SKCT. 4] SOUND IN THE SEA 491 



Cavitation is the most serious but not the only cause of emission of noise by 

 ships. At certain speeds some propellers emit a loud pure note due to vibration 

 of one or more blades, an effect known as "singing" ; moreover, machinery 

 inside the ship causes the hull to vibrate and thus to radiate sound. 



Lastly, vibrations of the ground produced either by the activities of man or 

 by earthquakes or volcanoes or other natural causes can be transmitted 

 through the ground to the sea-bed and thence to points in the sea. Vibrational 

 energy from earthquakes and volcanic activity distant from the point of 

 reception is rich in low frequencies, probably because any high-frequency 

 energy has been dissipated during transmission through the earth. However, 

 nearby volcanic activity clearly radiates elastic energy having a broad spectrum, 

 much of it in sonic frequencies (e.g. see Snodgrass and Richards, 1956). Sharp 

 reports, apparently of volcanic explosions, were received over hydrophones 

 several miles from the cinder cone of Capellinhos off Fayal, Azores, by a Woods 

 Hole group aboard U.S.C.G.C. Yamacraw in 1958 (unpublished). Industrial 

 activities can produce vibrations of high frequency but the attenuation in the 

 ground then being high the resulting noise in open water is rarely of any 

 consequence. 



4. Instruments and Applications of Sound to Oceanography 



A. Detectors 



Although detectors of the electromagnetic, magnetostrictive and other types 

 are suitable in special circumstances, the piezoelectric detector is the one most 

 frequently used. 



Materials are said to be piezoelectric if electric charges are liberated on them 

 by compression. Quartz, Rochelle salt, ammonium dihydrogen phosphate and 

 barium titanate are well-known examples. In principle the detector consists of 

 a plate of the material with metal coatings, called electrodes, on two opposite 

 faces. One electrode is grounded, the other connected to the grid of a thermionic 

 tube which forms the first stage of an amplifier ; when the plate is subjected to 

 stress the output of the amplifier changes in proportion. The faces exposed to 

 the pressure wave of the signal are not necessarily those on which the electric 

 charges appear ; whether they are or not depends on the material and on the 

 orientation of the surfaces to the crystal axes. 



If the detector is to cope with a wide range of frequencies, its own natural 

 frequencies of vibration must be well above the highest frequency to be ob- 

 served. In order that this condition may be fulfilled, it is necessary that the 

 dimensions of the plate be not greater than the least wavelength, a require- 

 ment also necessitated by another consideration, which is that all parts of the 

 active faces of the detector should be exposed to the same pressure. It is not, 

 however, always possible to fulfil this condition at high frequency and at the 

 same time preserve adequate sensitivity, thus calibration of detectors over the 

 whole range of frequencies for which they are intended is always a wise precau- 

 tion to take. 



