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Acoustic treatment to reduce fan noise levels in a wind tunnel circuit consists of fibrous materials such as fibreglass or rockwool. Open cell foam materials are also used as acoustic treatments. The acoustic treatments are conventionally applied at fan tail cone regions, tunnel walls along fan diffuser section, cross-legs, test section diffuser, and nozzle contraction areas. However, conventional treatments are not possible in cryogenic wind tunnels, since bulk absorber materials with required resistivity, when operating at cryogenic temperatures, are not available. One possible solution is to design reactive silencers tuned to dominant frequencies. One such approach was used as noise control technique so as to satisfy test section noise specifications. The sound power spectrum of the compressor at different speeds were evaluated. The estimated test section sound pressure levels showed noise reduction at two dominant frequencies were required. The acoustic treatment, therefore, resulted in a double layer reactive design tuned to the two dominant frequencies. The design process will be highlighted in the presentation. The final treatment details will also be presented.

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