A. B. Wood 171 
arrangement could easily be calibrated by a displacement of the metal strip 1 mm 
from its normal position above the water surface. Using the double-beam cathode 
ray oscillograph, records have been made illustrating how waves of small ampli- 
tude modulate the received sound signal when running with their wavefronts 
parallel or at right angles to the direction of sound propagation (see Fig. 10.9). 
Using omnidirectional transducers, as in the examples illustrated, the wave ef- 
fects are very striking, but are barely observable when directional transducers 
(4.5°, semiangle) are used. 
10.2.5. Temperature Effects 
Two cases arise: (a) isothermal effects—change of uniform temperature and 
(b) temperature-gradient effects. In case (a), small changes of temperature, of 
the order of a few degrees centigrade such as are experienced between winter 
and summer conditions in the sea aroundthe British Isles, result in correspond- 
ing changes of sound velocity Cinthe water. At a constant transmission frequency 
WN this implies a corresponding change of wavelength C/N. The same result may 
of course be obtained at a constant water temperature by an equivalent small 
(0) 15-2 @ ss-astc ® Pc 
WO = 000)» 
Ga ee = 
5 
@ 8.7 (gorrom) 92° (sottom) @) 227° (BoTToM) 
SOUND AMPLITUDE 
TEMPERATURE RECORD (f° C. CALIBRATION) 
(© 22.1°c (Borrom) @ aes (sottom) = ((2)_—s9.6°c (soTtoM) 
Fig. 10.10. Isothermal temperature, and temperature-gradient effects. 
