Impact of Perforated Sheet Geometry on the Insertion Loss of Absorptive Silencers

Abstract

This study evaluates the influence of perforated sheet geometry on the acoustic and aerodynamic performance of absorptive silencers. A modular silencer is developed, enabling the installation of six different perforated metal sheets with varying hole shapes (round, square, elongated), sizes (2mm to 20 mm), and open area ratios (22% to 45 %). Glass wool is used as the sound-absorbing filling. Insertion loss, self-noise, and pressure drop are measured in a large reverberation chamber, within the frequency range from 50 Hz to 10 000 Hz, for airflow velocities of 4 m/s, 6 m/s, and 8 m/s. The results indicate that all configurations provide comparable attenuation at low frequencies. Silencers with small round perforations (diameter 2mm to 6mm) ensure higher insertion loss and lower self-noise in the mid-frequency range of 1000 Hz to 5000 Hz, without any measurable increase in pressure drop compared to variants with larger or elongated holes. For frequencies above 6300 Hz, perforated sheets with larger holes perform better. Pressure loss differences between all configurations do not exceed 1Pa at a given flow velocity. The results confirm that aperture size is the primary parameter affecting silencer acoustic effectiveness, while aperture shape and perforation ratio are secondary. These findings provide practical guidelines for optimal silencer design in ventilation systems, ensuring maximum noise reduction with minimal airflow resistance.