Specific vs Monopole mobility ?

[ChuckM]

It seems that the terms Specific Mobility and Monopole Mobility are used interchangeably in the book. Is this the case or did I miss something ?

[Trevor Gore]

They have the same technical definition in that they both have the formula 1/sqrt(km). What the k and m specifically refers to makes the difference. When k refers to the stiffness of the top in the way that I recommend measuring it and m refers to the equivalent mass of the top, then you have monopole mobility. Hopefully, I was consistent in the book!

[Dominic]

Hey Trevor, I am wanting to measure the monople mobility of 2 classical guitars I have just finished and I have a questions I could not find answer to in the books.

1. should this test be made with strings on or off? If the strings are left on, and they pull the top up a bit the measured deflection may be higher because the top is not in a static state. I might also be lower if the strings counter act the load at the bridge somewhat.

Cheers
Dom

[Trevor Gore]

I've mostly done it with strings on. On the deflection test, the principles of linear elastic structures and superposition apply, so it doesn't matter. On the tap test I get pretty much the same result, strings on to strings off. You need the saddle in because of the mass and the strings add a bit of mass also. The tension doesn't seem to affect things except that a bit of string tension makes sure the saddle travels with the top.

[Dominic]

Thanks, these guitars were closed up before I got your books so it will be interesting to see how they go in terms of efficiency. And I love being able to put numbers on things. Math does not lie.
The modes need a bit of trimming but not too much and I will compensate the saddle in a week or so. Didn't do nut compensation this time. But I'm set up to do the whole thing next time from string testing on. Should be fun.
Cheers
Dom