462 MECHANICAL AND ACOUSTICAL SENSES 



Table 5. Electrical signal 

 levels expressed as dB rel- 

 ative to 1 V. 



gives a better signal-to-noise performance and therefore a greater signal- 

 detection range. However, if the bandwidth is too narrow, certain problems 

 can arise. One is that missing the signal becomes more likely if the tuning 

 dial is set just slightly off the precise frequency required. A second problem 

 involves the time it takes the relatively high-Q circuit (Q = center frequency/ 

 bandwidth) to resonate up to full amplitude in response to an incoming 

 signal. The narrower the bandwidth, the longer it takes the circuit to respond 

 fully, and this puts a practical limit on the shortness of pulse that can be 

 effectively received. As a general rule for a 1-kHz bandwidth, the signal pulse 

 (tone burst) should be no shorter than 1 ms; for 500-Hz bandwidth, 2 ms; 

 for 100-Hz bandwidth, 10 ms, etc. This effect is important when choosing 

 transmitter pulse length, especially if one is considering reducing pulse length 

 in an effort to extend battery life. A third problem with some narrowband 

 receivers is that they are prone to "ringing" if hit with a high-energy pulse, 

 even a very short one, such as from a nearby snapping shrimp. Such a shrimp 

 click can then sound longer and more "tonal" than it really is, making it 

 more difficult to discriminate from a transmitter pulse. 



Hydrophone directivity— Another important noise-reducing charac- 

 teristic is the degree of directionality of the receiver hydrophone. The 

 beamwidth is usually defined as the total angle to both the right and left of 

 the beam axis to where the response drops to the —3 dB points. For the 

 DuKane hydrophone, this is about 30° (15° on each side of axis). The effect 

 of beamwidth on total noise reduction is given by the directivity index, 

 which expresses in decibels the relative improvement in signal-to-noise ratio 

 of a given beamwidth as compared to omnidirectionality. Table 6 gives some 

 representative values of directivity index for conical beam patterns and 

 shows that the 30° DuKane beam yields a DI of about 16 dB. 



