Receipes to augment the amplitude of the A0 (Helmholtz) resonance?

[Andreas Preuss]

This is again a sideline to my thread about the New Concept Violin. 
 

I want to have a stronger air resonance (so its pitch should remain the same) and was wondering which alteration could make this happen. I was thinking about thinning down the upper and lower bouts of the back but not touch the area between the C bouts. Maybe it could also help to reduce the height of the ‘bass bar’ in the center?

[Don Noon]

A0 is not like the other resonances, and hard to affect much in a given instrument. 

The frequency is primarily determined by body volume and F-hole effective area (with length far more important than width), and secondarily by body flexibility.  In this case, "flexibility" can be imagined as sealing the F-holes and pressurizing the body, and see how much the volume changes.

Amplitude will be determined by how well coupled the string movement is to the change in volume.  Again, imagine pressurizing the body, and see how much the G string moves.  It will mostly be due to the top plate movement.

I have done a number of regrads on various fiddles, with the most extreme effects coming from excessively thick student fiddles and taking them down to normal-ish grads.  The most change I have seen in A0 is a gain of 1.7dB and a drop of 12Hz.  These are not very big changes.  Far bigger changes happen in the CBR adn B1- amplitudes... and you won't be able to just diddle with A0 without affecting other things.

Anyway, if I had to guess, I'd guess that your proposed regrad of the back would lower the A0 frequency a small amount, and have even less effect on the amplitude.

[Marty Kasprzyk]

15 hours ago, Andreas Preuss said:

This is again a sideline to my thread about the New Concept Violin. 
 

I want to have a stronger air resonance (so its pitch should remain the same) and was wondering which alteration could make this happen. I was thinking about thinning down the upper and lower bouts of the back but not touch the area between the C bouts. Maybe it could also help to reduce the height of the ‘bass bar’ in the center?

Increasing the f hole area increases the air resonance A0 frequency and increases its amplitude. Decreasing the cavity body stiffness lowers the A0 frequency while also increasing its amplitude.

So if you want to keep the same pitch with a stronger air resonance I therefore suggest you make two changes:  increase the f hole area and make the plates and ribs thinner.  Their frequency effects could be adjusted to cancel while their amplitude increases would add together.

Attached are two papers which describe the A0 behaviour.  If you are in the US it would be "behavior" not "behaviour".

JMC-Helmholtz_Resonance--Re-ct2 copy.pdf https:::acta-acustica.edpsciences.org:articles:aacus:pdf:2024:01:aacus240054.pdf

[Andreas Preuss]

On 2/6/2025 at 4:41 PM, Marty Kasprzyk said:

Increasing the f hole area increases the air resonance A0 frequency and increases its amplitude. Decreasing the cavity body stiffness lowers the A0 frequency while also increasing its amplitude.

So if you want to keep the same pitch with a stronger air resonance I therefore suggest you make two changes:  increase the f hole area and make the plates and ribs thinner.  Their frequency effects could be adjusted to cancel while their amplitude increases would add together.

Attached are two papers which describe the A0 behaviour.  If you are in the US it would be "behavior" not "behaviour".

JMC-Helmholtz_Resonance--Re-ct2 copy.pdf 5.07 MB · 4 downloads https:::acta-acustica.edpsciences.org:articles:aacus:pdf:2024:01:aacus240054.pdf 2.05 MB · 5 downloads

Thanks for forwarding the articles. Quiet interesting because it gives me some ideas on the next construction of the new concept violin. 
 

This time I am hesitant just to enlarge the f holes just for optical reasons. It would also drive the already high A0 resonance higher up to a degree where I think something gets out of balance. Maybe I have some other unconventional ideas which function well.
 
What concerns  my Engrish spelling (the way I adopted it in Japan)  I am living in the countly named ‘auto collect’ rokated somewhere between the North Pole and the South Pole. Rokal people here call it the Nethellands .

[LCF]

The Helmholtz mode isn't exactly a classic Helmholtz mode since it resembles pipe type resonance modes also but ignoring that, one thing to bear in mind about A0 is that the terminology is misleading. The Helmholtz resonance in isolation is not the same as the mode known as A0 rather it ( Helmholtz)  is located approximately in the dip between A0 and the lowest apparent wood peak. 

In reality the A0 + dip + lowest wood peak are a composite system where A0 is caused by the interaction between the  Helmholtz mode and the wood mode. 

The simplest way to play around with this on an existing instrument is to mask off one or both ff holes to various degrees using stiff card or a scrap of rib, taped over. Play/listen/tap/plot. 

 

Edited February 10 by LCF
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