No abstract

An investigation of low-frequency footfall noise in multifamily, wood-frame residential construction has led to the conclusion that, at present, there is no economically practical method of avoiding the perception of "thuds" and "thumps" in rooms beneath the walking surface. The IIC rating of a floor system is meaningless with respect to the perception of these low-frequency components of footfall noise, because the methodology ignores the frequency spectrum below 100 Hz; the peak energy in a footfall spectrum occurs at the fundamental natural frequency of the floor/ceiling system, which with typical light-weight structural framing is usually between 15 and 30 Hz. Although the construction of floated floors, or the addition of carpeting, is effective in attenuating mid-to high-frequency components of footfall noise, the data indicate that the amplitude of floor response at the natural frequency is actually increased. It is believed that this occurs because walking on a more resilient system results in a longer rise-time of the footfall waveform, which permits more low-frequency energy to be coupled into the system. The principal factor controlling the perception of low-frequency footfall noise is the point-stiffness of the structural floor system. In normal light-weight residential construction, the stiffness is usually much less than that required to avoid an audible disturbance. However, such problems are relatively rare in concrete structural floor systems, due to their substantially increased stiffness.

This paper reviews current methods of rating the noise produced by HVAC systems and explains why these ratings fail to be correlated with subjective opinion in many cases. An entirely new method of assigning noise ratings is proposed which is expected to provide a significantly better correlation between objective measurements and subjective response. The proposed new rating method makes use of a revised set of noise criterion curves (RC curves) which appeared for the first time in Chap. 35 of the 1980 Systems Volume of the ASHRAE Handbook. This paper also discusses the technical considerations leading to the development of the RC curves as a replacement for the NC curves which have been used in the past.

The noise criterion (NC) curves were developed both as a method for evaluating existing noise situations and as a means for specifying design goals for noise control. They have also influenced the N ratings used abroad. It has been demonstrated that if a background noise whose spectrum conforms to an NC-curve shape is deliberately generated, it does not sound to a listener as a pleasant or neutral noise, but is both “hissy” and “rumbly.” A recent tendency in consulting practice, therefore, has been to specify noise levels that are lower than NC curves at both low and high frequencies. Also, the original NC curves were derived using Stevens's Mk. I method of calculating loudness level from noise levels measured with the old octave bands, the lowest of which was the “below 75 Hz” band. A reexamination of the noise criterion curves has been undertaken using Stevens's Mk. VI method and the new octave bands. The result is a new set of curves, called “preferred noise criterion (PNC) curves,” having levels in the bands below 125 Hz and above 1000 Hz that are lower than those of the 1957 NC curves by 2–5 dB. The new set also takes into account the engineering realities of achieving the specified noise levels with conventional air-handling equipment. The results of a number of recent noise-control projects in building are presented for comparison.

The impact insulation class (IIC) rating of floor/ceiling constructions fails to identify low-frequency footfall noise problems common in contemporary wood-frame multifamily construction. Because the IIC procedure ignores the impact noise spectrum below 100 Hz, the rating is not influenced by the low-frequency impact spectra produced by a typical live-walker, which usually peak in the region of 15 to 30 Hz and are associated with the ‘‘thuds’’ and ‘‘thumps’’ commonly observed. Data comparing the shape of the typical low-frequency impact noise spectrum produced by a live-walker and that of the standard ISO tapping machine indicate that the tapping machine might potentially be used as the basis for obtaining low-frequency impact noise ratings [W. E. Blazier, Jr. and R. B. DuPree, J. Acoust. Soc. Am. 96, 1521–1532 (1994)]. However, a comparison made between the two sources in the range above 100 Hz shows significant differences in the spectra produced by the tapping machine and a live-walker. This suggests that a more meaningful IIC rating of mid- to high-frequency impact noise might be obtained by cushioning the tips of the hammers in the standard ISO tapping machine, in order to better align the impact spectrum with that of a live-walker.