Active control of sound transmission through an aperture
- Autores: Teresa Pàmies Gómez
- Directores de la Tesis: J. Romeu (dir. tes.)
- Lectura: En la Universitat Politècnica de Catalunya (UPC) ( España ) en 2011
- Idioma: español
- Tribunal Calificador de la Tesis: R. Capdevilla Pages (presid.), María Antonia de los Santos (secret.), Juana María Mayo Núñez (voc.), Francisco David Denia Guzmán (voc.), Manuel Recuero López (voc.)
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- Resumen
This thesis focuses in the reduction on sound transmission through an aperture by active means. Two different cases are studied: sound transmitted through an aperture placed in a wall dividing two homogeneous half spaces and sound transmitted through an aperture placed in a wall of an enclosure. In the first case, the primary field is composed by a plane wave that arrives at a rectangular aperture placed in an infinite rigid thin wall. The sound transmission to the other side is calculated by the Rayleigh radiation equation after continuity conditions are applied in the aperture plane. The secondary field is generated by a monopole source placed close to the aperture in the incidence side and it is tuned to get zero acoustic pressure at the error microphone, placed in the center of the aperture. The sound transmitted after active control is calculated by the superposition of the primary and secondary fields. Using this model, investigation on the effect of wavelength, size of the aperture, distance between secondary source and error microphone and incidence angle of the primary and secondary sound fields has been carried out. The results show that the sound transmission attenuation strongly depend on all these variables, with better results for low frequency sounds and when the secondary source is placed far from the aperture. The dependence on the incidence angle of both fields shows more complicated behavior.
In the second case, the incidence field is composed by the enclosed sound field which is calculated using the modal model theory. Active cancellation is intended in the low modal range and it is carried out by including a secondary source and proceeding as in the free-free case, but considering both reverberant and direct field of the secondary source. The model is experimentally validated by measuring the directivity and sound pressure radiated by an aperture placed in a rectangular box. Since the walls of the enclosure are not rigid, an experimental procedure to determine its admittance is also presented. The experiments have been carried out for the first four modes of the enclosed sound field, and good agreement is found with the theoretical results. It has been found that not only the admittance of the aperture, its radiation efficiency and directivity depends on the predominant mode shape and on the frequency, but also depend on the location and shape of the aperture regarding the predominant enclosed mode shape. Regarding the active control, results confirm that the best approach to avoid sound transmission consists of achieving a global control inside the enclosure, rather than look for a local cancellation at the aperture. Finally, the principle of active control of sound transmission is applied successfully to a real case of incoming aircraft noise.
