Investigations of an impedance tube technique to determine the transmission loss of materials under angular incidence
Invited paper
FEMTO-ST Applied Mechanics
Tuesday 2 june, 2015, 17:40 - 18:00
0.7 Lisbon (47)
Abstract:
The most popular technique for transmission loss characterization of porous materials is undoubtedly the impedance tube method based on the transfer matrix method (described in ASTM E2611) which is valid for frequencies of a few kilo hertz, depending on the tube’s size. A mono dimensional harmonic plane wave is created at one tube’s extremity; this wave propagates inside the tube and is partially transmitted through an absorbing material to the opposite side of the tube. Using proper boundary conditions and measuring the sound pressure yields to the estimation of the transmission loss after a few algebraic manipulations. All measurement systems developed until now are based on a normal sound incidence. Yet, some modeling methods used for these materials are based on excitations with variable incidence angle and thus the validation of the developed models requires to use a facility system for which the characterization can be done using a variable wave incident angle on the material. The work proposed here details the design of such an experimental system. This system is based on a new design of the classical impedance tube: a numerical study is carried out to identify the range of validity of the proposed technique. The possibilities that this new system offers as well as its limits in terms of incident angle or frequency are illustrated on specific samples.