Resonators

For quite a few number of years now, the Structural Dynamics and Acoustics group at the University of Twente studies wave propagation in narrow gaps (e.g. thermal-viscous wave propagation in narrow tubes and thin air layers). The waves propagating in these narrow gaps can be described elegantly by means of the so-called ‘low reduced frequency’ models, based on the model of prof. Zwikker and prof. Kosten (honorary members of NAG). Recently, the low reduced frequency model has been applied, adapted and extended to various onfigurations by H. Tijdeman, M. Beltman, T. Basten and F. v.d. Eerden, M.H.C. Hannink, M. Nijhof and the author. Despite its simplicity (often an analytical solution be obtained), the model is very accurate and suitable to optimize the various parameters to optimally reduce or reflect sound or minimize radiated sound. Needless to say, as the model describes acoustic waves, it also describes resonance features. This paper is intended to show some examples of the use of the model to resonance applications. In the current paper only the basic equations of the low reduced frequency model will be given and the reader is referred to the literature for additional information. The paper focuses on three applications which can be described by the low reduced frequency model, i.e. the use of resonators for broadband sound absorption with non-absorbing material (e.g. suitable for sound absorption at high temperatures and/or corrosive environments), the ‘acoustical mirror’ for the reduction of fan-noise while retaining the cooling flow and the use of resonators to reduce radiated sound (and to increase transmission loss), introducing λ/2-resonators.