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Single-walled carbon nanotubes (SWNTs) were synthesized via a novel chemical vapor deposition (CVD) technique incorporating iron catalysts. Stable aqueous solutions of the nanotubes using the anionic surfactant sodium dodecylbenzene sulfonate were also obtained, and the properties of as-produced SWNTs were documented through atomic force microscopy and Raman spectroscopy and compared with purified HiPCO SWNTs as a reference.

Crosstalk in electrostatic actuator calibrations is defined as the ratio of the microphone response to the actuator excitation voltage at a given frequency with the actuator polarization voltage turned off to the response, at the excitation frequency, with the polarization voltage turned on. It consequently contributes to the uncertainty of electrostatic actuator calibrations. Two sources of crosstalk are analyzed: the first attributed to the stray capacitance between the actuator electrode and the microphone backplate, and the second to the ground resistance appearing as a common element in the actuator excitation and microphone input loops. Measurements conducted on 1/4, 1/2, and 1 in. air condenser microphones reveal that the crosstalk has no frequency dependence up to the membrane resonance frequency and that the level of crosstalk lies at about -60 dB for all three microphones-conclusions that are consistent with theory. The measurements support the stray capacitance model. The contribution of crosstalk to the measurement standard uncertainty of an electrostatic actuator calibration is therewith 0.01 dB.

A static extended trailing edge attached to a NACA0012 airfoil section is studied for achieving lift enhancement at a small drag penalty. It is indicated that the thin extended trailing edge can enhance the lift, whereas the zero-lift drag is not significantly increased. Experiments and calculations are conducted to compare the aerodynamic characteristics of the extended trailing edge with those of the Gurney flap and the conventional flap. The extended trailing edge, as a simple mechanical device added on a wing without altering the basic configuration, has a good potential to improve the cruise flight efficiency.

Infrasonic windscreens, designed for service at frequencies below 20Hz, were fabricated from a variety of materials having a low acoustic impedance, and tested against four specifications (the first three in a small wind tunnel): (1) wind-generated noise reduction (“insertion loss”) at a free-stream wind speed of 9.3m∕s, (2) transmission of low-frequency sound from a known source (subwoofer), (3) spectrum of sound generated from trailing vortices (aeolian tones), and (4) water absorption (to determine suitability for all-weather service). The operating principle is based on the high penetrating capability of infrasound through solid barriers. Windscreen materials included three woods (pine, cedar, and balsa), closed-cell polyurethane foam, and Space Shuttle tile material. The windscreen inside diameter ranged from 0.0254to0.1016m (1to4in.), and wall thickness from 0.003175to0.01905m (18to34in.). A windscreen made of closed-cell polyurethane foam revealed a wind noise reduction of 10–20dB from 0.7to25Hz, t...

Wide-temperature electronically scanned pressure (ESP) measurement modules have been developed for windtunnelapplicationsforserviceovera widetemperaturerange[50 ±C(122±F)toi175±C(i283±F)].Thesemodules are designed to account for offset voltage drift, operate without thermal protection in cryogenic temperature environments,andmaintainastablecalibrationformonths.ConventionalESPmodulescannotoperateincryogenic environments without thermal protection, for example, a heater box, which reduces available space for module installation, demands labor-intensive and time-consuming installation, and requires heavy electrical wiring that can shunt the wind-tunnel force balance. To account for force-balance shunting, separate wind-tunnel tests must be conducted to collect pressure and force-balance measurements, which devour both time and money. Because of offset voltage drift, conventional modules also require frequent test interruptions for online calibration, which can consume as much as 15% of total test time. Wide-temperature ESP modules eliminate these shortcomings while providing total uncertainty in pressure measurement less than 0.1% of full-scale output. Design, fabrication, and calibration of these modules are described. Test results from data gathered using two 16-port wide-temperature modules in the National Transonic Facility wind tunnel at NASA Langley Research Center are also provided.

Clear air turbulence (CAT) is the leading cause of in-flight injuries and in severe cases can result in fatalities. The purpose of this work is to design and develop an infrasonic array network for early warning of clear air turbulence. The infrasonic system consists of an infrasonic three-microphone array, compact windscreens, and data management system. Past experimental efforts to detect acoustic emissions from CAT have been limited. An array of three infrasonic microphones, operating in the field at NASA Langley Research Center, on several occasions received signals interpreted as infrasonic emissions from CAT. Following comparison with current lidar and other past methods, the principle of operation, the experimental methods, and experimental data are presented for case studies and confirmed by pilot reports. The power spectral density of the received signals was found to fit a power law having an exponent of −6 to −7, which is found to be characteristics of infrasonic emissions from CAT, in contrast to findings of the past.

Measurements of wind noise reduction were conducted on a box-shaped, subsurface windscreen made of closed cell polyurethane foam. The windscreen was installed in the ground with the lid flush with the ground surface. The wind was generated by means of a fan, situated on the ground, and the wind speed was measured at the center of the windscreen lid with an ultrasonic anemometer. The wind speed was controlled by moving the fan to selected distances from the windscreen. The wind noise was measured on a PCB Piezotronics 3" electret microphone. Wind noise spectra were measured with the microphone exposed directly to the wind (atop the windscreen lid) and with the microphone installed inside the windscreen. The difference between the two spectra comprises the wind noise reduction. At wind speeds of 3, 5, and 7 m/s, the wind noise reduction is typically 15 dB over the frequency range 0.1-20 Hz.

In this project, quasi-static and flexible trailing edge devices and fins on airfoils were studied for lift enhancement in cruising flight and drag reduction and oscillation suppression in deep stall. The aerodynamics of a NACA0012 airfoil with a static extended trailing edge was studied systematically using a combination of experimental, computational and theoretical methods. Compared with Gurney flap and conventional flap, this device enhanced lift at a smaller drag penalty, indicating a good potential to improve the cruise flight efficiency. Furthermore, drag reduction and low-frequency oscillation suppression of a NACA0012 airfoil model in deep stall were achieved by using a flexible fin attached at a suitable location on the airfoil. Detailed measurements of the velocity fields and fin kinematics revealed the significant effects of the flexible fin on the development of the flow structures in the separated flow region and the physical mechanism of the natural low-frequency oscillation. The coupled computational fluid dynamics and structural dynamics methods were developed and computations were conducted to study the corresponding problems in the experimental studies. The theoretical models were also used to provide insights into the relevant aspects of the problems. The MEMS sensors and actuators embedded on flexible elements were developed and characterized for active flow control.

This paper explores a new concept of post-stall flow control for airfoils by using a thin flexible fin attached on the upper surface of an airfoil to passively manipulate flow structures in fully separated flows for drag reduction and oscillation/flutter suppression. The relevant similarity parameters are given for aerodynamic and aeroelastic scaling of an airfoil with a flexible fin. Experiments are conducted on a NACA0012 airfoil section in a water tunnel. Force measurements are made by using a balance and velocity fields are measured by using particle image velocimetry. Drag reduction and oscillation suppression particularly for the natural lowfrequency oscillation in deep stall are achieved by using a flexible fin attached at a suitable location on the airfoil. The analysis of the flow fields and the connection between the velocity fields and the fin kinematics provides insight into the physical mechanisms of the low-frequency oscillation and the post-stall flow control.

This paper presents work to date on investigating chemical detection using a recently developed method for designing, powering and interrogating sensors as electrically open circuits having no electrical connections. In lieu of having each sensor formed from a closed circuit with multiple components electrically connected, an electrically conductive geometric pattern powered using oscillating magnetic fields and capable of storing an electric field and a magnetic field without the need of a closed circuit or electrical connections is used. When electrically active, the sensors (conductive patterns) respond with their own magnetic field whose frequency, amplitude and bandwidth can be correlated with the magnitude of the physical quantities being measured. Preliminary results of using two different detection approaches will be presented. In one method, a thin film of a reactant is deposited on the surface of the open-circuit sensor. Exposure to a specific targeted reactant shifts the resonant frequency of the sensor. In the second method, a coating of conductive material is placed on a thin non-conductive plastic sheet that is placed over the surface of the sensor. There is no physical contact between the sensor and the electrically conductive material. When the conductive material is exposed to a targeted reactant, a chemical reaction occurs that renders the material non-conductive. The change in the materialpsilas electrical resistance within the magnetic field of the sensor alters the sensor response bandwidth and amplitude allowing detection of the reaction without having the reactants in physical contact with the sensor.

Design and testing of a multi-layer thermal insulation system that also provides debris and micrometeorite damage detection is presented. One layer of the insulation is designed as an array of passive open-circuit electrically conductive spiral trace sensors. The sensors are a new class of sensors that are electrically open-circuits that have no electrical connections thereby eliminating one cause of failure to circuits. The sensors are powered using external oscillating magnetic fields. Once electrically active, they produce their own harmonic magnetic fields. The responding field frequency changes if any sensor is damaged. When the sensors are used together in close proximity, the inductive coupling between sensors provides a means of telemetry. The spiral trace design using reflective electrically conductive material provides sufficient area coverage for the sensor array to serves as a layer of thermal insulation. The other insulation layers are designed to allow the sensor s magnetic field to permeate the insulation layers while having total reflective surface area to reduce thermal energy transfer. Results of characterizing individual sensors and the sensor array s response to punctures are presented. Results of hypervelocity impact testing using projectiles of 1-3.6 millimeter diameter having speeds ranging from 6.7-7.1 kilometers per second are also presented.

The research reported herein includes the fabrication of a tunable optical fiber Bragg grating (FBG) fiber ring laser (FRL)1 from commercially available components as a high-speed alternative tunable laser source for NASA Langley s optical frequency domain reflectometer (OFDR) interrogator, which reads low reflectivity FBG sensors. A Macro-Fiber Composite (MFC) actuator invented at NASA Langley Research Center (LaRC) was selected to tune the laser. MFC actuators use a piezoelectric sheet cut into uniaxially aligned rectangular piezo-fibers surrounded by a polymer matrix and incorporate interdigitated electrodes to deliver electric fields along the length of the piezo-fibers. This configuration enables MFC actuators to produce displacements larger than the original uncut piezoelectric sheet. The FBG filter was sandwiched between two MFC actuators, and when strained, produced approximately 3.62 nm of wavelength shift in the FRL when biasing the MFC actuators from 500 V to 2000 V. This tunability range is comparable to that of other tunable lasers and is adequate for interrogating FBG sensors using OFDR technology. Three different FRL configurations were studied. Configuration A examined the importance of erbium-doped fiber length and output coupling. Configuration B demonstrated the importance of the FBG filter. Configuration C added an output coupler to increase the output power and to isolate the filter. Only configuration C was tuned because it offered the best optical power output of the three configurations. Use of Plastic Optical Fiber (POF) FBG s holds promise for enhanced tunability in future research.

The demand for high safety and reliability standards for aerospace vehicles has resulted in time-consuming periodic on-ground inspections. These inspections usually call for the disassembling and reassembling of the vehicle, which can lead to damage or degradation of structures or auxiliary systems. In order to increase aerospace vehicle safety and reliability while reducing the cost of inspection, an on-board real-time structural health monitoring sensing system is required. There are a number of systems that can be used to monitor the structures of aerospace vehicles. Fiber optic sensors have been at the forefront of the health monitoring sensing system research. Most of the research has been focused on the development of Bragg grating-based fiber optic sensors. Along with the development of fiber Bragg grating sensors has been the development of a grating measurement technique based on the principle of optical frequency domain reflectometry (OFDR), which enables the interrogation of hundreds of low reflectivity Bragg gratings. One drawback of these measurement systems is the 1 - 3 Hz measurement speed, which is limited by commercially available tunable lasers. The development of high-speed fiber stretching mechanisms to provide high rate tunable Erbium-doped optical fiber lasers can alleviate this drawback. One successful approach used a thin-layer composite unimorph ferroelectric driver and sensor (THUNDER) piezoelectric actuator, and obtained 5.3-nm wavelength shift. To eliminate the mechanical complexity of the THUNDER actuator, the research reported herein uses the NASA Langley Research Center (LaRC) Macro-Fiber Composite (MFC) actuator to tune Bragg grating based optical fibers.

Development of highly capable small UAVs present unique challenges for technology protagonists. Size constraints, the desire for ultra low cost and/or disposable platforms, lack of capable design and analysis tools, and unique mission requirements all add to the level of difficulty in creating state-of-the-art small UAVs. This paper presents the results of several small UAV developments, the difficulties encountered, and proposes a list of technology shortfalls that need to be addressed.

This paper describes a measurement acquisition method for aerospace systems that eliminates the need for sensors to have physical connection to a power source (i.e., no lead wires) or to data acquisition equipment. Furthermore, the method does not require the sensors to be in proximity to any form of acquisition hardware. Multiple sensors can be interrogated using this method. The sensors consist of a capacitor, C(p), whose capacitance changes with changes to a physical property, p, electrically connected to an inductor, L. The method uses an antenna to broadcast electromagnetic energy that electrically excites one or more inductive-capacitive sensors via Faraday induction. This method facilitates measurements that were not previously possible because there was no practical means of providing power and data acquisition electrical connections to a sensor. Unlike traditional sensors, which measure only a single physical property, the manner in which the sensing element is interrogated simultaneously allows measurement of at least two unrelated physical properties (e.g., displacement rate and fluid level) by using each constituent of the L-C element. The key to using the method for aerospace applications is to increase the distance between the L-C elements and interrogating antenna; develop all key components to be non-obtrusive and to develop sensing elements that can easily be implemented. Techniques that have resulted in increased distance between antenna and sensor will be presented. Fluid-level measurements and pressure measurements using the acquisition method are demonstrated in the paper.

The control of u wanted structural vibration is implicit in most of NASA's programs. Currently several approaches to control vibrations in large, lightweight, deployable structures and twin tail aircraft at high angles of attack are being evaluated. The Air Force has been examining a vertical tail buffet load alleviation system that can be integrated onboard an F/A-18 and flown. Previous wind tunnel and full-scale ground tests using distributed actuators have shown that the concept works; however, there is insufficient rom available onboard an F/A-18 to store current state-of- the-art system components such as amplifiers, DC-to-DC converter and a computer for performing vibration suppression. Sensor processing, power electronics, DC-to-DC converters, and control electronics that may be collocated with distributed actuators, are particularly desirable. Such electronic system would obviate the need for complex, centralized, control processing and power distribution components that will eliminate the weight associated with lengthy wiring and cabling networks. Several small and lightweight power amplifiers ranging from 300V pp to 650V pp have been designed using off the shelf components for different applications. In this paper, the design and testing of these amplifiers will be presented under various electrical loads.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

This paper describes the development of a Micro/Mini Air Reconnaissance Vehicle for advanced sensors and electronics at NASA Langley Research Center over the last year. This vehicle is expected to have a total weight of less than four pounds, a design velocity of 40 mph, an endurance of 15-20 minutes, and a maximum range of 5km. The vehicle has wings that are simple to detach yet retain the correct alignment. The upper fuselage surface has a quick release hatch used to access the interior and also to mount the varying propulsion systems. The sensor suite developed for this vehicle consists of a Pitot-static measurement system for determining air speed, an absolute pressure measurement for determining altitude, magnetic direction measurement, and three orthogonal gyros to determine body angular rates. Swarming GPS-guidance and in-flight maneuvering is discussed, as well as design and installation of some other advance sensors like MEMS microphones, infrared cameras, GPS, humidity sensors, and an ultrasonic sonar sensor. Also low cost, small size, high performance control and navigation system for the Micro Air Vehicle is discussed. At the end, laboratory characterization of different sensors, motors, propellers, and batteries will be discussed.

Glenn Research Center and Kulite Semiconductor Products have demonstrated, through their preliminary work, applicability of SiC for high-temperature pressure sensing. These experiments conducted on a non-hermetic package have shown survivability and stability up to 500 °C in a turbine engine environment. These pressure sensors have been fabricated for an upper limit pressure of 1000 psia. For space applications such as Mars Missions, the pressure sensor requirements are stringent in accuracy at a lower range of pressure (25 psia), temperature requirements up to 1000 °C, and tolerance to radiation. To achieve this goal, new SiC sensors are being developed which will operate at low pressures, 25 psia maximum. This paper will describe the developmental efforts of the low pressure SiC pressure sensor and its preliminary performance characteristics.

An infrasonic field installation was set up at Newport News-Williamsburg International Airport in early 2013. The system is made up of three PCB 377M06 microphones installed into non-porous subsurface windscreens [POMA 1pNS9, 18, 040005 (2013)], which limit the bandwidth to 100 Hz. The microphones are placed 250 ft (76.2 m) orthogonal to the runway and 200 ft (60.96 m) apart. The data acquisition system is the B&K Pulse, from which time histories, spectra, and coherence between microphone channels are derived. The system is placed inside an instrumentation vehicle just behind the center microphone. Perforated drainage hoses are installed from the subsurface windscreens to adjacent drainage ditches and weight is added to the windscreens for additional stability. The drainage system has proved successful even on occasions of heavy downpour, revealing a truly all-weather system. A pistonphone calibration at 14 Hz in the field reveals that the three channels are matched to within 2 dB. This capability permits...

Infrasonic emissions from aircraft wake vortices were investigated at the Newport News-Williamsburg International Airport early in the year 2013. Signals received by the microphones situated along an airport runway were processed in 10-s intervals. As an aircraft accelerates toward takeoff, it produces a large pressure burst as it passes each microphone. Following the burst, there appear low-frequency signals of high coherence among microphone pairs. These are interpreted as emissions from the aircraft wake vortices, as suggested by theory. In successive 10-s intervals, the coherence gradually diminishes to background levels, signifying the disappearance of the vortices. On landing the intervals of high coherence precede the bursts at aircraft touchdown, and then diminish. The pressure burst serves as a time stamp for the ensuing vortex emissions and thereby permits the tracking of successive takeoff or landing events on the same runway or on adjacent runways. The emission spectrum is essentially broadband, lacking spectral features (e.g., tones). Data were taken for takeoff of Airbus 319, DC-9, MD-88, CRJ, Lear Jet, Corporate Jet, and Dash-8 aircraft, and for landing of the Airbus 319. The pattern of pressure burst, high-coherence intervals, and diminishing-coherence intervals was observed for all takeoff and landing events without exception.

An array of three infrasonic microphones (0.2–20 Hz), operating continuously in the field at NASA Langley Research Center, on several occasions received a class of signals interpreted as infrasonic emissions from clear air turbulence. The presence and location of the turbulence were confirmed by pilot reports (PIREPS), and the direction of emitted signals toward the array was determined by slowness mapping. The coherence of the signals among the three microphone pairs in the array was close to unity. The amplitude spectrum of the received signals was found to fit a power law having an exponent of −7/2, which disagrees with the exponent of −7/4 of Meecham and Ford [J. Acoust. Soc. Am. 30, 318–322 (1958)], based on turbulence self‐noise and with the exponent of −1 of Meecham [J. Acoust. Soc. Am. 33, 149–155 (1971)], based on mean shear fluctuations. Thus the above models do not account for the observed spectrum. Two case histories are described in detail.

A field test on a three‐microphone array at NASA Langley Research Center was conducted using a mobile controlled infrasonic source. A Helmholtz resonator, used to provide a simulated point source for infrasonic propagation studies, had an output SPL of 99 dB (at 1 m) at its resonance frequency of 9.45 Hz. The three‐microphone array was arranged as an equilateral triangle with microphone spacing of 30.48 m (100 ft) and at a distance of more than 85.3 m (280 ft) from the source. The signal level was 40 dB above the background noise in a 1‐Hz band. Measurements of the acoustical response for each of the array microphones were recorded, and the received signal was measured at the nearest microphone to be 60 dB (6 dB per doubling of distance).

Slowness mapping is a method to estimate the angle of arrival of plane waves propagating across a sensor array. A review of time‐delay estimation and its application to slowness vector estimation, the forward model, the inverse model, azimuth estimation, and elevation estimation will be presented. A method for performance grading with “out‐of‐bounds” conditions is described, and in the special case of subsurface acoustic sensors, a method for discriminating against seismic signals. The method has been applied to locate the direction of Space Shuttle and other rocket launches, infrasonic emissions from clear air turbulence, and incidental sources found in the environment.

A windscreen has been developed that features two advantages favorable for the measurement of outdoor infrasound. First, the sub-surface location, with the top of the windscreen flush with the ground surface, minimizes the mean velocity of the impinging wind. Secondly, the windscreen material (closed cell polyurethane foam) has a sufficiently low acoustic impedance (222 times that of air) and wall thickness (0.0127 m) to provide a transmission coefficient of nearly unity over the infrasonic frequency range (0-20 Hz). The windscreen, a tightly-sealed box having internal dimensions of 0.3048 x 0.3048 x 0.3556 m, contains a microphone, preamplifier, and a cable feed thru to an external power supply. Provisions are made for rain drainage and seismic isolation. A three-element array, configured as an equilateral triangle with 30.48 m spacing and operating continuously in the field, periodically receives highly coherent signals attributed to emissions from atmospheric turbulence. The time delays between infrasonic signals received at the microphones permit determination of the bearing and elevation of the sources, which correlate well with locations of pilot reports (PIREPS) within a 320 km radius about the array. The test results are interpreted to yield spectral information on infrasonic emissions from clear air turbulence.

Calibration of the pressure sensitivity of measurement microphones in a coupler requires by definition that the incident pressure be uniform over the surface of the microphone diaphragm. This requirement is readily satisfied at frequencies up to the cut in frequencies of the radial modes and, if the microphone is not azimuthally symmetric, the spin modes. Above the lowest natural frequency of either of these modes, the uniformity of the pressure distribution will be destroyed. This effect is alluded to in current standards, ANSI S1.15‐2005/Part 2 and IEC 61094‐5:2001, but the suggested countermeasure “to use more than one coupler with different dimensions” or to compare “calibrations performed in a variety of other jigs and couplers” does not resolve the problem. Specification of the maximum excitation frequency to preserve pressure uniformity in a coupler is presented for several laboratory standard and working standard microphones.

Membrane‐based microphones are subject to seismic excitation, which interferes with signals from propagating infrasound. An experiment was designed to decouple seismic and infrasonic signals. The microphone was rigidly attached to a base plate on the ground with a fixture that permitted the orientation of the microphone axis to be varied. An accelerometer was mounted to respond to seismic stimuli normal to the base plate. Subsequent field testing included the dropping of various weights from a height of 2–3 m vertical drop at distances of 2 to 15 m from the system. In addition, a low frequency Helmholtz resonator (17 Hz) was used to provide an acoustic test signal. The intent of the experiment was to identify the best microphone orientation to reduce the effects of local seismic disturbances in the presence of low‐frequency acoustic monitoring. Additional experiments were performed in the laboratory using a vibration exciter. The results of decoupling of seismic and infrasonic sources, based upon laboratory and field experiments, will be presented.

A condenser microphone has been fabricated for measuring low‐frequency sound pressure. The goal of this design is to keep the background noise as low as possible. The microphone features a high membrane compliance with a large backchamber volume, a prepolarized backplane, and a high impedance preamplifier located inside the backchamber. Methods for characterizing the performance of the microphone will be presented including background noise levels, which will be compared to commercially available microphones.

The background noise of a condenser microphone originates from external sources (electromagnetic interference, vibration) and internal sources (self‐noise). The leading internal sources include the membrane damping resistance, the polarization voltage resistance (air condenser microphone only), the input resistance to the preamplifier, the capillary vent resistance, and a source of 1/f noise. These sources are modeled in a noise‐equivalent circuit, from which an expression for the noise power spectral density is derived. This expression is compared to measurements on a low‐frequency electret condenser microphone taken in an acoustic isolation vessel. The capillary vent and 1/f noises vary inversely with frequency and, thus, impact the background noise levels in the infrasonic region.

A compact nonporous windscreen described previously [J. Acoust. Soc. Am. 114, No. 4, Pt. 2, 2323 (2003)] was tested in the field against a soaker hose array to compare performance at infrasonic frequencies. The cylindrically shaped compact windscreen, made of closed‐cell polyurethane foam, had dimensions 0.0762 m i.d. ×0.2286 m height ×0.0127 m wall (3×9×0.5 in.). The low acoustic impedance of the foam permits the propagation of infrasound through the walls of the windscreen with a transmission coefficient near unity. The soaker hoses were 15.24‐m (50 ft.) long and coupled to a Chaparral model 5 low‐frequency microphone. The hose plenum was removed and replaced with the compact windscreen for testing. A sonic boom simulator, located at a distance of 400 m (1/4 mile) from the microphone, generated tones at 3, 4, 5, and 6 Hz. Analysis of the signals received by the interior microphone revealed that the tones transmitted through the windscreen, as well as the reduction of background noise due to naturally oc...

An improved and compact windscreen was conceived for a microphone of a type used outdoors to detect atmospheric infrasound from a variety of natural and artificial sources. This cylindrically shaped compact windscreen, made of closed‐cell polyurethane foam, had dimensions of 0.0762 m i.d. ×0.2286 m height ×0.0127 m wall (3×9×0.5 in.). The low acoustic impedance of the foam showed a transmission coefficient near unity. The same closed‐cell polyurethane foam was used to fabricate a spherical windscreen of 0.254 m (10 in.) diameter with 0.0127‐m (0.5 in.) wall thickness. In this paper the effectiveness of cylindrical and spherical windscreens is evaluated using a low‐speed wind tunnel facility that enables controlled and repeatable experiments. Wind noise levels are characterized for both windscreens as a function of frequency and mean wind velocity.

A new approach to aeroacoustic microphone array design and implementation is described and demonstrated. Using commercially available, low‐cost MEMS microphones exhibiting a suitable low‐frequency response, a series of 128‐channel arrays were constructed on flexible Kapton circuit boards which were bonded to rigid aluminum backplates. Cover panels with precision cutouts for the microphones were bonded on top of the Kapton circuit boards to create a smooth surface providing flush‐mounting for all microphones. Connections for the microphones were created by extending strips of Kapton containing power and signal busses to the rear of the backplates. All channels were powered from a common 3 V power source, and all signals were conditioned using custom‐manufactured filtering and line‐driving hardware. The conditioned signals were digitized and processed in near real‐time using both commercially available and customized data acquisition and analysis hardware. This new type of array construction addresses two challenges which currently limit the widespread use of large channel‐count arrays for aeroacoustic applications, namely by providing a lower cost‐per‐channel solution and by providing a simpler method for mounting microphones in wind tunnels. The MEMS arrays have been extensively tested in anechoic and hard‐walled facilities, and their performance has been found comparable to that of condenser microphone arrays.

Surface‐mount microphones based on MEMS (micro‐electromechanical system) technologies have recently become viable as component‐level engineering solutions for acoustic measurements. In addition, advances in microelectronics, flexible circuitry, and array processing have motivated the design of a high‐speed, low‐cost acoustic array system for aeroacoustic measurements. A variety of microphones are available in the market today. Each type of microphone has its benefits and drawbacks. For example, standard condenser microphones have excellent sensitivity, stability, and high frequency response, but tend to be unwieldy, expensive, and require relatively high operational voltages. Electret microphones are small and fairly inexpensive but their performance deteriorates if exposed to moderately elevated temperatures. MEMS microphones combine the best features of the electret and condenser microphones while occupying a volume of less than 20 cubic millimeters. This paper details the custom‐made 128‐channel MEMS‐based acoustic array for wind tunnel applications as well as the electrical, mechanical, and acoustic properties of the MEMS microphones utilized here.