A strong reason against the duplication of pipes of similar tone in an organ is that curious acoustical phenomenon, the _bête noir_ of the organ-builder, known as _sympathy_, or interference of sound waves. When two pipes of exactly the same pitch and scale are so placed that the pulsations of air from the one pass into the other, if blown separately the tone of each is clear; blown together there is practically no sound heard, the waves of the one streaming into the other, and a listener hears only the rushing of the air. That the conditions which produce sound are all present may be demonstrated by conveying a tube from the mouth of either of the pipes to a listener's ear, when its tone will be distinctly heard. In other words, one sound destroys the other. Helmholtz explains this phenomenon by saying that "when two equal sound waves are in opposition the one nullifies the effect of the other and the result is a straight line," that is, no wave, no sound. "If a wave crest of a particular size and form coincides with another exactly like it, the result will be a crest double the height of each one" (that is, the sound will be augmented). * * * "If a crest coincides with a trough the result will be that the one will unify the other," and the sound will be destroyed.[1] That is why in the old-style organs the builder, when he used more than one Diapason, tried to avoid this sympathy by using pipes of different scale, but even then the results were seldom satisfactory; the big pipes seemed to swallow the little ones. In the big organ in Leeds Town Hall, England, there was one pipe in the Principal which nobody could tune. The tuner turned it every possible way in its socket without avail, and at last succeeded by removing it from the socket and mounting it on a block at a considerable distance from its proper place, the wind being conveyed to it by a tube. This is only one instance of what frequently occurred.
In the Hope-Jones organ the usual plan of putting all the C pipes on one side of the organ and all the C# pipes on the other, is departed from. The pipes are alternated and in this ingenious way sympathy is largely avoided.
[1] Broadhouse: "Musical Acoustics," p. 261.