Predictions of structural intensity fields using solid finite elements

A structural intensity capability for solid finite elements is presented. The capability is validated using a structure with a known solution for structure-borne sound-the INCE standard Lexan T-beam. Although beam finite elements may be used to model structure-borne sound in the T-beam accurately for low wave numbers, the existing beam model is used to help validate the intensity fields computed in a solid element model. The structure-borne power through a cross section in the T-beam and the spatial power distributions along the stem of the T-beam are examined for frequencies up to 1 kHz. Excellent agreement between measured and computed structure-borne power amplitudes is shown. It was found that accurate modeling of the total cross-sectional power carried by flexural waves requires at least two, and preferably three or four solid elements through the thickness of the cross section carrying the waves. If the structural intensity field is dominated by longitudinal waves, a single element should be sufficient to compute the total power flow through a cross section. Also, intensity fields due to co-existing longitudinal and flexural waves can vary considerably over a beam cross section, often reversing sign from top to bottom.