Designing reverberation chambers for improved low frequency response

Abstract

Most reverberation rooms are able to provide good diffusion and spatial uniformity in the reverberant field of the source at mid- and high-frequencies. Meeting the broadband spatial uniformity requirements for ANSI S12.51 (ISO 3741) is relatively easy to achieve in the frequency bands 100 Hz to 10,000 Hz provided that the recommended room volumes are used. However, meeting the spatial uniformity requirements becomes much more difficult when the desired frequency range includes frequency bands below 100 Hz and the room must meet the pure tone qualification criteria (as is required in AHRI 220). There are two techniques that can be used to improve the low frequency room response of a reverberation room. One is to select room dimensions that will provide near uniform spacing of the room modes in the low frequency bands. The second technique is to install low frequency absorbers to the surfaces of the reverberation room to lower the “Q” of the individual modes. This paper will re-introduce a new quantity (first introduced at Internoise 2012 in New York City) called the modal distribution factor, D2, and illustrate how it can be used to optimize the dimensions of any reverberation room within the space constraints of the proposed location. In addition, the paper will illustrate the design and acoustical performance of two very successful reverberation rooms that were conceived and constructed using these techniques.