Tone near plate struts

[Ron Overs]

---------------------- multipart/alternative attachment
Phil, Ron and all,

>  > Ron Nossaman wrote:
>>>
>>>It is, after all, a bridge. It's primary job is load distribution.
>>>Stiffness affects assembly impedance, as is most obvious where a bridge is
>>>notched to go under a strut, making that section of bridge below minimum
>>>stiffness, and the tone quality of the unisons immediately on either side
>>>of the notch are reminiscent of mini killer octaves. This lack of stiffness
>>>is also evident at the low tenor, where the bridge (structural support)
>  >>ends, making the assembly more flexible than it is just a few 
>notes up scale.

Indeed, and while the practice of using a ring bridge is an attempt 
to alleviate this problem, it has the disadvantage of reducing the 
fundamental tone from the lowest bass notes. In the past few years we 
have been extending the low end of the long bridge to beyond the 
lowest tenor note, to provide support on the 'lower' side of the 
note. In our piano no. 003, it extends such that it terminates over 
an auxiliary sound board rib (one which does not extend to as far as 
the bass bridge or the inner rim on the straight side). This has the 
effect of forcing the sound board panel area immediately adjacent to 
the lowest tenor note, to move more as a unit. For many pianos, there 
is quite an area of sound board in the back-bass corner where there 
is little or no rib support for the panel. Very often the tone is 
found to be wanting here.

>. . . But on pianos that do not have a cut down bridge there is 
>still a deterioration, or at least a change, of tone adjacent to the 
>plate struts or braces.  It seems to be worse on some pianos than on 
>others.  What do you think is the reason for this?  I'd be 
>interested in hearing some conjectures (or authoritative 
>explanations if someone out there has some).
>
>Phil F

Phil, this is an interesting phenomena. We design the contact of the 
bridge with the sound board such that it forms a uniform curve as it 
approaches and leaves the immediate area under a plate strut (I do 
not allow a dog-leg of bridge contact with the panel - dog leg 
contacts are certainly a no no on my computer). However, since I am 
not prepared to compromise the speaking lengths of the scale for any 
reason, the top of the bridge must therefore lay back, and then 
forward of the board contact as it goes down under a plate strut, 
particularly with regard to the second break down. Theoretically at 
least, we build the piano such that the percentage of breaking strain 
is uniform. Therefore, we would expect the tuning to be relatively 
stable also at the plate struts. However, while the stability is 
superior to those pianos of the straight bridge under the strut 
variety, some instability with temperature and humidity variations 
continue to occur - as it does also at the string gauge changes. 
Tonally, there seems to be more risk of variation if the note to not 
spacing across the strut is wider. Since the wider the space between 
the adjacent note centres (at the struts), the greater will be 
requirement for the bridges to lean over as they approach the struts. 
I believe it is important to minimise the bridge lean as much a 
possible.

Now while I hate to raise the topic of bridge rocking with respect to 
this matter (since it has nearly been done to death over recent 
months), I believe it must be considered with respect to bridges 
leaning (adjacent to plate struts). Let's assume that we have built 
the long bridge as described above, and that it leans back on the 
treble side of the strut and vice versa on the bass side.

Check the second top break on the overhead image of no. 003 at; 
http://overspianos.com.au/OS003.html
This bridge cap leans back significantly on the treble side of the 
strut. I have got better images showing the detail of leaning 
bridges, but they're not on my site at present.

As the bridge rocks during the cycle (and if you don't believe that 
the bridge rocks then that's OK, but I happen to believe that it 
does), the rear leaning section will tend to increase sound board 
panel depression as the speaking length tension increases, while the 
forward leaning section will tend to reduce panel compression. This 
may indeed cause a difference in the tonality on either side of the 
strut. It should cause the board impedance to be lower on the treble 
side and higher on the bass side. Now I'm not saying that this is the 
magic factor which causes tone variability immediately adjacent to 
the struts. But I suspect that it might be an important consideration.

Any other thoughts on this matter? Its an interesting question Phil.

Ron O.
-- 
                  OVERS PIANOS
        Grand Piano Manufacturers
_____________________________

Web:    http://overspianos.com.au
Email:  mailto:ron@overspianos.com.au
_____________________________
---------------------- multipart/alternative attachment
An HTML attachment was scrubbed...
URL: https://www.moypiano.com/ptg/pianotech.php/attachments/02/37/51/b3/attachment.htm

---------------------- multipart/alternative attachment--