External Acoustic Noise Research Articles

Self-Oscillators Based on Surface Acoustic Waves (A Review)

Introduction. Modern precision radio systems impose stringent requirements on the quality of the reference signal sources used. Various approaches are used to create sources of reference signals in the microwave range – micro-wave self-oscillators (SO). A promising direction in the development of such SO is SO with frequency-setting elements based on surface acoustic waves (SAW).Aim. A review of international achievements in the development of frequency-setting elements based on SAW and respective SO.Materials and methods. The selection of materials for a comparative analysis and generalization was carried out using available sources published over the past 30 years in well-known engineering journals, advertising brochures and websites of the manufacturers of devices based on SAW. The selection criteria included low values of the power spectral density of the frequency fluctuations of the generated signal, the presence of vibration protection, the presence of a thermostat, as well as the miniaturization and originality of the design.Results. Specific features of various methods used for constructing microwave oscillators were analyzed. It is shown that the achievement of the best values of the power spectral density of frequency fluctuations in SO with frequency-setting elements on SAW is possible only with the use of two-port resonators. An analysis of the main technical characteristics of temperature-controlled vibration-resistant SO was carried out.Conclusion. Despite the large number of different manufacturers on the world market (more than 20 companies) and the variety of different models of oscillators with frequency-setting elements on SAW (more than a 100 different models), only two companies produce oscillators resistant to external vibrations and acoustic noise.

ПРОСТРАНСТВЕННОЕ РАЗДЕЛЕНИЕ ИНФОРМАЦИИ В САМОЛЕТНОМ ПЕРЕГОВОРНОМ УСТРОЙСТВЕ

The article presents some results of research aimed at the design of human-machine interfaces,taking into account the multimodal nature of human perception, for use in the on-boardequipment of an aircraft. In particular, we are talking about the possibility of a wider use of audiochannels for input and output of information. The advantages of sound interfaces in relation tovisual and tactile ones are, first of all, in the absence of the need for directed attention of the pilot,in the ability to create auditory objects in three-dimensional space and indicate the direction toseveral different objects at the same time. In the experiments, the possibilities of spatial separationof speech information flows in an aircraft intercom in situations where the level of interferencesignificantly exceeded the level of the target speech message were tested. The indicators of targetmessage recognition were evaluated in the presence of two types of sound interference: the sound ofanother speech message and the noise of an aircraft engine. The results showed that spatial separationof audio messages significantly improves the operator’s ability to recognize their content, regardlessof the type of interference. The maximum number of errors when recognizing a target messagecorresponds to its spatial position in the same direction as the noise of the interference. At thesame time, message recognition is significantly better if it is pronounced in a female voice. The fact ofspatial asymmetry of correct recognitions was also revealed: messages arriving from the right are recognized better than in cases of their arrival from the left. The practical significance of the researchconcerns the possibility of creating intercom with increased security against conflicts betweendifferent information flows, as well as against the impact of external acoustic noise. The prospect isseen in the use of three-dimensional audio interfaces not only as part of an intercom, but also fornavigation and aircraft control systems, as well as monitoring its state.

Effects of Surface Waviness on Fan Blade Boundary Layer Transition and Profile Loss—Part I: Methodology and Computational Results

AbstractThis two-part paper describes a new approach to determine the effect of surface waviness, arising from manufacture of composite fan blades, on transition onset location movement and hence fan profile losses. The approach includes analysis and computations of unsteady disturbances in boundary layers over a wavy surface, assessed and supported by wind tunnel measurements of these disturbances and the transition location. An integrated framework is developed for analysis of surface waviness effects on natural transition. The framework, referred to as the extended eN method, traces the evolution of disturbance energy transfer in flow over a wavy surface, from external acoustic noise through exponential growth of Tollmien–Schlichting (TS) waves, to the start and end of the transition process. The computational results show that surface waviness affects the transition onset location due to the interaction between the surface waviness and the TS boundary layer instability and that the interaction is strongest when the geometric and TS wavelengths match. The condition at which this occurs, and the initial amplitude of the boundary layer disturbances that grow to create the transition onset is maximized, is called receptivity amplification. The results provide first-of-a-kind descriptions of the mechanism for the changes in transition onset location as well as quantitative calculations for the effects of surface waviness on fan performance due to changes in surface wavelength, surface wave amplitude, and the location at which the waviness is initiated on the fan blade.

Exploring soundscaping options for the cognitive environment in an open-plan office

A major source of noise complaints in open-plan offices has consistently been co-workers talking nearby or talking on the phone. Various masking sounds such as white noise and pink noise have been explored to reduce the intelligibility level of speech from adjacent co-workers in open-plan offices. Recently, nature soundscapes such as water, rainfall or birdsong have been introduced instead of conventional white or pink noises. Water sounds with limited acoustic variation have shown more effectiveness than nature sounds with high acoustic variation such as birdsong. This study examined four types of background sounds in a controlled lab environment that simulated a typical open-plan office in a large pharmaceutical company in the United Kingdom. The purpose was to understand cognitive performance, satisfaction and preference, and physiological responses among the four types of background sounds: no external acoustic noise, typical daily office noise, white noise and spring water sound. The study employed three methods: an assessment of cognitive performance to measure memory, distractibility and decision-making; a survey to understand subjective satisfaction and preference; and use of wearable devices to monitor physiological responses. Two different types of wearable devices were used: pulse oximeter and electrodermal activity sensor to compare physiological responses to the four types of sounds. This article discusses discrepancies found between participants’ satisfaction/preference and their cognitive performance under the four background sounds and potential future implications of masking sounds in open-plan offices.

Baseline restoration method based on mathematical morphology for high-pressure xenon detectors

The shift of baseline has always existed in high-pressure xenon (HPXe) detectors because of the influence of external acoustic noise. This problem seriously influences the spectrometric characteristics of these detectors. A special baseline restoration method is required to remove the shift of baseline in real time. This study demonstrated that the top hat transform based on mathematical morphology can restrain the shift of baseline in digital multichannel analyzer. An improved baseline restoration (IBLR) method based on mathematical morphology was proposed and implemented in field programmable gate array (FPGA) to obtain superior baseline restoration. The structuring element of the IBLR method can be adjusted automatically in real time according to the characteristics of the baseline. Experimental results show that the IBLR method can maintain an excellent energy resolution over a wide shift range of baseline. To strengthen our conclusion, we carried out an external acoustic noise experiment on a HPXe detector. The HPXe detector with IBLR was virtually unaffected by external acoustic noise.

Improvement in QEPAS system utilizing a second harmonic based wavelength calibration technique

A simple laser wavelength calibration technique, based on second harmonic signal, is demonstrated in this paper to improve the performance of quartz enhanced photoacoustic spectroscopy (QEPAS) gas sensing system, e.g. improving the signal to noise ratio (SNR), detection limit and long-term stability. Constant current, corresponding to the gas absorption line, combining f/2 frequency sinusoidal signal are used to drive the laser (constant driving mode), a software based real-time wavelength calibration technique is developed to eliminate the wavelength drift due to ambient fluctuations. Compared to conventional wavelength modulation spectroscopy (WMS), this method allows lower filtering bandwidth and averaging algorithm applied to QEPAS system, improving SNR and detection limit. In addition, the real-time wavelength calibration technique guarantees the laser output is modulated steadily at gas absorption line. Water vapor is chosen as an objective gas to evaluate its performance compared to constant driving mode and conventional WMS system. The water vapor sensor was designed insensitive to the incoherent external acoustic noise by the numerical averaging technique. As a result, the SNR increases 12.87 times in wavelength calibration technique based system compared to conventional WMS system. The new system achieved a better linear response (R2=0.9995) in concentration range from 300 to 2000 ppmv, and achieved a minimum detection limit (MDL) of 630 ppbv.

Wavelength-stabilization-based photoacoustic spectroscopy for methane detection

A compact and portable photoacoustic gas sensor was developed for sensitive methane (CH4) detection at 1.6 µm using a software-based wavelength stabilization scheme. A transmission-type photoacoustic cell was connected in series with a reference gas cell to measure the photoacoustic signal and the reference gas absorption for wavelength stabilization simultaneously. The central wavelength of the diode laser was locked to the target CH4 line with a fluctuation of less than 10.6 MHz using a digital proportional–integral–derivative controller. The CH4 sensor was designed to be insensitive to the incoherent external acoustic noise by the cumulative average of the demodulated photoacoustic signal by a digital lock-in amplifier. With an incident laser power of 6 mW, our CH4 sensor achieved a minimum detection limit of 11.5 ppm at 10 s response time and an excellent linearity (R2 = 0.9999) in the concentration range of 400–6300 ppm.

Phase-sensitive noise suppression in a photoacoustic sensor based on acoustic circular membrane modes

A photoacoustic (PA) sensor based on higher order acoustic modes is demonstrated. The PA sensor is designed to enhance the gas-detection performance and simultaneously suppress ambient noise sources (e.g. flow noise, electrical noise and external acoustic noise). Two microphones are used and positioned such that the PA signals are ($\pi$) out of phase. Ambient acoustic noise are approximately in the same phase and will be subtracted and thus improve the SNR. In addition, by placing the gas in- and outlets so that the gas flows through the node of the first higher order membrane mode the coupling of flow noise is approximately 20 dB lower compared with flow through the fundamental mode at 5 L/min. The noise reduction and thus the increase in sensitivity is demonstrated by measuring vibrational lines of methanol and methane using a broadband interband cascade laser emitting radiation at 3.38 $\mu$m. A signal-to-noise improvement of 20 (26 dB) using higher order modes are demonstrated compared with the fundamental mode. The maximum normalized noise equivalent absorption coefficient is 1.1 $\times 10^{-6}$ W cm$^{-1}$ Hz$^{1/2}$ for a flow of 5 L/min. It is anticipated that the noise cancelation strategy may find use in many practical industrial and environmental PA sensors, where ambient noise sources plays a crucial role for the absolute sensitive.

Photoacoustic Cell with Digital Differential Detection

Solutions which reduce the impact of the external acoustic noise on the photoacoustic signal rely often on the appropriate modification of the photoacoustic cell structure. The goal is to obtain a frequency response of the cell which suppresses the external noise as much as possible. Another approach is differential detection, in which two microphones are used, and assuming that the external noise signal components from both microphones are identical, their subtraction should result in canceling the external noise. The main difficulty of that solution is that both microphone signal paths should be calibrated to have virtually identical characteristics. The paper presents a differential photoacoustic cell with digital differential detection.

Parametric Analysis of a Differential Photoacoustic Helmholtz Cell

The paper describes how mechanical dimensions of a differential open photoacoustic Helmholtz cell affect its operation. The influence of the diameter and length of the internal and external ducts and the volume of the acoustic buffers on the frequency response of the cell, as well as on the attenuation of the external acoustic noise, was studied. The analysis was performed by means of computer simulations based on a model in which loss-corrected transmission line duct definitions were used. The results showed that under proper selection of the cell dimensions, its frequency response at the frequencies around the main resonance is nearly identical as in the case of the conventional, closed Helmholtz resonator. The length of the ducts is only slightly related to the frequency of resonance, and does not noticeably affect the Q- factor of the cell. A decrease of the duct diameters and an increase of the buffer volumes improve attenuation of the external acoustic noise. It should be possible to obtain external acoustic noise rejection as high as 80 dB, which is a substantial improvement in comparison to the previous designs, which reported this value at the level of 40 dB to 50 dB.

Differential Open Photoacoustic Helmholtz Cell

There are very few designs of the open photoacoustic Helmholtz cells, and most of them exhibit very strong penetration of the external acoustic noise inside the cell. So far the best values of external acoustic noise suppression obtained in such cells were reported at the level of about 40 dB to 50 dB. This paper presents an open photoacoustic Helmholtz cell design with a differential signal detection. Both Helmholtz resonator cavities are equipped with microphones and connected with the exterior via duct-buffer-duct structures. The length and diameter of the ducts as well as volume of the acoustic buffers are selected in such a way that the acoustic impedance of the duct-buffer-duct structure at the frequency of light modulation is relatively high. As a result, the resonance of the cell is not damped, while penetration of the external acoustic noise inside the cell is strongly reduced. Preliminary analysis predicts attenuation of the external acoustic noise at the resonance frequency of the cell with a single microphone to be at the level of at least 60 dB. Additional rejection of the external acoustic noise can be obtained with differential detection, which simultaneously doubles the photoacoustic signal component; as in the Helmholtz resonator, pressure changes in the cavities are in counterphase.

New modification of xenon gamma-ray detector with high energy resolution

Performance of a new modification of xenon gamma-ray detector (XGD) is presented. This detector differs from the previous ones by virtue of improved energy resolution (1.7±0.1)% at 662 keV and the ability to function in the presence of external acoustic noise (up to 100 dB) with virtually no degradation of spectrometric characteristics. These results have been achieved by developing a digital method of processing every electric signal coming from the XGD. For this method, digital electronics based on field-programmable gate array has also been developed.

Parametric Analysis of the Improved Open Photoacoustic Helmholtz Cell

Many photoacoustic cells use a Helmholtz structure for amplification of the photoacoustic signal, but very few of them are open cells. Open Helmholtz cells known from the literature exhibit very poor attenuation of the external acoustic noise, which severely limits their applications. Recent research showed that this problem can be overcome in a so-called improved open photoacoustic Helmholtz cell structure. The paper describes how mechanical dimensions of such a cell affect the frequency response of the cell and its penetration by the external acoustic noise. Properties of the cell were investigated by means of computer simulations based on the loss-improved transmission line model. The analysis showed that appropriate selection of the duct dimensions and buffer volumes should result in over 50 dB attenuation of the external acoustic noise, without noticeably affecting the Q-factor and resonance frequency of the cell.

Improved Open Photoacoustic Helmholtz Cell

This paper presents the design of an open photoacoustic Helmholtz cell in which high acoustic volumes with quarter-wave ducts act as high-impedance separators between the main Helmholtz cell structure and the exterior. As a result, penetration of the external acoustic noise into the cell was substantially reduced in comparison to earlier open Helmholtz cell designs. Although the presented cell is not windowless, the photoacoustic background signal resulting from the absorption of the light by the windows is significantly lower in comparison to standard Helmholtz cells. Such an effect was obtained by locating the windows not at the relatively small sample cavity, but at the acoustic buffers, for which the volumes are two orders of magnitude higher. The proposed cell is dedicated for gas or liquid measurements, and its design allows for constant flow of the fluid. Hence, it can be used in continuous, real-time photoacoustic measurements.

Two-way voice communication device having external acoustic noise reduction

A two-way voice communication device which provides a high-level of acoustic noise reduction is disclosed. A resilient ear tip provides acoustic isolation and secures the earphone within the ear canal of the user. The ability to rotate the earphone within the ear canal allows the user to easily position an attached microphone. The audio signals may be conveyed by an included radio frequency receiver and radio frequency transmitter, or the earphone may be connected to an electrical audio device using a multi-conductor cable. Audio signals may be conveyed to one or both ears of a user. A switch to mute the audio signal from the microphone, or for supporting a push-to-talk mode of operation, may be included in the earphone housing, or on the cable.

Combining an energy boundary element analysis with an energy finite element analysis for computing airborne noise inside a flexible structure due to an exterior acoustic source

The energy boundary element analysis (EBEA) has been utilized in the past for computing the exterior acoustic field at high frequencies (above ∼400 Hz) around vehicle structures and numerical results have been compared successfully to test data [Wang et. al., J. Sound Vib. 126, 278–283 (2004); Internoise (2002)]. The EFEA has been developed for computing the structural vibration of complex structures at high frequencies and validation has been presented [Zhang et. al., J. Sound Vib. 282, 679–700 (2005); 41, 11–12 (2005); Finite Elements Anal. Des. 41(11-12) (2005); Wang et. al., SAE Paper No. 2005-01-2372]. In this presentation the EBEA is utilized for computing the acoustic field around a cylindrical structure (similar to the NASA aluminum test-bed cylinder) from an external acoustic noise source. The computed exterior acoustic field comprises the excitation for the EFEA analysis. The structural vibration and the interior noise levels are computed, demonstrating how the EBEA and the EFEA can be combined in airborne noise applications for performing a complete vibro-acoustic analysis. [Work supported by ONR Code 334, SBIR program.]

Essential roles of noise in neural coding and in studies of neural coding

We present examples of results from our studies of auditory primary afferent nerve fibers and populations of such fibers in the frog and gerbil. We take advantage of the natural dithering effect of internal noise, where it is sufficient, to construct highly predictive descriptive models (based on the Wiener series with kernels derived from white-noise analysis). Where the internal noise is insufficient, we enhance dithering by applying external acoustic noise together with our stimuli. Using acoustic noise as a background sound, orthogonal to the stimulus waveform, we show that under some circumstances such background sound can enhance the ability of individual fibers and populations of fibers to encode the stimulus waveform.

Fabrication of a silicon-based superfluid oscillator

We have constructed an integrated superfluid oscillator using various silicon processing techniques, including micromachining and electron beam lithography. This device has the advantage of a very small internal volume (0.72 mm/sup 3/). This makes it insensitive to spurious external acoustic noise which has limited the performance of larger experiments. We have tested the performance of this device in two configurations, one with a single micro-aperture and another with an additional fine tube. Both configurations demonstrate macroscopic quantum phenomena in superfluid /sup 4/He at low temperatures (0.25 K<T<2.2 K) and have been used to study these effects in detail.

Pressure pulsations on an obstacle in a jet

At the present time, much attention is devoted to auto-oscillations that arise from the interaction between a supersonic underexpanded jet and an obstacle that it encounters at right angles [1, 2]. There are far fewer data on the pressure pulsations on an obstacle in the absence of auto-oscillations [3–6]. However, in many cases the highest total levels of the pressure pulsations are observed when the barrier is situated at fairly large distances from the nozzle opening and the pressure pulsations have a random nature. We have investigated the pressure pulsations on a plate normal to a supersonic strongly underexpanded jet. The pulsation characteristics were measured for an arrangement of the obstacle when auto-oscillations are absent. We have established dependences that generalize the results of measurement of the pulsation characteristics at both subsonic and supersonic velocities on the jet axis directly in front of the obstacle. We have also investigated the correlation between the pressure pulsations on the plate and external acoustic noise. We have obtained the dependence of the level of the acoustic noise on the value of the maximal pressure pulsations on the plate.