Center for the Built Environment

The use and interest in underfloor ventilation systems has been growing due to the systems’ energy savings and inherent flexibility. Because the air distribution openings are in the floor, there has been concern over excessive sound transfer between adjacent spaces through the floor plenum, which could lead to unacceptable levels of speech privacy and other acoustical problems. Charles M. Salter Associates was hired to provide acoustical input for room acoustics and sound isolation between spaces in an office building incorporating an underfloor air distribution system. The office spaces had differing acoustical requirements, for which five wall types were specified based on the levels of noise reduction and speech privacy required between spaces. Standard offices had walls that terminated at the top of the raised floor and the floor plenum was left open, while executive offices and conference rooms had an acoustical septum added in the floor plenum at the location of the common wall. This septum was installed to reduce the transfer of noise in the floor plenum between adjacent spaces. Finally, full-height walls were specified for adjacent spaces requiring the highest level of noise reduction. All partitions were extended to the slab above, with offset return air transfer grilles installed on the corridor or open plan side of the offices, to minimize the potential flanking sound path. After construction was completed, acoustical measurements were conducted to quantify the level of noise reduction and speech privacy between adjacent spaces. The overall level of speech privacy between all the various tested spaces met normal privacy standards. The noise reduction performance of the walls without acoustical septums was generally a few points lower than the noise reduction of the same wall as tested in a laboratory without a raised floor. The noise reduction performance of the walls with the acoustical septum was generally the same as the same wall’s performance in the laboratory. This data seems to indicate that the acoustical septum was effective in reducing noise transfer through the floor plenum, and an acoustical septum may be warranted for spaces requiring high levels of speech privacy.