Gregory C. Herring

Calibration of the pressure sensitivity of microphones by a free-field method at frequencies up to 80 kHz

A free-field (FF) substitution method for calibrating the pressure sensitivity of microphones at frequencies up to 80 kHz is demonstrated with both grazing and normal-incidence geometries. The substitution-based method, as opposed to a simultaneous method, avoids problems associated with the nonuniformity of the sound field and, as applied here, uses a 1/4-in. air-condenser pressure microphone as a known reference. Best results were obtained with a centrifugal fan, which is used as a random, broadband sound source. A broadband source minimizes reflection-related interferences that can plague FF measurements. Calibrations were performed on 1/4-in. FF air-condenser, electret, and microelectromechanical systems (MEMS) microphones in an anechoic chamber. The uncertainty of this FF method is estimated by comparing the pressure sensitivity of an air-condenser FF microphone, as derived from the FF measurement, with that of an electrostatic actuator calibration. The root-mean-square difference is found to be +/- 0.3 dB over the range 1-80 kHz, and the combined standard uncertainty of the FF method, including other significant contributions, is +/- 0.41 dB.

Focal-plane imaging of crossed beams in nonlinear optics experiments

An application of focal-plane imaging that can be used as a real time diagnostic of beam crossing in various optical techniques is reported. We discuss two specific versions and demonstrate the capability of maximizing system performance with an example in a combined dual-pump coherent anti-Stokes Raman scattering-interferometric Rayleigh scattering experiment (CARS-IRS). We find that this imaging diagnostic significantly reduces beam alignment time and loss of CARS-IRS signals due to inadvertent misalignments.

Performance Analysis of a Cost-Effective Electret Condenser Microphone Directional Array

Microphone directional array technology continues to be a critical part of the overall instrumentation suite for experimental aeroacoustics. Unfortunately, high sensor cost remains one of the limiting factors in the construction of very high-density arrays (i.e., arrays containing several hundred channels or more) which could be used to implement advanced beamforming algorithms. In an effort to reduce the implementation cost of such arrays, the authors have undertaken a systematic performance analysis of a prototype 35-microphone array populated with commercial electret condenser microphones. An ensemble of microphones coupling commercially available electret cartridges with passive signal conditioning circuitry was fabricated for use with the Langley Large Aperture Directional Array (LADA). A performance analysis consisting of three phases was then performed: (1) characterize the acoustic response of the microphones via laboratory testing and calibration, (2) evaluate the beamforming capability of the electret-based LADA using a series of independently controlled point sources in an anechoic environment, and (3) demonstrate the utility of an electret-based directional array in a real-world application, in this case a cold flow jet operating at high subsonic velocities. The results of the investigation revealed a microphone frequency response suitable for directional array use over a range of 250 Hz - 40 kHz, a successful beamforming evaluation using the electret-populated LADA to measure simple point sources at frequencies up to 20 kHz, and a successful demonstration using the array to measure noise generated by the cold flow jet. This paper presents an overview of the tests conducted along with sample data obtained from those tests.

Observation of in a flame by two-colour laser-induced-grating spectroscopy

By using two-colour laser-induced-grating spectroscopy (TC-LIGS), we observed the third-overtone spectrum of the O - H stretch of water vapour at a point in a stoichiometric - air flame. We also demonstrated the extension of these point measurements to a line image in a flame. Only thermal gratings could be observed. The reasons for this and the difficulties in making a practical combustion diagnostic are discussed.

Raman shifting a tunable ArF laser to wavelengths of 190–240 nm

Tunable radiation at far-ultraviolet wavelengths is produced by Raman shifting a modified (285 mJ at 1–100 Hz repetition rate) ArF excimer laser. Multiple Stokes outputs are observed in H2(20%), CH4(22%), D2(53%), N2(21%), SF6(2.1%), and CF4(0.35%). Numbers in parentheses are the first vibrational Stokes energy conversion efficiencies. We can access 70% of the frequency range 42 000–52 000 cm−1 (190–240 nm), with Stokes pulse energies that vary from 0.2 μJ to 58 mJ inside the Raman cell. One of our better results, using 110 mJ of pump energy and D2, is an energy conversion efficiency of 53% and a tunable Stokes energy that varies over the range 29–58 mJ/pulse for the wavelength range 204–206 nm.

Effect of Off-Body Laser Discharge on Drag Reduction of Hemisphere Cylinder in Supersonic Flow

The interaction of an off-body laser discharge with a hemisphere cylinder in supersonic flow is investigated. The objectives are 1) experimental determination of the drag reduction and energetic efficiency of the laser discharge, and 2) assessment of the capability for accurate simulation of the interaction. The combined computational and experimental study comprises two phases. In the first phase, laser discharge in quiescent air was examined. The temporal behavior of the shock wave formed by the laser discharge was compared between experiment and simulation and good agreement is observed. In the second phase, the interaction of the laser discharge with a hemisphere cylinder was investigated numerically. Details of the pressure drag reduction and the physics of the interaction of the heated region with the bow shock are included. The drag reduction due to this interaction persisted for about five characteristic times where one characteristic time represents the time for the flow to move a distance equal to the hemisphere radius. The energetic efficiency of laser discharge for the case with 50 mJ energy absorbed by the gas is calculated as 3.22.

Application of laser-induced thermal acoustics to a high-lift configuration

Laser-Induced Thermal Acoustics (LITA) has been used to measure the flow field in the slat region of a two-dimensional, high-lift system in the NASA Langley Basic Aerodynamics Research Tunnel (BART). Unlike other point-wise, non-intrusive measurement techniques, LITA does not require the addition of molecular or particulate seed to the flow. This provides an opportunity to obtain additional insight and detailed flow-field information in complex flows where seeding may be insufficient or detection is problematic. Based on the successful use of LITA to measure the flow over a backward-facing step, the goal of this study was to further evaluate the technique by applying it to a more relevant and challenging flow field such as the slat wake on a high-lift system. Streamwise velocities were measured in the slat wake and over the main element at 11.3 degrees angle of attack and a freestream Mach Number of 0.17. The single-component LITA system is described and velocity profiles obtained using LITA are compared to profiles obtained using two-dimensional, Digital Particle Image Velocimetry (DPIV) and a steady, Reynolds-Averaged Navier-Stokes (RANS) flow solver for the same configuration. The normalized data show good agreement where the number of measurement locations had sufficient density to capture the pertinent flow phenomena.

Measurements of Freestream Fluctuations in the NASA Langley 20-Inch Mach 6 Tunnel

An experimental campaign was conducted to measure and to characterize the freestream disturbance levels in the NASA Langley Research Center 20-Inch Mach 6 Wind Tunnel. A pitot rake was instrumented with fast pressure transducers, hot wires, and an atomic layer thermopile to quantify the fluctuation levels of pressure, mass flux, and heat flux, respectively. In conjunction with these probe-based measurements, focused laser differential interferometry was used to optically measure density fluctuations. Measurements were made at five nominal different unit Reynolds numbers ranging from 3.28–26.5×10/m. The rake was positioned at two different streamwise locations and several different roll angles to measure flow uniformity within the test section. In general, noise levels were spatially consistent within the tested region. Pitot pressure fluctuation levels ranged from 0.84% at the highest Reynolds number tested to 1.89% at the lowest Reynolds number tested. Freestream mass-flux fluctuations remained relatively constant between 1.8–2.5% of the freestream. The pressure transducers were also used to determine the dominant disturbance speed and angle of propagation. The disturbances were estimated to travel at approximately 54–81% of the freestream speed at an angle of approximately 21–44° from the freestream direction, but these measurements had a significant amount of uncertainty. A comparison to previous measurements of pressure made in 2012 and of mass flux made in 1994 show almost no change in the RMS fluctuation of these flow quantities.

Free‐field calibration of the pressure sensitivity of air‐condenser, electret, MEMS, and piezoresistive microphones at frequencies up to 80 kHz

A free‐field substitution method for calibrating measurement microphones at frequencies up to 80 kHz is demonstrated with both grazing and normal‐incidence geometries. The method is suitable for newer technology microphones that cannot be calibrated with the electrostatic actuator (EA), currently the industry standard for high frequency calibrations. A substitution‐based method, as opposed to a simultaneous method, avoids problems associated with the nonuniformity of the sound field and uses a 1/4‐inch pressure sensitive microphone as a known reference. A commercially available reference sound source (centrifugal fan) is used as a broadband acoustic source. Although the broadband excitation produces smaller instantaneous signal‐to‐noise ratios than tonal excitation, it minimizes reflection‐related interferences that often plague free‐field measurements. Calibrations were performed on 1/4‐in. air‐condenser, electret, piezoresistive, and MEMS microphones in an anechoic chamber. Five repetitions of a single ...

Free‐field calibration of measurement microphones at frequencies up to 80 kHz

Civil‐aviation noise‐reduction programs, that make use of scaled‐down aircraft models in wind tunnel tests, require knowledge of microphone pressure (i.e., not free‐field) sensitivities beyond 20 kHz—since noise wavelengths also scale down with decreasing model size. Furthermore, not all microphone types (e.g., electrets) are easily calibrated with the electrostatic technique, while enclosed cavity calibrations typically have an upper limit for the useful frequency range. Thus, work was initiated to perform a high‐frequency pressure calibration of Panasonic electret microphones using a substitution free‐field method in a small anechoic chamber. First, a standard variable‐frequency pistonphone was used to obtain the pressure calibration up to 16 kHz. Above 16 kHz, to avoid spatially irregular sound fields due to dephasing of loudspeaker diaphragms, a series of resonant ceramic piezoelectric crystals was used at five specific ultrasonic frequencies as the free‐field calibration sound source. Then, the free‐...