How do strings work? (probably one for Trevor)

[ProfChris]

I've got in my head a mental picture that strings work by transferring their energy to the top in two ways:

1. By rocking the bridge back and forward (if a fixed bridge).

2. By pushing it down and relaxing (for both fixed and floating bridges).

This helpfully explains (to me) why a higher saddle = more volume (greater rocking leverage for (1), greater downforce for (2)). It also explains why floating bridges tend to produce less volume than fixed (mainly (2), little or no (1))

But I'm fairly sure this is wrong, either wholly or in part. So, primarily, I just want to know.

This question is partly because I recently made a pair of (silly but fun) 7 inch scale ukuleles:




I used a floating bridge because I had no idea what compensation might be required, and they turned out to be surprisingly loud for their size.

One day I might try a tenor guitar with a floating bridge, and if I knew how strings worked this might help me work out how thick to make the top and howlightly to brace it.

[Trevor Gore]

That's about right, Chris. The pumping action is a lot more effective than the rocking action, especially on nylon strings. It's a tension thing. The more tension, the larger the proportion of rocking force compared to pumping force, but the pumping is always at least 4 times larger in terms of the force exerted on the top and it is exerted in a much more efficient way, so the pumping action produces most of the sound.

Floating bridges may be less inclined to rock the soundboard, as you suggest, but the main reason why floating bridges produce less sound (in general) is that the tops of the instruments that have floating bridges are generally much stiffer and much heavier than typical flat-top, fixed bridge instruments and so have a much lower monopole mobility than a broadly equivalent flat top.

The ukes look like fun!

[ProfChris]

Floating bridges may be less inclined to rock the soundboard, as you suggest, but the main reason why floating bridges produce less sound (in general) is that the tops of the instruments that have floating bridges are generally much stiffer and much heavier than typical flat-top, fixed bridge instruments and so have a much lower monopole mobility than a broadly equivalent flat top.

Many thanks Trevor - I'm surprised (and delighted) to find I had some understanding after all.

I know that mandolins and archtop guitars have heavier soundboards than flat tops. But what about flat top guitars with floating bridges, eg those 20s/30s small bodied Regals? I had the impression they were more lightly built than guitars with conventional bridges, because there was less torsional force acting on the top.

The ukes are fun indeed - only just playable, with a little practice.