A SIMPLE DEMONSTRATION OF THE DOPPLER EFFECT IN SOUND. 
BY FRANK F. ALMY. 
The example which is perhaps most frequently cited as an illustra- 
tion of the Doppler effect is the change of pitch of a locomotive hell or 
whistle or of a street-car gong in rapid motion, particularly as heard by 
a person on board a car passing rapidly in the opposite direction on a 
parallel track. To the use of this illustration Professor R. W. Wood' 
objects on the ground that “scarcely one person in ten has any distinct 
recollection of having noticed the phenomena unless his attention has 
been directed to it”. 
Professor Wood proposes to attach a pitch pipe to the tip of a hollow 
bamboo rod and a rubber hose to the hand end of the rod so that by this 
means the pipe may be sounded and waved to and fro in the “line-of- 
sight”. This simple device may be effective if blown by compressed air, 
but complications are introduced if the demonstrator attempts to blow 
the pipe while he waves the rod. 
While considering the experiment as suggested by Professor Wood, 
it occurred to my assistant, Mr. H. D. Way, that the motion in the “line- 
of-sight” could be produced by attaching the pitch pipe to the arm of 
the rotating apparatus. The additional suggestion immediately occurred 
that the reed could be placed at the base of a funnel faced in the direc- 
tion of rotation and be made to sound by the air pressure caused by the 
rotation. Following out these suggestions we produced a simple device 
which very effectively demonstrates the Doppler effect. The tone has its 
highest pitch when moving directly toward the observer, slurs continu- 
ously through its natural pitch as it moves across the “line-of-sight” to 
the lowest pitch when moving directly away from the observer. 
i’hys. Rev. 4 ; 504. 
( 229 ) 
