What Does a Tremulant Do?

A tremulant intentionally introduces a fluctuation into the wind supply, and that fluctuation creates a wavering quality in the sound of the pipes. James H. Cook, in a tutorial published by the Organ Historical Society, describes the device in exactly those terms: a deliberate disturbance put into the wind, not a defect that lets one in. The distinction matters because the rest of the wind system exists to deliver wind that is steady, and the tremulant is the one component whose purpose is to make it periodically unsteady, on demand and to a controlled degree. When the organist draws the tremulant, the wind becomes deliberately unsteady, so a wavering heard with the tremulant on is the effect working as intended. Unwanted unsteadiness with the tremulant off is a different subject entirely, and the two should never be described in each other’s terms.

The Cinema Organ Society puts the same idea in a single phrase: a tremulant provides a periodic disturbance to the wind supply to the pipework. Periodic is the operative word, because a regular, repeating disturbance is what turns a steady tone into a wavering one.

With the tremulant drawn, the wind pressure rises above and falls below the normal regulated pressure in a steady beat. The Garden State Theatre Organ Society describes the action in just those words. The regulated pressure is the working pressure the regulator holds for the pipes while they are speaking; the tremulant swings the actual wind pressure around that level, alternately a little higher and a little lower, and the wind pressure returns to its regulated level when the tremulant is shut off. Each swing reaches the pipes as a change in the wind that feeds them, and the pipes answer with a change in pitch and loudness. Because the swing repeats at a fixed rate, the ear hears it as a beat laid over the tone rather than as a drift.

In its usual forms the tremulant works on the wind pressure itself, which means it can only modulate what the surrounding system gives it to work with; the same tremulant on a different regulator and a different windchest produces a different result, a point developed later on this page.

Speed is the rate at which the tremulant’s cycles or beats occur, and depth is the size of the change between the high and low points of each cycle. The OrganTutor teaching resource published by Brigham Young University defines tremulant frequency as the speed with which the cycles or beats occur and gives about six cycles per second as a relatively fast example. It defines depth as the amount of pitch or volume change occurring between the points of highest and lowest wind pressure in the tremulant cycle. That definition repays a careful reading: depth is measured between the pressure extremes and is heard as both pitch change and volume change, so a tremulant does not produce one without the other.

On most organs both speed and depth can be adjusted by an organ technician. The American Institute of Organbuilders (AIO), in its guidelines for new organbuilding work, asks that the speed and depth of pulsation be adjustable to suit the taste of the builder or organist, that tremulants other than the fan type give an even pulsation within the wind supply under normal playing conditions, and that the tremulant be muffled for quiet operation. The values chosen on a given instrument are therefore matters of setup and taste rather than fixed properties of the device, and they are set by a technician as part of regulating the instrument; how that adjustment is carried out is not described here.

Waveform is the shape of each pressure cycle, and it is the third parameter alongside speed and depth. The Cinema Organ Society names all three as the adjustment parameters of a tremulant: speed, depth and the shape of the waveform. Two tremulants running at the same speed and depth can still sound different if one rises and falls smoothly while the other moves abruptly between its extremes, because the pipes follow the wind pressure through the whole cycle and not only at its ends.

How much depth matters shows plainly in the difference between theatre-organ and classical practice. The Cinema Organ Society states that theatre-organ tremulants are generally much deeper than classical ones and produce a much more significant change to the sound because of the pressure changes during the tremulant cycle. That is a description of two established traditions rather than a judgment between them, and nothing on this page should be read as a recommendation of any particular depth or setting for an instrument.

A tremulant may be self-powered, using a bellows, or electrically operated with magnets. The Garden State Theatre Organ Society describes both constructions: in the first the motion comes from a bellows, in the second from magnets. Beyond that division by what drives the unit, tremulants are also classified by the way they act on the wind. The AIO’s guidelines for new organbuilding work recognize four families, providing that tremulants may be of the exhaust, pressure, inertial or fan type as appropriate to the type of wind-regulating system.

The qualifying phrase carries weight: the choice of tremulant type follows from how the instrument’s wind is regulated, so a tremulant is not a freely interchangeable accessory that can be moved from one winding scheme to another. Because the constructions differ, what a given tremulant’s behavior means depends on which type is fitted, and that is a matter for a technician examining the instrument rather than for this page.

An exhaust tremulant lets wind out at regular intervals, which fluctuates the pressure at the pipes and sets the regulators bouncing as they attempt to steady the wind. That description comes from Organ Piper Pizza’s account of its own instrument, but the principle is general: an exhaust tremulant produces its effect by defeating the regulator’s own correction loop. The regulator’s purpose is to hold the wind pressure steady; the tremulant’s purpose is to keep upsetting it; and the audible beat is the product of the two working against each other.

James H. Cook, in the same Organ Historical Society tutorial cited above, records that the usual method of making a tremulant is to move the regulator so as to disturb the wind pressure. The two devices pursue opposite ends in the same system, and a tremulant is normally built to act on the regulator rather than around it.

A different scheme controls the wind pressure directly instead of exhausting it. Johan Liljencrants, an engineer describing a design for his own instruments, feeds an intermittent control pressure to the regulator so that the regulated pressure oscillates between 100 percent and 89 percent of normal, driven by a two-valve pneumatic oscillator with bleed screws set for about 7 Hz. That is one engineer’s design rather than common practice, but it gives a rare explicit figure for tremulant depth expressed as a percentage of the regulated pressure.

Commercially supplied units fall into the same families. Organ Supply Industries lists bellows-style tremolos whose built-in electro-pneumatic stop action can be regulated to control air exhaust, a dual pneumatic pressure-rise tremolo described as installable on almost any wind system up to 6 inches wind pressure, and a built-in pressure-rise tremolo pneumatic available on reservoirs 20 by 30 inches and larger. Those are catalog statements from a 2006 listing and would need to be confirmed against a current catalog, but the pressure limit on the dual pneumatic unit illustrates a general point: tremulant devices are matched to a pressure regime and a reservoir size rather than being universal. Naming these products describes the market; it is not a recommendation of any of them.

The fan tremulant acts at the pipes rather than at the reservoir. In an Austin Universal Air Chest the volume of air in the chest is so large that varying the wind pressure fast enough to be heard is impractical, so the usual method of moving the regulator cannot be used; instead, large wooden fan blades driven by an electric motor are placed above the pipes. The rotating blades force an unsteady current of air against the upper ends of the pipes, so that the difference between the pressure inside the chest and the pressure outside it fluctuates and the wavering sound is produced.

Cook’s tutorial presents the arrangement as a consequence of the chest design rather than a matter of preference. The AIO’s even-pulsation requirement applies to tremulants other than the fan type; the guideline does not say why, but a fan reaches the pipes by a different route from the other three families, and the exclusion reflects that difference.

The overall tremulant effect is determined not only by adjustments to the tremulant itself but also by the layout and setup of the trunking, windchests and regulators, and the speaking properties of the pipes themselves affect the end result. That statement from the Cinema Organ Society describes the tremulant as a property of the whole wind system rather than of one device.

The pressure disturbance the unit creates must pass along the wind conductors and into the windchests before it reaches the pipes, and the regulators it passes act on it along the way, as the exhaust mechanism above showed. The layout of the wind conductors, the volume of air in each windchest and the way each regulator responds all shape how much of the disturbance survives and what shape it has when it arrives, and the pipes, with their own speaking characteristics, then decide how audibly they render a given pressure swing as pitch and loudness change. A tremulant moved to another instrument will therefore not sound the same, because the system it speaks through has changed. How reservoirs, regulators and schwimmers do their work is explained on their own page; the point here is only that they stand between the tremulant and the pipes.

The reach of a tremulant is also a design choice. Taylor and Boody state that on their Opus 83 the tremulant affects the entire organ and that wind stabilizers can be engaged to calm the lively wind. That is documented for this one instrument, and the source does not describe how the stabilizers are built or how much they change the wind pressure, but it shows that a tremulant can be made to affect a whole instrument and that the flexibility of the wind can be made a switchable option rather than a fixed characteristic. The rest of the wind system the tremulant disturbs is described on the wind system overview.

A report that the tremulant has changed, or no longer sounds as it once did, does not localize to the tremulant unit. Because the effect is distributed across the wind system, a change in the condition of a regulator or a change in the wind pressure can alter the tremulant effect without the tremulant having failed or having been touched. The unit may be doing exactly what it did before while the wind it disturbs, or the regulator it works against, no longer presents the conditions the tremulant was set up for. It follows that replacing a tremulant unit is not assured to restore an effect that has weakened or altered, since the speed, depth and waveform regulated into the instrument were regulated against the surrounding wind system as it then stood.

The distinguishing evidence is whether the wind system around the tremulant still presents the conditions the tremulant was regulated against, and gathering that evidence is a technician’s task on the instrument itself. This caution follows from the documented dependence of the effect on its surroundings rather than from a rule stated by any one authority, and it concerns an effect that has weakened or altered rather than one that has stopped entirely.

None of this identifies what has changed on any particular instrument. A changed effect can sit in the tremulant, in a regulator, in the wind pressure or in the pipes, and this page does not rank those possibilities or suggest what a reader should look at. It explains only that, where the effect is concerned, the tremulant and the wind system are one subject, so that a question about the tremulant is understood from the start as a question about the wind system as a whole.

Where a tremulant effect on a particular instrument has changed, establishing whether the change sits in the tremulant or in the wind system around it is work done on the organ itself.