New research on low-frequency membrane absorbers

Abstract

Low-frequency (LF) room modes are one of the greatest issues for accurate sound recording and reproduction. Effective LF absorbers can mitigate modes in professional and consumer audio rooms. However, fiber- and foam-based absorbers act on sound velocity; membrane absorbers act on sound pressure (greatest at hard surfaces and corners). Velocity at hard surfaces is zero; thus, fiber and foam absorbers work far less effectively than membrane absorbers under 200Hz. Additionally, most independent testing laboratories are only large enough to accurately measure absorption results above 160Hz (per Schroeder frequency) but not below. Only one lab is large enough to be accurate down to 40Hz. A new LF membrane-based absorber product was designed to compliment the frequency range of an existing product. Both were separately tested for LF absorption down to 40Hz at the above-referenced lab. Ten LF absorber tests revealed that the type of absorber, and its location and orientation in a room, are critical to LF absorber effectiveness. Some unexpected results, however, showed clearly that without standardized laboratory absorption testing in a lab capable of accurately testing down to 40Hz, it is not possible to state conclusively that low-frequency absorber products perform as claimed.