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Harpsichords and Related Topics

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Subject:
From:
Andrew Bernard <[log in to unmask]>
Reply To:
Harpsichords and Related Topics <[log in to unmask]>
Date:
Fri, 7 Sep 2018 20:12:18 +1000
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Hi Huw,

I find that fascinating in the extreme. Had I laid a wager on that, I would
have lost. My standard bet is a large pizza, so I will send you one. I
truly would not have credited that.

There's an acoustic research paper quite well done, the result of 13 years
work, of which Kottick is one of the authors. [Air and structural modes of
a harpsichord.] In it, the cavity is successfully modelled as a wedge,
successful because the theory matches the measurements pretty nicely, as
Newton would say. One point they make is that the aperture at the belly
rail, although long and wide and apparently large, actually has a high
acoustic impedance, and rather than finding a pressure minimum there, it
behaves as a pressure maximum (I am not using strict acoustic terminology
here, but simplifying.), meaning, it does not really 'port' the cavity.
Which is why I find drilling some relatively small diameter holes amazing.

I don't for a minute doubt your result! I have actually found in my study
of acoustics of these darn instruments (!) that despite papers full of
impressive Bessel equations and so, that sometime they miss the point
entirely. Not always, but sometimes. As in any science. I think the paper
mentioned is quite good, but unfortunately the effect of a soundhole was
not studied.

On your other point, if it is the case that the belly rail aperture of such
large size has a high impedance, I would think that cracks in the bottom
would have even higher impedance and not contribute any porting or venting
that would alter the tone. Unless the crack was huge, in which case you may
just have other problems on your hands. On the top deck, wide open cracks
that let air in and out in the soundboard don't seem to affect the sound
greatly either, I have noticed, consistent with my view that having a
soundhole or not makes no difference.

Andrew



On Fri, 7 Sep 2018 at 05:54, Huw Saunders <[log in to unmask]>
wrote:

> I don't have an acoustic theory but I do have an experience: a few years
> ago I made 2 versions of the same instrument which doesn't have a rose and
> has a solid belly rail. The first was solid walnut, the second poplar and
> painted. When I was setting up the second I was tearing my hair with
> frustration because it just wouldn't sing. Lying awake in bed one night I
> remebered a remark of Chris Nobbs' about sound holes. The next morning I
> took out all the action and took my biggest drill and drilled 2 holes in
> the belly rail (about 32mm diameter I think). The instrument immediately
> began to sing as I wanted. One thing that may help us in this regard is
> that most bottoms (except Italians), if they are made up in the usual way
> from butt jointed boards, will eventually split. That won't happen with
> plywood of course.
>
>

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