Designing, Building and Validating a Test Rig to Measure Two-port Data of Small Fans

1.6 Test Rig & Instrumentation

There is a great need in the industry for measurement methods to characterize acoustic sources in ducts. One way to obtain a complete description of a source is to measure 2-port data, comprising of the source scattering matrix and the source vector. The resulting model can then be used to predict the source properties, e.g., radiated sound power, in the plane wave range for all installation conditions. Methods to measure the two-port model have been developed over the last few decades and can today be efficiently used for industrial purposes. The present paper offers a review of the main guidelines for designing and building a two-port rig to measure high speed small fans, as well as an example of how to use the data to predict the noise emission of a product.
All rig elements have been designed after a literature review and an analysis of the physical principles governing the behavior of the rig. Guidelines on microphone spacing, loudspeaker mounting, rig terminations and overall rig dimensions are given.
The theory behind the measurement method of the active two-port in a duct is presented. Additionally, a number of different post-process methods are evaluated with respect to the properties of the experimental setup used i.e. the number of available microphones, the magnitude of the reflection coefficient at the rig terminations and the type of test object measured. The standard method that is most widely used nowadays is shown to become singular when the reflection coefficients at the rig terminations are high. A new post-processing method is suggested, and tested against the standard one. It is shown to behave better in highly reflective cases.