Bald Ham Pitch Swing

[Ron Overs]

Hi Gina and all,

My original post contained:

>> The lower tenor and either side of the treble (strut) break are, in many
>> pianos, designed with string lengths which produce wide variations in the
>> percentage of breaking strain when at pitch. >>
>

Gina Carter's reply contained:

>
>This is the first time I've heard the term "breaking strain." I am assuming
>that it means something like near to its maximum tensile strength. Am I
>close?
>
>Gina

You are indeed.

Measure the string speaking lengths, core and cover diameters for all 
notes on a given piano. Record these figures into a suitable spreadsheet. 
Utilising a formula derived from Youngs law, you can calculate the 
tension of the unison strings of each note. Each  note when at pitch, 
will have a string tension which can be expressed as a percentage of the 
breaking strain (ie. the tension at which the wire will, in all 
probability, break) of the core wire for that note.

If you construct a graph within your spreadsheet, with the percentage of 
breaking strain for each note on the vertical axis, and the note number 
on the horizontal axis, you will find that the variation in percentage of 
breaking strain is indeed considerable across the compass of the 
instrument.

If you graph a typical 6 foot grand piano (say a C3 Yamaha) you will find 
that:

1) The extreme top treble is strung at around 65% to 70% of breaking 
strain (C88 typically of 5.3 cm [2.086"] speaking length, strung with 0.8 
mm [31.5 thou - Westphalia gauge] dia. wire)

2) As the break sections are approached, the percentage of breaking 
strain will tend to fall noticeably adjacent to the treble side of the 
plate bar, whereas it will be much higher on the bass side of that bar - 
hence the typical tuning instability adjacent to the frame bars.

3) At the last note on the long bridge before the crossover (often at, 
say, note B27 - a poor choice - D30 with a string length of 117 cm would 
be better), the piano designer will have 'run out of piano' and the 
bridge, which was running nicely down the centre of the soundboard, will 
now turn perhaps six notes before the break, abruptly towards the bass 
side of the rim. The tension in this area will fall disastrously, perhaps 
to as low as 25% of breaking strain at the break note. This typical 
scenario is also commonplace on seven foot grands, where many designers 
insist on running the last long bridge note down to as low as F21 (just 
because it works on a concert grand with an F21 of 183 cm [72"] speaking 
length, doesn't mean automatically that we should 'put our head in the 
sand' and build a seven foot grand with an F21 speaking length of 145 cm 
[57.1"] - it never works - except in piano brochures).

If you graph the percentage of breaking strain for several different 
pianos, you will notice that the percentage of breaking strain curves 
vary considerably from piano to piano.

I first discovered this phenomenon in the late seventies when doing a lot 
of recording studio tuning. Most of the studios in Sydney were equipped 
with Yamaha C7s (Bs primarily). This model has a tuning instability at 
note F21 which is as poor as anything in the business. Fortunately, the 
C7Fs are a marked improvement in this regard (these break at G23-G#24).

I noticed that these Sydney studio C7s tended to be unstable on 
particular weeks - all of them together. Now if the instability was due 
to pianists thrashing the pianos, then the problem would tend to be 
random. But they all tended to be 'out' on the same week, while at other 
times they would be fine. I realised that the instability occurred only 
when there was a change in the weather from humid and wet to fine and 
dry, or vice versa.

Not only was climate associated with this tuning instability, but it 
tended to occur only in specific areas of the scale, ie. those areas 
where the percentage of breaking strain deviated most.

If you build a data base of scales from pianos with good and poor tuning 
stability, you will find that the percentage of breaking strain deviation 
is a critical stability indicator. After some experience, you will be 
able to predict the inherent stability for a given piano, just by looking 
'in the lid'.

Sorry for the length of this post.

Regards to all,

Ron E. Overs

Email: ron@overspianos.com.au
Website: www.overspianos.com.au