This post discusses the partial change locations in an FAC tuning.

An FAC tuning contains 3 partial changes.

From top down, here are their locations:
B5/C6   The Treble Partial Change  
B4/C5    The Midrange Partial Change
B2/C3    The Tenor Partial Change: 

However, there can be significant advantages to moving a those partial changes to slightly different locations. Since we’re working from the top down, we’ll start with the treble partial change location, and save the best until last!

Lowering the Treble Partial Change from B5/C6 to G#5/A5:

The treble partial change is lowered so A5 uses the fundamental instead of the 2nd Partial.  Using the fundamental @ A5 makes tweaking the treble stretch easier and allows us to see the exact width of the A3/A5 double octave.   It also makes it easier to use the D4/A5 12th for checking or adjusting the FAC’s stretch in the treble.

Using an already tuned D4 as a reference note, both A5 and A6 can be checked for adjusting the stretch in the treble.   it is easier to tweak the treble using D4 as the reference note and see and hear the widths of the D4/A5 12th, and the D4/A6 19th when determining a new stretch for the newly tweaked FAC tuning.

A simple Double Octave Beat (DOB) adjustment can be used for adjusting the treble stretch.

If the treble partial change is NOT lowered to this new location, A5 is using it’s 2nd partial for the tuning instead of it’s fundamental.   A conversion is necessary to  tweak the treble accurately and easily for a good sounding location for both A5 and A6.   Without lowering the partial change so that A5 is using it’s fundamental rather than it’s 2nd Partial, the conversion that’s necessary is complicated, less accurate, and takes morfe time than simply lowering the treble partial change in the first place.  Lowering the treble partial change so that the FAC tuning uses the fundamental for A5 makes tweaking the treble much faster, easier, and more accurate.

Lowering the Midrange Partial Change from B4/C5 to A4/A#4

The midrange partial change is easily the least important of the three.   After lowering this partial change and the treble partial change, the 2nd Partials will be used form A#4 – G#5.  Again, this partial change is the least important of the three, but if you’re doing the other two, why not do this one?   All three will need to be corrected anyway.  Lowering each partial change will only take a few seconds.

Lowering the tenor partial change from B2/C3 to G#2/A2

The tenor partial change is the most important one with the biggest bang for the buck!  If you only do one, do this one!
Lowering the Tenor Partial change from B2/C3 to G#2/A2 on many grand pianos, puts the tenor partial change on the bass bridge, instead of near the low end of the long bridge.   Of course on the long scales, the G#2/A2 partial change will be on the long bridge, but it is higher up on that bridge and the potential error/correction is generally less of a problem.

Most verticals and virtually all grands under 6′ the FAC partial change of B2/C3 ends up at the low end of the long bridge.   This area of the piano is fraught with scaling issues.   The transition from plain wire to wound bichords, as well as the transition from the long bridge to the bass bridge can be problematic.   A partial change in there, can make things worse.   We can’t fix the scaling problems, but we can get the partial change out of the way.  Just getting the partial change out of this area, will do good things for the tuning.

Another advantage involves aural checks in the temperament area.  All the notes in what most consider the temperament octaves (A2 – a4) are using the 4th partials.  Aural checks are more reliable without having a partial change between B2 and C3, which would have A2, A#2, and B2 using the 6h Partials instead of the 4th.

Another benefit of lowering the tenor partial change is with A2 using it’s 4th P instead of the 6th, it’s easy to see the width of the A2/A4 octave.   Al Sanderson said that when the the A2/A4 double octave is more than about 4 c. wide, it’s probably starting to get too wide.   with the tenor partial change lowered so that A2 is now using the 4th P. it’s easy to ‘see’ what the width of that double octave, temperament area double octave width is.   If it is lower than -4, it might be time to use some DOB to contract the octave a little bit.  Let your ear then be your guide, but knowing what the A2/A4 double octave width is, can be a big help.

So, why not just just get into the habit of lowering them all?   And once they are relocated, the next step is to correct any potential partial change errors.  The AccuTuner has a feature to help do this:  the Partial Change Correction (PCC).   PCC takes care of the math and makes the correction by pressing only 2 buttons.   It’s easy and fast.

(Of course PCC can be used to correct the default FAC partial changes too!!)

Once the partial changes have been lowered AND corrected, the tunings will sound better.  The potential errors at the partial changes have been eliminated.   Also, being able to easily adjust the treble stretch will make the tunings sound better.     All this is unique to the Sanderson AccuTuner.  None of the other tuning software programs can do this.

The AccuTuner 3 partial changes whereas most of the other software ETDs, will have 5 partial changes in their tunings!!   A system incorporating 5 partial changes has more of a need to correct their partial changes, than an FAC tuning does.   But those other systems have no way to do it.

Lowering and correcting the partial changes, and accurately tweaking the treble stretch, and the width of the temperament double octave when needed can set your tunings apart from those NOT doing these tweaks!