SONIC TRANSMISSION 



157 



loss can be most adequately represented by an equa- 

 tioii liaviiif;; the form, 



// = 15 log/- + a, (5) 



where a, the coefficient of attenuation in decibels per 

 kiloyard, depends on /, the frequency in kilocycles, 

 according to 



a = 0.25(/-2) (6) 



above 2 kc. Below 2 kc the attenuation is very small. 

 Equation (6) is believed to be adequate up to about 

 20 kc. 



Equation (6) represents merely the average de- 

 pendence of the attenuation coefficient on frequency. 



In the portion of the sea fairly near to the surface, 

 which is the only region of interest in sonic listening, 

 the absorption coefficient probably depends on highly 



variable factors, .such as bubble content; thus large 

 deviations from equation (6) may be expected to 

 occur quite frequently. 



There appears to bo little correlation at .sonic fre- 

 quencies between transmission loss and refraction 

 conditions, depth of the water, and surface rough- 

 ness. With strong upward refraction, an increase of 

 attenuation with increasing sea state has been ob- 

 served, undoubtedly caused by the poor reflectivity 

 of a rough and aerated surface. 



At short ranges, out to approximately the range 

 equal to the depth of the water, image interference 

 maxima and minima have frequently been measured. 

 However, except possibly at very low frequencies, 

 the inverse fourth power decay has not been observed 

 because of the disruptive effect of bottom-reflected 

 sound at the ranges where the fourth power decay 

 might be expected. 



In general, reliable information on sonic transmis- 

 sion is scanty and is less consistent than the informa- 

 tion on the transmission of 24-kc sound. In the future 

 an increasing amount of stress is likely to be laid on 

 the investigation of sonic transmission. However, 

 sonic transmission will probably remain a more 

 difficult field for investigation than supersonic trans- 

 mission, because of the low directivity of most 

 sources of sonic sound. 



