"killer octave:

[Delwin D Fandrich]

----- Original Message -----
From: Ron Nossaman <nossaman@SOUTHWIND.NET>
To: <pianotech@ptg.org>
Sent: Saturday, December 25, 1999 9:40 PM
Subject: Re: "killer octave:


> >, tho I'd like a bit
> >more detail
> >as to what soundboard design problems cause this, and what can be done
about
> >it.
> >
> >Richard Brekne
>
>
> Assembly impedance is too low in this area. The fix is to raise the
> impedance, and the resonant frequency. I won't get into the details, since
> I'm not sure how close what I'm doing is to what Del's doing (I haven't
yet
> seen or heard one of his), and since Del still has patent paperwork oozing
> through the system at glacial speed, I don't intend to shoot him in the
> foot (or anywhere else). Suffice it to say, for now, that there is a
> logical, understandable, and quite rational cause and effect relationship
> between the strings and soundboard assembly going on here, that can be
made
> much more efficient in the design stage.
>
> Ron N
> -----------------------------------------------------

I have not observed that the "killer-octave" phenomena is particularly
related to tuning...although there may well be a relationship there.  I have
not really looked into that aspect of the problem.  As Ron has pointed out
it is essentially an acoustical problem that, at the risk of offending those
who don't believe in them, is specifically related to the physical and
structural characteristics of the compression-crowned soundboard design.

The problem can be -- and is -- substantially reduced simply by switching
from a compression-crowned soundboard design to an appropriately designed
crowned-rib design.  For the most part, this can be done even with the new
ribs installed in their original locations.  However, it can usually be
improved even further by adding a few ribs to the system.  Most pianos in
the 160 to 200 cm range typically have somewhere around 9 to 11 ribs, from
200 to 220 cm they may have up to 13 or 14 ribs and the traditional concert
grand will usually have 17 ribs.  There is nothing magic in these numbers,
indeed, these ribbing convention are part of the problem.  Given my choice I
would use several more ribs in most soundboard scales (properly calibrated
to reflect the larger number of ribs), crown them appropriately and raise
the moisture content of the soundboard panel at the time the ribs are glued
to the panel to a more reasonable level.  Even with no other changes made to
the overall design of the piano, this will reduce the killer-octave
effect -- at least its acoustical component -- to an acceptable level in
most pianos.

Del