Comparison between design and installed acoustic characteristics of the NASA Lewis 9‐ × 15‐ft low‐speed wind‐tunnel acoustic treatment

The test section of the NASA Lewis 9‐ × 15‐ft low‐speed wind tunnel was acoustically treated to allow the measurement of sound under simulated free‐field conditions. The treatment was designed for high sound absorption at frequencies above 250 Hz and to withstand the environmental conditions that exist in the test section. To achieve the design requirements, a fibrous bulk absorbing material was packed into removable panel sections. Each section was divided into two equal‐depth layers that were packed with material to different bulk densities. The lower density was next to the face of the treatment. The facing consisted of a perforated plate and screening material layered together. Sample tests for normal incidence acoustic absorption were conducted in a small rectangular duct. Tests with no air flow involving the measurement of the absorptive properties of the installed treatment combined the use of time delay spectrometry with a previously established free field measurement method. These measurements we...