94 
IOWA ACADEMY OF SCIENCE Voi.. XXVIII, 1921 
is the length between openings of the side tubes. 
1 2 is the length of the side tubes. 
V 2 is the volume of the chamber surrounding each side tube. 
a is the velocity of sound. 
The computed frequencies are shown by vertical lines, one pair 
for each filter. 
Each filter consists of three cells. 
It is found theoretically that corresponds to the resonance 
frequency of the volume V 2 and neck 1 2 and also experimentally 
that : 
(1) The peaks correspond to closely. 
(2) The cut off frequency corresponds to /a closely. 
(3) Filtering action from to a frequency almost ten times 
as great is very good, the transmission being less than one- 
millionth part. 
It is supposed that the band at the higher frequency is caused by 
the fact that, as in a Helmholtz resonator, the resonating frequen- 
cy has more than one value. 
Physical Laboratory, 
State University oe Iowa. 
