Chapter 9 



ACOUSTIC EOUIPMENT ASSOCIATED WITH 



UNDERWATER SOUND DEVICES: 



DOMES AND BAFFLES 



By Henry Primakoff and Joseph B. Keller 



The Underwater Sound Reference Laboratories 

 has calibrated acoustic equipment auxiliary to 

 electroacoustic transducers. Among the most impor- 

 tant auxiliary equipment tested have been stream- 

 lined domes and baffles. 



APPARENT RELATIVE- 

 BEARING OF TARGET 



9.1 



DOMES' 



In general, a streamlined dome is necessary for an 

 echo-ranging or listening device to minimize noise by 

 reducing the turbulence and cavitation about its ac- 

 tive face arising from its passage through the water. 

 The alternative possibility of streamlining the device 

 itself has not been widely adopted. 



The dome should be properly streamlined, that is, 

 it should be of such a size and shape that turbulence 

 and cavitation noises are eliminated, or at least do 

 not set in until high speeds of the vessels are reached. 

 Further, the dome structure should have sufficient 

 mechanical strength to resist the hydrodynamic pres- 

 sure and drag forces on it and should be constructed 

 from a noncorrosive, sea-resistant material. Finally, 

 the dome should be acoustically transparent, causing 

 as little disturbance as possible in the magnitude and 

 directivity of the response of the enclosed acoustic- 

 device. 



To be acoustically transparent, the dome must ful- 

 fill three requirements. 



First, the dome must introduce only a small trans- 

 mission loss. By such a loss is meant the reduction in 

 the magnitude of the response of the transducer 

 caused by placing it in the dome; usually this is meas- 

 ured along the transducer axis. Thus, for an echo- 

 ranging projector a 3-db one-way transmission loss 

 means a 50 per cent decrease in the pressure ampli- 

 tude of any echo received by reflection from a target. 



Second, the dome must introduce no large side 

 lobes in the directivity pattern of the enclosed trans- 

 ducer. Such lobes may arise as a result of internal 

 specular reflections from the dome and cause false 



_jl__ , ACTUAL RELATIVE 



r~>*-~y / BEARING OF TARGET 



SPECULARLY 

 REFLECTED 

 BEAM 



NORMAL TO 

 DOME WALL 



Figure 1. Specular reflection in domes. 



bearings to be taken in echo ranging or directional 

 listening. For example, if a listening or echo-ranging 

 device on an antisubmarine vessel is trained in the 

 direction of the transducer axis as indicated in Fig- 

 ure 1, and an enemy submarine is present in the 

 direction of the internal specular reflection, a rela- 

 tively strong signal will be received. The vessel may 

 then assume that the signal is being received on the 

 main lobe, and head into the indicated bearing. Ac- 

 tually, in this case, the vessel should head in the 

 direction of the specular reflection. 



In addition to introducing one relatively strong 

 internal specular reflection, domes also distort the 

 transducer directivity patterns by giving rise to vari- 

 ous second order effects such as multiple reflections. 

 Multiple reflections introduce additional side lobes 



153 



