Some mandolin thoughts

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peter.coombe
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Some mandolin thoughts

Post by peter.coombe » Mon Dec 31, 2018 9:39 pm

In a thread a few months ago I presented a pancake type of mandolin with a Gidgee back and sides, and asked if any had used Gidgee because it sounded better than any other pancake flattop mandolin I had made. Trevor Gore commented that with Gidgee, the back would be a dead back because it was so heavy. He is right of course, but it got me thinking, and I started taking some measurements of completed mandolins. Cohen and Rossing have shown that mandolins vibrate similar to a guitar, but the frequencies are higher, so why not apply the same principles to my flattop mandolins. The measurements showed that all my flattop mandolins had the main back mode about 3 semitones lower than the main top mode. I did measure an original Gibson Army Navy mandolin that my Pancake mandolins are based on and that also had the main back mode about 3 semitones lower than the main top mode. I also measured 2 vintage Lyon and Healy style A mandolins (archtop) and the main back mode is about 3 semitones higher than the main top mode. So, why not deliberately make a pancake mandolin with the main back mode 3 semitones higher than the main top mode, and thus deliberately create a dead back with a different back wood. Shifting the main back mode up 6 semitones is huge, so required a thicker back and much beefier bracing to stiffen the back. How much stiffer was an educated guess, but in the end I nailed it pretty close.

The results have been very interesting to say the least. All of the 3 main modes went up in frequency, and the node of the main top mode moved out significantly towards the sides. I made two mandolins and one mandola using the same principle to make sure it was not a fluke. The change in sound is quite significanly very much for the better. So, to copy what Gibson did was wrong wrong wrong. The mandola is astonishingly good sounding for a flattop mandola. It does sound different , but gives nothing away to my archtop mandolas in terms of overall sound quality.

Here are pictures of the Chladni patterns of the main top mode. The Gidgee mandolin is obvious and it looks very similar to all the other flattop mandolins I made beforehand.
Attachments
Gidgee top mode 474Hz.JPG
Gidgee top mode 474Hz.JPG (100.47 KiB) Viewed 1140 times
KBP main top 456Hz.JPG
KBP main top 456Hz.JPG (116.39 KiB) Viewed 1140 times
Walnut 465Hz Top mode.JPG
Walnut 465Hz Top mode.JPG (88.38 KiB) Viewed 1140 times
Peter Coombe - mandolin, mandola and guitar maker
http://www.petercoombe.com

Dave M
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Re: Some mandolin thoughts

Post by Dave M » Tue Jan 01, 2019 3:21 am

Peter that's very interesting. Trying to understand what is going on.

Presumably the actual frequency of the back shouldn't matter because we are not hearing it. If the node line of the main top has moved outward presumably this would suggest an effect like Trevor's extra side mass...? Perhaps the extra mass of the back is doing something similar. The extra vibrating surface should give you more volume I guess. What do you think?
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Trevor Gore
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Re: Some mandolin thoughts

Post by Trevor Gore » Wed Jan 02, 2019 9:58 am

Nice work, Peter. It seems that the principles in the book can be applied across a whole range of stringed instruments comprising a ported box and strings - which isn't surprising, really. All that changes is the scale (as in size).

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peter.coombe
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Re: Some mandolin thoughts

Post by peter.coombe » Wed Jan 02, 2019 7:55 pm

Thanks Trevor Yep the principles in the book have now been applied in the mandolin world, but whether or not anyone in the mandolin world actually takes any notice is another question. Tradition is a powerful thing. It has been a bit of, duh why didn't I think of that before. I have read the books, so applying the same principles should have been a no brainer. The main difference in sound I have noticed is the new instruments are more even and sweeter sounding, and the difference is not small, they blow away my earlier pancake mandolins. From my point of view, a big breakthrough. However, it now makes it difficult to sell the older mandolins that don't have this technology! The latest mandolin was finished on 30th December, King Billy Pine and Blackwood (picture in the middle), and the sound is amazing for a flattop, I can't put it down, which was confirmation it definitely works. With a bit more experience, that one has the modal frequencies pretty much bang on what I was aiming for, and I think it is slightly better sounding than the Qld Walnut mandolin (last picture).
Peter Coombe - mandolin, mandola and guitar maker
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