Fast accurate non-destructive measurement of absorber impedance and absorption 2019-01-1584
Cabin acoustic comfort is a major contributor to the potential sales success of new aircraft, cars, trucks, and trains. Recent design challenges have included the increased use of composites, and the switch to electrically powered vehicles, each of which change the interior noise spectral content and level. The role of acoustic absorption in cabins is key to the optimisation of cabin acoustic comfort for modern vehicles, with acoustic impedance data needed in order to assess and optimise the impact of each component of a given lay-up.
Measurements of absorbing interior trim are traditionally performed using either sample holder tests in a static impedance tube (impedance and absorption), or through tests in reverberation rooms (absorption only). Both of these procedures present challenges. In-tube absorption and impedance measurements are destructive, requiring highly accurate sample cutting and sealing. Reverberation room absorption measurements are subject to the effects of varying room diffusion, along with the impact of edge diffraction, sample geometry, and location. Finally, while non-destructive methods using hand-held probes also measure absorption, they are not able to measure impedance accurately.
This paper describes fast non-destructive tests using a portable flanged impedance tube, and how they be used to quantify and optimise the absorption of interior trims. Measurements are made on non-locally reacting lay-ups, with the results corrected to equivalent in-tube results using a flanged-to-sample holder correction factor. The corrected flanged tube results are then compared with baseline in-tube measurements. Discussions address data quality and how the non-destructive measurements may be used to optimise lay-ups for increased absorption.
Paul B. Murray, Jon Alexander, Jason Kunio, Flemming Larsen