But of course, your organ can't wait that long if it sounds out of tune - the reed stops are known to be very sensible in terms of tuning.
In a normal situation, the tuning of the reeds is the responsibility of the resident organist (if he/she has the necessary qualifications and experience, of course).
Imagine playing a church service with the reeds out of tune. How would your congregation react? Or how about having to play a recital but knowing that the reeds need to be tuned? Your registration choices would surely be much more limited if the reeds are out of tune.
So the question is this: how often should you tune your reed stops?
From the experience with the organ at Vilnius University St. John's church I can say that reeds with the shorter resonators (Vox Humana, Krummhorn etc.) tend to go out of tune more easily than the ones with the longer resonators (Trompette, Posaune, Bombarde etc.)
I recommend tuning the out of tune reeds before the any important public use of them. Not before every public use of the organ, though. Just tune the reed if you really know that it is going to be used on that occasion.
You may want to plan your registrations ahead of time for church services. Or consult with the guest organist which reeds will he/she need for the recital?
It also depends on the size of your organ and the number of reeds it has. If it is a small organ with only 3 or less reed stops, you can easily tune them all before the performance (if you know how, of course).
But if it is a large organ with, say 12 reed stops (like the one at St. John's in Vilnius), then chances are not all of them are going to be used in every occasion.
In situations when all of the reeds needs tuning, then you might want to split your tuning sessions in two several parts and tune only a few of them in one sitting because of the impact of the sound on your hearing.
And don't forget to wear protective heavy duty headphones when you tune any organ stop.
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