Two-microphone Technique Research Articles

Criteria for diagnosing the state of the hearing organ based on the results of the study of acoustic reflex

Introduction. This study is relevant due to the practice’s need for physiologically adequate criteria for diagnosing the state of the hearing organ of personnel exposed to the cumulative effects of high-intensity pulsed low-frequency noise. 
 The aim of the study was the theoretical and experimental substantiation of the criteria for diagnosing the state of the hearing organ according to the results of the study of the acoustic reflex.
 Materials and methods. Using the two-microphone technique of acoustic measurements, experimental studies of changes in the acoustic reflex were carried out in representatives of the experimental (subjected to cumulative effects of high-intensity pulsed low-frequency noise, n=5) and control (not subjected to such effects, n=12) groups. A total of 256 measurements were made within 6 days (191 in the control group and 65 in the experimental group) before exposure (background) and at the end of the working day. The measurement results were processed using methods of mathematical statistics: tests to determine outliers in the data set, estimates of the coefficient of variation, standard deviation, and boundaries of the 99% confidence interval of changes in the studied indicators were calculated.
 Results. Studies of the effect of high-intensity impulse noise on the organ of hearing showed that with minor changes in the threshold of the acoustic reflex, significant changes in the similarity coefficient were recorded in the experimental group, compared with the control group, which indicates its diagnostic informativeness. In the future, to establish new hygienic standards for regulation the load of high-intensity impulse noise, more extensive studies of the acoustic reflex threshold and the similarity coefficient are needed, as the most informative and sensitive methods for assessing adverse effects.
 The limitations of the study are determined by its focus on use in screening medical examinations of personnel exposed to the cumulative effects of high-intensity low-frequency impulse noise.
 Conclusion. The obtained results demonstrate the potential for studying the acoustic reflex threshold as an objective approach to early detection and prediction of negative changes in the state of the hearing organ of personnel exposed to the cumulative effect of high-intensity low-frequency impulse noise.

Investigation of changes in lung volume caused by high-intensity acoustic oscillations at the resonant frequency of the respiratory system

This paper presents an experimentally determined dependency of an increase in the volume of the lungs of laboratory animals on the level of high-intensity low-frequency sound at the resonant frequency of their respiratory system. Experiments were conducted on 16 rabbits. A modified two-microphone technique was used for the measurement of the resonant frequency of the respiratory system in the frequency range from 5 Hz to 100 Hz for a step in frequency of 3 Hz. Having determined the resonant frequency, a tone signal was delivered to the lungs at this frequency with an adjustable sound amplitude from 120 to 138 dB. The measurement results indicate that the resonant frequencies of the respiratory system in rabbits decreased by an average of 13%, that corresponds to an increase in lung volume by 33% due to the opening of the alveolar oxygen reserve. The observed phenomenon relating to lung volume increase under the influence of a stimulating high-intensity sound signal at a resonant frequency can be used to enhance the effectiveness of bioacoustic stimulation of human lungs in order to increase the volume of the respiratory system.

Characterization of acoustic materials at arbitrary incidence angle using sound field synthesis

Standardized methods for measuring sound absorption such as the impedance tube and reverberation chamber methods are limited to normal or diffuse incidence, respectively. Two research axes have been generally followed in the literature to develop alternative techniques, the first one focusing on the measurement part, that is from the two-microphone technique to the use of microphone arrays or pressure-velocity sensors. The second axis focuses on the excitation part with for instance the use of sound field synthesis techniques. Since acoustic impedance and sound absorption coefficient of materials are classically defined under normal and oblique plane wave excitation, synthesizing an “ideal” plane wave using a loudspeaker array would allow measuring these acoustics quantities using a simple microphone pair. In this article, the effect of the different parameters of a loudspeaker array on acoustic plane waves reproduction on a material’s surface is first numerically studied. Then, numerical and experimental results for the estimation of both impedance and absorption coefficients are reported. These results show that sound field synthesis allows to characterize a material for arbitrary incidence angles over a wide frequency range, thus offering an alternative method to standard techniques and an improvement over existing works.

Effect of tree species and season on the ability of forest floors to abate environmental noise

The nature of the ground beneath a belt of trees along transport infrastructure can contribute significantly to the associated noise abatement. To get more insight in this topic, repeated measurements were performed, both in winter and summer time, in a common garden of six monoculture broadleaved tree stands (ash, cherry, lime, maple, beech and oak, all of the same age). A two-microphone technique was used to derive the ground surface impedance and soil properties describing the interaction between the forest floor and sound waves. The slit-pore and Zwikker and Kosten (phenomenological) ground impedance model, both in rigid backing configuration, were found to adequately predict the measured short-range level differences. The much higher moisture content of the forest floor during the winter campaign resulted in a strong increase in the deduced surface impedance. A clear overall tree species effect seemed absent, despite large differences in litter degradation rates. Additional measurements after raking away the organic layer allowed concluding that a pronounced forest floor might compensate for an acoustically harder mineral soil beneath. In tree belts aiming at noise reduction, litter should not be removed and care is needed not to compact the mineral soil, especially in case of stands with rapidly decaying litter and a thin organic horizon.

Analysis of impedance characteristics of the respiratory system of animals and humans

The aim of the study was to modify the method of pulsed oscillometry of the respiratory system of biological objects in the interests of the possibility of its application to extrapolate the medical and biological effects of extreme physical factors of the conditions of professional activity from animals to humans. The method is based on a verified algorithm for determining the impedance characteristics of the human lungs (resonant frequency, active and reactive frequency–dependent components of the impedance) using a two-microphone technique. The impedance characteristics of the respiratory system of animals (30 rats, 15 rabbits, and 15 sheep) in the frequency range from 5 to 100 Hz were measured on decapitated animals when the trachea was connected to an experimental setup including a loudspeaker, waveguide, two measuring microphones, an impedance wall, and a connecting tube designed to connect the installation to the trachea. The impedance characteristics of the respiratory system in humans (40 athletes aged 20–30 years) in the same frequency range were measured during free breathing: a standard mouthpiece was installed as a perforated panel at the end of the waveguide. As a result of the experiment, the resonance frequency of the respiratory system, the absorption coefficient of acoustic vibrations and the resistance at the resonance frequency and the acoustic quality factor of the respiratory system were determined. The results of theoretical and experimental studies indicate that the resonant frequencies of the respiratory system in humans, rabbits and sheep are close to each other, and the resonant frequencies of the respiratory system of rats and humans are significantly different. This allows the use of rabbits and sheep as an adequate biological model for extrapolating the effects of extreme factors from animals on humans in terms of the state of the respiratory system. Extrapolation of similar results from rats to humans should be carried out taking into account the ratio of the impedance characteristics of the respiratory systems.

Development of Kenaf Nonwoven as Automotive Noise Absorption

Getting rid of unwanted noise in car compartment is necessary and really significant measure for automotive makers. This research was directed to produce nonwoven material from Kenaf fiber using the needle-punching machine and the ability of the produced material to absorb sound will be tested. The performance in sound absorption of the sample was analyzed by the sound absorption coefficient (SAC) and noise reduction coefficient (NRC) using the impedance tube test referring to ASTM E1050-98. The sound absorption frequencies were evaluated utilizing the two-microphone transfer function technique in the impedance tube that has a 100 mm diameter for low frequency and 28 mm for high frequency, 0 Hz to 4000 Hz respectively. The physical examination also was executed according to ASTM D1772 to determine the density and the thickness of each sample. The parameter verified in this research is the number of layers to form the samples and it also were compared with the commercial products. As the outcome of this research, the sound absorption coefficient (SAC) showed that the sound reduction coefficient value was increased as the number of layers of the sample increase. In summation, the result also proved that the denser the samples, the higher the absorption coefficient value. On the other hand, for the noise reduction coefficient (NRC), overall result showed slight differences between each sample. The result is due to the rating is an average, two materials with the same rating might not perform the same. Referring to the previous research, the sample with higher areal density is dependable for higher sound reduction and there is a negative relationship between area density and bulk density of needle-punched nonwoven and sound reduction. With the rise in the number of density of nonwoven fabric, the sound reduction through the fabric increases at first but after the maximum it remains almost unaffected.

A correction method for the two-microphone transfer function technique in the free field using numerical modeling

The two-microphone transfer function technique of measuring absorption coefficient in a free field has remained unchanged since its development in the 1980s. The free field technique has remained scarcely used due to usage restrictions caused by sound field contributions from diffraction of the test sample edge. Currently, the technique is only valid for instances where field contributions from edge diffraction is sufficiently minimized. This research uses acoustic numerical modeling to study the effects of error sources on the technique. Numerical models have been developed and used to quantify the effects of “image source deviation” and edge diffraction on the implementation of the free field technique. Each error source is quantified independently. Updated guidance on the usage restrictions of the free field technique is provided. Additionally, an improvement to the free field technique using a correction method is proposed. An experimental validation of the correction method was performed. The correct...

Analysis of the error sources of the two-microphone transfer function method for measuring absorption coefficient in the free field using numerical modeling

Extension of the two-microphone transfer function absorption coefficient measurement technique from impedance tubes to the free field introduces several error sources. The three-dimensional nature of the sound field necessitates consideration of factors that are not relevant in an enclosed tube below the cutoff frequency. The impedance tube technique has been modified to account for non-planar wave propagation due to an acoustic point source. The sound field contamination from sample edge diffraction has generally restricted use of the technique to frequencies such that wavelengths are small relative to sample dimensions. This requires the use of very large test panels for low frequencies. Numerical models of the two-microphone free field technique have been created to quantify these effects. Each effect was isolated to better understand its independent impact on the accuracy of the technique. Finally, a series of experimental tests were conducted to validate the numerical modeling results.

Experimental investigation of the acoustic damping of In-duct orifices with bias flow

Geometry of orifice is investigated by the experiments to evaluate the acoustic damping capacity of orifice plates in a duct. Four kinds of plates with complex orifice shapes are fabricated by using modern 3D printing technology. To characterize acoustic damping performance of these plates, sound absorption coefficient is used as an index determined by using the classical two-microphone technique. It is found that the geometric shapes of the perforated orifices can affect their sound absorption performances, and the damping performances of different shaped orifices depend on the frequency range. The length of downstream duct is also proven to determine the damping performance of perforated plates. The shorter the downstream pipe length, the narrower frequency range corresponding to lower power absorption. Moreover, the bias flow is shown to play a critical role on the sound absorption capacity of orifice plate in the experiments. Sound absorption coefficient is found to increase first and then decreased with increased Mach number.

Estimation of impedance model input parameters from in situ measurements: Principles and applications

The impact of ground physical parameters variations on the previsions of recent outdoor sound propagation numerical models can be substantial. Among these parameters, the air flow resistivity and the effective thickness of numerous soil types (in particular grass lawns) have to be characterized in situ. Thus, a two-microphone technique and a semi-automatic procedure have been developed in order to estimate the ground impedance parameters. The estimation method is based on a narrow-band fitting between the experimental sound level attenuation spectra and the analytical solutions for the measurement configuration. This approach is applied to three types of ground, both in winter and summer conditions. Results are analyzed and discussed in terms of space and time variabilities as well as the impact of such spatial and seasonal variations on sound pressure levels.

Exploration into the sources of error in the two-microphone transfer function impedance tube method

The two-microphone transfer function method has become the most widely used method of impedance tube testing. Due to its measurement speed and ease of implementation, it has surpassed the standing-wave ratio method in popularity despite inherent frequency limitations due to tube geometry. Currently, the two-microphone technique is described in test standards ASTM E1050 and ISO 10534-2 to ensure accurate measurement. However, while detailed for correct test execution, the standards contain vague recommendations for a variety of measurement parameters. For instance, it is only stated in ASTM E1050 that “tube construction shall be massive so sound transmission through the tube wall is negligible.” To quantify this requirement, damping of the tube was varied to determine how different loss factor values effect measured absorption coefficient values. Additional sources of error explored are the amount of required absorbing material within the tube for reflective material measurements, additional calibration methods needed for test of excessive reflective materials, and alternate methods of combating microphone phase error and tube attenuation.

Spatial Analysis on Forced Heat Release Response of Turbulent Stratified Flames

The local equivalence ratio distribution in a flame affects its shape and response under velocity perturbations. The forced heat release response of stratified lean-premixed flames to acoustic velocity fluctuations is investigated via chemiluminescence measurements and spatial Fourier transfer analysis. A laboratory scale burner and its boundary conditions were designed to generate high-amplitude acoustic velocity fluctuations in flames. These flames are subject to inlet radial equivalence ratio distributions created via a split annular fuel delivery system outfitted with a swirling stabilizer. Simultaneous measurements on the oscillations of inlet velocity and heat release rate were carried out via a two-microphone technique and OH* chemiluminescence. The measurements show that, for a given mean total power and equivalence ratio (φg=0.60), the flame responses vary significantly on the equivalence ratio split, forcing frequency, and velocity fluctuation amplitude, with significant nonlinearities with respect to forcing amplitude and stratification ratio (SR). The spatial Fourier transfer analysis shows how the dependence is affected by the underlying changes in the rate of heat release, including the direction and speed of the perturbation within the flame.

Experimental investigations of sound reflection from hot and subsonic flow duct termination

Abstract The knowledge of the reflection properties of open end jet ducts is important for different applications, where the flow and high temperature conditions are involved and add complexity to the problem. In this paper, the magnitude of the reflection coefficients together with the respective end-corrections is experimentally determined for hot flow duct openings. A Mach number range up to 0.3 for cold jets and up to 0.12 for a jet temperature of 200 °C is treated. The experimental results are compared with the numerical model proposed by Munt (Acoustic transmission properties of a jet duct with subsonic jet flow: 1. The cold jet reflection coefficient, Journal of Sound and Vibration 142 (1990) 413–436) and a good correlation in plane-wave region is demonstrated. To reduce experimental uncertainty, the sound reflection properties at the duct opening are obtained by using an overdetermined two-microphone technique with the implementation of a three pressure transducer array. By introducing a modified multistep version of the stepped sine excitation, the accuracy of data acquisition process is improved without compromising the measurement time.

English

This paper presents the application of an in situ measurement technique of sound absorption characteristics, namely the “Ensemble Averaged Method”. Previously, a measurement technique of material surface normal impedance by using a twomicrophone (p-p) technique and ambient noise was proposed by some of the authors. This includes the concept of “ensemble averaged” surface normal impedance to extend the usage of the obtained values to various applications, such as architectural acoustics and computational simulations. With the development of a particle velocity (p-u) sensor, the practicality of techniques for the direct measurement of impedance becomes feasible. A concept of the ensemble averaged surface normal impedance is addressed, including a basic technique to measure it by using p-p and p-u sensors. Then, the agreements between p-p and p-u sensors are discussed and the discrepancies of absorption coefficients are shown to be small. The paper reveals the reliability, applicability and feasibility of the method throughout the investigation as an in situ measurement technique.

Adaptive Concepts for Herschel–Quincke Waveguides

The enhancement of Herschel–Quincke (HQ) waveguides to incorporate adaptive capabilities is investigated. Passive HQ waveguides are known to provide noise attenuation in pipes and ducts at selective narrow frequency bands associated with their resonances. The approach to achieve adaptation is to produce a frequency shift in these resonances to allow targeting incoming tonal noise of variable frequency. The frequency shift is obtained by placing a variable cross-section constriction along the interior of the waveguide. Two adaptive devices are considered. The first consists of a ball with fixed diameter that can be axially displaced inside the waveguide. Then, the frequency tuning is obtained as a function of the ball position. The second device consists of a diaphragm at fixed axial location which can be deformed to obtain a variable cross section. In this case, the frequency shift is obtained as a function of the diaphragm deflection. The internal acoustic dynamics of the two devices are investigated both analytically and experimentally. The created constriction inside the HQ waveguide is modeled as a series of constant cross-section tube elements with small finite area jump between adjacent pieces. The model is validated by comparing the predicted dynamics with experimental data from an extended impedance tube setup based on the two-microphone technique. Finally, attenuation predictions on a one-dimensional pipe are presented in order to illustrate the performance of the proposed adaptive HQ waveguides.

Tuned Passive Control of Acoustic Damping of Perforated Liners

To suppress combustion instabilities, perforated liners can be fitted along the bounding walls of a combustor to provide acoustic damping. These liners are typically subjected to a low-Mach-number bias flow (a cooling flow through perforated holes), and they tend to be effective only over narrow frequency ranges. To investigate the damping effect of perforated liners on plane acoustic waves and to increase their effective frequency range, experiments and numerical simulations are carried out. An acoustically driven pipe system containing a lined section was designed and experimentally tested. length of the pipe system, along with the bias flow rate, could be varied. experimental results showed that the liner damping depended on both the pipe length and the bias flow rate, in agreement with predictions from the numerical model presented by (Eldredge, J. D., and Dowling, A. P., The Absorption of Axial Acoustic Waves by a Perforated Liner with Bias Flow, Journal of Fluid Mechanics, Vol. 485, No. , 2003, pp. 307-335.). To maintain the acoustic damping of the liner in the presence of large frequency changes (corresponding to instability frequency changes in a combustor), real-time tuning of perforated liners was experimentally investigated. Both a pipe length parameter and the bias flow rate were sequentially tuned using a multiple-parameter tuning scheme. scheme required two algorithms to be developed: one for characterizing the finer's acoustic damping in real time and another for sequentially determining the two optimum actuation signals for the damper tuning. former involved developing a real-time version of the two-microphone technique for resolving the two plane acoustic wave strengths, which is widely applicable. On implementing these algorithms in the pipe system, optimal damping of the liner was achieved and maintained over a broad frequency range.

Source Height Determination for Several Sources at the Same Height

Equivalent acoustic source height is an important parameter for accurate predictions of the acoustic impact of transportation infrastructures. Research has shown that this parameter can be obtained from a two-microphone technique in the case of one broadband omnidirectional point source above an homogeneous ground, modeled as a Delany-Bazley absorber. It is shown in this paper that this method can be generalized to n broadband sources with same height, above an homogeneous ground or a ground with an impedance discontinuity. All sources are considered to have the same emission spectrum. The method is based on the comparison of the level difference spectrum between two microphones - hereinafter called attenuation spectrum - measured and computed. Numerical and experimental validations confirm the reliability of the method for controlled noise sources. Furthermore, it is demonstrated that qualitative assessment of ground parameters is sufficient to obtain an accurate source height determination.

Spatially distributed flame transfer functions for predicting combustion dynamics in lean premixed gas turbine combustors

The present paper describes a methodology to improve the accuracy of prediction of the eigenfrequencies and growth rates of self-induced instabilities and demonstrates its application to a laboratory-scale, swirl-stabilized, lean-premixed, gas turbine combustor. The influence of the spatial heat release distribution is accounted for using local flame transfer function (FTF) measurements. The two-microphone technique and CH ∗ chemiluminescence intensity measurements are used to determine the input (inlet velocity perturbation) and the output functions (heat release oscillation), respectively, for the local flame transfer functions. The experimentally determined local flame transfer functions are superposed using the flame transfer function superposition principle, and the result is incorporated into an analytic thermoacoustic model, in order to predict the linear stability characteristics of a given system. Results show that when the flame length is not acoustically compact the model prediction calculated using the local flame transfer functions is better than the prediction made using the global flame transfer function. In the case of a flame in the compact flame regime, accurate predictions of eigenfrequencies and growth rates can be obtained using the global flame transfer function. It was also found that the general response characteristics of the local FTF (gain and phase) are qualitatively the same as those of the global FTF.

Characterization of Forced Flame Response of Swirl-Stabilized Turbulent Lean-Premixed Flames in a Gas Turbine Combustor

Flame transfer function measurements of turbulent premixed flames are made in a model lean-premixed, swirl-stabilized, gas turbine combustor. OH∗, CH∗, and CO2∗ chemiluminescence emissions are measured to determine heat release oscillation from a whole flame, and the two-microphone technique is used to measure inlet velocity fluctuation. 2D CH∗ chemiluminescence imaging is used to characterize the flame shape: the flame length (LCH∗ max) and flame angle (α). Using H2-natural gas composite fuels, XH2=0.00–0.60, a very short flame is obtained and hydrogen enrichment of natural gas is found to have a significant impact on the flame structure and flame attachment points. For a pure natural gas flame, the flames exhibit a “V” structure, whereas H2-enriched natural gas flames have an “M” structure. Results show that the gain of M flames is much smaller than that of V flames. Similar to results of analytic and experimental investigations on the flame transfer function of laminar premixed flames, it shows that the dynamics of a turbulent premixed flame is governed by three relevant parameters: the Strouhal number (St=LCH∗ max/Lconv), the flame length (LCH∗ max), and the flame angle (α). Two flames with the same flame shape exhibit very similar forced responses, regardless of their inlet flow conditions. This is significant because the forced flame response of a highly turbulent, practical gas turbine combustor can be quantitatively generalized using the nondimensional parameters, which collapse all relevant input conditions into the flame shape and the Strouhal number.

Azimuthal source localization using interaural coherence in a robotic dog: modeling and application

SUMMARYIn nature, sounds from multiple sources, as well as reflections from the surfaces of the physical surroundings, arrive concurrently from different directions at the ears of a listener. Despite the fact that all of these waveforms sum at the eardrums, humans with normal hearing can effortlessly segregate interesting sounds from echoes and other sources of background noises. This paper presents a two-microphone technique for localization of sound sources to effectively guide robotic navigation. Its fundamental structure is adopted from a binaural signal-processing scheme employed in biological systems for the localization of sources using interaural time differences (ITDs). The two input signals are analyzed for coincidences along left/right-channel delay-line pairs. The coincidence time instants are presented as a function of the interaural coherence (IC). Specifically, we build a sphere head model for the selected robot and apply the mechanism of binaural cues selection observed in mammalian hearing system to mitigate the effects of sound echoes. The sound source is found by determining the azimuth at which the maximum of probability density function (PDF) of ITD cues occurs. This eliminates the localization artifacts found during tests. The experimental results of a systematic evaluation demonstrate the superior performance of the proposed method.