Determination of the characteristics of a linear ducted sound source

TL;DR

This paper develops a two-port model to include higher-order acoustic modes in ducted sound source characterization, offering a shorter setup than ISO standards and analyzing the impact of higher modes on measurement accuracy.

Contribution

It introduces a novel two-port model for higher-order modes in ducted sound sources, reducing setup size and improving accuracy over ISO methods.

Findings

Two-port model effectively captures higher-order modes.

Shorter experimental setup compared to ISO rig.

Higher-order modes significantly affect sound power measurements.

Abstract

Ducted flow devices for a range of purposes, such as air-moving fans, are routinely characterised experimentally to understand their acoustic performance as part of the continuing trend for quiet, high efficiency design. The International Organization for Standardization (ISO) method 5136 is widely used in industry and academia to determine the sound radiated into a duct. This involves placing the device at the centre of a long cylindrical duct with anechoic terminations at each end to eliminate reflections. A single off-axis microphone is used on the inlet and outlet sides that can theoretically capture the plane-wave mode amplitudes but this does not provide enough information to fully account for higher-order modes. In this study, the 'two-port' source model is formulated to include higher-order modes and applied for the first three modes as a proof of concept. This requires six independent surface pressure measurements on each side or 'port'. The resulting experimental set-up is much shorter than the ISO rig and does not require anechoic terminations. The relative importance of the higher-order modes has been considered and the inaccuracies when using the ISO method to find source sound power has been analysed.