M. P. Fodch 107 
Sh en db 
1 n i OW OAH i ul Ir AD 
100 1000 
1 ug f en Hz 
Fig. 6.3. Comparison of different calibration methods. (Type H6 Tel. No. 23 hydrophone); +Electrostatic 
Method, @ Electromagnetic method, a Ballistic galvanometer method, x Barymetric method, 
Voltmetre 
Méthode de comparaison en bruit Filtre” 
Analyseur 
bre Sao (Gas Go hee y 
dhe thas (ere eolots Ampli 60 db 1-100 kHz 
Af=250 Hz 
mpli 
Bruiteur rotatif 
4 billes |- 25 kHz 
__Générateur de __\ 
souffle 10 - 100 kHz 
Fig. 6.4. Anechoic 
tank with instru- 
mentation, 
Hydrophones 
Tour de directivite 
standing-wave ratio; therefore the calibrating signal used consists of a white 
noise which is filtered at the receiving end by a narrow-band filter only wide 
enough for filtering out the standing waves, Sometimes the noise source is a 
mechanical set consisting of a rotating drum filled with bearing balls producing 
a noise in a spectral band from 1 to 35 kcps and sometimes a ceramic projector 
driven by a white-noise source. 
The hydrophone to be calibrated and the standard hydrophone are placed 
successively in the same position, their signals being picked up by a heterodyne 
analyzer between 2 and 100 kcps. Anautomatic processing device permits simul- 
taneous analysis and recording of the response curve. 
