190 Lecture 10 
ments have excessively large diameters on the scale under consideration, they 
have proved very useful in providing known directional characteristics for cer- 
tain propagation studies. 
10.4.6, Calibration of Scan Pictures of Sound Fields 
The recorded sound fields illustrating this paper show light and shade pat- 
terns representing the variations of sound intensity in vertical cross sections of 
the water. The areas of high intensity are seen as white (bright) spots while 
those of very low intensity appear dark on the records. There are two possible 
sources of error in regarding the brightness of the spots as a measure of sound 
intensity: (1) the motion of the scanning transducer is SHM, and as a consequence 
undue emphasis is given tothe brightness of the CRO spot and to the photographic 
exposure at the turning points of the motion near the surface and bottom of the 
water layer, and (2) there is also a possibility that the brightness modulation 
by the sound signal applied to the grid of the CRO may not be linear. In the 
method of intensity calibration to be described, these two unknowns are taken 
into account to some extent. The record brightness in this method is varied 
by introducing known changes in the transmitter voltage in 2/1 (6-db) steps or 
by introducing a db attenuator in the receiver circuit. Typical records of such 
calibrations are shown in Fig. 10.30. The upper record shows a scan at con- 
stant range reduced in strength by 6-db steps, the lower intensities being 
thereby gradually eliminated from the record. The two lower records show the 
effect of repeating the same longitudinal scan of the tank with a 6-db reduction 
of signal strength in the second record. The predominant features and over-all 
character of the records of course remain the same. 
10.5. THEORETICAL WORK - FULL-SCALE TRIALS 
As indicated at the outset of this paper, the theoretical treatment of the 
complete problem of sound transmission in shallow water, even under idealized 
conditions, is very difficult. A brief reference was made to the work of C.L. 
Pekeris, particularly in regard to his development of the mode theory in 1948. 
More recently T.G. Weale [8] has dealt with the mathematical aspect of the 
0 db 6 db 12 db 18 db 
Fig. 10.30. Intensity cali- 
bration records. (1) Con- 
stant range intensity re- 
duced in 6-db steps. (2) 
and (3) Longitudinal scan 
with a 6-db reduction in 
record 3, 
om we 
? > i ; 
a “ee 2 heats 9 
Nn 
