Abstract This paper discusses issues regarding in-situ methods of obtaining sound power at a point in a heating, ventilating and air-conditioning (HVAC) duct system. Such a method is being used as part of a larger investigation on acoustical prediction methods to allow for comparison of measured data to results from attenuation predictions of inpidual duct elements, such as elbows or dampers. Sound propagation in HVAC duct work is complex. Any measurements of sound energy in the duct must address the characteristics of sound propagation in ducts, end reflections, and air turbulence. Investigations are being conducted to understand the extents to which these acoustical issues affect measurement results. The study provides a better understanding of sound propagation in HVAC ducts for future investigation of acoustical prediction methods.53443

Introduction

The study outlined in this paper is part of a larger investigation of acoustical prediction software for heating, ventilating, and air-conditioning (HVAC) systems. The authors have previously investigated prediction software for HVAC systems along entire acoustical paths from the fan source to the receiver in a room (Ryherd & Wang, 2005). Such verification of the prediction software required controlled environments with well-documented information about the actual sound source power, duct elements, and receiving room characteristics. In an attempt to verify the algorithms used against actual data measured in field installations, though, it is desired to limit other potential sources of error.

This paper presents the issues surrounding an in-situ method of investigating the acoustical influence of each element in the duct path separately. To verify each duct element contribution, there must be dependable means of obtaining a sound power level at the inlet and outlet of the specified element. These sound power levels could then be compared to the expected attenuation of that element, as currently

projected in HVAC noise prediction software. However, in-situ measurements of this kind are not common, especially in installed HVAC systems.

Background

Sound prediction software for noise in HVAC systems utilize a collection of algorithms that calculate the attenuation contributions of each element of a system. In general, the algorithm is an empirical black box that takes an incoming sound power level and characteristic information of the element to produce an output sound power level. For example, if a fan is used as the sound source at the beginning of a length of duct, the sound power levels at each octave band are put into the algorithm for the specific type of duct (e.g. rectangular, circular, etc.) along with the duct dimensions, duct length and amount of absorption. The algorithm calculates the amount of sound attenuation and projects the output sound power level. This process continues down the path of the HVAC system for each element of the path (silencers, elbows, branches, etc.) until the sound reaches the receiver room. At the receiving room, correction factors are applied to the estimated sound power level to calculate the equivalent sound pressure level perceived by a receiver in the space.

Although the example is simply stated, the process of predicting the noise in HVAC systems is complex with many potential sources of error. There is inherent error when algorithms based on empirical data made in controlled environments are used for in-situ applications; and unfortunately, users of software programs often are not able to access even what algorithm is being used to know if its application is appropriate. Additionally, the initial source data from a fan is not always reliable and may vary based on operating conditions and installation. Ultimately, any inaccuracies at one point in the analysis of the system can compound errors further down the system path. To improve acoustical predictions along HVAC ductwork, one should first be sure to understand sound propagation in ducts.