1902.] on Interference of Sound. 3 



every two or three seconds by a small hammer. Although in every 

 position the flame shows some uneasiness at the stroke of the hammer, 

 the distinction of loops and nodes is perfectly evident, and the 

 measurement of wave length can be efifected with an accuracy of 

 about 1 per cent. In the actual experiment the wave length was 

 nearly 8 cm. 



The formation of stationary waves with nodes and loops by per- 

 pendicular reflection illustrates interference to a certain extent, but 

 for the full development of the phenomenon the interfering sounds 

 should be travelling in the same, or nearly the same, direction. The 

 next example illustrates the theory of Huyghens' zones. Between 

 the bird-call and the flame is placed a glass screen perforated with 

 a circular hole. The size of the hole, the distances, and the wave 

 length are so related to one another that the aperture just includes 

 the first and second zones. The operation of the sounds passing 

 these zones is antagonistic, and the flame shows no response until a 

 part of the aperture is blocked ofi*. The part blocked off may be 

 either the central circle or the annular region defined as the 

 secord zone. In either case the flame flares, affording complete 

 proof of interference of the parts of the sound transmitted by the 

 aperture. 



From a practical point of view the passage of sound through 

 apertures in walls is not of importance, but similar considerations 

 apply to its issue from the mouths of horns, at least when the dia- 

 meter of the mouth exceeds the half wave-length. The various parts 

 of the sound are approximately in the same phase when they leave 

 the aperture, but the effect upon an observer depends upon the phases 

 of the souncls, not as they leave, but as they arrive. If one part has 

 further to go than another, a phase discrepancy sets in. To a point 

 in the axis of the horn, supposed to be directed horizontally, the dis- 

 tances to be travelled are the same, so that here the full effect is pro- 

 duced, but in oblique directions it is otherwise. When the obliquity 

 is such that the nearest and furthest parts of the mouth differ in 

 distance by rather more than one complete wave-length, the sound 

 may disappear altogether through antagonism of equal and opposite 

 effects. In practice the attainment of a complete silence would bo 

 interfered with by reflections, and in many cases by a composite 

 character of sound, viz. by the simultaneous occurrence of more than 

 one wave-length. 



In the fog-signals established on our coasts the sound of powerful 

 sirens issues from conical horns of circular cross-section. The in- 

 fluence of obliquity is usually very marked. When the sound is 

 observed from a sufficient distance at sea, a deviation of even 20° 

 from the axial line entails a considerable loss, to be further increased 

 as the deviation rises to 40° or 60°. The difficulty thence arising is 

 met, in the practice of the Trinity House, by the use of two distinct 

 sirens and horns, the axes of the latter being inclined to one another 

 at 120°. In this way an arc of 180° or more can be efficiently 



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