Free reed vibrator - Encyclopedia




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FREE REED VIBRATOR (Fr. anche libre, Ger. durchschlagende Zunge, Ital. ancia or lingua libera), in musical instruments, a thin metal tongue fixed at one end and vibrating freely either in surrounding space, as in the accordion and concertina, or enclosed in a pipe or channel, as in certain reed stops of the organ or in the harmonium. The enclosed reed, in its typical and theoretical form, is fixed over an aperture of the same shape but just large enough to allow it to swing freely backwards and forwards, alternately opening and closing the aperture, when driven by a current of compressed air. We have to deal with air under three different conditions in considering the phenomenon of the sound produced by free reeds. (r) The stationary column or stratum in pipe or channel containing the reed, which is normally at rest. (2) The wind or current of air fed from the bellows with a variable velocity and pressure, which is broken up into periodic air puffs as its entrance into pipe or channel is alternately checked or allowed by the vibrator. (3) The disturbed condition of No. i when acted upon by the metal vibrator and by No 2, whereby the air within the pipe is forced into alternate pulses of condensation and rarefaction. The free reed is therefore not the tone-producer but only the exciting agent, that is to say, the sound is not produced by the communication of the free reed's vibrations to the surrounding air,' as in the case of a vibrating string, but by the series of air puffs punctuated by infinitesimal pauses, which it produces by alternately opening and almost closing the aperture.' A musical sound is thus produced the pitch of which depends on the length and thickness of the metal tongue; the greater the length, the slower the vibrations and the lower the pitch, while on the contrary, the thicker the reed near the shoulder at the fixed end, the higher the pitch. It must be borne in mind that the periodic vibrations of the reed determine the pitch of the sound solely by the frequency per second they impose upon the pulses of rarefaction and condensation within the pipe.

The most valuable characteristic of the free reed is its power of producing all the delicate gradations of tone between forte and piano by virtue of a law of acoustics governing the vibration of free reeds, whereby increased pressure of wind produces a proportional increase in the volume of tone. The pitch of any sound depends upon the frequency of the sound-waves, that is, the number per second which reach the ear; the fullness of sound depends upon the amplitude of the waves, or, more strictly speaking, of the swing of the transmitting particles of the medium - greater pressure in the air current (No. 2 above) which sets the vibrator in motion producing amplitude of vibration in the air within the receptacle (No. 3 above) serving as resonating medium. The sound produced by the free reed itself is weak and requires to be reinforced by means of an additional stationary column or stratum of air. Free reed instruments are therefore classified according to the nature of the resonant medium provided: (i) Free reeds vibrating in pipes, such as the reed stops of church organs on the continent of Europe (in England the reed pipes are generally provided with beating reeds, see Reed Instruments and Clarinet). (2) Free reeds vibrating in reed compartments and reinforced by air chambers of various shapes and sizes as in the harmonium (q.v.). (3) Instruments like the accordion and concertina having the free reed set in vibration through a valve, but having no reinforcing medium.

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 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.

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 10th 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. 's 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.)

Current Understanding of the motion of free reeds

Although much of what has been writen here about the free reed vibrator is very good, there is one important fundamental process in the sound production that has been shown to be different from previous theory. Many wind instruments that use a reed rely on the harmonic excitation of the reed coupled with the enclosed mass of air to produce a forced resonator. In free reed acoustics however the reed is not coupled to the air and the sound is produced entirely by the reed. This is very interesting because it is unlike any other wind instrument and relies on the shape of the construction of the free reed plate. The motion can be explained through fluid mechanics and a good explanation is given in the Journal of Fluid Mechanics, Volume 49, part 4, pp.803 - 816. The important aspect of the reed is that it only vibrates when air passes in one direction and this is the reason that harmonicas have a different note on the blow and the suck (because a different reed is excited).

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