COMPARISON OF MODAL ANALYSIS MEASUREMENTS WITH MICROPHONE AND ACCELEROMETER ON HAMMER-IMPACTED STRUCTURES

摘要

Very light structures such as string instruments (violin, viola, cello) bridges are significantly mass-loaded with even the lowest mass accelerometers. On structures where the accelerometer mass is greater than 10% of the total mass of the structure, not only are the mode frequencies significantly changed, but the mode shapes can be strongly affected. Use of a microphone as a "no load" alternative to an accelerometer is an obvious - but not straightforward - solution to the mass loading problem. To determine in practice the applicability of microphone measurements for modal analysis, microphone and accelerometer transfer functions at a common driving point under identical conditions were measured for hammer-impact excitation on various freely suspended shapes, including a cylindrical aluminum (Al) bar, a flat rectangular wood plate, a flat aluminum rectangular plate, and a cello bridge. Considerable agreement between accelerometer and microphone measurements was found for mode frequencies and dampings, although some amplitudes were (relatively) quite different. Comparative results between the two methods will be presented.