Methods to Improve the Accuracy of Acoustic Measurements in Small Scale High Speed Jets

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.