M. P. Foadch 111 
A = Fig. 6.8. Transducer- 
calibration tank with 
fittings. 
6.2.2.1. Separation Distance 
In the vicinity of the transducer, intensity fluctuations due to the Fresnel 
zone do not allow measurement of the sound level. For a plane transducer, 
with maximum dimension d, the distance has to be more than d?/\ in order to 
have a discrepancy of less than 3% from the spherical divergence law. 
Figure 6.9 gives the minimum separation distance tobe used as a function of 
frequency and transducer size for a given accuracy to be obtained. 
6.2.2.2. Pulse Length 
When the pulse length Tis suchthat T equals 2D/c, where D is the separation 
distance between transducers and c is the velocity in the medium, interferences 
due to reflections betweenthe surfaces of transducer and hydrophone are avoided. 
We have plotted for our measuring tankthe basic diagram of Fig. 6.10 establish- 
ing a pulse length such that the direct pulse has arrived before the pulses due 
to reflection. 
On the other hand, the Q factor of the transducer calls for a minimum pulse 
length, the duration of the transient state increasing with the value ofQ. The 
diagram in Fig. 6.11 determines the pulse length for a given value of Q cor- 
responding to an amplitude equal to 87, 95, and 99% of the signal value on con- 
tinuous waves. When measuring transducers at a frequency far from the 
resonance, the pulse length has to be twice as great as that fixed by Fig. 6.11. 
6.2.2.3. Pulse Rate 
Pulse rate must be such as to introduce an appreciable difference from 
continuous wave measurement. It is therefore a function of cathode-ray-tube 
remanence or of the integration time constant of the meter. 
We commonly use a pulse separation of 20 msec for measurements carried 
