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Acoustics of Organ Pipes
Flue Pipes
Although the main features of the sound of flue organ pipes have been investigated extensively (Yoshikawa and Saneyo- shi, 1980; Verge et al., 1994; Fletcher, 1996), the connection between sound character and pipe shape and the dimen- sions are still not well understood. In the tradition of organ building, however, the sound character of the different ranks is unambiguously associated with pipe shape, material, and dimensions (Töpfer, 1888; Mahrenholz, 1987). Although the timbre, and especially the speech (attack) of the pipe (the very beginning of the pipe sound), may be changed signifi- cantly by voicing adjustments (changing the geometrical pa- rameters of the pipe such as the diameter of the foothole and the width of the flue and cutting up the mouth [upper lip]), the main characteristics of the sound are quite stable for a given rank and primarily depend on the form and progress of dimensions with note (scaling) of the pipes. It is of inter- est to scientists that only a very narrow range of all the pos- sible dimensions (diameter, wall thickness, cut-up height, flue width) and materials are actually used for organ pipes. Some of these limitations can be explained by technological reasons, but most of them have no basis in science.
Experimental Results
Although flue pipes offer a very wide variety of sounds, the measured properties of these sounds contain several com- mon elements that can be used to characterize them. To de- termine such characteristics, three measurements are used: the stationary spectra (the spectrum of the sound of a con- tinuously sounding pipe) at both the mouth and the open end and the attack transient at the mouth. To do this, sta- tionary spectra are measured by microphones placed close (~3-5 cm) to the two openings of the pipes and the attack transients at the mouth are analyzed using a special com- puter program (Angster and Miklós, 1995).
Steady-Sound Characteristic Features and Related Physical Phenomena
The stationary spectra of a flue pipe and the characteristic features of the sound spectra can be seen in Figure 4.
The flue pipe ranks are divided into three groups according to their characteristic sound. The widest flue pipes (flutes) produce tones with the most fundamental and the least har- monics among flue pipes, and they start to speak the fastest
Figure 4. Typical stationary spectrum of a flue organ pipe at the open end (a) and at the mouth (b). See text for details.
(fast attack). The Diapason or principal family produces the characteristic sound of the pipe organ and is not intended to imitate any other instrument or sound. They are medium scaled and are often prominently featured in the façades of pipe organs. They can be characterized by their strong second partial, especially in the attack. String pipes are the narrowest flue pipes. They produce a bright sound that is low in fundamentals and rich in upper partials. One of the most common string stops is named Salicional. String stops are often named after bowed string instruments such as the Violoncello, the Gamba, and the Geigen (from the German Geige, for violin; see http://acousticstoday.org/flue). They have very bright sounds with more than 20 harmonic par- tials but with a slow attack (Miklós and Angster, 2000).
The characteristic features of the sound spectra of a flue or- gan pipe can be listed and the related physical phenomena can be explained as follows.
A Series of Harmonic Partials
It is well-known from the elements of the Fourier theory (Korn and Korn, 1975) that the spectrum of a periodic sig- nal contains a series of harmonic components (partials). These partials can be seen in Figure 4.
12 | Acoustics Today | Spring 2017