Wherever there is a partial change, there is potential for an error at that partial change.
For a smoother tuning in the A2-A4 range for tuning a 2-Octave Temperament, all the notes within that range should use the same partial. This eliminates any possibility of a partial change error in that A2-A4 range.
Most tuning systems have at least one partial change below A4. Some systems have 2 or even 3! In order for aural checks to be accurate, errors at the partial changes need to be corrected.
An FAC tuning has one partial change in the A2-A4 range. The tenor partial change in an FAC tuning is between B2 and C3. An FAC tuning uses the 4th partials from C3 to B4.
In order to get a full Two Octave Temperament range using the same 4th partials, all that is needed is to lower it’s B2/C3 partial change down to G#2/A2.
Once we’ve lowered the tenor partial change location from B2/C3 down to G#2/A2, there will be a complete 2-Octave range of notes from A2-A4 all using their 4th partials.
This improves the reliability and accuracy of our aural checks because they will be free of any issues caused by different partials being used and/or a partial change error.
It is up to the technician to know where the partial changes are in whatever system being used, and how to fix them when they cause their noticeable ‘hiccups’ in the tuning.
When using an AccuTuner to do an FAC tuning, its easy to move the tenor partial change a few notes lower.
Another advantage to lowering the tenor partial change is, that on all but the longer scale pianos, lowering the partial change down to G#2/A2 places the tenor partial change onto the bass bridge.
This is an advantage on the ’26 bass’ grands whose lowest note on the long bridge is B2 and the highest note on the bass bridge is A#2.
Here is what we are doing in graph form:
First of all, here is an FAC tuning with the 3 partial changes circled in red.
The partial change we are going to lower is of course the one in the tenor (B2/C3).
(Click on graph to enlarge.)
Here’s how it will look when done:
(The vertical lines in these graphs are the ‘C’s.)
(Click on graph to enlarge.)
Here are a few blowups’ of what’s done when lowering the partial change:
(Click on graph to enlarge.)
This next graph is a 10 – 9 – 8 FAC tuning but with the .8 multiplier default. Instead of A2 @-4.5, the .8 multiplier puts A2 @ -3.7 c.
(Click on graph to enlarge.)
Here is a video on how to lower the tenor partial change: