Measuring the absorption coefficient of panels in free field by using inverse filtered MLS signals

This paper describes a method for the measurement of the absorption coefficient of a panel at oblique incidence based upon a new procedure which combines inverse filtered MLS signals with the subtraction technique. This allows for the use of shorter pulses which better discriminate between the different events usually present in measurements, e.g. direct, reflected, and diffracted signals. Therefore, the windows that select the direct and reflected events from the measured trace, which are then used to calculate the absorption coefficient, can be better set up. This in turn optimizes the frequency bandwidth of interest. The method has been validated by applying it to a Microperforated Insertion Unit panel (MIU) in an anechoic room. The MIU consists of a 2.44 m×2.44 m× 1 mm steel panel, properly perforated to provide a suitable absorption bandwidth, in front of a 2.44 m×2.44 m ×2 cm wooden panel with an 5-cm air gap between the steel and wooden panels. The inverse filter of the electroacoustic response has been designed so that the radiated pulse has a minimum-phase cosine-magnitude spectrum. Experimental absorption curves agree with the predicted ones.