A new experimental procedure, analogous to that used in structural modal analysis, has been developed to perform acoustic modal analysis in three-dimensional cavities. The procedure involves measurement of the vector components of the acoustic particle acceleration using two closely-spaced microphones. Particle acceleration transfer functions are then determined using these measurements referenced to either a third microphone mounted near the source driving the cavity or an accelerometer mounted on this source. Since these transfer functions contain both magnitude and single axis directional information, acoustic mode shape results can be determined unambiguously. This is not the case if acoustic pressure transfer functions are measured.
Standard FFT equipment is used to perform the measurements, and the procedure is easily implemented as a “user data acquisition procedure” in standard structural modal analysis software. With this software, the acoustic modal parameters can be extracted using the standard structural curve-fitters, and the acoustic mode shapes can be animated.
The procedure has been verified to work well in extracting modal parameters above 30 Hz in regular and complex cavities as long as the acoustic damping is low to moderate (below 8% of critical) and the modes are not too closely-spaced. In cavities with high damping (above 10-12% of critical, as in a fully-dressed vehicle interior), the procedure only works for well-separated modes, these typically being below 150 Hz in a full-size vehicle. Several potential sources of error and limitations of the procedure have been identified and ways to minimize these are discussed.