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Some Thoughts On The Theory And Practice Of Nicking organ pipes

Robert Lamb, M.I.S.O.B.

Nicking is a device that enables the voicer to control the way in which the energy is applied to the pipe.

When a cricketer catches a fast ball off the bat, he moves his hands with the ball before stopping its flight in order to lessen the force of the blow. This amount of movement is an analogy of the effect of nicking.

When we have a small child on a swing, we graduate the force of our push, otherwise we distort unduly the natural movement of the swing by jerking it and causing alarm. This is also an analogy of the effect of nicking.

I have heard the lateral streams of air, caused by the nicks, described as being little springs which form a buffer between the energy and the resonance. The pianoforte toner, softening his hammer-felts, is doing much the same thing, as he is graduating the force of the blow on the strings.

There is nothing simple in nature; everything is exceedingly complex. When an organ pipe speaks, the quality of the tone produced is the result of innumerable factors and tendencies consisting of a range of frequencies from the fundamental to the molecular. There is probably a point where the intervals between the harmonics are so relatively minute and the amplitude so small that there is virtually a common frequency. This point might vary with the initial amplitude.

There is a fluctuation of pressure at the node. An alteration in the pressure of a gas is assumed to affect the speed of the molecular movement. The maintenance of the vibration depends on elasticity and this is a matter of pressure and molecular movement.

The higher frequencies, which are past the audible range, and the lower frequencies, which are too small in amplitude to be heard, all play their part, collectively and differentially, in shaping the wave form depicted on the screen of the oscilloscope.

If it were possible for all the harmonics to be correctly proportioned in amplitude, they would all augment the simple wave form of the fundamental without distorting it, and by so merging lose their identity. White light contains all the frequencies of the spectrum, but the colours are not evident until refraction takes place.

When the wave form on the screen approaches a sine wave, the tone is pure fluty and uninteresting. When the wave form is complex and irregular, the tone producing it is rich in audible harmonics. If the wave form alters, the tone producing it has altered, unless it is a matter of interference from the reflection off the walls of the room in which the tests are being made.

To simplify the following argument, I am neglecting the all-important effect that the material of the pipe has on tonal quality; in the same way that students of Mechanics assume complete rigidity in order to simplify their calculations concerning machines, etc. Afterwards they introduce factors of safety which take care of any discrepancy.

The difference between a Strad and an ordinary violin cannot be calculated and put down on paper, but there is absolutely no doubt that a difference exists. In tonal matters we are dealing with the resultants of very minute forces and the effect that these have on that discriminating and sensitive organ, the human ear.

A contemplation of what we imagine happens when a pipe is nicked, must obviously be hypothetical since in that small area of expanding and vibrant air at the mouth, the movement of the particles may be compounded of all the systems of motion known to science.

Since sound travels in all directions, a spherical resonator favours the production of pure tone, while a small scale cylindrical pipe favours the production of the upper audible harmonics. In a cylindrical pipe, the character of the longitudinal motion is affected by the lateral motion. The wave form in this case is the resultant of two wave motions interfering with one another at right angles. When the lateral timing is considerably quicker than the longitudinal timing, there is a considerable distortion of the simple fundamental wave. In larger scaled pipes, there is less distortion, and they favour the production of purer tone.

When we voice a pipe, we first determine the amount of cut-up required for producing the desired result in regard to power and quality. We generally know this from past experience, or we experiment until we are satisfied. The amount of cut-up determines the pitch tendency of the eddy note, which, when the pipe is speaking steadily, plays its part in augmenting the production of the upper harmonics heard in the tone. For any particular cut-up, the pitch tendency at the mouth is controlled by the air speed of the applied pressure, which we modify by regulating the size of the hole in the foot.

When the initial energy is applied too suddenly, there is not time for the inertia of the air in the pipe to be overcome and full resonance to take place. This results in temporary high pitched compressions and rarefactions causing ripples which distort the character of the natural unforced resonance of the pipe. The eddy note, or some harmonic of it temporarily sounds and we get the well-known chiffy attack. When we nick the flue of the pipe, we graduate the application of the energy in much the same way as we graduate our push in the analogy previously quoted. We thus minimise the initial distortion of the resonance and also we allow the two systems of vibratory motion (viz, the eddying at the mouth and the resonance) to couple up smoothly.

When we voice a stop without using the device of nicking, we have to keep the flues narrow and the power down. The pipes have to be made of tin or light metal and carefully regulated for thickness. This tends to lessen the effect of the inertia of the metal, both at the start of the note, and also, when the pipe is sounding, it modifies the hardness of the quality and the inevitable edginess becomes more musical. There are many charming flutes and quintatons, etc., voiced without nicks. In some of these stops, a certain amount of chiff adds to their interest, especially if they are of ancient origin. The "Princess" organ (1603) in Carisbrooke Castle, in the Isle of Wight, is an example of this.

In some un-nicked pipes, with normal flues, there is a tendency for the air to sidle into the pipe, first one corner and then the other, thus producing a parasitic periodicity which produces windiness which makes the tone hopeless. By nicking the pipe, the energy can enter and leave the pipe gracefully, and the tone immediately becomes satisfactory.

Nicking large pedal wooden pipes is, in most cases, unnecessary as the vibrations are slower and the air, in greater mass, is self-cushioning. We naturally do not, as a rule, nick violones, since we want the effect of the grippy attack. We usually, adjust the width of the flue to modify the chiff.

For normal practice, nicking is essential. I do not think that there are many practical men who will dispute this statement. Every voicer is an individualist, and rightly so; each has his own particular methods and ideas as to the best treatment of different stops, and it must be left to his personal judgment as to the type of nicking that he employs.

The chemist can give useful tips to the artist as to the permanence of certain mixtures of pigments, etc., yet with all this knowledge, it still requires the skill of the artist to paint the picture. This principle applies to voicing and to all professions that call for craftsmanship.

For more information on organ pipe nicking and other related subjects vist The Tonal Structure of Organ String Stops

Extract from :- Journal of the Incorporated Society of Organ Builders. 1952