Petošiæ A., Ivanèeviæ B., Jambrošiæ K.

The purpose of this work is to investigate the possibility of using broadband OFDM acoustic signals for making absorption coefficients measurements in a simple acoustic circular waveguide (Kundt pipe). First, the broadband Gauss pulse in the frequency range for plane wave propagation (significant energy below cut-off frequency) has been generated and transmitted through the pipe, reflected from appropriate termination impedance (rigid, unflanged and unknown impedance) and collected with a small microphone located in the pipe. The unknown loading impedance has been found knowing the frequency characteristics of transmitted and reflected signals including parameters of attenuation. Next step has been the construction of transmitting acoustic signals using OFDM modulation scheme using FFT algorithm in MATLAB with appropriate symbols depending on the frequency range of investigated termination impedance. These signals are transmitted using a loudspeaker with known frequency characteristic. Reflected signals from the end of pipe, which is terminated with unknown characteristic acoustic wave impedance (absorption coefficient), are recorded with single microphone and converted in digital form and comapred with original transmitted signal. Comparing these two signals, attenuation coefficient has been found in frequency range of interest with desired frequency resolution. Simulation of the described method has been made first solving equation of propagation (Fourier space propagator) in frequency domain and then solving lossy 2-D wave with appropriate boundary conditions and at inlet surface (vibrating loudspeaker membrane in time domain), and at outlet surface (termination with measured acoustic wave impedance in defined frequency range transformed in time domain). At the end of the simulation, the results have been compared with experimentaly obtained results for Gauss pulse and OFDM propagation methods.