The mysticetes are known to emit sounds in the low-frequency range, 

 approximately between 10 and 1000 Hz, with particular emphasis on the region 

 around 20 Hz. At these low frequencies, the wave length is very long (500 

 feet at 10 Hz and 50 feet at 100 Hz). Since physical considerations require 

 that a receiving body have dimensions at least approximating the wavelength in 

 order to have appreciable directivity, and since even the head of a large 

 whale does not reach 50 feet in size, it is assumed in this report that the 

 animals' hearing sensitivity is equal in all directions, yielding a receiver 

 directivity indexl of zero. This is a useful assumption for the purpose of 

 simplifying the analytic treatment. It will be essentially true for the 

 lowest frequencies, up to about 100 Hz. Above this some directivity will be 

 expected, but the directivity index is not expected to exceed 10 dB at 

 frequencies below 500 Hz. For the general estimates of this report this will 

 be ignored. It should be pointed out that a directivity index of zero does 

 not mean that the acoustic receiver is not capable of determining the 

 direction of a sound source. The human ear, for example, by using interaural 

 phase information, makes excellent determinations of direction of sounds at 

 low frequencies whre the ear-head size is less than 0.1 wavelength, so is 

 functioning as an omni-directional system with directivity index near zero. 



iDirectivity index of a receiving sensor system, such as a hydrophone or 

 hydrophone array is a number used to quantify the discrimination of the system 

 against omnidirectional noise as a result of its directional sensitivity 

 pattern. A receiver which is equally sensitive in all directions has no 

 capability to discriminate directional ly against noise, so it has a 

 directivity index of zero. 



G-23 



