[pianotech] Tuning stability problem

[Ron Nossaman]

On 9/22/2010 5:54 PM, Ron Overs wrote:
> Hello all,
>
> Low tension in the back scale is very common, and Ron N's description of
> it rings true with me. When a piano hasn't been tuned for while, or when
> it has been tuned recently but there are a number of unisons which are
> more wild than they should be, it is worth giving a couple of unisons a
> rub on the speaking length with a piece of hammer shank to see what
> happens. If there is more than a little pitch change, I rub down the
> whole piano. This really will help tuning stability. A quick rub down
> takes less than 10 minutes. Its well worth it.

I expect that's right, though I'd question that anything below mid scale 
is necessary or effective.


> It is also worth doing the rub down check with new pianos as well. It
> seems that today more and more pianos are being built, without any
> attempt being made at the factory to rub down the speaking lengths to
> get the back scale tension up. This is very likely placing huge lateral
> forces on the bridges. You may be surprised as the speaking length pitch
> plummets, when you rub down many new pianos, but it will significantly
> improve stability. If there is a huge drop in pitch when you do the rub
> down, there may be benefit in doing a first rub down, then a pitch raise
> followed by a second rub down.

Also likely the case. The thing that's universally missing in these 
observations is that the back scale isn't necessarily always low in 
tension. In a seasoned piano, it will, contingent on climate control and 
the timing of the tuning in the cycle, be as likely to be high as low. 
We don't become aware of this because neither whacking, nor massaging, 
changes speaking length pitch when the back scale is of higher tension, 
so our test only tells us half of the information needed. Remember all 
those puzzled observations of strings climbing sharp after tuning? This 
phenomenon is bipolar and essentially symmetric. It's not absolutely 
symmetric because cumulative wood compaction trends toward ever lower 
pitch, but there are still cyclic highs as well as lows in the back 
scale, and the highs are currently undiagnosed by a simple test. This 
back scale tension differing from front scale is, incidentally, *THE* 
argument against the validity of tuned rear duplexes. Never mind that 
there's no direct relationship between speaking lengths and rear 
duplexes in the first place, the tension differences across the bridge 
on any given day ((since they're always moving with humidity shifts) 
mean that there won't be a rational tuning relationship between speaking 
length and rear duplex except by accidental circumstance, momentarily at 
best.


> Furthermore, I suspect that there is a small improvement in tone
> following a rub down, but its often hard to distinguish between what's
> real and what's imagined with some aspects of our work. It might be that
> large lateral forces on the bridge have a slight limiting effect on the
> ability of the down-bearing vector force to act on the soundboard panel.

My take here is that the rub down seated strings on the bridge, jamming 
any flagpoling pins into temporary immobility. I think you likely didn't 
imagine it, but that the perceived result was independent of segment 
tensions.

So how's things, mate?
Ron N