1902.] on Interference of Sound. R 



already described, illustrates what are known in Optics as Lloyd's 

 bands. These bands are formed by the interference of the direct 

 vibration with its very oblique reflection. If the bird-call is pointed 

 toward the flame, flaring ensues. It is only necessary to hold a long 

 board horizontally under the direct line to obtain a reflection. The 

 effect depends upon the precise height at which the board is held. 

 In some positions the direct and reflected vibrations co-operate at the 

 flame and the flarin<^ is more pronounced than when the board is 

 away. In other positions the waves are antagonistic and the flame 

 recovers as if no sound were reaching it at all. This experiment 

 was made many years ago by Tyndall who instituted it in order to 

 explain the very puzzling phenomenon of the " silent area." In 

 listening to fog-siguals from the sea it is not unfrequcntly found that 

 the signal is lost at a distance of a mile or two and recovered at a 

 greater distance in the same direction. During the recent experi- 

 ments the Committee of the Elder Brethren of the Trinity House had 

 several opportunities of making this observatif)n. That the surface 

 of the sea must act in the manner supposed by Tyndall cannot be 

 doubted, but there are two difficulties in the way of accepting the 

 simple explanation as complete. According to it the interference 

 should always be the same, which is certainly not the case. Usually 

 there is no silent area. Again, although according to the analogy of 

 Lloyd's bands there might be a dark or silent place at a particular 

 height above the water, say on the bridge of the Irene, the effect 

 should be limited to the neighbourhood of the particular heigiit. At 

 a height above the water twice as great, or near the water level itself, 

 the sound should be heard again. In the latter case there were some 

 difficulties, arising from disturbing noises, in making a satisfactory 

 trial ; but as a matter of fact, neither by an observer up the mast uor 

 by one near the water level, was a sound lost on the bridge ever 

 recovered. 



The interference bands of Fresnel's experiment may be imitated 

 by a bifurcation of the sound issuing from A (Fig. 1). For this 

 purpose a sort of T-tube is fitted, the free ends being provided with 

 small elliptical cones, similar to that already described, whose axes 

 are parallel and distant from another by about 40 cm. The whole 

 is constructed with regard to symmetry, so that sounds of equal in- 

 tensity and of the same phase issue from the two cones whose long 

 diameters are vertical. If the distances of the burner from the mouths 

 of the cones be precisely equal, the sounds arrive in the si me phase 

 and the flame flares vigorously. If, as by the hand held between, 

 one of the sounds is cut off, the flaring is reduced, showing that with 

 this adjustment tl e two sounds are more powerful than one. By an 

 almost imperceptible slewing round of the apparatus on its base- 

 board the adjustment above spoken of is upset and the flame is induced 

 to recover its tall equilibrium condition. The sounds now reach the 

 flame in opposition of phase and practically neutralise one another. 

 That this is so is proved in a moment. If the hand be introduced 



