Development and assessment of a low frequency acoustic liner design for landing systems noise minimization

Acoustic liners for aeronautics have mostly a Helmholtz resonator behavior provided by perforated sheets backed by honeycombs. They may be used, for example, at the surface of a landing gear door to reduce the noise generated by a landing gear at landing. However, their acoustic absorption ability is naturally limited to medium and high frequencies due to thickness constraints. The LEONAR concept is proposed to overcome the problem of available space in case of “low frequency” range. The design consists in a meta-surface in which a perforated plate is coupled with tubes of variable lengths. This meta-surface covers a back cavity with limited volume and generate a significant shift in the frequency range of absorption, towards lower frequencies. First, an optimization problem is carried out to obtain the LEONAR meta-surface design that allows a maximal absorption coefficient between 400 and 1000 Hz, for a low thickness and a grazing flow up to Mach 0.2. Then the effect of impedance surface is checked by simulation of the radiated pressure field produced by a monopole source in presence of a landing gear door and compared with wind tunnel tests. The measurements showed significant attenuation within the prescribed range (D(OASPL) up to 1.5dBA).