The PIPE ORGAN

Good singers employ vibrato in their singing and players of most musical instruments do likewise. Vibrato is a periodic slight variation in the instantaneous pitch of a musical sound such that the pitch changes at a regular rate slightly above and slightly below the steady state pitch, in such a way that the average pitch over time is the same as the steady state pitch. Vibrato usually sounds best if it occurs at a rate of 360 to 400 variations per minute or between 6 to just under 7 per second. Along with vibrato there is usually a little tremolo as well. Tremolo is an instantaneous variation of the volume level of a musical tone, and once again, it is such that the average volume remains the same as steady-state volume when there is no tremolo present.

I'm not exactly sure how singers impart vibrato to their voices, but in a pipe organ, the most successful way to do this is by very slightly and regularly varying the pressure of the air supplied to the pipes. As is true with a good singer's vibrato, the pipe organ effect likewise sounds best if the rate is in the range of 360 to 400 variations per minute. If you slow the rate too much, then it becomes more like a disturbing wavering or uncertainty, and even worse, if you speed it up too much, say 430 or more per minute, then the result becomes very unpleasant. I think we've all heard poor singers who do this. I can think of several well-known singers whose vibrato is too fast and which seriously spoils their voice quality and to whom I'd like to say, "I wish you'd shut up!"

The pitch and the volume of a sounding organ pipe are both dependent on the pressure supplied. When you lower the pressure, both pitch and volume decrease, and they also increase with an increase in pressure. Of the two effects, vibrato is much more interesting to listen to than tremolo, and if you haven't already done so, read our article about vibrato to find out exactly what vibrato and tremolo are and to see diagrams and animations that show what they look like and do to musical tones.

Different types of organ pipes change their pitch and volume by differing amounts with changes in operating air pressure, so some pipes will have more vibrato than tremolo, and others will have more tremolo than vibrato, but in all pipe tones, variation of the air pressure by means of a tremulant device will produce both effects.

As you may infer, the device in a pipe organ which produces vibrato and tremolo in the tones of the pipes is called a tremulant. There are several different types of tremulants that have been used, but the most effective ones have some form of a big valve that opens and shuts at the desired vibrato rate (360-400 times per minute) and when open dumps large puffs of air out of the regulated side of a pipe organ pressure regulator. What happens is this.

The tremulant exhausts a large amount of air from the regulated side of a pressure regulator. The top immediately descends and opens the internal valve to make up the difference. Because the amount of air that the tremulant dumps is large, this amounts to a slight instantaneous drop in the pressure as the regulator immediately begins to correct for this sudden big demand. As soon as the tremulant has dumped this large puff of air, its valve closes, and now the regulator top immediately rebounds, closing off the supply valve as it senses the loss of demand and a slight increase in internal pressure.

As the tremulant continues to operate, the regulator top will bounce up and down slightly at the same rate, that is 360 to 400 times per minute as it constantly corrects for the periodic dumpings of air from the regulated side of the system. In order for the tremulant to be effective, it is necessary for there to be a slight time lag in the operation of the pressure regulator so that it will allow the regulated pressure to vary slightly above and below design pressure. The best pressure regulators, however, will respond so well that they can filter out and practically neutralize the effect of the tremulant entirely. Therefore, it is necessary to slow their response slightly so that they will not correct the pressure too well which would then essentially eliminate the vibrato effect entirely.

Therefore, the pipe organ builder will often place some weights on the tops of those pressure regulators where there is an associated tremulant. As mentioned previously, the weights will slow the corrective action of the regulator by adding inertia and momentum to the motion of the regulator top so when the tremulant is in use, the regulator will slightly lag behind the instantaneous demand and allow a small amount of pressure variation to develop.

For the pipe organ builder, this can require a degree of experimentation to get just the right ratio of spring tension vs weight on a given pressure regulator so that he gets the correct output pressure, the regulator responds fast enough to correct for normal playing demand changes, and yet the regulator does not respond so quickly that it neutralizes the effect of the tremulant.