DRS. GUY BARLOW AND H. B. KEENE ON THE ANALYSIS OF SOUND. 
149 
not be an efficient radiator for low frequencies in consequence of short-circuiting action 
referred to above, and also it would be bi-directional. 
(4) Evinrude Row-boat Motor .—To obtain a more powerful and complex source an 
Evinrude row-boat motor was used. 
This was a 2-3 h.p. single-cylinder, two-cycle petrol motor fitted with a two-blade 
propeller. In most of the experiments it was attached to the stern of a 13-ft. centre¬ 
board sailing dinghy, which either circled round the receiving instrument or was kept 
in a fixed position, about 100 feet out, by steel wires made fast to posts on the bank 
of the reservoir. The frequency of the piston, which must be regarded as the funda¬ 
mental frequency, could be varied from about 10-11 /sec. when the boat was fixed, 
while a frequency of 14-5/sec. could be obtained when the boat was in motion. During 
a single experiment the motor was found to run at a very constant frequency, and 
proved to be a very convenient source of vibration for the purpose of analysis. 
(h) Receivers. 
In all the water experiments described below attention was usually confined to the 
range of frequency 5-150/sec. 
In order to obtain a faithful analysis the receiver should be either non-resonant or 
its resonance frequency should be well above the range under investigation. In the 
latter case the receiver will be insensitive in this range, and the results of the analysis 
will be complicated by the disturbance of subharmonics of the diaphragm frequency 
(see p. 144). We did not succeed in devising an ideal receiver for low-frequency vibra¬ 
tions, although fairly satisfactory results were obtained with the Rubber Diaphragm 
Receiver. Owing to damping the latter did not give unduly sharp resonance, and its 
resonance frequency could be varied to eliminate selective action. 
(1) Metal Diaphragm Receiver .—The construction of this instrument is shown in 
fig. 10. Owing to the thinness of the diaphragm, it is 
necessary to compensate the external water-pressure. 
This was done by putting the air cavity in communica¬ 
tion with a cylindrical reservoir the lower end of which 
was provided with a small hole to admit the water. If 
the cross-section of the cylinder is sufficiently great, then 
it is easily seen that the level of the water in it will not 
alter much with change in depth, and hence the air- 
pressure in the instrument will remain approximately at 
the pressure of the water outside. By adjusting the level 
of the reservoir with respect to the diaphragm, the 
compensation can be made exact for any particular 
depth. 
Fig. 10. Metal diaphragm 
receiver (section). 
The microphone v 7 as attached by its base to the centre of the diaphragm, and enclosed 
y 2 
