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Although many standards exist for the measuring of sound power in ducts, the one most pertinent to this research is ISO 5136^Determination of sound power radiated into a duct by fans and other air-m^^ing devices^In-duct. This standard requires that the duct be oriented in a straight line with minimal transitions to reduce turbulence, and measurements are taken at a specified distance from the source to assure that the air has laminar flow. Turbulence effects are further reduced by using a foam ball, nose cone, or turbulence screen on the end of the microphone. End reflection factors are minimized by having only one inlet and outlet of the duct with an anechoic termination. The specially designed termination limits the ability for a sound wave to reflect back into the duct by eliminating the plane of reflection and flaring the edges of the remaining duct. Finally, the acoustical characteristics are addressed by varying the location of measurements in the duct, limiting the frequency range measured, and using a modal correction factor.
Unfortunately, in-situ measurements of HVAC duct systems do not provide the necessary controlled environment; however, consideration of all three issues can be addressed. The effects of the modal characteristics in the duct can be minimized by varying the location of measurements and averaging data for an equivalent value. The measurement locations will vary within the cross-sectional area of the duct and along the length of a duct. Also, measurements must be taken at a considerable distance from any major disturbance both upstream and downstream. The standard suggests approximately 6 feet or four duct widths to ensure undisturbed flow conditions, and this distance requirement should also be observed for in-situ measurements. The issue of end reflections can also be limited by observing the recommended distances. The influence of the reflected sound would be diminished by the distance to the measurement location.
Finally, the turbulence effects can be further diminished by utilizing one of the microphone protection devices specified by the standard. The foam ball is designed for measurements in air velocities up to 3000 feet/minute (fpm), and the nose cone is designed for up to 4000 fpm. Both of these devices are considered to maintain the omni-directional characteristics of the microphone. The third protective device is referred to as a sampling tube which is a long cylinder that encases the microphone with a slit down the side and a nose cone on the end. The sampling tube is designed for flow velocities of approximately 7800 fpm and is strongly suggested for measurements at or below the 125 Hz octave band.
Conclusion
Pursuing a reliable in-situ method of obtaining sound power at a point in an HVAC duct will provide a way of verifying algorithms used to account for the acoustic behavior of inpidual duct elements. The proposed method is part of a
larger study to test such algorithms used to predict HVAC acoustics in many software programs. The nature of such a prediction is very complex with many sources of error. By developing a method of measuring sound pressure to obtain sound power, verification of the algorithms can be obtained for any element of the ducted system.
Three main issues must be addressed to obtain reliable sound energy data from a duct. Those three issuesharacteristics of acoustical energy, end reflections, and turbulence, affect sound propagation in ducts and any measurements made in the duct. To address these concerns, measurements must be made at a great enough distance from a disturbance upstream or downstream from the duct; they must be made in various locations; and they must be made with appropriate protection from turbulence. Further investigation is required to determine the best approach to accomplish the goal of obtaining sound power at a point in the duct while addressing each issue.
References
Bies, D. A. and Hansen, C. H. (2003). Engineering Noise Controlheory and Practice, Spon Press, New York.