Sound-Absorbing Thermoplastic Composite with Helmholtz Resonance

This work concerns a thermoplastic composite manufactured by thermal pressing of a nonwoven layered system. The composite structure consists of two layers: a thicker porous fibrous/plastic layer that responds to sound absorption and a thinner, rigid polymer (plastic) layer on the outside, enhancing sound absorption at selected frequencies. The novelty of this work is the creation of two interconnected holes of different diameters in a real fibrous composite, passing through both composite layers, acting as the neck and cavity of a Helmholtz resonator. Both the neck and the cavity are located within the composite structure, creating the shape of interconnected cylinders. The sound absorption of a composite with a varying neck diameter and a constant cavity diameter was studied using an impedance tube. The effect of the neck diameter on the resonant frequency and the corresponding sound absorption value was investigated. For various neck diameter variants, the resonant frequency was calculated using not constant, but a variable effective neck length. To achieve this, appropriate neck length corrections have been made for each neck diameter. The experimental results are consistent with the calculated results. These studies show that based on the relationship between the resonant frequency and the resonator neck diameter determined for a given composite, the resonant frequency (absorption peak) can be controlled by changing the neck diameter.