LECTURE 17 
SOME EXPERIMENTS ON THE REDUCTION OF 
STRUCTURE-BORNE NOISE 
J.H. Janssen 
Technisch Physische Dienst T.N.O. en T.H. 
Delft, Netherlands 
17.1. INTRODUCTION 
"The acoustic stealth of a ship or torpedo is determined by the external 
noise it generates" [1]. Four different sources of underwater ship noise may be 
distinguished: (1) cavitation, (2) hydroelastic forces, (3) flow, and (4) machinery. 
This paper is concerned with some problems of machinery noise reduction. 
Noise control is basically a system problem. The system consists of a ship, 
water, and detection apparatus. It contains a source, a path, and a receiver for 
sound signals. The aim of noise control in this instance is to prevent detection 
by the enemy. In underwater noise reduction, more often than not a restriction 
is imposed on the measures to be taken: only the sound paths are available for 
blocking. 
It is for this reason that only the path is considered here, or rather the 
multiplicity of paths. Of course, not all types of ships, sources, and structures 
can be reviewed with respect to their acoustical behavior. A considerable 
simplification is obtained if only the structures of small ships like submarines 
Or minesweepers are studied. Only two typical noise paths seem to be important 
then; viz., the path via the structure only and the path via the air and the struc- 
ture. In the latter instance, for example, the noise is radiated by a diesel engine 
directly into the air and excites the shell, which in turn radiates sound into the 
water. The pure structure-borne noise follows the path through springs, foun- 
dation, and hull into the water. At large distances, hydrophones may detect the 
pressure fluctuations due to the radiated sound power anyway. 
In Fig. 17.1 a noise path together with associated levels is illustrated. In 
this paper some experiments, both theoretical and practical, relating to the 
steps in the path (machinery to foundation, foundationto shell, and shell to water) 
will be discussed. 
17.2. UNITS AND LEVELS 
Sound power manifests itself by way of fluctuations in pressure and particle 
velocity. In fluids and gases, these pressure fluctuations are easily measured 
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