Laboratory measurements of sound transmission at low frequencies.

Abstract

The ability of acoustic materials to attenuate sound is usually determined in accordance with ASTM E-90 or ISO 140-3, both based on the diffuse field theory. However, the reality is that the size of typical reverberant chambers does not provide a sufficiently diffuse field at low frequencies. As a result, there is significant variation in sound transmission data of tested materials below 150 Hz. Several attempts were undertaken to improve the accuracy of measuring TL such as increasing field diffuseness by using reflecting surfaces, installing near-field array of loudspeakers, and using sound intensity probes on the receiving sides. None of these proved totally satisfactory. This work presents studies on using various approaches to more accurately determine acoustic mitigation of low-frequency transmitted noise. A metal box consisting of a very rigid frame supporting elastic panels on all sides was acoustically excited by a loudspeaker suspended inside of the box. The panels and box interior were designed to have very low-fundamental frequencies. The box was suspended by its frame in an anechoic room. An intensity probe was used to measure the transmitted sound from the box panels. The acoustic performance of various acoustic materials was compared for frequencies below 500 Hz.