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BELL SYSTEM TECHNICAL JOURNAL 



Thus far only the sound generated by the orchestra itself has been 

 considered. However, it is well known that the esthetic value of 

 orchestral music in a concert hall is dependent to a very great extent 

 upon the acoustic properties of the hall. At first thought one might 

 be inclined to leave this out of account in considering the reproduction 

 by a loud speaking system, as one should normally choose a hall known 

 to have satisfactory acoustics for an actual orchestra. There would 

 be no further problem in this if the orchestral instruments and the 

 loud speaker radiated the sound uniformly in all directions, but some 

 of the important instruments are quite directive; i.e., they radiate 

 much the greater portion of their sound through a relatively small 

 angle. As an example, a polar diagram giving the relative intensities 

 of the sound radiated in various directions by the violin is given in 

 Fig. 3, which is taken from a paper published by Backhaus.^ The 



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Fig. 3 — Variation of intensity with direction of the sound radiated by a violin 



(660 c.p.s.). 



directional characteristics of some of the instruments is one of the 

 chief reasons why the music from an orchestra does not sound the 

 same in all parts of a concert hall. The music which we hear comes to 

 us in part directly and in. part indirectly; i.e., after one or more 

 reflections from the walls. Both contribute to the esthetic value of 

 the music. The ratio of the direct to the indirect sound, which has 

 been designated by Hughes '' as the acoustic ratio, is to a first approx- 

 imation inversely proportional to the product of the reverberation 

 time and the angle through which the sound is radiated.^ For a 

 steady tone by far the greater part of the intensity at a given point in 

 a hall remote from the source is attributable to the indirect sound. 

 However, inasmuch as many of the tones of a musical .selection are 



