Vibration characteristics of pipe organ reed tongues and the effect of the shallot, resonator, and reed curvature

Pipe organ reed pipes sound when a fixed-free curved brass reed mounted on a shallot connected to a resonator is forced to vibrate by an impressed static air pressure. Five sets of experiments were performed in order to investigate the influence of the most important parameters which could affect the tone of a reed pipe. First, the phase difference between the pressure variation in the shallot and the boot, and its relationship to the motion of the reed tongue were analyzed to compare their phases and their spectra. Next, the frequency dependence of the reed on three basic parameters (reed thickness, its vibrating length, and the imposed static air pressure) was investigated in an attempt to determine an empirical equation for the frequency. For each trial, two of the variables were kept constant while the third was altered in order to construct an equation giving frequency as a function of the three variables. Third, experiments were conducted using three different types of shallots: the American (or English) style, the French style, and the German style. The results show that for each shallot, the frequency increases linearly with thickness and linearly with air pressure (over the normal operating range of the reed). For each of the shallots, frequency varies inversely with length when the other variables are held constant. The effect on the reed spectrum of using the three different types of shallot was also investigated, as was the effect of reducing the interior volume of each type. Progressively filling the shallot interior generally decreases the frequency of the vibrating reed. The effect of the resonators on frequency and spectrum was studied because the resonator is an integral part of the resulting tone; virtually every reed stop has some type of resonator. The resonator tends to raise the Q of the impedance peaks and reduce the fundamental frequency. Finally, the influence of the type and degree of curvature on reed vibration was briefly examined; increasing the reed curvature tends to decrease the vibration frequency and increase the sound intensity by creating a richer spectrum.