156 



ASSOCIATED EQUIPMENT: DOMES AND BAFFLES 



mum strength for given thickness should also be cho- 

 sen, that is, the dome should be torpedo-shaped rather 

 than straight-sided. Added strength can be obtained, 

 if necessary, by expanded metal or possibly by corru- 

 gated sheet construction in preference to increased 

 wall thickness. 



It is seen from equations (5) that the specular re- 

 flection is not only minimized by minimizing the 

 transmission loss but also is least for a given transmis- 

 sion loss when the projector is as directive as possible, 

 the transverse dimensions of the dome are no larger 

 than required to accommodate the projector, and the 

 dome surface is as highly curved as possible, particu- 

 larly in the vertical cross section. 



In this last respect the doubly curved torpedo- 

 shaped domes again have a decided advantage over 

 the singly curved straight-sided domes. For example, 

 at 25 kc, for a 40-mil thick torpedo-shaped dome of 

 intermediate dimensions, the specular reflection is 

 about 20 db below the main beam; for a straight-sided 

 dome of the same sound window thickness and the 

 same dimensions the specular reflection is only 10 db 

 below the main beam. 48 In addition, the torpedo 

 shapes have a considerable advantage from the stand- 

 point of mechanical strength and streamlining. 23 Ex- 

 cessive noises due to eddying turbulence and to cavi- 

 tation set in at much higher speeds for doubly curved 

 torpedo-shaped domes than for singly curved straight- 

 sided domes. Also, much of the self or ship's noise 

 within a dome at supersonic frequencies, for a de- 

 stroyer moving at relatively high speeds, arises from 

 streaming turbulent water, particularly in instances 

 where bubbles detached from the prow strike the 

 dome and set the dome shell into vibration. This 

 bubble noise is probably smaller for torpedo-shaped 

 than for straight-sided domes because of the small 

 front area and greater streamlining of the latter. 



In regard to the effect of modern thin-wall (20 to 

 40 mil) stainless steel domes on echo ranging and lis- 

 tening, it may be shown that the decrease in the 

 signal and the increase in the noise due to the dome 

 transmission loss and the associated change in the 

 directivity index affect detection ranges for various 

 types of targets by relatively small amounts. A side 

 lobe introduced by specular reflection in the directiv- 

 ity pattern may not be troublesome at first contact 

 (except for a 60-mil, straight-sided dome), but it may 

 give false bearings and thus make it difficult to regain 

 contact at short range. The use of 20- to 40-mil stain- 

 less steel torpedo-shaped domes, with specular reflec- 



tions 25-20 db below the main beam, overcomes this 

 difficulty and in addition minimizes any mutual in- 

 terference between beams from different craft. 



9.2 



BAFFLES 08 



The self noise picked up by a transducer on a mov- 

 ing vessel is due in part to turbulence and cavitation 

 at the propeller screws; therefore, it might be ex- 

 pected that this noise is directional and has a maxi- 

 mum in the direction of the screws. For small vessels 

 this is the case, necessitating additional acoustic 

 shielding of the screws; on the other hand, on larger 

 vessels the hull partially shields the transducer from 

 the screws. The available information on the extent 

 of acoustic shielding by the hull of various types of 

 craft is meager; from the evidence at hand it seems de- 

 sirable to provide additional shielding both on small 

 and large vessels. On vessels where the self noise has a 

 maximum in the direction of the screws, arising pre- 

 dominantly from propeller motion, such shielding 

 will be advantageous. Even where noise is apparently 

 nondirectional, a shield may lower the average noise 

 level.* 



A common method of acoustic shielding is to place 

 a baffle inside the dome between the transducer and 

 screws. There are two possible disadvantages in this 

 method: First, sound incident on the transducer side 

 of the baffle may be reflected to the transducer, in- 

 creasing the noise level or giving spurious indications. 

 This difficulty is eliminated by covering the front of 

 the baffle with an absorbing material. The second dis- 

 advantage is the prevention of echo ranging or listen- 

 ing in the sector subtended by the baffle. Usually this 

 is not a serious objection, since the baffle is always to 

 the rear of the transducer and screw noise plus wake 

 make it useless, in general, to listen or range toward 

 the rear. In some cases the noise or echo of the sub- 

 marine may be intense enough to be detectable in the 

 rear sector; in this case the baffle actually prevents 

 detection. This disadvantage may be great enough to 

 offset any of the possible advantages of a baffle; the 

 difficulty can be overcome by the use of an additional 

 transducer without a baffle to sweep the rear sector. 



t Nondirectional or isotropic self noise has been observed on 

 large antisubmarine craft with dome-enclosed transducers 

 shielded by baffles from the screws. Whether the noise still ap- 

 pears isotropic in the absence of the baffle is not known; if it 

 does, the supposition that the noise arises largely from bubbles 

 striking the dome obtains support.*0a 



