LECTURE 18 
SIGNAL PROCESSING OF UNDERWATER 
ACOUSTIC FIELDS 
P.L. Stocklin 
Office of Naval Research 
Washington, D.C. 
U.S.A. 
18.1. INTRODUCTION 
Within the Acoustics Branch of the Office of Naval Research, we have a 
program of applied research in underwater acoustic signal processing. Since 
a thorough treatment would require far more space than is at my disposal, I 
will attempt first to summarize the rationale behind this program, and then to 
treat in detail one research area which may be both novel and of interest to you, 
that of space—time decision-theory applications in acoustic signal processing. 
18.2. RATIONALE 
Signal processing is defined as the art of using physical information to make 
decisions. The basic physical fact of life around which modern signal-processing 
research is being built is the instantaneous acoustic field, in contrast to the 
basis of power averages of the field which has led in the past to such notions 
as power-pattern formation and correlation analysis. With a detailed knowledge 
of the acoustic field for situations of interest, we can make intelligent surmises 
about processor design and behavior and formulate from these a meaningful 
program. This detailed knowledge may in itself be only statistical; but the point 
is that, precisely to the extent we can describe the field in time and space, to 
that extent we can design an efficient processing system. The field description 
and the design of an efficient processor are in fact one and the same. 
A useful starting point for this discussion is the well-known sonar equation 
for passive detection: 
(F)_ = Ge Mw) — Gn - Non (1) 
N]sp 
where 
(S) = Signal differential 
SD 
L, = radiated source level, db/ipb/1 yard 
NV, = propagation loss, db 
L, = ambient noise level (local), db/1pb 
Np, = Girectivity index, db 
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