P. M. Kendig 247 
connected in series in order to achieve a high value of the sensitivity and con- 
sequently a high impedance. (The hydrophone has successfully withstood hy- 
drostatic pressures of 300 psi.) The four cylinders could be connected in several 
different series—parallel combinations which would provide various other im- 
pedances. Since the free-field voltage response varies as the square root of the 
impedance, this device may be used for adjusting the response. However, all 
such combinations will still give the same equivalent noise pressure. In this 
case, the cylinders were all series connected so as to achieve a high response 
and a high signal-to-noise ratio for the hydrophone amplifier combination. One 
very versatile means of controlling the impedance is to polarize the cylinder 
circumferentially in sections by means of electrical conducting stripes inside 
and outside the cylinder parallel to the cylinder axis. Since both the polarization 
and the field are now in the same direction (circumferential), the transducer 
also has a higher coupling coefficient than when it is plated inside and outside 
in the more usual manner (as was the case for the transducer described here). 
Following is a summary of the most significant acoustic characteristics of 
the hydrophone shown in Fig. 13.7, all of which appear to meet the design re- 
quirements over the specified range of 100 to 3000 cps. 
1. Free-Field Voltage Response. -78+1.5 db re 1 v per pbar over 
the frequency range of 100 to 8000 cps. 
2. Directionality. Omnidirectional within +1 db in all planes for fre- 
quencies up to 3000 cps. 
3. Equivalent Noise Pressure. Varies from —91 to -102db rel 
pbar for a 1-cps band from 100 to 3000 cps, which is about 52 to 38 db, 
respectively, below the ambient noise levels of zero sea state. 
4. Hydrophone Impedance. The impedance is essentially that due to 
a Capacity of approximately 0.01 uf. 
This section presented guide lines for designing a hydrophone with a low 
equivalent noise pressure and means for varying the hydrophone impedance in 
order to match the amplifier. 
Now the question is, can one detect a target that has a level over a band of 
frequencies lower than the ambient noise over the same band ? Here we will not 
consider correlation techniques or other similar devices, but just straight 
listening. The answer to this question is definitely affirmative, provided the 
target is localized in space. Hydrophones with radiating areas whose dimen- 
sions are large compared with the wavelength of sound in the medium have 
directional properties. The best example is the circular piston which has the 
so-called searchlight pattern. The directivity factor is the ratio of the intensity 
on the main beam of such a transducer at a fixed distance r to the average in- 
tensity over a sphere with a radius equal to r. For a circular piston with a 
diameter greater than the wavelength the directivity factor is approximately 
= aa 
DAG: 
where d is the diameter of the circular piston and A is the wavelength of the 
sound. If one substitutes this value in Eq. (10) of Section 13.3, this rather re- 
markable expression is obtained: 
