Oakeley, Sir Herbert (Stanley)



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III. Pipework

There are several classes of organ pipes, the two oldest and most integral to the development of the organ being flue pipes and reed pipes. More common by far, though not necessarily more varied, are flue pipes. Both types operate on the coupled-air system of sound production common to flutes, recorders, oboes, clarinets etc.

1. Flue pipes.

2. Reed pipes.

3. Free reeds.

4. Diaphones (valvular reeds).

Organ, §III: Pipework

1. Flue pipes.

Air under pressure from the chest passes through the foot-hole (bore) at the base of the pipe-foot (fig.16) and so through the flue or windway, to issue in a flat sheet of wind striking the edge of the upper lip; the refracted wind causes eddies to form at the mouth, first on one side of the upper lip, then on the other. The natural frequency of the pipe’s body is coupled to the note of the ‘edge tones’ produced at the upper lip and gives to the eddies a rate of production that becomes the frequency of the note produced. Thus the effective length of the pipe is the principal factor in the pitch of the note.

Pitch and timbre are affected by several other factors, few of which, however, are variable outside narrow limits. A narrow pipe, to produce a certain pitch, must be longer than a wide one; a conical one must likewise be longer if it narrows towards the top, but shorter if it tapers outwards. Such variations in shape, however, are generally more important for their effect on a pipe’s timbre than on its pitch. A cylindrical pipe stopped at the end will sound approximately an octave lower than if it were open; for a conical pipe the difference is not quite so great. A half-stopped cylindrical pipe (i.e. with its cap pierced and usually a tube passing through the hole) speaks at a somewhat higher pitch than a stopped pipe.

The narrower the mouth or the smaller the flue, then the smaller the volume of air (at any given pressure) striking the upper lip and the softer the sound; the higher the mouth in relation to its width (i.e. the greater the ‘cut-up’), then the rounder, duller or more flute-like the tone (hence the designation ‘flute stops’); the narrower the pipe as a whole, the richer the harmonic spectrum and the more string-like the tone (hence ‘string stops’). It was said at one time that the harder the metal, the richer the harmonic spectrum; or the more lead contained in the pipe-alloy, the ‘duller’ the sound. But Backus and Hundley (C1966) established from theoretical and experimental evidence that ‘the steady tone of a pipe does not depend on the material of the pipewall. The belief that the use of tin in constructing pipes gives a better tone appears to be a myth unsupported by the evidence’. Experienced voicers, however, will aver that the composition of pipe metal does affect tone quality, and that it is impossible to match exactly the tone quality of two otherwise identical pipes made of very different alloys. More to the point, perhaps, is that tin-lead alloys are easy to work and shape, thus allowing the builder a high degree of adjustment at the parts of the pipe crucial to voicing processes.

Most of these factors can be used only to a certain degree: a point is soon reached when a pipe will not speak at all, even when other factors are altered, e.g. increasing or decreasing the wind pressure. Consequently the various interrelated factors involved in voicing a pipe require pragmatic expertise in their manipulation.



In addition to its more general usage, the term ‘scale’ can refer to a pipe’s diameter in relation to a norm (‘wide’ or ‘narrow’ scale), and the relationship or ratio between one pipe’s diameter and that of its octave below in the same rank (3:5 etc.; see Scaling). One well-known norm is the Normprinzipal suggested at the German Organ Reform (Orgelbewegung) conferences in the 1920s; this norm is ‘one pipe larger’ than the Normalmensur promulgated by J.G. Töpfer about 1845 (thus the diameter of Töpfer’s C pipe is that of the Normprinzipal C). G.A. Sorge had been the first to use logarithms to find constant scalings for organ pipes (c1760), calculating pipe diameter, pipe length, mouth width and mouth height by this method. Other 17th- and 18th-century theorists (such as Mersenne and Bédos de Celles) suggested scaling-figures by means of tables culled from practical experience and from the empiricism of organ builders themselves. Only two generations after Sorge did Töpfer develop the idea of arithmetical calculation for pipes (with immense influence on builders of his time): he calculated the cross-sectional area of a pipe an octave higher than the given pipe by applying the ratio 1:√8. Thus a pipe with half the diameter of a given pipe is not an octave (12 pipes) above but 16 or 17 pipes above. Such a factor as 1:√8 was itself reasonable, and many older builders had worked more or less to it, though empirically and not rigidly; indeed, Töpfer’s formula can be deplored for the encouragement it gave to 19th-century ‘organ-factory builders’ who applied a constant scale irrespective of the acoustics of the church or indeed any other variable of importance to organ tone. The Orgelbewegung’s Normprinzipal was similarly abused by some of the less imaginative builders of the neo-Baroque era in the early 20th century.

Fig.17 shows some flue-pipe shapes and is scaled to indicate the relative sizes of different types all producing the same C
. (The Normprinzipal diameter of the C pipe at a pitch standard of a' = 435 is 155·5 mm; at a pitch standard of a' = 440, the diameter for C would be reduced to 154·17 mm – a fine point of difference since variations in temperature will change the pitch this much). Most historic types of English Open Diapason, French Montre and Venetian Principale have been wider in scale than the Normprinzipal, and for many builders it remains merely one of the possible norms. It must also be remembered that the diagram does not refer to factors other than scaling, such as wind pressure. Mouth widths are usually expressed as proportions of the circumference, and those ordinarily used range from 2:7 down to 1:6, though 1:4 remains common for Principal pipes, and further extremes have been used for special effects. The cut-up is expressed as a fraction of the mouth width, ‘quarter cut-up’ indicating that the mouth is a quarter as high as it is wide.

Wooden pipes are either stopped (most commonly 8', then 16' and 4') or open (16', 8', 4', 2'); sometimes half-stopped wooden pipes (i.e. with a pierced stopper) of the Rohrflöte (Chimney Flute) type are found, especially in small organs. Metal or wood conical pipes narrowing towards the top have been found in the largest Dutch, German and Spanish organs since about 1540. Metal pipes with ‘pavilions’ or ‘bells’ (inverted conical caps) were made especially by French and English builders for about a century from about 1840, both on the flute and string side of tone-colour, as well as in Principals. Overblowing pipes have also been popular in large organs and in special instruments made for colourful secular use; the most common during the period c1600–1800 was the narrow-scaled, narrow-mouthed open cylindrical pipe, overblowing to the 2nd partial or ‘at the octave’ above. Such pipes require to be twice as long as the pitch length (8' for 4' pitch). Stopped pipes overblow to the 3rd partial or ‘at the 12th’ above, and require to be three times as long as the normal stopped length (6' for 4' pitch); they are fairly rare. Overblowing flute pipes (Flûte harmonique, etc.) became widely used after the middle of the 19th century, having been developed to a high degree in France. Such pipes are of double length but of the scale of a normal-length open flute, and are pierced at the node (approximately halfway up from the mouth) with one or two small holes. Given full wind, such pipes will overblow, giving a strong, sweet and rather fundamental tone not unlike that of the modern orchestral flute, but are not usually found below 13/5' e' in pitch, the lower part of the stop consisting of wide-scaled open pipes of normal length. Alternatively, to prevent overblowing in narrow-scaled string-toned pipes, or to aid tuning at the mouth of stopped pipes, ‘ears’ or ‘beards’ are often added: these are short metal plates or rods of metal or wood soldered or held to the sides of (and sometimes below) the mouth, protruding from it and helping to direct the vortices of wind on to the edge of the upper lip.



Organ, §III: Pipework

2. Reed pipes.


Air under pressure from the chest passes through the bore into the boot and so through the opening in the shallot (fig.18); in so doing the wind sets the thin, flexible brass reed-tongue into vibration against the shallot; this in turn sets the air column in the pipe or resonator into vibration, producing a coupled system. The frequency of the note produced is determined by the length of the air column in the resonator and by the length, mass and stiffness of the reed-tongue.

The pitch and tone of the pipe are affected by many factors; if all the factors are constant, then the longer the reed-tongue and shallot, the lower the pitch. To produce a required pitch in reed pipes with either cylindrical or conical resonators, the resonator must be shorter the longer the tongue. But in practice this property is used within only a small margin, as the tone is more immediately and strikingly affected by a change in the relationship between tongue length and resonator length. Natural ‘full-length’ (‘harmonic-length’) cylindrical resonators correspond roughly in length to stopped pipes of the same pitch; for natural ‘full-length’ conical resonators the ‘resonance length’ is as little as three-quarters of the pitch length (i.e. 6' or 7' for an 8' Trumpet). A reed pipe will speak (although weakly and without fundamental) without its resonator, whose purpose is to reinforce certain partials, to ‘give tone’ to the pipe. But in a reed with a resonator a point is soon reached, if the reed-resonator relationship is altered, when the pipe will either fly off its speech or not speak at all. This is particularly true of double-cone reeds such as Oboes and Schalmeys.

The thinner the tongue, the richer the harmonics in the tone it produces; the thicker the tongue, the smoother and more fundamental the tone. Wider resonators produce stronger tone; conical resonators have a ‘thicker’ partial-content than cylindrical ones. The resonator gives its air column its own natural frequency; when this is greater than that of the tongue (i.e. when the pipe is shorter than the tongue requires for both to respond naturally to the same pitch) the tone becomes brighter, richer in partials. The more open the shallot, the louder and richer the tone; to obtain brilliance from partly closed shallots, higher wind pressure is required; to obtain a rounder, more horn-like tone, 19th-century builders placed the opening higher on the face of the shallot, the curved tongue thus closing the opening before its travel was complete. As in the case of flue pipes, it has been established recently that the hardness of the resonator material (this can be, in order of decreasing hardness, brass, tin, lead or wood) is unlikely to influence the tone – tradition and hearsay notwithstanding. However, the hardness of the tongue material is a definite factor in tone quality. The commonest material used by modern builders is what is known as ‘half-hard’ brass, but soft brass, hard brass and even (the very hard) phosphor bronze are also used in certain instances. The thickness of the tongue likewise has an effect on tone.

Reeds with very short resonators (whatever their shape), and usually of small scale, are called Regal stops and were known from at least about 1475. In practice, most Regals are either predominantly conical in shape or predominantly cylindrical; they also exhibit an inconstant scale (i.e. relative to the reed-tongues, the resonators in the treble are progressively longer than in the bass). Reed stops with resonators of twice or even four times natural length were sometimes made in the later 19th century, especially by French and English builders, and became equivalent to overblowing flue pipes, although such overlength resonators are generally used only above the pitch of 2' c'. 19th-century builders, particularly in those two countries, very often placed their reeds on higher wind pressure than the flue stops (18 cm upwards) by means of divided windboxes and double pallets, in the chest. The desire to supply ‘carrying power’ by such means, particularly in the treble, had grown in France from about the second third of the 18th century onwards.


Fig.19 shows models of some of the more popular reeds of the early 17th century (PraetoriusSM, ii; fig.19a) and the late 19th (Audsley, B1905; fig.19b). A great deal depends on the use of various shapes and proportions of shallots, and these, like the tube, block and boot, may be made of wood (though this is more often a feature of low-pitched pedal reeds than a general alternative).

Organ, §III: Pipework

3. Free reeds.

Free reeds were developed in Europe (probably after the Asian sheng) towards the end of the 18th century in several areas around the Baltic (see Free reed), and offered the first radically different type of organ pipe since flues and reeds had been perfected. Instead of a shallot with an orifice against which the tongue beats when wind excites it, a thick, oblong plate of brass is perforated with a narrow opening through which vibrates the close-fitting brass tongue (fig.20). It swings freely, hence ‘free reed’. The boot needs to be larger than that of a corresponding reed stop to allow copious winding. When made by German builders about 1825 and French builders about 1850, free reeds had resonators of various types and tone-colour, thus being legitimate ranks of organ pipes. However, some stops, such as the Physharmonika, had instead of individual pipe-resonators one resonating chamber common to all notes of the rank, thus taking less room on the chest. It was such pipeless free reeds that led to the various kinds of harmonium, or Reed organ, of the 19th century. Free reeds could be mass-produced more easily than the so-called beating-reed stops, although in itself the workmanship was not inferior. The best builders by no means regarded them as easy alternatives to beating reeds, and the best examples, especially when used at 16' pitch in the pedal, could sound deceptively like beating reeds.

Though less incisive in articulation and weaker in volume than the beating reed, the free reed had a quality highly favoured by its period: it could be made ‘expressive’. On admitting more wind to a free reed, the amplitude, but not the frequency, of the swinging tongue is increased; it can thus produce a louder tone without rising in pitch, like a more or less excited tuning-fork but unlike a beating reed. When the free reed was a separate stop in a large organ, however, this property could not easily be exploited. Rarely outside the period 1810–1910, and then most often only in parts of northern France, central Germany and northern Italy, did the free reed achieve much popularity.


Organ, §III: Pipework

4. Diaphones (valvular reeds).


In 1894 Robert Hope-Jones took out a patent for a pipe, making use of the fact that any device allowing puffs of compressed air to be projected into a tube or resonating box (i.e. into a chamber holding a column of non-pressurized air) will create a sound if the frequency becomes audible (fig.21). On activation from the keyboard, air under pressure is admitted through the bore and sets the thin ‘vibrator’ into motion, whereupon the pallet-like disc attached to its free end admits a rapid and regular succession of puffs of air into the resonator (i.e. the pipe standing above). As with the free reed, the tone increases in volume but not in frequency as the wind pressure is increased; but, as is not the case with the free reed, greater wind pressure can make for much power. The tone itself is smooth and powerful, but always ‘unblending’ and useful only in organs (chiefly cinema organs) conceived on ideals current in a few areas of Europe and the USA between 1900 and 1930. The most enduring application of the diaphone principle has been fog-signalling, and many lighthouse diaphones were in regular use in the USA and elsewhere until late in the 20th century.

See also Organ stop.

Organ




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