Coming back to our Flute again and applying the knowledge we have gained to an organ pipe, we observe:
1. That the pitch of the sound depends on the length of the tube.
2. That the pitch of the sound also depends on the amount of wind pressure.
From this last will be seen how important it is that the pressure of the wind in an organ should be steady and uniform. Otherwise the pipes will speak a harmonic instead of the sound intended—as, indeed, frequently happens.
When a stop is labeled "8 ft.," that means that the bottom pipe, CC is 8 feet long and the pitch will be that of the key struck. A "16-ft." stop will sound an octave lower; a "4-ft." stop an octave higher. These measurements refer to pipes which are open at the top and are only correct in the case of very narrow pipes, such as the stop called Dulciana. Wider pipes do not require to be so long in order to produce 8-ft. tone.
"If a tube * * * open at both ends be blown across at one end, the fundamental tone of the tube will be sounded; but if the hand be placed at one end of the tube, so as to effectually close it, and the open end be blown across as before, a sound will be heard exactly one octave below that which was heard when both ends of the tube were open. One of these pipes was an open pipe, the other a stopped pipe; and the difference between the two is that which constitutes the two great classes into which the flue pipes of organs are divided." [1]
Thus by stopping up the end of an organ pipe we get 8-ft. tone from a pipe only 4 ft. long, 16-ft. tone from a pipe 8 ft. long, and so on, but with loss of power and volume. The harmonics produced from stopped pipes are entirely different from those of the open ones; their harmonic scale is produced by vibrations which are as 1, 2, 3, 4, etc., those of a stopped pipe by vibrations which are as 1, 3, 5, 7. All these harmonics are also called upper partials.
The Estey Organ Company claim to have discovered a new principle in acoustics in their Open Bass pipes, of which we show a drawing opposite. This invention (by William E. Haskell) enables the builders to supply open bass tone in organ chambers and swell boxes where there is not room for full-length pipes.
