26 HAYES— DETECTION OF SUBMARINES. 



tion will not reach the listener's ears in phase and as a result will not 

 be magnified in the same proportion. Indeed, the sum total may be 

 less than that given by a single receiver through destructive inter- 

 ference. 



Although several types of detectors employing a single receiver 

 for each ear, as in the " 3-Spot," have been developed at the Naval 

 Experimental Station for special purposes such as equipping light- 

 ships, hydroplanes, dirigible balloons, etc., yet the major part of the 

 efforts of this Station have been directed toward the development of 

 the so-called " multi-unit " detector devices. 



Except that the " 4-Spot " arrangement of receivers has been 

 used throughout in preference to the " 3-Spot " arrangement, these 

 special single-unit types of detectors are very similar to the " 3- 

 Spot " type which has been described. Therefore a detailed descrip- 

 tion will not be given. The principle and operation of the more ef- 

 fective devices, those employing multiple units, follows : 



The so-called " AI-B Tube" is a rotating listening device employ- 

 ing multiple unit receivers. The principle may be understood by 

 considering Plate X. In Fig. i let the numerals 1,2, 3, and 4 repre- 

 sent four similar acoustic receivers equally spaced in a line and con- 

 necting through equal length tubes with the stethoscope leads R and 

 L at the common junction (A). Sound coming from a direction 

 perpendicular to the line of receivers actuates all the receivers simul- 

 taneously and the response from all four reaches the ears in phase. 

 Under such conditions the intensity of the sound heard is four times 

 the intensity from a single receiver. 



Sound from any other direction, such as represented by arrow 2, 

 does not reach the receivers simultaneously and, as a result, the re- 

 sponses from the various receivers do not arrive at the ears in phase. 

 The intensity of the resulting sound will therefore be less than four 

 times that from a single receiver. The difference will vary for the 

 different components of the sound depending upon the wave-length. 



Such an instrument is capable of determining direction by means 

 of the maximum-minimum principle except for an ambiguity of 180 

 degrees. If, as in Fig. 2, half of the receivers is connected to each 

 ear respectively, then advantage can be taken of both the maximum- 

 minimum and the binaural principles. The sound response from 



