Double resonant porous structure backed by air cavity for low frequency sound absorption improvement

In this study, multi band sound absorbing structure with double resonant porous structure employing a slit porous medium was proposed. Sound absorbing materials are widely used for noise reduction in various industrial fields. When the thickness of the porous medium increases, better absorption efficiency occurs at low frequency bands. Since the thickness should be small, the noise reduction performance at low frequencies is unacceptable for practical applications. The proposed structure was composed of a slit absorber in helical shape attached to a perforated membrane backed by an air cavity. Its acoustic characteristics were controlled by adjusting the design parameters that are essential for specific noise control applications. To confirm the acoustic characteristics, three samples were made and comparison analysis was performed. Sound absorption coefficients were measured using two-microphone impedance tube to verify the acoustic performance. The proposed structure exhibited better low-frequency sound absorption compared with the results of a conventional porous medium. The measurements showed significant sound absorption improvement at low frequency bands without sacrificing performance in the mid- and high-frequency bands. Acoustic impedance of the DRPS was predicted and compared with the measured results. The sound absorption mechanism was composed of a Helmholtz resonator, a perforated membrane and the effect of the slit porous medium. A parametric study was conducted for achieving optimal sound absorption efficiency.