Full Characterization of Fans as Aero-Acoustic Sources Using Multi-port Models

B1 - Signal processing for noise source location/characterization

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 that 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 condition systems in aircrafts is investigated in detail in order to develop strategies to abate sound emission and hence augmenting safety and comfort within the air-plane. In air condition systems noise is mainly emitted by a combination of a fan and downstream obstacles in the form of orifices and valves. One approach to handle such numerically and experimentally sophisticated 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. These parameters may be ascertained in two steps and either numerically or experimentally.
In a first step, a number of external sound fields dominating the existing sound filed are applied, in order to determine the system scattering. As the second step, the reflection free source strength can be computed. Once those 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. This paper documents an approach to measure the multi-port field created by an axial fan connected to a straight duct. For the application at hand it was decided to use a model with 8 propagating modes on each side of the fan creating a multi-port of order 16. To separate the modes in up- and downstream propagating waves an array with 16 microphones was used on each side together with 12 external sources (loudspeakers) to excite different incident modes. The microphone and source positions was selected in order to minimize the condition number for the matrix inversions involved in the extraction of the multi-port data. Here the results of the measurements on the tested fan will be presented and discussed in relation to the numerical modelling also done within "Idealvent".