Another factor that enters into acoustic design is the minimum 

 detectable signal (MDS) . MDS is a function of the ambient noise level, 

 the band width of the receiving system, the directivity index of the 

 hydrophone, and the recognition differential between the signal and the 

 ambient noise. This function is expressed in decibel terminology as: 



MDS = N + 10 log BW - DI + RD (3) 



s 



where N = spectrum or ambient noise level in decibels 

 s ■■ 



referred to 1 ybar 



BW = bandwidth, cycles 



DI = directivity index, db 



RD = recognition differential, db 



If the MDS is higher than the source pressure level at the hydrophone, 

 the signal cannot be detected. Conversely, if the MDS is lower than the 

 source pressure level at the hydrophone, the signal can be detected. 



To solve Equation 3 the factors in the right-hand side must be 

 determined. First, an assumption needs to be made about RD between the 

 signal and the ambient noise. In acoustic signal processing an RD of 

 20 db is often used. This RD provides a conservative factor of safety 

 because the signal has an amplitude 10 times the noise. The characteristics 

 of the receiver -hydrophone system are usually known. For the depth sounder- 

 receiver hydrophone on the standard ship, BW is 1,000 cycles and DI is 

 about 15 db (60-degree beam width). The ambient noise level is a function 

 of many different factors that are difficult to separate. Myers, Holm, 

 and McAllister (1969) provide a chart of noise level for different 

 conditions at sea which include rain, wind force (sea-state), ship 

 noise, and other factors in relation to specific frequencies. At the 

 operational frequency of the penetrometer (12.5 kHz) this chart gives the 

 ambient noise as -53 db referenced to 1 ybar at sea state 4, which is a 

 likely maximum operable sea state for using the penetrometer with presently 

 available launching systems. Using these numbers in Equation 3 gives an 

 MDS of -18 db referenced 1 ybar. 



Referring to the sound pressure level required at the hydrophone 

 (-19 db referenced to 1 pbar) it is apparent that the minimum detectable 

 signal of the receiver-hydrophone is about the same as the required 

 sound pressure level. If all necessary assumptions are reasonable, 

 providing an acoustic source with a pressure level 84.5 db above 1 ubar 

 at 1 yard and a projector DI of 9 db is adequate. 



It is anticipated that development of the penetrometer will include 

 launching systems so that the penetrometer can be deployed at sea state 

 4 and above. With deployment at an upper sea state 6 (average wave 

 height = 11 feet, average tenth highest wave = 23 feet) as a goal, 

 ambient noise levels will increase by 8 db. Therefore, an equal improvement 

 in the pressure level of the acoustic source or hydrophone-receiver 

 system would be required to detect the signal. An 8-db improvement 

 would be relatively easy to achieve in the receiver-hydrophone alone by 



12 



