170 



INTENSITY FLUCTUATIONS 



BOTTOM 

 REFLECTION 



<SmC_ 



Figure 10. Oscillograph trace showing the effect of bottom-refiecteci sound. 



fereiices were random and if the average number of 

 paths were sufficiently great (at least five or six), the 

 resulting distribution of intensities should approach 

 the Rayleigh distribution very closely. 



Against this proposed mechanism two principal ob- 

 jections have been raised: one, experimental, the 

 other, theoretical. The experimental objection is 

 simply that later research has revealed that the Ray- 

 leigh distribution is only occasionally a very good fit 

 to observed transmitted intensities. 



The theoretical objection concerns the phase shifts 

 expected from the observed microstructure. it is pos- 

 sible to compute the root-mean-square difference in 

 acoustical path length between two alternative paths 

 through the interior of the ocean on the basis of the 

 average parameters of the observed microstructure.' 

 It turns out that the magnitude of this variation in 

 path length is too small to produce the random phase 

 shifts as required for Rayleigh distribution. This 

 argument is not entirely conclusive, because the 

 microstructure parameters reported in Section 5.1.3 

 were obtained on a single run and have not been 

 confirmed by a repetition of the experiment. 



No critical evaluation has as yet been made of the 

 multiple path hypothesis on the basis of wave 

 acoustics. The multiple path hypothesis is based, 

 conceptually, on ray acoustics, and it may be that 

 the ray concept has been stretched in this case be- 



yond the limits of its validity. A similar analysis for 

 a different problem has been made by CUDWR.^ 



7.2.3 Lens Action of Microstructure 



If light passes through a medium with variable 

 index of refraction, such as the turbulent heated air 

 above a tarred road on a hot summer day, objects 

 seen through this medium are often blurred. If sun- 

 light falls on a white screen after having passed 

 through such a medium, say the hot gases surround- 

 ing an open flame, the surface of the screen is mottled, 

 with bright and dark patches alternating and chang- 

 ing rapidly as the thermal microstructure of the trans- 

 mitting medium is varied. This random fluctuation 

 in the brightness of the illuminated screen can be ex- 

 plained by means of the lens action of patches of 

 above-average and below-average velocity of propa- 

 gation of light. A similar explanation has been sug- 

 gested to account for part of the fluctuation of trans- 

 mitted sound intensity in the sea.' This role of the 

 thermal microstructure in fluctuation is quite dif- 

 ferent from the hypothetical interference effect dis- 

 cussed in Section 7.2.2. While the interference effect 

 is based on the coexistence of several distinct paths 

 through the interior of the ocean, fluctuation because 

 of refraction will be produced even over a single path. 

 The theoretical treatment, not reproduced here. 



