Proceedings of the International Symposium on Music Acoustics (Associated Meeting of the International Congress on Acoustics) 25-31 August 2010, Sydney and Katoomba, Australia Flow acoustical determinants of historic flue organ pipe voicing practices D.Steenbrugge Ghent University College, Ghent, Belgium PACS: 43.75.NP ABSTRACT Pipe organs sound differently depending on the regional and contemporary circumstances of their genesis. In order to adequately conserve them it is important, due to the scarcity of sources providing direct information on the building process and particularly on pipe voicing practices, to be able to specify as much as possible the factors that determine their sound signature. To this purpose an organized overview of some of these practices is proposed as a starting framework, exposed with reference to the classical flow-a- coustical flue pipe models developed by, among others, Cremer & Ising, Coltman, Elder and Fletcher. On the other hand, a method is presented to assess the tonal and musical effects of voicing configurations, using a 'voicing jack'-like system featuring a complete stop rank of experimental pipes with electronically controlled, continuously variable flue, mouth and toe hole geometry. On this play- able system the voicing (and tuning) of the pipes can be varied easily, rapidly, precisely and reversibly. In order to characterize spe - cific voicing strategies both approaches are combined by relating the musical significance associated with the test voicing configura - tions to corresponding values of a set of characteristic dimensionless ratios. Well-known voicing methods like open or closed toe and over- or underblown voicing are identified accordingly, as well as less common or builder specific procedures. INTRODUCTION • various tuning devices near the open end A better understanding of the operation of organ pipes can be • wall material and thickness and surface finish of considerable help in developing better conservation • upper labium geometry strategies of historic pipe organs. Although differences in • languid edge geometry tonal performance between instruments from different re- • nicks gions and builders are easily perceived, it's more difficult to • wind supply transients track these differences down to the material evidence. Pipe • various ear and roller beard geometries voicers traditionally are experts in just a few particular styles and feel uncomfortable and ignorant outside of these. On the other hand, physical and acoustical analysis of flue instru- ments operation is more involved with fundamental operating mechanisms and their modeling, and less with the esthetics BASIC VOICING PRINCIPLES involved with the differentiation of the model parameters. This paper , which attempts to give an overview of the great Determinants of flue organ pipe voicing variety of historic flue organ pipe voicing practices as found Although pipe organs seem to have a common origin, going in different regions and periods, does so by combining ele- back to the Greeks, by the end of the Middle Ages, when they ments from both worlds. A double approach will be used: seem to have been firmly established all over Western Europe voicing practices will be described in terms of the classical [Williams 2005 ], differences in climate and society culture flue instruments physical descriptions that have been de- inevitably led to differentiation in organ design. They were veloped ever since the XIXth century, with particularly im- mainly conceived for the churches, first for the smaller and portant progress in the 60's, 70's and 90's of last century. Spe- less reverberant Roman, later for the large stone-and-glass cifically, a number of dimensionless parameters will be selec- Gothic edifices. Accordingly they seem to have had to pro- ted with which voicing practices will be classified. And on duce rather large sound volumes, as testified by the early ap- the other hand, because the sensible ear remains a valuable pearance of the Blockwerk, which by the simultaneously measuring device in correlating or distinguishing sounds, an sounding many pipes per note of widely differing pitch (su- experimental setup will be presented which focuses on the peroctaves and -fifths), fully took into account the masking perceptive and esthetic qualities of voicing schemes. This ap- properties of the ear. In this period they all had in common proach might make up for some of the difficulties en- the use of low wind pressure and flow rate, because of eco- countered in trying to identify with confidence certain sound nomies of number of necessary bellows and man-power and qualities in the extant historic evidence. less problems with leaks which tend to get worse with in- The scope of this paper is limited to flue organ pipes, more creasing pressure. specifically the so-called medium-scaled (open) principals Low wind pressure has more advantages: which form the heart of every organ. Attention will be given – Lower action force necessary mainly to the voicing practices associated with the mouth re- gion, thus excluding influences of: ISMA 2010, associated meeting of ICA 2010 1 – Easier to voice because irregularities tend to be diameter halving occurring at the 16th to 18th pipe instead of smoother. the 13th. Disadvantages however were: Quick voicing opposes rich timbre – A less powerful sound, countered as much as pos- Another law which is reminiscent of similar observations in sible by the 'widely different pitches' principle other instruments, like in strings or percussion. As to its rel- – Attempting to minimize air consumption meant evance in flue organ pipe voicing, it will be studied further in voicing with narrow flues, so that it's more difficult conjunction with the jet angle parameter. to obtain both equalized loudness and timbre throughout the range – Dust accumulation in narrow flues disturbs opera- tion FLOW-ACOUSTICAL DETERMINANTS OF – The loudness and brilliance of reeds suffers particu- FLUE ORGAN PIPE VOICING larly Accordingly the early pipe organs, like the Blockwerk, min- Introduction imized wind consumption by: – In this section a set of dimensionless parameters is defined, using a 'synthetic' tonal design which not necessar- by means of which an attempt will be done to characterise ily contained reeds – various voicing practices. They are based on the classical implementing relatively weak basses (often with models of flue instrument operation that have been developed short octave) since the 1960's. A thorough evaluation of their validity and – using open foot and narrow flue voicing utility, by means of extensive measurements on the variable The gradual evolution towards organs with more 'gravitas' is mouth geometry pipes set, will be the subject of future re- thus another demonstration of the close interaction between ports. esthetics and technology. Voicing practices, as was every part of the organ design, were also determined by the intended use of the instruments. Inharmonicity parameter Dinharm In Italy and France the organ served to embellish the liturgic- The inharmonicity of the passive pipe resonances is determ- al service, for instance through the alternatim practice. In the ined by the ratio a/L, where a is the pipe's radius and L its former country the organ was treated rather as an alternative length, and the ratio of pipe cross-sectional area to mouth to the voice, in the latter is was precisely supposed to provide area Sopenend/Smouth. a/L can equivalently be written as Sopen- 1/2 a contrast to the sung parts. With the advent of the Lutheran end /L, so that combining both leads to the introduction of the church in Northern Germany an instrument capable of ac- inharmonicity determinant: 1/2 companying the community singing was needed. Besides Dinharm= Sopenend/(Smouth L) these liturgical conditions the sound of the organ was also Because the end corrections increase with increasing a, and impregnated with the treats peculiar to a people's culture: the are smaller for the higher resonances, wider pipes and pipes classical French organ reflected the splendor and wealth of having small mouths will sound less rich than narrower pipes. the Sun-king's court, the North German organ the order and Accordingly, because their radiated acoustic energy is spread multilayered-ness of this society consisting of Church and over a larger spectrum the latter will therefore tend to sound State, patricians and peasants. louder. On the other hand the sound of the former, with their acoustic energy more concentrated in the very first harmon- ics, will carry a larger distance. The fundamental voicing laws Establishing this determinant for a pipe register constitutes a Certain voicing laws seem to be universally applicable and fundamental problem in pipe organ design, the goal being to find their equivalents elsewhere in music practice. Before find a row of pipes which is sufficiently balanced in loudness turning into the details of pipe voicing adjustments it's inter- and timbre throughout its range. Assuming a to scale like f -x esting to briefly mention some of the more widely accepted and with the usual pipe geometries, an equalized loudness of these laws and possibly try to interpret them from an level throughout the range can be obtained when x varies acoustical viewpoint. from .87 to .67 going from bass to treble [Fletcher & Rossing In highly reverberant rooms bass notes are favored 1998, Steenbrugge 1997]. These values lie around the so-c- In accordance with the rule of thumb, that the longest (open) alled normal scale with x=.75. pipe of the organ should fit about 16 times in the largest dis- As both the mouth [Ingerslev & Frobenius 1974] and open tance the sound can travel uninterrupted, the ability of a end corrections [Levine & Schwinger 1948] scale with a in sound wave to perform sufficient reflections seems to 1-x pipes with the usual proportions, Dinharm scales like f . This sharpen the aural attention to this frequency. In small rooms implies that only geometrically similar pipes, with x=1, will this becomes difficult for low frequencies and they are per- show the same resonance positions relative to their respective ceived with less acuity than the treble sounds.