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HYDROGRAPHIC MANUAL 



energy and is most sensitive to its recep- 

 tion. Transducers have been designed for a 

 wide range of frequencies from near sub- 

 audible, below 50 cycles per second, to super- 

 sonic frequencies of 200 kilocycles or more. 

 Each range of frequencies has distinct ad- 

 vantages for use in a certain depth range. 



3-97 Sonic frequencies. — Sonic frequen- 

 cies can be generated in the water at a high- 

 energy level. Most echo sounders used in 

 navigation employ sonic frequencies. Because 

 of the low absorption at sonic frequency their 

 high penetrating power makes them useful 

 for deep soundings. Sonic frequencies have 

 certain restrictive limitations. These fre- 

 quencies cannot be used to measure extremely 

 shoal depths with a high degree of accuracy. 

 Most of the energy of water and ship noises 

 is in the sonic-frequency range and, there- 

 fore, sonic soundings are more susceptible 

 to interference from strays of this kind. Be- 

 cause of their long wave lengths, these low 

 frequencies cannot be directed, or beamed, 

 to an advantageous degree without using 

 transmitting and receiving units of a pro- 

 hibitive size. 



3-98 Supersonic frequencies. — Supersonic 

 frequencies overcome, to a large extent, 

 most of the disadvantages of sonic frequen- 

 cies. The advantages of supersonic frequen- 

 cies are: high directivity with small trans- 

 mitting and receiving units and resulting 

 concentration of sound energy; less inter- 

 ference from ship and water noises ; shoal 

 depths can be measured ; a more detailed pro- 

 file of irregular bottom can be obtained ; and 

 side echoes are reduced. On the other hand, 

 there is greater attenuation of sound, and 

 strays may be recorded when sounding in 

 turbulent water, or in areas where there are 

 sharply defined changes in water tempera- 

 ture or density at various depths. With pres- 

 ent designs of transducers it is not possible 

 to sound in very deep water when using 

 high frequencies. 



3-99 Acoustic elements. — There are two 

 principal parts of a transducer: the acoustic 

 element and the mechanical support of the 



element which may or may not function as 

 part of the acoustic system. There are nu- 

 merous types of acoustic element materials 

 which are constructed in a variety of shapes 

 and sizes. Only the materials and designs 

 used by the Coast and Geodetic Survey will 

 be described in detail. 



In general, the acoustic elements may be 

 grouped in two classes, magnetic and electro- 

 static. Of the magnetic class, the magneto- 

 striction type is used most in echo sounding. 

 When certain metals, such as iron or nickel, 

 are placed in a magnetic field they will 

 change dimension. If the metal is in the 

 field of a coil which is energized by an al- 

 ternating electric current, the metal will al- 

 ternately expand or contract along the axis 

 of the coll and in unison with the exciting 

 current. When the alternating field corre- 

 sponds in frequency with that of the me- 

 chanical resonance of the metal, maximum 

 vibration occurs. The transducer is usually 

 mounted so that a vibrating face is exposed 

 to the sea water and facing the sea bottom. 

 This face acts as a piston which, through 

 its vibration, generates sound waves in the 

 water. Conversely, when sound waves strike 

 the exposed face it is set into vibration which 

 changes the magnetic flux to produce a cur- 

 rent in the coil surrounding the metal. 



There are a number of different materials 

 used in electrostatic transducers. Those used 

 most are Barium Titanate, Quartz, Ammo- 

 nium Dihydrogen Phosphate (ADP) and 

 Rochelle Salts. The material is placed in an 

 electric field rather than a magnetic field, 

 but the effects are the same. It is normal to 

 have opposite faces of the material coated 

 with a thin metallic foil and the transmitter 

 or oscillator is connected to these faces. The 

 material changes its dimensions in unison 

 with the applied signal from the signal gen- 

 erator. Some of the materials used in elec- 

 trostatic transducers are cut from natural 

 crystals, and they must be cut carefully with 

 regard to the axis. Some of the new plastic 

 materials now in use, such as Barium Ti- 

 tanate, can be molded which adds enormously 

 to transducer design possibilities. For this 

 and other reasons. Barium Titanate is being 



