Design of the sound source centered on the amount of acoustic 

 output required to send the signal through at least 20 feet of sediment 

 (30+ feet of penetration minus the 10- foot penetrometer length) and 

 20,000 feet of water and to have a usable signal at the water surface. 

 This design required consideration of acoustic losses in the trans- 

 mission mediums, the acoustic characteristics of the sound source, and 

 the acoustic characteristics of the listening system at the ship. 



Expected transmission losses for 30 feet of penetration and 20,000 

 feet of water are: 



Sediment attenuation loss 20 decibels (db) 



Seawater attenuation loss +16 db 



Spreading loss for 20,000 feet +76.5 db 



zt 



!- 3.7.J 



Total Losses 112.5 db Zo • 



All of these values are pressure levels referenced to 1 ybar at 1 yard. 

 The sediment loss was calculated after Hamilton's (1972) data, and the 

 spreading and water attenuation are those given by Horton (1959). 



The required sound pressure level at the receiving hydrophone can 

 be calculated from the required signal at the receiver and the sensitivity 

 of' the hydrophone. The signal required at the receiver of the depth 

 sounder-receiver is 20 yV which can be translated to -94 dbV. The 

 sensitivity of the hydrophone of the depth sounder-receiver is about -75 

 db referenced to 1 volt at 1 ybar of pressure. The sensitivity of the 

 hydrophone can be subtracted from the required signal level' at the 

 receiver to determine the sound pressure level required at the hydrophone. 



Required signal level -94 db (20 yV) 



Hydrophone sensitivity -75 dbV 



Sound pressure level required 



referenced to 1 ybar -19 db 



Knowing the transmission losses and the sound pressure level 

 required at the receiving hydrophone allows the required source level of 

 the acoustic projector on the penetrometer to be calculated. Since the 

 hydrophone can detect a pressure 19 db below the reference pressure 

 1 ybar at 1 yard, and since the losses are equal to 112.5 db referenced 

 to 1 ybar at 1 yard, these-values can be added to determine the source 

 pressure level. The source pressure level is then 93.5 db above 1 ybar 

 at 1 yard for a nondirectional source. However, the directivity index 

 of the projector, which is related to its beam width, will increase the 

 pressure on the axis of the main lobe when compared to a nondirectional 

 source. Assuming a directivity index of nine, the required pressure 

 level is 84.5 db referenced to 1 ybar at 1 yard. 



For the prototype sound sources built at CEL, on-hand acoustic 

 projectors were used. Their acoustic output was insufficient for operation 

 to a water depth greater than several thousand feet. 



11 



