An uncertainty estimation for a higher order multi-port characterization in ducts with flow

Regular paper

Stefan Sack


Wednesday 3 june, 2015, 09:00 - 09:20

0.3 Copenhagen (49)

The generation and scattering behaviour of fluid machines in connected duct or pipe systems is of great interest to minimize disturbing and harmful sound emission, for instance of air condition systems. Such systems may be described as networks of passive elements, which only scatter existing sound fields and active elements, which emit noise themselves. Within the framework of the European project “IdealVent“ the acoustic behaviour of air conditioner systems in aircraft are investigated in detail in order to develop strategies to abate sound emission and hence augmenting safety and comfort within the air-plane and at the aircraft ramp. In air conditioner systems, noise is mainly emitted by a fan combined with downstream obstacles in the form of orifices and valves. One approach to handle such systems is to apply a linear multi-port model that includes direction-depending transmission and reflection coefficients for the propagating wave modes and the sound generation amplitude per mode. Those parameters may be ascertained in two steps and either numerically or experimentally. Once this characteristic data are determined for all elements of interest, the sound scattering and emission behaviour of every considerable combination of those elements can be calculated easily. In a first step, a number of external sound fields dominating the existing sound filed are applied, in order to terminate the system scattering. As the second step, the reflection-free source strength can be computed. Both operations consist of matrix inversions and wave-number assumptions, which amplify uncertainties induced by the measurement procedure. The paper in hand shows a method to estimate the uncertainties of a full characterization of a multi- port of the order 6 by applying a condition number approach and a Monte-Carlo simulation.

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