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May 1984

Volume 75, Issue S1, pp. S1-S93

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back to top Session UU. Architectural Acoustics IV: Recent Advances in HVAC System Noise Control
Invited Papers
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Current developments in heating, ventilating, and air conditioning system noise control (A)

Howard R. Kingsbury

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S90-S90 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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Proper understanding of the sound generating and sound absorbing characteristics of the numerous elements in a typical ducted building air conditioning system has been an elusive goal. In some cases the data base is very thin, in others nonexistent. In addition, the technology of air conditions system design has been advancing faster than our understanding of the acoustical implications of such technology advances. This paper will review our current understanding of the acoustics of system elements and point out some of the areas where additional information is needed.
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Revised noise criteria for design and rating of HVAC systems (A)

Warren E. Blazier, Jr.

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S90-S90 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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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.
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Recent advances in the understanding of acoustic transmission through the walls of air conditioning ductwork (A)

Alan Cummings

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S90-S90 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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Acoustic noise transmission through the walls of air conditioning ductwork has long been a neglected aspect of HVAC acoustics, but two recent ASHRAE‐sponsored research projects have done much to rectify the situation. The results of this work are described here, and include both experimental data and prediction methods concerning interior/exterior noise transmission (“breakout”) through the walls of plain, unlined rectangular, flat‐oval, and circular ducts; the measured and predicted breakout insertion loss (IL) of external acoustic wall lagging on ducts of these three geometries is also discussed. Noise transmission into ducts (termed “breakin”) is of importance too, and a simple theoretical treatment is available, enabling the breakin transmission loss (TL) of the duct walls to be found from the breakout TL. In addition, the calculation of sound pressure levels in rooms, from duct wall radiation, is discussed here. The prediction methods of the wall TL of rectangular and flat‐oval ducts are simple to use and are fairly reliable; circular ducts are, however, difficult to cope with at present. The prediction of the IL of lagging on all duct geometries is straightforward and reasonably accurate. The “state of the art” in duct wall radiation is also discussed here.
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Predicting sound pressure levels in furnished dwellings and offices (A)

Theodore John Schultz

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S90-S91 (1984); (2 pages)

Online Publication Date: 12 Aug 2005

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The Diffuse Field Theory of sound propagation indoors, embodied in the formula, [Lp  =  Lw + 10 log(Q/4πr2 + 4/A)], does not work well for typical furnished rooms in dwellings and offices. An alternative simple empirical relationship has been found that predicts with considerable accuracy the sound pressure level in such rooms, based on the sound power level of the source, the room volume, the frequency, and the distance from the source: Lp  =  Lw − 10 log r(ft) − 5 log V(cu ft) − 3 log f(Hz) + 25 (dB). If this new relationship is accepted, there are serious implications for our current standard test procedures for field measurements in occupied buildings. Since the room absorption A does not appear explicitly in the formula, the customary normalization with terms like 10 log A in transmission loss and impact noise field tests is probably incorrect. In addition, since no uniform reverberant sound field occurs in normally furnished dwelling and office spaces, it is not clear what one ought to measure for the receiving‐room sound pressure level in field tests of transmission loss, noise reduction, and impact noise. [Work supported by ASHRAE.]
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An experimental investigation of active noise control in ducts (A)

C. D. Smith and R. J. Silcox

J. Acoust. Soc. Am. Volume 75, Issue S1, pp. S91-S91 (1984); (1 page)

Online Publication Date: 12 Aug 2005

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This study was directed to gaining an understanding of the physical mechanism of active noise suppression in ducts for both plane waves and higher order modes. Previous work has indicated large suppressions are attainable using active control concepts for plane progressive wave propagation. However, there is little evidence suggesting the mechanism of suppression. Furthermore, it was of interest to determine whether this concept could be applied to sound carried by higher order modes in ducts. This work utilized NASA Langley's Spinning Mode Synthesizer duct facility to provide the number of control points necessary for higher order mode propagation. Pressure measurement probes were installed between the noise source and control point to provide data to define the suppression mechanism. Results indicated that for all modes suppressions of 25 dB or greater were attainable over a broad frequency range. The mechanism of suppression appeared to be a pressure minimum being forced at the control plane which caused a near perfect reflection to occur, This was true for both primary and control sources, as little net acoustic energy was actually developed.
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