108 Lecture 6 
90° 
180° Fig.'6.5. Vertical directivity pattern. 
HP5OAN°OI1 directivité en site 
These measurements are carried out in an anechoic tank 3.5 m long, 2.2 m 
wide, and 1.5 m deep. The inner tankwalls are lined with wedges of a cork-and- 
rubber mixture especially effective in the range above 10 kcps where a reflection 
factor of less than -10 db has beenmeasured. Figure 6.4 shows the experimental 
arrangement. 
A shaft rotating by remote control is used for the recording of the horizontal 
and vertical directional patterns of the hydrophones shown in Fig. 6.5. 
By averaging the results obtained by a series of successive measurements, an 
accuracy better than 1.5 db has been achieved, supposing that the frequency 
response of the standard hydrophone is known to +1 db. 
6.2. TRANSDUCER CALIBRATION 
For some purposes transducers are calibrated by comparison with a standard 
transducer using the filtered noise method, but most commonly we apply the pulse 
technique or the reciprocity method. Measurements are carried out in an 
anechoic tank 4.60 m long, 3.15 m wide, and 3.15 m deep. ; 
The inner walls and bottom are lined with the same cork-and-rubber wedges 
described above for the filtered noise method. The damping value of this lining 
measured at normal incidence is given by Fig. 6.6 in the range from 8 to 40 kcps. 
Through the use of this lining the inside size of the tank has been reduced to 
3.80 by 2.40 by 2.80 m. 
