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HYDRODYNAMIC SOURCES OF SOUND 



H. M. Fitzpatrick and M. Strasberg 

 David Taylor Model Basin 



Abstract 



The paper describes the various flow phenomena that result in underwater 

 sound in flowing water, and reviews the recent experimental and theoretical investiga- 

 tions of the sound sources. Four distinct sources of hydrodynamic noise are discussed: 

 entrained air bubbles; vaporous cavitation; surface disturbances; and unsteady flows. 

 For each of these, brief descriptions are given of 1. the physical mechanisms involved 

 in the noise generation; 2. the available experimental data; 3. the theoretical methods of 

 analysis; and 4. the relations between the flow parameters and the amplitude and spectral 

 distribution of the sound. 



I. Introduction 



Sounds associated with flowing air and flowing water are everyday experiences. 

 Examples are the splash of water and the proverbial babble of the brook, the howl of 

 the wind, the hiss of escaping air, and indeed all the sibilant and aspirate sounds of 

 speech itself. 



Despite their familiarity, sounds from flowing fluids have received comparatively 

 little scientific attention. Although most text books on acoustics discuss at length the 

 sounds produced by vibrating bodies, the equally-common sounds associated with flow 

 are usually ignored. 



Within the past decade, however, interest in hydrodynamic and aerodynamic 

 sources of sound has increased. At the same time, a theoretical foundation for dealing 

 with these acoustical problems has been provided by recent developments in fluid 

 dynamics, particularly in the study of turbulence. As a result, important advances in 

 our understanding of hydrodynamic noise have been made since Folsom, Howe and 

 O'Brien [1] reviewed the subject in 1942. 



The study of hydrodynamic noise involves a combination of hydrodynamics 

 and acoustics. These two subjects were closely allied in the classical physics of the 

 nineteenth century; Stokes, Helmholz, Rayleigh, and Lamb, for example, contributed 

 to both. But in recent years they have grown apart; acoustics becoming associated 

 with electronics, while hydrodynamics concentrated on problems of steady motion. 

 As a result, many workers in each of the fields are scarcely aware of developments 

 in the other. The study of hydrodynamic noise requires a reunion of the two disciplines. 



The object of this paper is to describe the various flow phenomena that can 

 result in sound, and to review the recent theoretical and experimental investigations that 

 have led to a partial understanding of the relations between the flow parameters and 

 the intensity and spectral distribution of the resulting sound. Because this symposium 

 is concerned with naval problems, it is appropriate to limit the discussion to underwater 

 sounds associated with the flow of water. The discussion will also be limited to those 

 sounds which can be considered to be "noise" in the sense that the sound is either 

 undesirable or else an incidental concomitant of the flow.* 



* The generation of sound by underwater sirens and other devices specifically designed 

 to act as sound sources is discussed by Bouyoucos.(2) 



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