BAFFLES 



CD 

 O 





2 4 6 B 10 2 4 



TRANSDUCER BAFFLE SEPARATION IN WAVE LENGTHS 



Figure 6A. Decrease in rear transducer response due to baffle. 



10 



Consider first &R. Figure 5 gives an experimentally 

 obtained plot of SR versus separation between baffle 

 and transducer for plane waves of various frequencies 

 incident upon the former. k The baffle was air-filled, 

 circular in shape, and had an 18-inch diameter; the 

 transducer was also circular and had a 1 1-inch geo- 

 metrical and a 10-inch acoustic diameter. It is seen 

 that, in general, SR decreases with increasing separa- 

 tion; the values of SR, particularly those at the lower 

 frequencies, are in rough agreement with theory. 1 



Consider now the effect of interposing a baffle on 

 the rear response of the transducer. Figure 6 (A, B, C) 

 gives theoretically obtained plots of 8R' versus sepa- 

 ration between baffle and transducer. The theory and 

 the plots here reproduced indicate that an air-filled 



k The data were obtained at the Orlando station of I'SRL. 

 l See reference 68, equation (44c): 



baffle with a transmission loss ^ 25 db decreases the 

 transducer rear response by 15-25 db provided that 

 the transducer-baffle separation x is considerably less 

 than the smaller of the two critical lengths, irab/1.2\ 

 and 8[(rt - b) 2 /X + (a - b) + (A/1)], where a is the 

 radius of the baffle and b the acoustic radius of the 

 transducer. 1 " The first critical distance corresponds 

 to the bright spot covering the rear of the projector, 

 the second, to the annular ring on the rear making an 

 important contribution. 



It is seen from the above relations that for a = /;, 

 that is, for a transducer of the same size as the baffle, 

 the baffle-transducer separation v must be less than 

 2a to have any shielding at all. On the other hand, 

 for a = b, and x as small as A, the relations given are 







where \ is the wave length, a the radius of the baffle, b the acous- 

 tic radius of the transducer, and \ the transducer-baffle separa- 

 tion (air-filled baffle). Equation (44c) docs not include the effect 

 of the transmitted wave. 



"| The plots in Figure 6 are based on equation (47) of reference 

 68: sR's [x/(ab/\) J r (2.4 \V + x"/a) ] for an air-filled baffle 

 (J 1 = first order Bessel function; x « jra6/1.2X and 8 [(a — b) 2 /\ 

 + (a — 6) + X/4]. Equation (47) and the plots of Figure 6 do 

 not include the effect of the transmitted wave. 



