76 



EXPERIMENTAL PROCEDURES 



the attenuator is automatically actuated when the 

 received signal level rises above or drops below certain 

 limits for several successive signals. All changes in 

 attenuator setting are recorded, either in a separate 

 log book, or automatically on the oscillograph record.^ 



4.3.2 



Field Procedures ^ 



In this section the field procedures used in trans- 

 mission runs will be described. First, a number of 

 oceanographic facts are ascertained and recorded, 

 either inamediately preceding or immediately follow- 

 ing each transmission run. These include the depth 

 of the ocean, the type of bottom (in shallow water), 

 the state of the sea, the swell, the wind strength, and, 

 most important, the vertical temperature distribu- 

 tion in the ocean. The bathythermograph, an instru- 

 ment which measures vertical temperature gradients, 

 is in general use in the Navy wherever echo ranging 

 is involved. It is a recording device which can be 

 lowered into the water down to considerable depth 

 (as much as 450 ft for the "deep" model) and which, 

 upon being returned to shipboard, indicates the 

 temperature versus depth distribution as a trace 

 marked on a smoked slide. Ordinarily, a bathyther- 

 mograph is lowered on each of the two vessels partici- 

 pating in a transmission run; the source vessel makes 

 its lowering at the point of greatest distance from the 

 receiving vessel and frequently one or two lowerings 

 at intermediate points. Figure 8 shows a blank which 

 contains the oceanographic information belonging to 

 a simple transmission run. This blank has been used 

 at UCDWR. 



Another subsidiary step is the calibration of equip- 

 ment. In this chapter the term calibration will be 

 used with a definite meaning. Calibration is a pro- 

 cedure which translates sound field data taken off the 

 oscillograph trace into the transmission loss. The 

 transmission loss was defined in Section 4.1 as the 

 difference in decibels between the source level S of the 

 Projector and the sound level L at the hydrophone. 

 Siiico the source level of the projector is defined in 

 turn as the sound pressure level at a range of 1 yd, 

 the transmission loss is then the difference in decibels 

 between the soimd levels at a range of 1 yd, and the 

 range r of the hydrophone. In theory, then, one would 

 obtain the transmission loss according to the follow- 

 ing formula. 



H = 101og^|, 



where a is the sound pressure amplitude in the water 

 at the hydrophone, and Ui is the sound pressure 

 amplitude at 1 yd. 



If it were possible to bring the receiving hydro- 

 phone up to a distance of 1 yd from the projector, the 

 absolute transmission loss could thus be readily de- 

 termined without knowing either the projector source 

 strength or the hydrophone response. The squared 

 ratio between the signal amplitude (on the oscillo- 

 gram or on the tube screen) at 1 yd and the signal 

 amplitude at R yards would give the transmission 

 loss, provided the design of the receiving stack guar- 

 antees proportionality between received pressure 

 amplitude and recorded trace amplitude. Actually, 

 it is next to impossible to bring the projector of the 

 sending vessel and the hydrophone of the receiving 

 vessel closer together than about 30 to 50 yd without 

 inviting a maritime catastrophe. Correction of the 

 observed signal level at 30 or 50 yd back to the pre- 

 sumed level at 1 yd has at times been done by 

 straightforward application of the inverse square 

 law. However, this method is probably too simple. 

 There is some evidence that, even at ranges of 50 yd, 

 the transmission loss cannot always be expected to 

 follow the inverse square law of spreading.^ 



Several more complicated methods have been em- 

 ployed, which in theory should enable determination 

 of the absolute transmission loss. vUthough none of 

 these suggested calibration procedures have proved 

 completely satisfactory, some may be preferable to 

 the simple correction by means of the inverse square 

 law. The following paragraphs are devoted to a 

 description of some of these more refined calibration 

 procedures. 



During a substantial part of its supersonic trans- 

 mission program, UCDWR carried out runs called 

 calibration runs at very short range, approximately 

 100 yd. During these runs, both the sending vessel 

 and the receiving vessel were permitted to drift. The 

 signal level at 100 yd, obtained from this run, was 

 arbitrarily assigned a transmission anomaly value 

 of zero; and all other transmission data obtained on 

 the same day were referred to the 100-yd level 

 obtained in the calibration run. Somewhat later, 

 these special runs were discontinued. Instead, an 

 average was taken of all the short-range data ac- 

 cumulated during the day, and a value of zero for 

 the transmission anomaly was assigned to this 

 average. In these two methods no attempt is made 

 to calibrate in terms of a distance of the order of 

 1 yd; that is, no test is made which would relate 



