The broadband component of fan noise has grown in relevance with the utilization of increased bypass ratio and advanced fan designs. Thus, while the attenuation of fan tones remains paramount, the ability to simultaneously reduce broadband fan noise levels has become more attractive. Advanced manufacturing techniques have also opened new possibilities for the implementation of broadband liner concepts. This innovation is an integrated method for the design and evaluation of novel broadband acoustic liner concepts for complex engine configurations.

An acoustic duct propagation and radiation code is used to predict optimum impedance spectra over operating conditions of interest. In the absence of fan source information, a statistical representation is used that allows for the generation of 95% confidence intervals for the predicted metric. The cost function for this impedance optimization is based on in-duct, near-field, and/or far-field results.

Given the optimum impedance spectra, acoustic liner modeling tools are used to identify geometric liner parameters (within manufacturing constraints) necessary to produce impedance spectra that most closely match those values. Design selection is based on acceptance criteria that provide the ability to apply increased weighting to specific frequencies and/or operating conditions.

Because of the geometric constraints, the resultant liners provide impedance values that differ from the optimum values. Thus, the next step in the process is to use the design impedance values in the propagation/radiation code to evaluate liner performance. This assessment may again be based on in-duct, near-field, and/or far-field results. As with the design impedance cost function, weighting of specific frequencies/operating conditions could also be introduced in this performance metric. If the assessment does not yield acceptable results, iteration between the liner design and evaluation stages is possible.

This innovation takes advantage of advanced manufacturing techniques that allow acoustic liners to be customized such that the surface impedance of each individual cell is independently controlled. Duct acoustic propagation/radiation and liner modeling tools are integrated to design broadband liners to achieve acceptable sound absorption over a wide frequency range. The ability to use a statistical source model also provides the added benefit of generating confidence intervals for the predicted liner performance.

The current method targets the entire broadband frequency spectrum simultaneously. This is a major advantage over current liner design approaches that focus on narrow-band attenuation spectra, and are generally not broadband in character. For situations in which acoustic source information is unavailable, the method also incorporates a statistical source model providing confidence intervals for the predicted performance.

This work was done by Douglas Nark and Michael Jones of Langley Research Center. NASA is actively seeking licensees to commercialize this technology. Please contact This email address is being protected from spambots. You need JavaScript enabled to view it. to initiate licensing discussions. Follow this link for more information: . LAR-18211-1

NASA Tech Briefs Magazine

This article first appeared in the August, 2016 issue of NASA Tech Briefs Magazine.

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