276 
PHYSICS: A. G. WEBSTER 
lag is between one and two right angles, as shown in figure 1. If we force air 
into a chamber of volume F, the compression s = X/V will be related to the 
excess pressure p by the relation p = es, where e is the modulus of elasticity 
of the air e = pa^, p being the density and a the velocity of sound. Conse- 
quently we have 
e pa 
^0 — ^ — "TT y 
(3) 
and the analogy is to a condenser. If we have air passing through an orifice 
or short tube of conductivity c its inertia gives an apparent mass p/c, and if it 
FIG. 1 
Xv 
FIG. 2 
FIG. 3 
FIG. 4 
X 
FIG. 5 
escapes from a circular hole in an infinite plane it dissipates energy so that the 
whole impedance credited to the hole is 
pn^ pn^ . ( k l\ n f.. 
Z2 = i = ek' {-i--\,^hev^k = -, (4) 
c Lira k2V c) a 
These three t3q)ical impedances will be at constant use in acoustics. It is to 
be remembered that systems in series have their impedances added and in 
parallel have the reciprocals of impedance added. Also that the free vibra- 
tions of a system are obtained by equating the impedances to zero. 
As a simple example consider the phone described in the previous article, 
figure 3. 
