STUDY OF FREE-RANGING SHARKS 463 



Table 6. Directivity indexes 

 for various beamwidths (to —3 

 dB points) of conical beam 

 patterns. 



Signal-Detection Level— Assuming that an acoustic signal arrives at 

 the hydrophone at a level high enough that absolute receiver sensitivity is 

 not limiting, the next question involves whether it can be recognized above 

 ambient noise in the environment. The signal strength in dB re 1 juPa neces- 

 sary for recognition is called the minimum detectable signal (MDS) and is 

 given by the equation 



MDS = Ns + 10 log BW—DI + RD, (2) 



where Ns is the spectrum-level noise in dB re 1 fJ.Fa at the frequency of the 

 signal, BW is the receiver bandwidth in hertz, DI is the directivity index of 

 the receiving hydrophone in decibels, and RD is the recognition differential 

 in decibels. 



The recognition differential is a measure of how far the signal must be 

 above noise at the receiver output in order to be recognized. For human 

 listeners, the RD can be considered to be in the area of to 3 dB, the human 

 ear being quite good at signal discrimination. This value partly depends on 

 the temporal nature of the noise. For example, a slow hammering on a metal 

 plate would sound very loud (and would produce a high reading on a 

 "slow-averaging" sound level meter), yet the faint sound of a telephone 

 ringing would still be audible in the periods between the hammer strikes. 

 Thus, the RD is really meant to indicate a signal-to-noise ratio more or less 

 just during the signal pulse. For nonhuman "listeners," such as the automatic 

 signal-recognition circuits of transponders, the RD may be considerably 

 higher, e.g., 10-15 dB for some simple systems. 



