choosing the appropriate accelerometer

[tmb2]

i'm doing modal analysis on violins and have been mounting the accelerometer onto the corner of the bridge.  the bridge weighs ~1.8gr and the accel i'm using weighs 0.2gr.  i thought this would be a good setup however the cable from the accel is pretty stiff so am wondering if i might be better off with a slightly heavier accel (maybe 0.5gr) but with an almost silk-like flexible cable.  any thoughts or comments would be appreciated, thanks in advance!

[johnsold]

I think a transducer with mass of 10% of the mass of the part you want to measure will have some effect on the acceleration. Can a good violinist hear the difference with and without the transducer?  That subjective test might be a good and quick indicator of whether the transducer and cable are modifying the results.

It would be considerably more expensive, but optical interferometry would have minimal effect on the motion.  Of course you would also need to measure and account for the gross motion of the violin while being played.  Perhaps for your tests it could be held in a position which is stable with respect to the optical bench.

Lynn 

[tmb2]

thanks for your thoughts lynn.  it's actually pretty difficult to do the test you mention for anything other than gross acoustical changes.  subtle things are difficult to track and i imagine this would fall into that category.  what i can do is put a violin in a testing rig, tap with an impact hammer at the bridge, and do a transfer function of the spl at the microphone.  i could do this without moving the instrument; just glue the accel on and re-test.  if it doesn't show a difference in the frf then it's probably a pretty safe bet that it won't effect the modal analysis of the body (typical weight ~400gr.)  thanks very much for the response, this is what makes these forums so helpful!