422 Lord Rayleigh on the Theory of Resonators. 



the diagram, which, though somewhat different in appearance, 

 are substantially identical. 



Fig. 1. Fig. 2. 



In both cases the sound enters, or rather is free to enter, at 

 A ; and at B finds itself at the mouth of a resonator B C, whose 

 natural pitch agrees with its own. Under these circumstances 

 the sound is absorbed, and there is no vibration propagated along 

 BD. It is clear that the cylindrical tube may be replaced by 

 any other resonator of the same pitch (fig. 3) without prejudice 



Fig. 3. 



to the action of the apparatus. 

 The ordinary explanation by the 

 interference (so called) of direct 

 and reflected waves would then 

 not apply, at any rate not without 

 extensive modification. 



The nature of the influence of 

 a resonator on a simple source of 

 sound in the immediate neigh- 

 bourhood of its mouth will now be 

 tolerably clear; the difficulty is 

 rather to explain why the result 

 is so different in the common experiment with the tuning- 

 fork. Yet it is evident that a vibrating tuning-fork is some- 

 thing very different from a simple source of sound. The ad- 

 vancing face of either prong may be treated as a diffused source 

 tending to produce a condensation ; but this is always accom- 

 panied by an equal and opposite source due to the retreating 

 face. Even if we confine ourselves to a single vibrating bar, the 

 source cannot be approximately represented as any thing simpler 

 than a double source, composed of two neighbouring equal and 



