BAFFLES 



ItiJS 



180' 



270" 



Figure 7. Change in response of dome-enclosed trans- 

 ducer due to interposition of baffle. 



KC 



Figure 8. Attenuation versus frequency for different sized 

 Nubbles. (R = bubble radius.) 



Few measurements have been made on the effect <>! 

 a baffle on the rear response of a transducer, particu- 

 larly for transducers enclosed in domes. Figure 7 

 shows the results of one such measurement. Referring 

 to this figure it is seen that for angles between the 

 transducer and dome axes varying from 90-270 de- 

 grees (180 degrees corresponds to the transducer fac- 

 ing away from the source) a 34x24-inch rectangular 

 and dome-enclosed baffle gives an average decrease of 

 only a few decibels in the response of a dome-enclosed 

 transducer 20 inches away compared to the response 

 of a bare projector at the same distance with no baffle 

 interposed. These measurements, however, also in- 

 clude the detrimental effect of reflections from the 

 dome wall which usually increase the response in the 

 90 to 270-degree sector by 5—15 db. Thus, in this case, 

 the effect of the baffle more or less cancels that of the 

 dome. Further experiments are particularly desired 

 which will compare the rear response (angles of 90- 

 270 degrees) of dome-enclosed and of bare transducers 

 with and without a baffle. 



Summing up, it has been shown that the diffraction 

 of sound around, rather than its transmission through. 



the baffle limits the latter's effectiveness. Both with 

 regard to transmission loss and to diffraction, air- 

 filled baffles are superior to steel. To minimize the dif- 

 fraction effect, the baffle should be appreciably larger 

 than the transducer and should be placed as close as 

 possible to it. The baffle-transducer distance should 

 always be considerably less than the smaller of the 

 two critical lengths given above. 



9.2.1 



Bubble Screens 



ft has been found that air bubbles resonate in water 

 at frequencies depending on their size and that at and 

 near that frequency they are very effective scatterers 

 and absorbers of sound. When, then, a layer of such 

 bubbles is inserted in the sound field, it offers very 

 high attenuation. Such a layer may be used as a baffle 

 and is called a bubble screen. 



The propagation of sound through water contain- 

 ing bubbles has been studied by USRL both theoreti- 

 cally and experimentally." 4 '"" The attenuation per 

 centimeter thickness of a bubble screen varies directly 

 as the number of bubbles per cubic centimeter. Fig- 



