Soundboard sizing

No scientific records here, only a subjective - albeit very strong - impression.

I have in my home “workshop” two harpsichords in construction:

  1. A Zuckermann/TPW French double kit. 2x8’, 1x4’. Two keyboards. 63 notes from F to g’‘’.
  2. A made from scratch big flemish my design derived from Ruckers but of course with modifications and strenghtenings to withstand 5 octaves. 2x8’. One keyboard. 61 notes from F to f’‘’.

They are both in more or less the same state: case done, still no bottom, the two soundboard made from the same wood (abete della Val di Fiemme, I think it’s named spruce in English) from the same planks. Both have their soundboard still not glued into the case.
However, n. 2 has the soundboard furniture glued (bridge, ribs, boudin), while the n. 1 is thinned but without ribs and bridges and boudin/4’ hitchpin rail).
The last difference is n. 1 is still bare wood, while n. 2 has been entirely sized up and down sides with very diluted hide glue.

When you finger-tap, or when you just caress the two soundboards (put inside the case but not glued into it yet) with your fingertips, n. 2 responds with a lively sound, very loud, deep tone, different at the right of the bridge than at the left, at the tail than just behind the gap.
N. 1, on the other hand, responds with a thinner sound, more acute, much less loud, shorter (this I am not entirely sure but seems shorter. Impossible to say when just tapping or caressing).

I haven’t measured the effect, I’d not even know how to, but it’a strong. I mean, it’s not a vague impression: the two sounds are very different. I had already registered this effect previously, but now I can compare having at the same moment two harpsichords at my disposal. I believe this effect is given by the hide glue sizing. I’ll tell you if the n. 1 will change its sound characteristics after I’ll size it with hide glue as well.

Lastly, this has been discussed by Grant O’Brien, I think in his Ruckers boom but who knows where. It could be Martin Skowroneck in his “Cembalobau”, sorry I am not precise but I can search if needed as I have both books at home.
Now, O’Brien - or Skowroneck - says that Ruckers probably didn’t size their soundboards with hide glue, because he (O’Brien) put his wet finger on the soundboard and didn’t feel it sticky as expected.
Well, I am sure I have sized my soundboard with hide glue, but be assured my wet finger doesn’t feel it sticky, neither.

Other famous candidates for historic sizing are: gumarabic, shellac, egg’s white. I have used shellac before, and it does keep the soundboard clean and does provide a good support for decoration, but hasn’t any consequence on sound, like hide glue has (a very good one).

Dear Domenico

I’m unsure how it is possible to even subjectively attribute any tonal difference you perceive between your two soundboards to the hide glue sizing of #2 when soundboard #1 is yet to have its furniture attached!



You think the difference is due to the furniture? Maybe, but the phenomenon is one I had observed before, hearing the before-and-after glue sizing on the same soundboard,. Now I have two soundboards at once so I can hear them in the same momenti, that’s why I decided to write.
However I’ll attach the furniture to harpsichord 2 as well, size its soundboard and re-check. I’ll tell you the results.

I don’t think Carey is saying the difference is the furniture. He is saying you cannot compare the boards until the furniture is on both of them, since that will have an important bearing on the sound. Think logically about this. The bridges ribs and boudin add mass and stiffness factors to the board, so even if the boards are truly identical and sized, they are not going to sound the same if one has the furniture and the other does not. And when you put the furniture on No.1, that will have an 4 foot bridge and boudin that No.2 doesn’t have. There is no way they will sound and act the same. BUT, with both having their furniture, you at least have a better basis for comparison!

I understand. Indeed, the added mass can be accounted for the deeper tone. However I felt I could write about that because it confirms an observation I had done previously on the same soundboard before and after sizing, but of course a couple days after to allow a proper drying. With shellac i never observed this phenomenon. Again: before and after sizing, with just a couple hours distance since the alcohol which the shellac is diluted in dries much faster.
I know these are only observations, not scientific analysis. However I’ll attach the 8’ bridge and redo the “test”.

@domenico.statuto I think you have to ask yourself what is the point of sizing a soundboard. The only thing I can think of is to ever so slightly stiffen it. We are not talking about varnishing for some sort of protection. Size will add a small amount of wood protection.

I think glue size would have a very small, but not negligible effect on stiffness. And now, what effect would increased stiffness have? In a free unmounted soundboard you may get some added higher partial response which you could hear in a knock test, but a lot would depend on how stiff the original spruce is. When the board is mounted and thoroughly stuck down all the whole periphery, all the stiffness characteristics change. So I think this is all a bit dubious. Most makers do like to size. We use (free range! organic!) egg white. We have also made many instruments totally unsized. We never do painted soundboards but I suppose glue size and egg white make a good ground for tempera to lock on to.

Just musing out loud.

I see your point, Andrew. Of course it’s dubious! No science, just an uneducated observation I wanted to share.

The most if the effect I observe comes not from the knock test but from the “caress test”, the sound the soundboard makes when just caressed. Of course this is not how the soundboards normally work. Probably it does no difference under normal operation circumstances (i.e. when normally playing).

I don’t want to start any new hoax such as the “secret varnish of Stradivari”! :slight_smile:
The secret of a good sounding harpsichord is to make a good harpsichord. Still there are visibly bad-made harpsichord that still have a good tone… who knows.

What can be observed about the effect of soundboard stiffness on the sound of a harpsichord?
To some degree, depending on stiffness, a soundboard dampens vibrations. Canvas does not make a good soundboard, it absorbs all the energy quickly.
To some degree, depending on stiffness, a soundboard reflects vibrations back into the string. A steel monochord can hardly be heard, but the string vibrates a long time.
Somewhere in between a soundboard produces a sustained audible tone, gradually using the energy of the vibrating string to move air molecules.
Piano soundboards that have lost compression stiffness due to humidity cycling have a more percussive prompt sound and weak sustain in the treble. They can be stiffened by epoxy sizing, giving some improvement to the sound. But pianos have string tensions and soundboard thicknesses much greater than harpsichords.
Can anyone comment on the sound of Italian harpsichords with maple soundboards? This might offer something of interest.
I have the capacity to make a small instrument that allows exchanging soundboards. Given some clear intention, I might make some comparisons.

@EdS Most of the instruments we made were never sized. It’s a very negligible thing. As I said, I think the best justification for it is as a ground for tempera paint. Egg white that we use sometimes probably has a small effect of stopping humidity putting some moisture in the board, but our climate in Australia is so dry I wouldn’t know! It’s in no way major or crucial to tone in the way violin varnish is. [I’m tempted to start a topic on violin varnishing, but that really is outside the forum remit.]

I think this topic is about the effect of sizing, not maple or cypress or spruce. We could start a new topic on that if people are interested.

I’ve read (here or elsewhere) that Italian harpsichords were sometimes made with maple soundboards.
Since maple is harder than spruce or pine, I have wondered if anyone can comment on the tonal characteristics of an Italian harpsichord with maple soundboard, since this might add some observations about the effect of stiff soundboards on the tone of harpsichords.
If you want to start a new topic “Soundboard Stiffness” might be helpful.

I reply here for the moment.
Soundboard stiffness is achieved by various means: material (usually quartersawn spruce/fir, but often the Italians used slab-sawn cypress and the Neapolitans maple); grain angling (not only Italians such as but some Frenchs as well); ribs etc. so indeed control of stiffness was considered important.
I’ve read (don’t remember where: Hubbard, maybe) that angled and slab-sawn soundboards were thinned more than the quartersawn spruce, in order to achieve about the same stiffness.

As for the characteristics of maple instruments, you can hear the RCM 175 Guarracino: Search | Royal College of Music

Descriptions and analisys:

Denzil Wraight:

Grant O’Brien: Guarracino RCM 175

I didn’t re-read now and I don’t remember precisely if the RCM 175 has a maple soundboard or just maple case sides, so please forgive if it isn’t relevant.

Escuse me, I messed up a copypasting. Here is the original sentence:
“not only Italians such as Grimaldi but some Frenchs as well”.

Indeed, I remembered wrongly, sorry. The soundboard is defined “fir” in the O’Brien article. The case sides, baseboard, bridge and nut are maple.
There are other Neapolitans with a maple soundboard.

Once installed, the soundboard is anchored along its perimeter. The boudin/4’ hitchpin rail acoustically divides the soundboard.

I suspect the chief role of the ribs is to prevent that portion of the soundboard from rising into the strings. A dried out case can shrink and bulge the soundboaard as happened to my ZHI Flemish I. The second owner had kept it in a church through Canadian Prairie winters. The third owner took my advice to cover it and run a humidifier underneath which remedied the problem.

An interesting experiment would be to install the soundboard and do taps before and after sizing. Double sided tape would allow temporary installation of the soundboard, YMMV, before installing the bottom and braces.

I’ve done that several times. Problem is, I sized the soundboard(s) before gluing them in, so I did the tapping-and-caressing experiment at many days distance, so the before-sizing results was all in my memory. Never thought of recording. This is why I was so happy to have both soundboards in the same moment to listen to.

What I can do now is: install and glue the complete soundboard non-sized; tap-and-caress in the various parts; size the soundboard already installed; repeat the tap-and-caress. Record everything, though I doubt the recording will capture the nuances, it should capture the stronger sound.
But then again, it will not be an experiment in controlled environment. The mics will be not in the precise same position, the levels will not be adjusted exactly the same, etc.

Hubert Bédard, who in the 1960s and 1970s restored (or lead others restoring) dozens of antique harpsichords (mostly but not only French) once told me that he found that the ribs, together with the bass bar, had the effect of greatly enhancing the bass response, as that part of the soundboard vibrates with no internal divisions.

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