HANDBOOK OF ACOUSTICS - online book

A complete view of Acoustical Science & its bearings on music, for musicians & music students.

Home | Just The Tune | Order | Contact




FLUE-PIPES AND REEDS.
101
pipe itself varies with change of temperature, increasing with a rise and shortening again with a fall of temperature. This will obviously have a contrary effect on the pitch, but to a very much smaller extent; in fact, in wooden pipes the expansion is quite inappreciable. Thus the general effect of rise of temperature in organ pipes is to sharpen them.
It is evident, from the above, that the wooden pipes of an organ will sharpen somewhat more than the metal ones, for the same rise of temperature. Furthermore, it is found that small pipes become relatively sharper than large ones, under the same increment of heat; and not only is this the case, but the change takes place much more rapidly in small pipes than in large ones, and in open than in closed pipes. On the other hand, although metal pipes do not sharpen quite so much as wooden ones, they are affected much more rapidly. According to Perronet Thompson, diminution of atmospheric pressure sharpens the tones of pipes, and vice versa. He states that a fall of an inch sharpens the tuning C by a comma.
The lowest note producible in the largest organ is=16, and is obtained from an open pipe about 32 feet long. This pipe together with those giving notes of lower pitch than C3 = 32, are said to belong to the 32 foot octave. C3 = 32 is produced by an open pipe about 16 feet long ; hence, from
Increase of intensity in the tones of an organ cannot be obtained by increase of force in blowing : for as we have just seen, a very slight increase in the wind pressure alters their pitch slightly, and still greater increase, as we shall presently see, would affect their quality also. Hence, increase of intensity on the organ has to be produced by bringing more pipes into action by means of stops, or by enclosing the pipes in a case, which can be opened or closed at pleasure, as in the swell organ.
We have now to turn our attention to the conditions that deter­mine the occurrence of overtones, in the tones of organ pipes. Procure an ordinary open wooden or metal organ pipe and blow