376 HAYES— U. S. NAVY MV TYPE OF HYDROPHONE. 



and dense portions of the sound waves will not remain perpendicu- 

 lar to this surface. If the sound travels from left to right the wave 

 front will be distorted to the left as shown by the full curved lines. 

 This distortion becomes greater as the sound travels farther from 

 the source, and at considerable distance it is conceivable that all 

 sound reaching the surface is travelling in a direction nearly perpen- 

 dicular to it and thus will be reflected nearly vertically downward. 

 If this distortion extends to a sufficient depth, neither the direct nor 

 the surface-reflected sound ray will reach the distant receiver placed 

 near the surface. Under such conditions only sound reflected from 

 the sea-bottom will be heard. 



Experiments for determining which of the above explanations is 

 valid have not been undertaken to date. However, when listening 

 to bell and oscillator signals, it has been noticed that the harmonics 

 are more prominent relative to the fundamental at the first instant 

 of response than during the rest of the signal. This fact tends to 

 substantiate the first explanation. The slight difference in path 

 length between the direct and surface-reflected sound introduces 

 more phase difference between the high-pitched components of the 

 sound than in the case of the fundamental and therefore the har- 

 monics are less perfectly neutralized by interference at the receiver 

 than is the fundamental. The sound at the receiver resulting from 

 these two paths consists largely of harmonics and this is heard an 

 instant before the sound which is reflected from the sea-bottom as 

 the latter has travelled a somewhat longer path, 



A New Method of Sounding. 



Since the only propeller sound heard by a hydrophone located 

 at a distance and near the surface is the component reflected from 

 the sea-bottom, it follows that the depth of water can be determined 

 from the angle which the reflected sound makes with a fixed line in 

 a plane determined by the sound source and the reflected ray — pro- 

 vided the distance between the hydrophone and sound-source is 

 known. Conversely, the distance between the hydrophone and the 

 sound-source can be determined if the depth of water is known. 



