A New System for the Measurement on Small Samples of the Normal-Incidence Insertion Loss of Acoustic Multilayers

摘要

This article presents a new system for the measurement on small samples of the normal-incidence Insertion Loss (IL) of multilayers used for the manufacturing of automotive sound package parts. The system consists of a rigid piston connected to an electrodynamic shaker and facing a tube positioned just above it. The measurement principle is based on the evaluation of the Transfer Matrix (TM) of the piston-and-sample assembly, from which the desired IL can be calculated. The TM can, in turn, be evaluated from the measurement of the piston velocity and of the acoustic pressure inside the tube, with two different tube termination conditions. The proposed measurement system is first of all aimed at simplifying the assessment of the acoustic insulation of multilayers used for the manufacturing of sound package parts. In the automotive field, this assessment is customarily done by measuring the diffuse-field Insertion Loss (IL) of flat samples having an area of around 1m2 in two-chambers facilities. A kind of test that, while being well-established, may be quite demanding and resource-consuming, in particular when many different multilayers (different materials, different constructions, different layers' thickness, etc.) have to be compared. With the proposed system, the amount of material and the time needed for a test is strongly reduced compared to this standard practice. Furthermore, the system intends to overcome the limitations of the 4-microphones test procedure based on the ASTM E2611 standard that cannot provide the actual IL of a package (but only its TL, without any support) and that suffers from the influence that samples' edge conditions may have on the results. In a first part, the article first discusses the principles on which the measurement system is based. In a second part it focuses on some aspects of the measurement hardware that are particularly delicate (e.g. sample's edge conditions). Eventually, the article describes how a prototype of the measurement system was built and validated.