DRS. GUY BARLOW AND H. B. KEENE ON THE ANALYSIS OF SOUND. 147 
of an electromagnet excited by a current of about 1 ampere from a spring-interrupter 
using 4 volts. Two sizes of this type of sounder were constructed. These were generally 
driven at their natural frequencies. 
Length ..... 
Diameter ..... 
Thickness ..... 
Natural frequency in water 
No. 1. 
12-8 cm. 
7 • 8 cm. 
0-68 mm. 
Ill /sec. 
No. 2. 
16-1 cm. 
9-4 cm. 
0 • 30 mm. 
21 /sec. 
With these sounders the intensity fell off exceedingly rapidly with the distance, probably 
owing to consecutive segments moving in opposite phases causing short-circuiting of 
the vibrations in the water. They have also directional properties, the sound being 
sent out radially with four symmetrically placed maxima. The rapid falling off of 
intensity with distance is an advantage in tank experiments, where multiple reflections 
are a source of disturbance. 
(2) Double Diaphragm Sounder .—This is non-directional, and on account of its great 
range has been used in most of the experiments carried out in the reservoir. 
It consisted of a shallow cylindrical cavity closed by two vertical metal diaphragms, 
one of which carried at its centre an electromagnet, and the other an equal mass of soft 
iron (fig. 8). The electromagnet was excited by an intermittent current from a spring- 
interrupter. The space between the diaphragms 
communicated with the water by a small hole at 
the bottom. The apparatus could be used either 
completely filled with air or filled with water, 
with the exception of a small chamber of about 
30 c.c. near the top of the cavity, in which the 
air remained trapped. The sounder was held by 
a 12-ft. iron rod, by means of which it could be 
lowered over the side of a boat to the desired 
depth. When used air-filled, a long piece of 
pressure tubing was attached to the cavity and 
air pumped in until the air-bubbles escaping from 
the hole at the bottom were seen rising in the 
water. In the boat from which the sounder was 
hung were the cells (4 volts) and the spring- 
interrupter, these being connected by long leads 
to the electromagnet within the sounder (current 
2-2| amperes). The boat could be anchored in 
jDb’ chamber 
30 cc 
-Diaphragm^ 
O’Srnm. 
A 
! 
/f- cm 
i 
i 
* 
Fig. 
Double diaphragm sounder 
(section). 
any desired position, and the apparatus when once adjusted would run for hours at 
a time without attention. 
The natural frequency of this sounder depended on the internal conditions. The 
VOL. CCXXII.-A. Y 
