| Fig. 2.—Organ pipe fitted with beating reed. |
| AL, Beating reed. R, Reed box. Ff, Tuning wire. TV, Feed pipe. VV, Conical foot. S, Hole through which compressed air is fed. |
The arrangement of the free reed in an organ pipe is simple, and does not differ greatly from that of the beating reed shown in fig. 2 for the purpose of comparison. The reed-box, a rectangular wooden pipe, is closed at the bottom and covered on one face with a thin plate of copper having a rectangular slit over which is fixed the thin metal vibrating tongue or reed as described above. The reed-box, itself open at the top, is enclosed in a feed pipe having a conical foot pierced with a small hole through which the air current is forced by the action of the bellows. The impact of the incoming compressed air against the reed tongue sets it swinging through the slit, thus causing a disturbance or series of pulsations within the reed-box. The air then finds an escape through the resonating medium of a pipe fitting over the reed-box and terminating in an inverted cone covered with a cap in the top of which is pierced a small hole or vent. The quality of tone of free reeds is due to the tendency of air set in periodic pulsations to divide into aliquot vibrations or loops, producing the phenomenon known as harmonic overtones or upper partials, which may, in the highly composite clang of free reeds, be discerned as far as the 16th or 20th of the series. The more intermittent and interrupted the air current becomes, the greater the number of the upper partials produced.[3] The power of the overtones and their relation to the fundamental note depend greatly upon the form of the tongue, its position and the amount of the clearance left as it swings through the aperture.
Free reeds not associated with resonating media as in the concertina are peculiarly rich in harmonics, but as the higher harmonics lie very close together, disagreeable dissonances and a harsh tone result. The resonating pipe or chamber when suitably accommodated to the reed greatly modifies the tone by reinforcing the harmonics proper to itself, the others sinking into comparative insignificance. In order to produce a full rich tone, a resonator should be chosen whose deepest note coincides with the fundamental tone of the reed. The other upper partials will also be reinforced thereby, but to a less degree the higher the harmonics.[4]
For the history of the application of the free reed to keyboard instruments see [Harmonium].
(K. S.)
[1] See H. Helmholtz, Die Lehre von den Tonempfindungen (Brunswick, 1877), p. 166.
[2] See also Ernst Heinrich and Wilhelm Weber, Wellenlehre (Leipzig, 1825), where a particularly lucid explanation of the phenomenon is given, pp. 526-530.
[3] See Helmholtz, op. cit. p. 167.
[4] These phenomena are clearly explained at greater length by Sedley Taylor in Sound and Music (London, 1896), pp. 134-153 and pp. 74-86. See also Friedrich Zamminer, Die Musik und die musikalischen Instrumente, &c. (Giessen, 1855), p. 261.