28 HAYES— DETECTION OF SUBMARINES. 



the receivers, the wave-length of the sound, and the angle by which 

 it is out of focus. 



In practice an eight foot line has been used carrying sixteen re- 

 ceivers, eight connecting with each ear. Fig. 3 gives the scheme by 

 which the receivers are connected. It is to be noticed that the path 

 from each receiver to the ear is the same. Care is also taken to pre- 

 serve the cross-section of the path. The cross-sectional area of the 

 tube joining each ear is twice that of the branching tube into which 

 it terminates. This branching tube has twice the cross-section of the 

 two branching tubes at its terminals, etc. Sound reflection within the 

 instrument, and hence resonance, is minimized by this means. 



Like the C-Tube, the M-B Tube has the disadvantage that it must 

 be lowered and raised when bearings are taken, but it possesses sev- 

 eral advantages over the C-Tube. It is most sensitive to sound from 

 a direction at right angles to the tube and is, therefore, relatively 

 insensitive to sounds from other directions. This makes it pos- 

 sible to pick a particular ship out of a mass of disturbing shipping 

 much more readily with the M-B Tube than with the C-Tube. The 

 M-B Tube hears the boat at which it is pointed with much greater 

 intensity than other boats, whereas the C-Tube, or any detector em- 

 ploying a single unit to each ear, hears all boats with the same rela- 

 tive intensity. Furthermore, the M-B Tube is much less disturbed by 

 local water noise than is the C-Tube as a great part of this noise is 

 out of focus. The M-B Tube is only focused on noise in a plane 

 perpendicular to the tube at its central point. 



It is obvious that two boats separated by 180 degrees will both 

 be in focus because of the bi-directional properties of the M-B Tube. 



The principle of the M-F Tube is shown in Plate XI. Suppose 

 (A) and (B), Fig. i, represents two receivers spaced a unit distance 

 apart and that sound is proceeding in the direction from (A) to (B), 

 as represented by the arrow. If the two receivers are joined by a 

 tube there is some point, (p), where the sound from the two re- 

 ceivers arrives in phase since the sound wave travels from A to B 

 through the water in less time than it travels from A to B through 

 the air in the tube. This point can readily be shown to be *^%oo of 

 the distance from A to B. A branch tube leading from point ( p) 

 will receive the impulses from both receivers in phase. 



