting 

 from the discharge of a condenser into an electric circuit. Piezoelectric oscillators are generall}- used 

 at supersonic frequencies, and may be used at frequencies much higher than is normally considered 

 practicable for most other types of acoustic transmitting units. 



D. HAMMER OR STRIKER TYPE 



Impact sound producers, commonly known as hammer or striker types, generate a damped signal 

 in the water. The essential operation is performed by a heavy metal plunger or hammer, forced by a 

 compressed spring, which strikes a sharp blow against one side of a metal diaphragm, the other side 

 of which is in direct contact with the water. The vibration of the diaphragm caused by the blow 

 sends acoustic waves into the water. After the blow, the plunger is raised from the diaphragm by 

 an electromagnet, the actuating spring being compressed during this operation. This leaves the 

 plunger poised in position to repeat the cycle. 



Instruments of the striker type are usually used only in moderate depths. Uncontrollable varia- 

 tions in the time of release and rate of fall of the impact plunger make this instrument unsatisfactory 

 for use in accurate hydrographic surveys. As a navigation instrument, where reasonable accuracy 

 is all that is necessary, its relative simplicity has contributed to its extensive use. 



5162. Acoustic Receiving Units 



The purpose of the echo receiver, generally known as the hydrophone, is to take 

 the acoustic energy of the echo from the medium and convert it efficiently into electric 

 energy. Echo receivers are composed of two parts; the sound collector or diaphragm, 

 and an element which converts acoustic energy to electric energy. 



All echo receivers have their diaphragms or soimd-collecting members submerged 

 in water. This may be the water outside the ship's hull, or water in a tank contiguous 

 to the inside skin of the ship. In the latter case the echo must penetrate through the 

 hull plates to reach the receiver. The electroacoustic converter may be inside a water- 

 tight cavity closed by a diaphragm, or in direct contact with the water. 



The type of echo receiver is usually determined by the frequency of the acoustic 

 waves employed, whether sonic or supersonic. To receive sonic frequencies either a 

 carbon button or an electromagnetic type of elem^ent is generally employed. Such 

 elements are normally attached to the inner side of a diaphragm that closes a watertight 

 cavity. The diaphragm may or may not be tuned to a definite frequency in water. 

 The carbon-button element is often mounted in a watertight rubber jacket, which is 

 submerged in a water-filled tank installed against the inside skin of the ship. 



Supersonic frequencies are most commonly detected on magnetostrictive or piezo- 

 electric receivers because of their more favorable response to these frequencies. 

 Magnetostrictive receivers are either operated in conjunction with a tuned diaphragm 

 or the magnetostrictive material is in direct contact with the water. Some piezo- 

 electric receivers have the crystals sandwiched between two metal plates, one of which 

 is in contact with the water. The plates and the crystals form an integral resonant 

 unit which is timed to the desired frequency. 



