Aprons and holey bridges

[John Delacour]

At 10:37 AM -0800 1/3/02, Delwin D Fandrich wrote:

>The bridge cantilever was introduced to allow bass strings of the longest
>possible speaking length on pianos of a given length while allowing for the
>attachment of the bridge to the soundboard further in where everything is
>somewhat more flexible than right next to the inner rim.

Yes, we know that.

>I dislike bridge cantilevers for a number of reasons:
>
>1)  The idea of placing strings having the longest possible speaking length
>on a piano of given size is itself flawed. It ignores the necessity of a
>reasonable backscale length.

Well I need to be convinced of this necessity too.  I've heard you 
and Ron several times emphasize the importance of the backscale 
length but I've not seen any evidence for your point of view.

This afternoon a colleague dropped off a very old 6'straight strung 
Kirkman that he was going to take to the tip because the veneer is 
lifting, but he knows I like Kirkmans and offered it me first.  The 
scale of this piano is exceptional, as I have found in other 
Kirkmans, and the bass is powerful and sustained right to the bottom 
although A1 is only 125 cm.  At A1 the back length is barely enough 
to accommodate the eye -- about 60 mm.  The bass bridge has an apron 
for all its length and this is tenoned into the long bridge.  As is 
usual on these pianos, the soundboard is free for about 30 cm. to the 
left of the bass bridge.  The inner rim round the bent side is 
laminated oak about 40mm wide and a good 15 cm deep, so very massive, 
but round the toe the support is much close to the outer rim, maybe 
only 20mm thick;  I haven't had a chance to examine it yet since 
night fell before I could take the frame out.  I should know tomorrow.

Your argument for the back length seems to be that a greater back 
length allows greater mobility of the system, but the system I have 
described above is quite mobile enough and can be made as mobile as 
you want by adjustment of the soundboard fixings, just as you are 
doing with your design 130 years later, and as I do myself.  Clearly 
if you have a broad solid bridge, a short back length AND a rigid 
fixing in that area, the soundboard's modal vibrations are going to 
be interfered with in such a way as to affect the tone badly, and 
that can happen whether there's an apron or not, though not to the 
same degree.

>2)  Bass bridge cantilevers reduce the efficiency of energy transfer between
>the strings and the soundboard assembly.

That would mean that notes on a suspended bridge sustain better but 
are not so powerful.  No such generalization can be made.  In certain 
combinations, the apron could "absorb energy" but it does not have to 
and there are many well-designed pianos where it does not and the 
English straight strung grands of the 1860s-70s -- eg. Collard and 
Kirkman, have a quality in the bass that is second to none and 
superior to many later overstrung grands with or without apons.

>
>3)  They absorbóand dissipate as heatólow frequency energy from the strings.

By what process?

>4)  They cause physical distortion in the soundboard. This is especially so
>with compression-crowned soundboard assemblies. (I published an article with
>a drawing describing and illustrating this some time back in the Journal but
>you can probably visualize what happens just by looking at the system.)

I have seen this effect only in Steinways, particularly the Model O 
and put it down purely to bad design.  Perhaps it is a common failing 
in American pianos that imitate the Steinway or make the same errors, 
but in my long experience the phenomenon is very rare.  Joint failure 
is marginally more common and some makers have trusted the quality of 
their glue too much.

>5)  There are better ways of accomplishing the same thing if it is required.
>One method is to simply undercut the tail end of the bass bridge.

Ditto the end of the long bridge sometimes, but still you are left 
with a piano 3" longer than you would have wished or strings 
significantly shorter than if you'd acquiesced in an apron.  To me 
there is nothing unseemly in wishing to accommodate the longest 
possible string length in a given case length and plenty of good 
makers have been very successful in achieving this by suspending the 
bass bridge.  This Kirkman that has just landed on me out of the blue 
is one example.  If they'd made it 6'7" instead of 6' in order to get 
a longer back length and a direct bridge with the same string 
lengths, they probably would not have sold it and it probably would 
not sound any better.

Incidentally, the Kirkman has even tension (about 155 lbf.) 
throughout the plain wire scale with no jumps at the frame bars 
(canted bridge) and the long bridge is beech over spruce throughout 
its length, so there's another for Joe to put in his pipe :-)

JD



JD