the surface and its course directed on the 

 underwater telephone. 



By and large, most submersibles employ 

 sonic (wireless) communications systems re- 

 lying on the water column, instead of a wire, 

 to carry the conversation. The carrier fre- 

 quencies employed in commercially available 

 devices commonly are 8, 28 or 42 kHz. Sev- 

 eral factors weigh heavily on the choice of 

 frequencies: range, ambient noise and com- 

 patibility with other communication sys- 

 tems. 



Range of an underwater telephone is then 

 a problem of signal strength versus refrac- 

 tion, reverberation, scattering and the am- 

 bient noise at the receiver. Ambient sea 

 noise has a large low frequency content; 

 hence, a low frequency communications sys- 

 tem must compete with this, but, on the 

 other hand, low frequency sound is less ab- 

 sorbed on its way to the receiver. The selec- 

 tion of the carrier frequency or communica- 

 tions system is a compromise of all the above 

 factors. Table 14.7 (Chap. 14) presents ranges 

 of various communicating systems; these are 

 advertised ranges, and, as the manufactur- 

 ers agree, they are not always attainable. 



The majority of submersibles use a system 

 operating at 8.0875 kHz for sub-to-surface 

 communications. This frequency is good for 

 range in areas of little ambient noise. The 

 higher frequencies (28, 42 kHz) provide good 

 short- to medium-range communications and 

 are usually used in sub-to-diver and diver-to- 

 diver systems. Because the submersible it- 

 self is a noise generator, one must consider it 

 as a negative factor in the selection of a 

 diver-to-sub wireless communication system. 



Chapter 2 briefly discusses sound propaga- 

 tion in the sea. A wide-ranging and compre- 

 hensive treatment of underwater acoustics 

 can be found in reference (12). For purposes 

 of this discussion, it is sufficient to note that 

 the velocity of sound in a liquid varies with 

 temperature, salinity and density. Previ- 

 ously, it was explained that sound waves 

 may be bent (refracted) and do not always 

 follow a straight path. To further complicate 

 its transmission, a sound can be scattered by 

 objects in the water and large objects (in- 

 cluding the bottom and the surface) can pro- 

 duce echoes (reverberation). The result is to 

 distort the signal such that intelligible com- 



munications are difficult. Added to this is 

 noise produced by animals, the sea surface or 

 ship traffic which may act to mask or drown 

 out the communications. Finally, the signal 

 itself spreads as it travels to a receiver and 

 this spreading loss also results in signal deg- 

 radation (with strength dropping as the cube 

 of the distance). Since all of these factors 

 vary constantly, communications can be ex- 

 cellent one day and, for all practical pur- 

 poses, non-existent the next. In some in- 

 stances communications can vary within the 

 tenure of a 5- or 6-hour dive depending upon 

 the terrain or environment over, within or 

 under which a vehicle may operate. 



The ambient diver confronts all of the 

 problems found in submersible communica- 

 tions and quite a few in addition: The diver 

 must have full freedom of mouth and lip 

 movements to enunciate words, the face 

 mask cavity into which he speaks often en- 

 hances lower frequencies and attenuates 

 higher frequencies, exhalation of bubbles in- 

 terferes with communications, and his hear- 

 ing sensitivity is less in water than in air. If 

 the diver is wearing a helmet the problems 

 also involve ambient noise caused by air 

 injected into the helmet and expired air es- 

 caping through the exhaust valve. When he- 

 lium (instead of nitrogen) is used with oxy- 

 gen a further complication arises in that this 

 lighter gas medium causes a shift in the 

 speech frequency and a resultant "Donald 

 Duck" effect, but converters are available 

 that can reconstruct the diver's voice so that 

 it is intelligible (13). 



A number of commercial diver communica- 

 tion systems is available, and tests have 

 shown their performance as adequate (14). 

 Where one offers good intelligibility it may 

 fail in reliability or vice versa. The improve- 

 ments in this area within the sixties have 

 been quite remarkable, and it is likely that 

 the increasing use of divers and lock-out 

 submersibles in the offshore oil industry will 

 focus more attention with subsequent solu- 

 tion of those problems that remain. 



Hardwire telephones to the surface have 

 been used since the advent of the hard-hat 

 diver, and the number of systems available is 

 large. All the tethered submersibles use a 

 hardwire telephone system, and in a few of 

 the very shallow submersibles (e.g., theiVAl/- 



493 



