100 



USRL TEST STATIONS 



PROJECTOR 



»l <X =0° 



, AND«=y360' > 



DOME 



BAFFLE 



•l ot = o , © = 0* 



i <x.=o°,e=ieo° 



«=/360°,e = l80° 



o ,30 o ,45°,ETC 

 ^'360' , ,G=/360 ,> 



•i <*=0 o ,e=/360» 

 c< =/ 360", 6 = 0" 



ANGLE NOTATION 



ex = ANGLE OF ACOUSTIC AXIS OF TRANSDUCER 



WITH RESPECT TO DIRECTION OF SOUND 



PROPAGATION 

 9 = ANGLE OF DOME WITH RESPECT TO DIRECTION 



OF SOUND PROPAGATION 

 <fc = ANGLE OF DOME WITH RESPECT TO ACOUSTIC 



AXIS OF TRANSDUCER 

 /= CONTINUOUS ROTATION THROUGH INDICATED 



ANGULAR RANGE 



Figure 36. Typical testing geometries tor dome and baffle measurements: (A) reference runs, (B) transmission through 

 nose of dome, (C) transmission through baffle, (D) dome loss versus angle, (E) specular reflection studies. 



ments are shown in the sketches of Figure 37. Com- 

 parison of the reference pressure obtained between 

 the hydrophone and projector with and without the 

 baffle gives the insertion loss characteristic of this 

 baffle. A sound pressure is present on the far side of 

 the baffle because of transmission through the baffle 

 and diffraction around it. This is more fully discussed 

 in Chapter 9. 



The reflecting and absorbing properties of the 

 baffle may be further studied by modifying the testing 

 setup as shown in Figure 37D. The baffle and trans- 

 ducers are positioned so that the over-all path length 

 of the reflected sound is equal to d. With the baffle 

 removed, a screen is placed between the projector and 

 hydrophone as in Figure 37C to minimize the direct 

 transmission. The magnitude of the sound received 

 determines the threshold in the reflection measure- 

 ments. If the sound pressure measured by the hydro- 

 phone, with baffle in place as in Figure 37D*is equal 

 to that measured in the reference run of 37A, the 



baffle is a perfect reflector. The amount by which the 

 two differ is a measure of the sound transmitted, ab- 

 sorbed, and diffracted by the baffle. 



Studies of Acoustic Properties of Various Materials. 

 Occasionally the acoustic properties of a particular 

 material are investigated to determine its suitability 

 for a proposed application. Reflection and transmis- 

 sion tests, similar to those made on baffles, are made 

 on samples of the material. These should be large 

 enough to avoid diffraction in the frequency range 

 under consideration. Before testing, the samples are 

 thoroughly washed, debubbled, and submerged until 

 they are in temperature equilibrium with the water. 

 Often additional tests are made to discover the criti- 

 cal angles at which incident sound will be completely 

 reflected by the material. The sample is rotated in the 

 setup of Figure 37B and the angles observed at which 

 minimum transmission occurs. This information is 

 particularly valuable in the design of dome windows 

 and in the selection of materials for them. 



