Optimizing broad-band passive absorbing structures for specified applications

In buildings there are many noise sources which are needed to be attenuated one way or another. One of these noise sources is typically air shafts. These can be passively attenuated by using absorbers and resonators or their combinations. With right design, these absorbent and resonator structures can attenuate desired frequencies efficiently. To accomplish this, however, knowledge of acoustical phenomena behind these structures and use of computer algorithms which use this knowledge is required. Structures which consist of absorbers and resonators are examined. Here they are theoretically optimized by a computer algorithm which is based on defining a structure by the transmission line model. With this kind of relatively accurate model it is possible to avoid errors made by commonly used approximate formulas. Since absorbers, which contains perforated plate and absorbent have many variable parameters their acoustical efficiencies flexibly can be optimized by finding the most suitable parameters. Optimized and modeled absorbent structures have been constructed and their respective absorption coefficients have been measured. Measured values correspond well with estimated theoretical values. With measurements it is also confirmed that optimized resonator structures are noticeably more effective than normal nonoptimized resonator structures considering acoustic functioning. Measurements of absorption coefficient have been made with a impedance tube using the twochannel method with excitation being a logarithmic sinesweep.