Comparison of the Two-Microphone Method and a Modal Decomposition Method for Acoustic Impedance Testing

Abstract

Accurate duct acoustic propagation models are required to predict and mitigate radiated aircraft engine noise. These models ultimately rely on acoustic impedance measurements to characterize candidate engine nacelle liner materials. This paper compares two different methods for normal incidence acoustic impedance testing: the two-microphone method (TMM) and a modal decomposition method (MDM). The two methods are compared using two different test specimens: a ceramic tubular honeycomb material with 65% porosity (CT65) and a single degree-of-freedom liner (SDOF) specimen. The MDM measurements are made from 0.3 to 13 kHz, and the TMM measurements are made from 0.3 to 6.7 kHz, using a 25.4-mm by 25.4-mm specimen. Additional TMM measurements of the CT65 specimen from 0.3 to 20 kHz are made using an 8.5-mm by 8.5-mm specimen in a smaller plane wave tube. The results show good agreement between the TMM and the MDM for the large specimen, but are inconsistent with the smaller specimen. The CT65 results showed a continuing series of resonances for the ceramic tubular material with a monotonic decrease in resonant peaks of the acoustic resistance with increasing frequency and significant mode scattering at the higher frequencies.