170 Lecture 10 
On a model scale, the effect of waves on the water surface in modulating the 
received sound amplitude is easily demonstrated. A wave 1 m high, full scale, 
will be 1 mmonthe model scale we are considering. Various methods of generat - 
ing waves (of lengths lying between 5 and 25 cm) and measuring their height 
have been tried, and very simple techniques have been used. Shaking the side 
of the tank proved very satisfactory for generating waves having wavefronts 
parallel to the line of sound propagation along the tank, while a simple "line- 
dipper" at the end of the tank would produce waves with fronts at right angles to 
the direction of sound propagation. A simple and convenient method of recording 
the wave amplitude and frequency was the use of an air condenser. This con- 
sisted of astripofmetal mounted parallelto the water surface about a centimeter 
above it, forming a condenser with the water surface, and varying in capacity 
as the water waves parallel to the length of the metal strip pass under it. This 
WEN EV WW 
a pM Aaa KR ROAK 
® SOUND TRANSMISSION IN DIRECTION 
PARALLEL TO WAVE CRESTS 
@ SOUND TRANSMISSION IN DIRECTION 
PARALLEL TO WAVE CRESTS 
@ SOUND TRANSMISSION IN DIRECTION wave caLiaration 
PARALLEL TO WAVE CRESTS Imm, 
Ave ify is 
a Seas 
@ SOUND TRANSMISSION AT RIGHT ANGLES 
TO WAVES 
@—SOUND AMPLITUDE RECORD 
$ — WAVE HEIGHT RECORD 
Fig. 10.9. Modulation of the sound amplitude by surface wives. 
