148 DRS. GUY BARLOW AND H. B. KEENE ON THE ANALYSIS OF SOUND. 
restoring force on the diaphragm was mainly due to the elasticity of the air in the cavity 
(the restoring force due to the elasticity of the diaphragm being relatively unimportant) 
giving when air-filled a natural frequency of about 67/sec., the exact value increasing 
with the depth of the sounder below the surface, the variation being roughly \ per cent, 
per foot. Although the effect of introducing water into the cavity is to increase the 
effective inertia of the diaphragms, the frequency is raised on account of the greatly 
increased restoring force given by the residual air. With the 30 c.c. of ah' generally 
used the natural frequency was about 123/sec. In practice the sounder was generally 
driven near the natural frequency corresponding to the working conditions ; the 
maximum energy is then emitted, and as a nearly pure tone. If driven at much lower 
frequencies the intensity was diminished, and, as would be expected, any harmonic near 
resonance with the natural frequency of the diaphragm became prominent. 
(3) Single Diaphragm Sounder .—A thin iron diaphragm was bolted on to the end 
of a massive cylindrical iron pot (fig. 9). A mass (380 gm.) of soft iron was attached 
to the centre of the diaphragm, behind which the exciting magnet was rigidly held. 
The cavity was always air-filled at the pressure of the surrounding water in order to 
avoid distortion of the diaphragm. The sounder was hung from a punt by a double 
chain at a depth of 4| feet. 
The instrument was found to be unsatisfactory in practice owing to the extreme 
sensitiveness of the diaphragm to changes of pressure due to small variations in depth. 
It had a natural frequency in water of 47/sec., a value much higher than was expected. 
The effect of the restoring force due to the air-cavity can be avoided by using a single 
diaphragm with both sides in contact with the water, but such an instrument would 
