THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

542 18. Music and Musical InstrumentsFigure 18.29. Two accordion versions: (a) button keyboard accordion and (b) piano keyboardaccordion. (Photographs of the Hohner Corona II diatonic accordion and HohnerAtlantic IV piano keyboard accordion courtesy of Hohner, Inc.)become smaller. The airflow then becomes reduced by the constricted passage,which causes the pressure on the flow side to increase, and the reed springs backto its original position. It then moves beyond its original position, under the effectof inertial energy, and the airflow becomes larger. The pressure again drops andthe reed returns to the normal position it had at the beginning, and the cycle ofevents repeats at the resonant frequency of the system. This action has the effectof converting a steady air stream into a saw-toothed pulsation that contains thefundamental and its harmonics.The harmonium or the free-reed organ operates through a series of air-actuatedfree reeds tuned to specific notes. The air supply driving the reeds is providedby two pedal-operated pedals connected to bellows that connects to a wind cheston the top of which reeds are mounted. This type of organ features a piano-likekeyboard. Each key operates a valve that controls the air supply to the reed, withone key for each reed. Reed organs are usually equipped with stops for connectingbanks of reeds. Thus, a single key can activate a number of reeds.The accordion shown in Figure 18.29 functions by the player’s arms working abellows which provides the air supply to the reeds, alternatively creating pressure(when the bellows is compressed) and a partial vacuum (when the bellows isexpanded). The air-actuated reeds, tuned to notes of a musical scale, are controlledthrough a keyboard. Each key connects to a valve that controls two separate reeds.One reed operates under pressure and the other under a vacuum. In some versionsof the instrument, two differently tuned reeds are assigned to each key that one toneis produced with expansion of the bellows and another tone with the compression.The vibrational action of the reed is that of a free-end bar. Accordions, which arereally portable versions of reed organs, are usually equipped with stops to connectindividual reeds so as to form banks of reeds. This enables operation of several