So, my chaotically creative mind started thinking about ways to mechanically amplify the sound out of an acoustic instrument (I'm really new at this). Something that would leverage a small movement in one place to a larger movement in another.
Then, after someone told me to move my bridge closer to the tail piece, I realized (hopefully correctly) that moving the bridge away from the center of the vibrating plane/top could accomplish the same thing. Bridge closer to edge, small vibration translates to a larger movement towards the center, like the difference between a diving board end where the diver propels off from (large movment), as opposed to the less movement back by the tower.
And I have no idea whether I'm right in all this, but it at least makes logical sense to me.
So, are there some rules of thumb, or ratios to consider for bridge placement?
My rule of thumb is place the bridge about 1/3 of the way along the box, closer to the tail. I've never really fooled around with moving it much, because I only have built instruments with frets, and once you decide scale and bridge location, you are locked in. But I do know for sure if you place the bridge at the end of the box, you'll need amplification.
I'll be honest, sometimes the bridge is placed exactly so, because of the graphics on the lid.
thanks to some of the feedback on this, I bought a tuning fork today at the music store, came home and messed around with the box I'm constructing from scratch. Sounds weak and reedy out in the middle. Moved it to within 1/2" of the bottom, and it sounded low but muted/dull, sort of like a fiddle where the sound post is too far from the bridge. Moved it up probably about 1.5" from the bottom max, and the sound was full and rich. This is a box that only has one face plate on so far. Then I started tap testing some of the wood I've got sitting around. Maple sounds good. Oak is "ok" but Maple has a broader range and richer tone.
Tried it on my already constructed fiberboard cigar box, and there's a rattling, which I assume is the lid not being screwed down at the corners or to the neck. The neck is not glued to the box yet, but has a very tight pressure joint, and the lid has a lesser pressure fit to the box top, but not enough. This is gonna help diagnose some things more easily than I was doing before.
Thanks. Y'all rock.
I've done further testing on this box I'm building. So far, I've got 4 sides (1/4" birch ply) and the back on (1/8" birch ply). size 8x12x3.
Besides just that sweet spot about 1.5" away from the bottom edge, there's also a band exactly 1/3 of the way from each end (4") where there's a sweet spot out at the two edges of that band, but not in the middle. So, seems I'd need to have an extra wide bridge (at least 6" wide) for the feet at each end to rest on a sweet spot.
I'm thinking about sprinkling some salt on the face of the box and hitting it with the tuning fork again to see if some other spots emerge. Salt should dance about at the sweet spots, and be visible to the eye. That way I can see what happens in other spots while holding the tuning fork in another area.
Meh, just tried it. not enough movement of the face plate to make the salt move.
Every box is different, so I go to the tail of the box and start thumping while moving slowly up. then place the bridge where the sound is loudest and brightest. That's what I do any way.
That's what I did, but then changed to putting it in my box tester and moving the bridge around, under string tension.