Index of content:
Volume 104, Issue 6, December 1998
- SELECTED RESEARCH ARTICLES 
Wave decomposition of the vibrations of a cylindrical shell with an automated scanning laser vibrometer104(1998); http://dx.doi.org/10.1121/1.423956View Description Hide Description
Elastic waves propagating in a cylindrical shell have been detected by an automated scanning laser vibrometer designed to record both in-plane and out-of-plane surface motion over the surface of the shell (32 points axially and 32 points circumferentially). The structure was freely suspended in air and excited radially by a shaker at a single frequency, either below or above the ring frequency of the shell. A wavevector analysis of the data was performed with a fast Fourier transform and an overdetermined modified extended Prony method. The results clearly show the presence of longitudinal, shear, and flexural waves above the ring frequency. In addition, the Prony method reveals the presence of evanescent waves due to mode conversion of the propagating waves near the ends of the shell. Below the ring frequency, two types of in-plane waves and flexural waves were identified. The results are in excellent agreement with predictions from the dispersion curves for thin shells.
Sound absorption in concert halls by seats, occupied and unoccupied, and by the hall’s interior surfaces104(1998); http://dx.doi.org/10.1121/1.423957View Description Hide Description
From experimental data in concert and opera halls, absorption coefficients were determined for audience seating, unoccupied and occupied, of different constructions, and for gypsum, wood, plaster, and concrete interior surfaces of various thicknesses and densities. A total of ten halls were involved in the bare hall (before seats were installed) analysis, yielding “residual” absorption coefficients, i.e., coefficients for those areas not including the areas to be covered by the seating. In ten halls reverberation times were measured after installation of the seats (unoccupied) and in seven of these halls at concerts with seats fully occupied. The seating absorption coefficients are presented for “acoustical” audience areas, i.e., with a 0.5-m-wide edge around each seating block. The results are compared with the data of Appendix 5 in Beranek [Concert and Opera Halls: How They Sound (Acoustical Society of America, Woodbury, NY, 1996)]. The sound absorption data presented for interior surfaces and audience areas should permit more accurate estimation of reverberation times as a function of frequency for large halls during the planning stage.