Hybrid acoustic materials through assembly of tubes and microchannels: design and experimental investigation

Purpose The purpose of this paper is the design and experimental investigation of compact hybrid sound-absorbing materials presenting lowfrequency and broadband sound absorption.

Design/methodology/approach – The hybrid materials combine microchannels and helical tubes. Microchannels provide broadband sound absorption in the middle frequency range. Helical tubes provide low-frequency absorption. Optimal configurations of microchannels are used and analytical equations are developed to guide the design of the helical tubes. Nine hybrid materials with 30 mm thickness are produced via additive manufacturing. They are combinations of one-, two- and four-layer microchannels and helical tubes with 110, 151 and 250 mm length. The sound absorption coefficient of the hybrid materials is measured using an impedance tube.

Findings The type of microchannels (i.e. one, two or four layers), the number of rotations and the number of tubes are key parameters affecting the acoustic performance. For instance, in the 500 Hz octave band (a500), sound absorption of a 30 mm thick hybrid material can reach 0.52 which is 5.7 times higher than the a500 of a typical periodic porous material with the same thickness. Moreover, the broadband sound absorption for midfrequencies is reasonably high with and a1000 > 0.7. The ratio of first absorption peak wavelength to structure thickness l/T can reach 17, which is characteristic of deep-subwavelength behaviour.

Originality/value – The concept and experimental validation of a compact hybrid material combining a periodic porous structure such as microchannels and long helical tubes are original. The ability to increase low-frequency sound absorption at constant depth is an asset for applications where volume and weight are constraints.

Uncontrolled Keywords

• sound-absorbing materials
• design
• additive manufacturing
• low frequency
• subwavelength absorption
• deep-subwavelength absorption