Methods to improve the accuracy of acoustic measurements in small scale high speed jets," AIAA Paper No

This paper focuses on methods to improve the accuracy of acoustic measurements in small scale high speed jet experiments. In such experiments the acoustic spectra contain significant energy up to 100 kHz which challenges the accuracy of present methods. Different physical effects alter the response of conventional 1/8 ” condenser microphones in this high frequency range. The electronic circuitry produces actuator variations in response with frequency. Refraction effects around the sensitive portion of the microphone produce free-field variations with frequency. Angulations of the microphone with the oncoming acoustic field also produce significant free-field variations. While examining these effects, an extensive series of measurements in the acoustic field of transonic and supersonic jets has demonstrated that for improved accuracy, the published free-field response of the microphones needs to be substantially altered to provide (significantly) improved accuracy of the measurements. This is required when the microphones are aligned for grazing incidence which can also provide some improvement in performance. The current study proposes to replace the published free-field response by an empirical free-field response experimentally determined. In addition the atmospheric attenuation correction is being simultaneously applied to all data. Application to measured data gives encouraging results validating the capability of the method to produce accurate measurements even at the highest response frequencies of the microphones. Nomenclature a = ambient speed of sound AR = exit-to-throat area ratio D = diameter of the nozzle Dj = diameter of the jet plume f = frequency fc = characteristic frequency Uj / Dj Ma = acoustic Mach number Uj / a Md = design Mach number of the nozzle Mj = average Mach number of the fully expanded jet NPR = nozzle pressure ratio OASPL = overall sound pressure level R = physical distance of the microphones from the jet exit Rprop = distance of propagation of the acoustic measurements SPL = sound pressure level St = Strouhal number Uj = mean exhaust velocity θ = polar angle, measured from the jet downstream directio