Assessment of Shipboard Sound Insulation in the Low Frequency Range Through a Laboratory Experimental Methodology Based on a Modal Approach

In recent years a growing need in naval acoustics for the characterization of low frequency materials below 100 Hz and particularly from 50 Hz has been observed. The problem related to airborne and impact acoustic insulation at low frequencies is becoming more and more relevant due to the considerable impact of sound sources at these frequencies, such as audio-video systems in theaters and discos as well as TV systems inside cabins. Current measurement procedures for identifying the sound insulation properties of bulkheads and floors are not sufficient to ensure repeatable and reproducible measurements, as accuracy and precision values are too low to be accepted when applied at frequencies below 100 Hz. In fact, at low frequencies and in ordinary laboratory or room volumes between 40 m3 and 80 m3, the acoustic field is no longer diffuse due to the dominant presence of standing waves or modes, which result in large fluctuations of the sound pressure level in space and frequency. In the present study, an experimental methodology based on the modal approach is proposed and applied for the study of low-frequency sound insulation phenomena below 100 Hz in the naval field. An extensive experimental activity has been conducted at CETENA laboratories in Riva Trigoso, which has allowed to validate the measurement procedure and compare it with classical theories of sound transmission. The development of this approach seems to be able to provide significant support in the design phase for the optimization of shipboard sound insulation performances.