Wide-band Microphone Research Articles

Experimental Analysis and Design Guidelines for Microphone Virtualization in Automotive Scenarios

In this paper, a performance analysis on the estimation of the so-called observation filter for the Virtual Microphone Technique (VMT) in a realistic automotive environment is presented. A performance comparison between adaptive and fixed observation filter estimation methods, namely Least Mean Square (LMS) and Minimum Mean Square Error (MMSE), respectively, was carried on. Two different experimental setups were implemented on a popular B-segment car. Eight microphones were placed at the monitoring and virtual positions in order to sense environmental acoustic noise propagating within the cabin of the car running at variable speed on a smooth asphalt. Our experimental results show that a large spectral coherence between monitoring and virtual microphone signals indicates a potentially effective and relatively wide-band virtual microphone signal reconstruction. The fixed observation filter estimation method achieves better performance than the adaptive one, guaranteeing remarkable broadband estimation accuracy. Moreover, for each considered setup, design guidelines are proposed to obtain a good trade-off between estimation accuracy and material costs.

Acoustic Optical Fiber Sensor Based on Graphene Oxide Membrane.

A Fabry–Pérot acoustic sensor based on a graphene oxide membrane was developed with the aim to achieve a faster and simpler fabrication procedure when compared to similar graphene-based acoustic sensors. In addition, the proposed sensor was fabricated using methods that reduce chemical hazards and environmental impacts. The developed sensor, with an optical cavity of around 246 µm, showed a constant reflected signal amplitude of 6.8 ± 0.1 dB for 100 nm wavelength range. The sensor attained a wideband operation range between 20 and 100 kHz, with a maximum signal-to-noise ratio (SNR) of 32.7 dB at 25 kHz. The stability and sensitivity to temperatures up to 90 °C was also studied. Moreover, the proposed sensor offers the possibility to be applied as a wideband microphone or to be applied in more complex systems for structural analysis or imaging.

Smart Phone based Snoring Sound analysis to Identify Upper Airway Obstructions.

Obstructive sleep apnea (OSA) is characterized by upper airway obstructions known as apnea/hypopnea events. Narrowing of the upper airway during or near the vicinity of apnea/hypopnea causes the spectrum of the snores to shift to higher frequencies. Using an instrumentation quality wideband (WB) microphone (4Hz-100kHz), we previously demonstrated that potentially diagnostically useful frequency shifts could be detected even in regions beyond the human hearing range. WB-microphone based systems are expensive and not available for home use or population screening application. In this paper we explore the feasibility of using smart phones to analyze snoring sounds in the 20Hz-22kHz band to identify events of upper airway obstructions. Modern smart phones have internal microphones with bandwidths up to 22kHz, above the nominal human hearing range, and provide a good platform for sound acquisition and processing. For the work of this paper we used a Samsung Galaxy S3 phone and recorded overnight respiratory sound data from 8 patients undergoing routine Polysomnography (PSG) study in a hospital. Our target was to develop models to classify each standard 30 second epoch of data as non-apnea or apnea. Using 700 epochs we developed logistic regression models with the input as snoring sound features and the outputs as the diagnostic classification of each event (apnea/non-apnea). Models developed within a 20Hz-15kHz band had accuracies of 89-93%, sensitivities 70-78% and kappa index ranging 0.75-0.83 on validation data set. When the same models were developed on the 20Hz-22kHz frequency band the improved performance shows accuracies 94- 97%, sensitivities 93-100%, and kappa ranging 0.86-0.91. The study shows that smart phones based high frequency band (15-22kHz) of snoring sounds carry information about the upper airway obstructions. Our non-contact, smart phone based snoring sound technology has potential to identify upper airway obstructions.

Frequency Selection Based Separation of Speech Signals with Reduced Computational Time Using Sparse NMF

Abstract Application of wavelet decomposition is described to speed up the mixed speech signal separation with the help of non-negative matrix factorisation (NMF). It is assumed that the basis vectors of training data of individual speakers had been recorded. In this paper, the spectrogram magnitude of a mixed signal has been factorised with the help of NMF with consideration of sparseness of speech signals. The high frequency components of signal contain very small amount of signal energy. By rejecting the high frequency components, the size of input signal is reduced, which reduces the computational time of matrix factorisation. The signal of lower energy has been separated by using wavelet decomposition. The present work is done for wideband microphone speech signal and standard audio signal from digital video equipment. This shows an improvement in the separation capability using the proposed model as compared with an existing one in terms of correlation between separated and original signals. Obtained signal to distortion ratio (SDR) and signal to interference ratio (SIR) are also larger as compare of the existing model. The proposed model also shows a reduction in computational time, which results in faster operation.

Graphene electrostatic microphone and ultrasonic radio

We present a graphene-based wideband microphone and a related ultrasonic radio that can be used for wireless communication. It is shown that graphene-based acoustic transmitters and receivers have a wide bandwidth, from the audible region (20∼20 kHz) to the ultrasonic region (20 kHz to at least 0.5 MHz). Using the graphene-based components, we demonstrate efficient high-fidelity information transmission using an ultrasonic band centered at 0.3 MHz. The graphene-based microphone is also shown to be capable of directly receiving ultrasound signals generated by bats in the field, and the ultrasonic radio, coupled to electromagnetic (EM) radio, is shown to function as a high-accuracy rangefinder. The ultrasonic radio could serve as a useful addition to wireless communication technology where the propagation of EM waves is difficult.

Ballistic shock wave localization estimation of shooter position and velocity using difference of time of arrival (DTOA) algorithm in orthogonally arranged discrete acoustic arrays: Part II

A mathematical algorithm was developed to estimate the parameter space involving both the displacement of the shooter with respect to the array and the velocity of the projectile using the difference in time of arrival, DTOA, of the ballistic shock wave cone at each position of an N-element array. [J. Acoust. Soc. Am. 133, p. 3506, May 2013.] The array geometry involves orthogonally arranged discrete point-like line arrays using wide-band microphone or piezo-electric elements. The algorithm utilizes a nonlinear least squares parameter fit by summing the squares of [DTOA (experimental)—DTOA (theoretical parameters)] values where the DTOA (theoretical) equation involves a lengthy Taylor series expansion of the exact “difference in time of arrival” theoretical equation. Earlier results, in the absence of noise (N = 7) showed that the model has good versatility in estimating displacement (location) and velocity in simulated computer trials. Here, near-field noise is simulated by a rotor blade resulting in uncertainty in the arrival time of the shockwave at each sensor, leading to uncertainty in the estimation of the fit parameters. For example, simulated scatter plots of the azimuthal angle vs the elevation angle parameters for different projectile miss distances become useful in computing parameter uncertainty for different signal-to-noise ratios.

Bulk micro-machined wide-band aero-acoustic microphone and its application to acoustic ranging

A wide-band aero-acoustic microphone was realized using a bulk micro-machining process based on the deep reactive-ion etching of silicon. The sensing diaphragm is completely sealed, thus eliminating the loss of low-frequency response resulting from pressure equalization through the release etch-holes present on the diaphragm of a previously reported microphone implemented using a surface-micro-machining process. A dynamic sensitivity of ∼0.33 µV/V/Pa was estimated using an acoustic shockwave (‘N-wave’) generated using a custom-built high-voltage electrical spark-discharge system. This value is comparable to the effective static sensitivity of ∼0.28 µV/V/Pa measured using a commercial nano-indenter system. The response of the microphone is relatively flat from 6 to 500 kHz, with a resonance frequency of ∼715 kHz. An array of three microphones was also constructed and tested to demonstrate the application of these microphones to the localization of high frequency and short duration acoustic sources.

Study of the On-Line Monitoring on the Effect of Laser Shock Processing for Residual Stress by the Properties of Acoustic Wave

In order to realize the on-line monitoring of the effect of laser shock processing(LSP),acoustic waves generated during the LSP of 45 steel are measured in real time by a wide-band microphone and digitized at a sampling frequency of 48KHz.The waveforms of the received acoustic signal imported to MATLAB are analyzed and in order to investigate the frequency characteristic of the acoustic wave, a power spectral density(PSD) analysis has been carried out. Based on the value of the largest peak of the PSD and compressive residual stress of metal materials induced by the laser shock processing, the correlation equation was established with the application of least square method for the on-line monitoring.

Rise times of low‐energy, free‐space propagating acoustic shocks.

Characteristics of the “ideal” shock, a one‐dimensional step change in pressure, are reasonably well established; however, the freely propagating shock with finite duration is challenging to model and measure. These challenges are reflected in an extensive body of literature over several decades. In this paper we present a method for producing free‐space propagating shocks, measurements of rise time with minimal contamination by turbulence, and model results that account for spreading, absorption (including molecular relaxation), and nonlinearity. The shocks are produced using an acoustic pipe filter to condition the irregular pressure release of either balloon pops or starter‐pistol shots. The shocks range in peak pressure from 10 to 1000 Pa and in rise time from 10 to 0.5 μs. Rise‐time measurements are made using a custom piezoelectric polymer wideband microphone with a measurement range that overlaps with that of a diffraction‐corrected 1/8‐inch measurement microphone. The exponential‐decay time constant beyond the pressure peak is typically less than 100 μs, which may be too rapid to permit development of characteristics associated with relaxation mechanisms: rise times over the entire range measured here are well predicted by accounting for classical and rotational absorption only.

Experimental Studies on Two-Dimensional Defect-Mode Waveguides in a Sonic/Phononic Crystal

Defect-mode waveguides (DMWGs) were constructed removing every second scatterer from a sonic/phononic crystal along an intended waveguide path. The host crystal was composed of a square array of acrylic resin cylinders in air with a diameter of 5.0 mm. The lattice constant was 6.25 mm with a Bragg reflection at around 27.6 kHz at room temperature. Various shapes of DMWGs were fabricated, and their transmission spectra were measured using a network analyzer with a high-frequency speaker, i.e., a tweeter, and a wideband microphone. The theoretically expected band-pass spectra of DMWGs were confirmed experimentally as follows. (1) A clear passband of the DMWG was observed at a normalized frequency of between 0.545 and 0.590 with a transmission per lattice constant of -0.52 dB. (2) The passband was not influenced significantly by the existence of a bend, a branch, or a crossroads in the DMWG. The DMWG was verified to work not only as a novel acoustic waveguide but also as a good band-pass filter.

Comparison of high‐frequency measurements of blast wave propagation to computational model predictions

In November 2005, the latest in a series of blast wave propagation experiments was conducted with the U.S. Army. Blast waveforms measured with a wideband microphone [W. M. Wright, ONR Report NR‐384‐321 (1971)] had shorter rise times than those captured with conventional1/8 ‐in. microphones. This indicates that the 1/8‐in. microphone setup did not have the high‐frequency bandwidth needed to capture such short rise times. The trend observed is that nonlinear effects steepen the waveform, thereby decreasing the shock rise time, up to a range of 50 m. At 100 m, the rise times had increased slightly, but they were still shorter than they were at 25 m. The experimental results are compared to predictions from a computational model. The model, based on the Pestorius/Anderson hybrid time‐frequency domain algorithm, accounts for nonlinearity, geometrical spreading, thermoviscous absorption, and absorption and dispersion due to molecular relaxation. It is found that the model predicts shorter rise times than what was measured with the wideband microphone, but the rise‐time variation with range is similar. Atmospheric turbulence present in the experiments, but not accounted for in the model, is a likely cause for the difference in rise times. [Work supported by U.S. Army ERDC CERL.]

Acoustic emission monitoring during laser shock cleaning of silicon wafers

A laser shock cleaning is a new dry cleaning methodology for the effective removal of submicron sized particles from solid surfaces. This technique uses a plasma shock wave produced by laser-induced air breakdown, which has applied to remove nano-scale silica particles from silicon wafer surfaces in this work. In order to characterize the laser shock cleaning process, acoustic waves generated during the shock process are measured in real time by a wide-band microphone and analyzed in the change of process parameters such as laser power density and gas species. It was found that the acoustic intensity is closely correlated with the shock wave intensity. From acoustic analysis, it is seen that acoustic intensity became stronger as incident laser power density increased. In addition, Ar gas has been found to be more effective to enhance the acoustic intensity, which allows higher cleaning performance compared with air or N 2 gas.

Laser ablation of solid substrates in a water-confined environment

Laser ablation of Si under a water surface has been investigated. The laser used is a KrF excimer laser, which has a wavelength of 248 nm and a pulse duration of 23 ns. It is found that the laser ablation rate of Si varies with the thickness of the water layer above the Si substrates. The laser ablation rate is the most highly enhanced with a water layer of 1.1 mm. It is assumed that the plasma generated in the water confinement regime with an optimal water layer thickness induces the strongest pressure. This high-pressure, high-temperature plasma results in the highest ablation rate. A wide-band microphone is used to detect the audible acoustic wave generated during the laser ablation. The amplitude of the acoustic wave is closely related to the ablation rate. It is found that the first peak-to-peak amplitude of the acoustic wave is the strongest when the water layer thickness is 1.1 mm above the substrate. Fast Fourier transform analysis of the wave forms shows that there are several frequency components included in the acoustic waves. The dominant frequency component decreases from 10.6 to 3.5 kHz as the water layer thickness varies from 1 to 2.2 mm. Diagnostics of the acoustic wave emission can be used to find the optimal water layer thickness to enhance the laser ablation rate. With proper calibration, acoustic-wave detection can be used as a real-time monitoring of the laser ablation.

Laser ablation of solid substrates in water and ambient air

Laser ablation of solid substrates in ambient air and under water is investigated. It is found that the laser ablation rate is highly enhanced by the water film. A wide-band microphone is used to detect the audible acoustic wave generated during laser ablation. Peak-to-peak amplitude of the acoustic wave recorded in water confinement regime (WCR) is greater than that recorded in ambient. It is assumed that the plasma generated in WCR induces a much stronger pressure. This high-pressure, high-temperature plasma results in a much higher ablation rate. Theoretical calculation is also carried out to verify this assumption. By proper calibration, acoustic wave detection can be used as a real-time monitoring of the laser ablation.

Prediction system of surface damage

During laser cleaning, surface damage on the substrate could easily occur by overexposure to a laser pulse of high fluence. The damage is especially serious in art conservation where recently the laser has found a successful field of application. Successful cleaning without surface damage can be achieved by skilled expert operators with long experience and good technique. This paper presents a fuzzy rule-based expert system to predict surface damage during laser cleaning like a human expert. In this work, a fuzzy rule base was used to embed the acoustic information including an indication for the progress in cleaning and the result. An inference process was conducted to predict whether and how much surface damage would be induced on the substrate. In order to detect the acoustic waves a wide-band microphone was utilised. Tests of the performance of the fuzzy expert system showed that the prediction of surface damage is well correlated with the actual results independent of initial surface conditions. Finally, a process control algorithm for laser cleaning has been developed on the basis of the surface damage prediction system.

Fuzzy Rule Based Prediction System of Surface Damage in the Laser Cleaning Process

During laser cleaning, surface damage on the substrate could easily occur by overexposure to a laser pulse of high fluence. Successful cleaning without surface damage can be achieved by skilled operators with long experience and good technique. This paper presents a fuzzy rule based expert system to predict surface damage during laser cleaning in the same way as a human expert. In this work, a fuzzy rule base was used to embed the acoustic information, which gives an indication of both progress in cleaning and the result. This is achieved by a wide-band microphone, An inference process was conducted to predict whether and how much surface damage would be induced on the substrate. Tests of the performance of the fuzzy system showed that the prediction of surface damage is well correlated with the actual results, independent of initial surface conditions. Based on the prediction of surface damage, the process control algorithm of laser cleaning has been developed.

Focusing of N waves in air by an ellipsoidal reflector

An experimental investigation of the focusing of intense, airborne pressure pulses by an ellipsoidal reflector is reported. Short-duration N waves were generated by weak sparks at the near focus F1 of four different prolate ellipsoidal surfaces. Reflection then concentrated the rays at the far focus F2. Measurements were made with a wide-band microphone, primarily along the axis of the reflector. A few transverse measurements in the plane of F2 were also made. In the axial measurements the reflected (upright N wave) and edge-diffracted (inverted N wave) components of the signal are distinct at points distant from F2, approach each other as the focus is neared, and merge at the focus. At the focus the waveform is U-shaped, or cusped. Beyond F2 a “droopy” N wave emerges and, where the edge wave can be resolved, it now arrives first and is phase inverted relative to its prefocal version. The more intense reflected wave does not experience phase inversion. Some features of the observed waveforms are consistent with recent calculations by Hamilton, which are based on lossless, linear theory. Other wave characteristics, including pulse amplitude and location of the head and tail shocks, are strongly influenced by nonlinear effects.

A study of blast waveforms detected simultaneously by a microphone and a laser probe during laser ablation

We examine blast waves generated in air during irradiation of absorbing samples with Nd: YAG laser pulses of fluences exceeding the ablation threshold. Blast waves were detected simultaneously by a wideband microphone and a laser beam deflection probe. By a comparative analysis of both signals in the time and frequency domain we investigate characteristic features of their nonlinear waveform evolution. To explain the observed phenomena we employ the weak shock solution of the point explosion model.

Investigation of blast waves generated by laser induced damage processes

Spherical blast waves generated by laser induced damage processes were studied in the weak shock region using wideband microphone detection. Shock wavefront propagation was examined and a comparison with the predictions of the point explosion model was made. Based on the agreement between the theory and experiment, blast wave energy and energy conversion efficiency were determined.

Focusing of an N Wave by a Spherical Mirror

An experimental and theoretical study of N-wave focusing has been carried out. An N-wave source (electric spark) was located on the principal axis of a spherical mirror so as to produce a focus (image point) beyond the center of curvature. The signals before, at, and beyond the focus were received by a wide-band condenser microphone of our own manufacture. Linear theory in the form of the Kirchhoff integral was used to obtain predicted waveforms. Corrections for known nonlinear propagation effects were applied. The data is explained adequately by the composite theory. Diffraction from the mirror's edge plays a prominent role. Before the focus, the reflected signal, a normal N wave, is followed in time by the diffracted signal, which is an inverted N. As the focus is approached, the amplitude becomes large and the delay between the two signals tends to zero, giving rise to a U-shaped wave, which approximates the derivative of the original N. Thus the well-known π/2 phase shift at the focus is explained as the superposition of the two signals. Beyond the focus, the phase shift of both signals is π; in addition, the order of arrival is reversed. Nonlinear effects cause distortion of the reflected signal, particularly after the focus, where the initial shape is an inverted N. [Work supported by ONR and AFOSR].