[pianotech] Soundboard Analysis

[David Love]

Sounds like a nice gauge but I’m trying to envision using it effectively
with the board still in the piano which is what this was and the analysis
was to outline a method of assessing boards before they come out not after
they’re out.  With a new board there are certainly more accurate ways to
measure crown along a rib but with the board in the piano this doesn’t seem
like it’s any easier, maybe harder given the access problems in some areas.

 

What I was getting at with the reason for the lack of change in crown does
have something to do with the difficulty in measuring with the string
however.  This board clearly started out with negative crown through that
area.  Hard to measure that with a string.  Even with some rise in the board
as indicated by the bearing change and the change at the struts it was not
enough to push the board into positive crown.  The board has oilcanned and
is toast.  

 

My question about restoring crown was rhetorical.  I agree that it is
useless as a method for restoring crown, though it is often cited as the
reason to do it.  Even though this board had been heavily shimmed there was
certainly no evidence of crown and, in fact, in many areas the crown was
negative.  

 

David Love

www.davidlovepianos.com

 

From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of Encore Pianos
Sent: Wednesday, June 22, 2011 5:17 PM
To: pianotech at ptg.org
Subject: Re: [pianotech] Soundboard Analysis

 

Hi David:

 

I’m going to throw in my 1 ½ cents here for what it’s worth to you and
others.  I do some things a little different than you do.

 

Let me start by saying that I think there are better ways to measure crown
than the method you describe.  I used to measure by way of the string method
more or less as you do.  It’s slow, awkward, and probably not terribly
precise.  

 

About a year ago, I bought a Crownulator from Jude Reveley.  It is made up
of a 1 ‘ long steel bar accurately flattened on one side.  In its center is
a hole through which passes the moveable foot of a dial gauge.  You lay the
bar square on the panel next to the rib.  As long as there is positive
bearing, the dial gauge foot will move a distance that will be measured on
the dial.  It’s a mirror image reading, so if the gauge is reading .080 (of
a full circle of .100), this means that you have .020 of crown over that 1’
span.  There is a formula by which you can drop the numbers in and calculate
the crown for the rib.

 

I use it when putting in new boards too.  After the board is glued in and
otherwise ready to go for stringing, I take a crown reading on as many ribs
as I can get at and record those measurements.  When setting bearing, drive
the board down and take crown readings til I get my target of 50%
deflection.  Record those measures.  Then string and tune the piano, measure
crown again and see how well I guessed in the process

 

I think you would find the Crownulator significantly more accurate, quicker,
and far easier to use.  

 

I am going to speculate that the reason the measured crown does not change
when there is a positive change in bearing is that the method of measuring
crown is insufficiently precise, it is too easy to deflect the string and
the gauge is too crude to reflect what may be happening.   By way of
contrast, the bar of the crownulator is rigid, and the dial is sensitive
enough to reflect small changes in your hand pressure on the tool and any
accidental roll you might introduce in your awkward state on the floor.  

 

I have a Wixey gauge and use that, but I have for 30 years or more used a
Fowler dial gauge that I bought from APSCO way back when.  I would take 13
downbearing readings  along the bridges, let the tension off the strings and
remove them except for my samples and measure again.  Plenty of boards out
there where the bearing did not change at all from loaded to unloaded, and
usually little or no crown down below.  More movement at the ends, although
the pattern of the readings would vary from piano to piano.  It doesn’t tell
you everything about downbearing, but if you use it consistently it can tell
you a lot quickly and precisely.  

I can combine that with my Wixey readings.

 

Shim to restore crown?  I doubt it does anything meaningful.  Has anyone who
advocates this as a rebuilding method taken precise before and after
measurements, and followed that up a few months later with more
measurements?  

 

Will Truitt

 

I hope this gets through.  Too many of my posts disappear into the ether in
this new but not improved system we have.

 

From: pianotech-bounces at ptg.org [mailto:pianotech-bounces at ptg.org] On Behalf
Of David Love
Sent: Wednesday, June 22, 2011 12:12 PM
To: pianotech at ptg.org
Subject: [pianotech] Soundboard Analysis

 

You might need html to view this.

 

Below is a chart that I use for soundboard analysis.  Measurements are taken
before and after taking the tension down.  Crown sections are measured
between the ribs using a string stretched as far as possible and a wedge
gauge (very low tech).  I try and correspond the measurement of the crown
with the downbearing measurements in terms of location.  An analysis of the
soundboard rib design can also be made using the rib dimensions—I haven’t
done that yet.  Downbearing measurements are Lowell gauge readings.  Note
that .0175 corresponds to about 1 degree.  

 

I won’t tell you what my conclusion was about this soundboard but you might
be able to guess.  (Steinway AIII)

 

Question to ponder:  Why would it be that although there is a positive
change in downbearing when the tension comes off there is no apparent change
in measured crown?  Also note (not included in the data below) that when
measuring the change in distance from the struts to the bridge in the upper
end of the piano (from about rib #6 to the top) the average change was about
1 mm (the board rose about 1 mm).  

 

BTW this board had received a lot of shimming perhaps with the idea of
restoring crown???  What do you think?

 

 

 

 

 

 

 

Rib#                       Crown                              DB w-ten/wo
Rib L    H        W  (mm)

Or section           w-ten/ wo-ten

 

1                              2/2
.006/.015                             600         17           28

 

2                              2/2
.006/.015                             790         20           27

 

3                              2/2
.009/.012                             940         20           27

 

4                              1.5/2.5
-.006/.003                            1040       20           27

 

5                              0/.5
-.003/.003                            1080       22           27

 

6                              0/0
-.009/.006                            1120       21           27

 

7                              0/0
.006/.018                             940         21           26

 

8                              0/0
.006/.021                             660         20           25

                                                                

9                              0/0
.006/.027                             560         20           18           

 

10                           0/0
.018/.042                             390         18           23

 

11                           0/0
.022/.045                             340         17           23

 

12                           0/0
.025/.050                             270         16           23

 

 

David Love

www.davidlovepianos.com

 

 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://www.moypiano.com/ptg/pianotech.php/attachments/20110622/fa19184b/attachment.htm>