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39ford
10-22-2017, 11:22 AM
I'm buying a tenor and it has a convex back. Are there concerns both for and against a convex back vs a flat back?

robedney
10-22-2017, 02:12 PM
In my humble opinion: Not if you like the way it sounds. A convex back will be stiffer than a similar flat section and will respond differently acoustically. In theory, you can also reduce bracing in an arched back (or top, for that matter). Bracing is both structural and aids in voicing the instrument, so it's the totality of the system that counts.

Convex backs and tops can also help avoid cracking/splitting in solid woods. Wood expands and contracts with variations in temperature/humidity, and a convex shape gives it somewhere to go with less stress. As an example, a flat back is pretty much trapped by the bracing and sides of the instrument. When subjected to low humidity the back wants to shrink -- particularly across the grain. At some point the wood is going to split (or part from the sides). In a convex back the same thing is happening, but there's more room for error because there's more wood there -- the back will flatten out some, but probably not to the point of splitting.

Pueo
10-22-2017, 02:24 PM
Yes, usually stronger. I have three ukuleles with not-flat backs, and I think, not just coincidentally, that they are the three best-sounding ukuleles I own.
It also takes a bit more skill to to have a rounded back, so you are likely getting a higher-quality instrument. That has been my experience anyway.

39ford
10-22-2017, 03:13 PM
Thank you , friends...I appreciate your keen insight

sequoia
10-22-2017, 07:06 PM
I build ukes with an arched, radiused (convex) back basically because that is what my borrowed design plans call for. I have never been able to get a straight answer from anyone why this is good. The answer is always, well that is the way it is done and it makes a better sounding and stronger ukulele. Think of a violin with its radical radiused back. So here is my theory: The back acts as a "reflector" of sound vibration coming from the top. If the back is flat, the sound waves would tend to dissipate into the corners instead of being concentrated (focused) and reflected back to the center of the top. Think of a parabolic mirror in a telescope magnifying and concentrating light rays. Basically the back is acting as an amplifier of sound vibration.

I am not an instrument designer at all, but I think about this a lot. Therefore lately I have increased the radius of my backs to see what happens. So far I'm down to an 18" radius and things are sounding great (tenor size). I think however that there is an endpoint where it becomes zero sum. Again, like a lot of things on an ukulele, there is a sweet spot and one can take things too far. It is finding that sweet spot that is the key. I have yet to find the sweet spot on back radius.

kohanmike
10-22-2017, 10:36 PM
Three people in my uke group have the Kala Thinline travel uke, which has a bowed back and to me they sound really good. I like them so much that I'm contemplating having a thinline made by my builder in Vietnam, Bruce Wei, with the bowed back, combined with the look of Ovation sound holes.

http://www.kohanmike.com/uploads/Ovation hole thinline 900.jpg