In small- and medium-sized rooms, room modes significantly affect low-frequency characteristics, often causing noticeable dips in loudspeaker responses. To address this, Helmholtz resonant absorbers were applied in the acoustic design of a mastering room. A mastering room is used for music production, where the sounds played back from loudspeakers are carefully listened to. First, eigenmode analysis was conducted to find the target mode which should be removed for improving the dip. Then, theHelmholtz absorber was designed whose resonant frequency matched the target eigenfrequency. Several absorbers were installed in sidewalls, which act as reflection boundaries of the target standing wave. Finally, measurements demonstrated improvement of the dips. A Helmholtz absorber provides a high absorption coefficient at the resonance frequency and also has a low specific acoustic impedance, less than ρc, at the port. Since the Helmholtz absorbers increased the room-average absorption coefficient only by 6%, we concluded that the lower impedance at the ports contributed to boost the dip. In short, the Helmholtz absorbers changed the boundary impedances, and node and antinode positions of the target mode were interchanged. Switching the antinode to a node at the listening position, the target mode was removed and the dip was improved.
Matsuo et al. (Wed,) studied this question.