r 



and Distribution of Sound. 297 



Trumj^ets of Elomiated Section. 



In the trumpets at present employed the section is of 

 circular form ami the greater part of the axis is vertical. This 

 disposition has its conveniences, hut it entails bending th<^ 

 axis at the wide end of the cone if the mouth is to face hori- 

 zontally. The effect of such a bending upon the propagation 

 of the wave within the trumpet is hard to estimate. When, 

 as in the case of certain rock-stations, the sound is required 

 to be heard in all directions, a symmetrical form is adopted 

 in the Trinity House Service, the mouth of the trumpet 

 which taces vertically being partially stopped by an obstacle 

 known as the '' mushroom.^" The intention is to cut off the 

 sound in a vertical direction while allowing it to spread in 

 horizontal directions through the annular aperture between 

 the bell- mouth of the trumpet and the mushroom. 



Considerino- the case of the axis horizontal throughout, we 

 may inquire into the probable distribution of the sound. The 

 ratio between the diameter of the mouth and the wave-length 

 is here of essential importance. If the diameter much 

 exceed the half wave-length, the sound is concentrated in 

 the prolongation of the axis. If on the other hand the 

 diameter do not exceed the above-mentioned quantity, we 

 may expect a tolerably equable distribution of sound_, at any 

 rate through angles with the axis less than 80°. It follows 

 that the behaviour of the various components of a compound 

 sound may be quite different. The fundamental tone may 

 spread fairly well, while the octave and higher elements are 

 unduly concentrated in the neighbourhood of the axis. 



It appears then that a limitation must be imposed upon 

 the size of the mouth, if it be desired that the sound should 

 spread. But since the spreading is required only in the 

 horizontal plane, the limitation applies only to the horizontal 

 dimension of the mouth. There is no corresponding limita- 

 tion upon the vertical diameter. We are thus led to prefer 

 an elongated form of section, the horizontal dimension being- 

 limited to the half wave-length, while the vertical dimension 

 may amount if desired to many wave-lengths. This subject 

 was explained and illustrated in a lecture before the Eoyal 

 Institution *, the source of (inaudible) sound being a ^' bird- 

 call'''' giving waves of 3 cms. length, w^hich issued from a 

 flattened trumpet whose mouth measured 5 cms. by IJ cm. 

 The indicator was a high-pressure sensitive flame, and it 

 appeared very clearly that when the long dimension of the 

 section stood vertical the sound was approximately limited 



* Proc. Roy. Inst. Jan. 1902 : Nature, 66. p. 4f> (1902). 



