Free-field Measurements Research Articles

Experimental characterization of the effective acoustic properties of anisotropic porous materials via free-field measurements

Porous media are commonly described as effective isotropic fluid materials, such that their acoustic properties can be adequately determined from their bulk modulus and dynamic density. Most acoustic absorbents, however, possess a marked anisotropy, which influences their acoustical behavior and the parameters necessary to deduce it. Specifically, the influence of anisotropy translates into a full symmetric density tensor (in place of a scalar). This study presents a method for retrieving the bulk modulus and all six components of the density tensor of a layer of porous material from reflection coefficients measured in free field with an array of microphones. The procedure consists in expressing the sound field measured in the vicinity of the layer as a superposition of plane waves, from which the surface impedance and the reflection coefficient are reconstructed. The reflection properties, as well as the pressure at the rigid backing interface, are estimated for various source positions (i.e., various angles of incidence) and an inverse problem is formulated to infer the effective fluid parameters. The validity of the method is examined experimentally on a manufactured porous material, and the resulting fluid parameters compared with experimental results obtained from reflection and transmission coefficients measured in an impedance tube.

A quarter wave infrasonic noise generator

The far-field prediction of infrasonic noise, e.g. from large emitters such as wind turbines, requires precise measurement of the sources. A method to determine the source power based on near-field holography techniques are currently under development in our group. However, tuning the method requires a calibrated infrasound source. In this work, we present the concept of a traceable loudspeaker driven infrasound source using the quarter wave resonator principle in a pipe. The power emitted by the source is measured in situ based on the two-port theory. The infrasonic frequencies can be tuned by adjusting the length of the resonator pipe. Results of the free-field measurements at multiple points at varying distances away from the source are presented. The results suggest that the infrasonic source can be used as a calibrated reference for method development and validation.

Front-back bias and the 1000-Hz interaural intensity anomaly

Free field measurements were made of the level differences, as measured in an ear canal, for a noise source in front of the head compared to a source in back. The measurements were made on a Kemar manikin for 19 lateral angles over the complete 180-degree range for front-back comparisons. There were 7 octave bands, 125–8000 Hz, and two horizontal planes. Front-back level differences were generally positive except for the dramatic case of 1000 Hz, where the differences were negative over an azimuth range of 120 degrees. When the head-related elevation of the sources was increased from zero to 10 degrees, the range of the negative front-back differences grew to 140 degrees. These physical results are consistent with the strong tendency for listeners to identify a one-third octave noise band in a narrow range near 1000 Hz as a source in back. This tendency may be the origin of the often-observed Grantham effect, wherein the difference limen for the interaural level difference is larger near 1000 Hz than at higher or lower frequencies. The connection is that sources in back of the listener, whatever their frequencies, are not localized nearly as well as sources in front.

CMUT as a Transmitter for Microbubble-Assisted Blood-Brain Barrier Opening.

Focused ultrasound (FUS) combined with microbubbles (MBs) has emerged as a promising strategy for transiently opening the blood-brain barrier (BBB) to enhance drug permeability in the brain. Current FUS systems for BBB opening use piezoelectric transducers as transmitters and receivers. While capacitive micromachined ultrasonic transducers (CMUTs) have been suggested as an FUS receiver alternative due to their broad bandwidth, their capabilities as transmitters have not been investigated. This is mainly due to the intrinsic nonlinear behavior of CMUTs, which complicates the detection of MB generated harmonic signals and their low-pressure output at FUS frequencies. Various methods have been proposed to mitigate CMUT nonlinearity; however, these approaches have primarily targeted contrast enhanced ultrasound imaging. In this study, we propose the use of polyphase modulation (PM) technique to isolate MB emissions when CMUTs are employed as transmitters for BBB opening. Our calculations for a human scale FUS system with multiple CMUT transmitters show that 10-kPa peak negative pressure (PNP) at 150-mm focal distance will be sufficient for MB excitation for BBB opening. Experimental findings indicate that this pressure level can be easily generated at 400-800 kHz using a readily available CMUT. Furthermore, more than 50-dB suppression of the fundamental harmonic signal is obtained in free field and transcranial hydrophone measurements by processing receive signals in response to phase-modulated transmit waveforms. In vitro validation of PM is also conducted using Definity MB flowing through a tube phantom. MB-filled tube phantoms show adequate nonlinear signal isolation and SNR for MB harmonic detection. Together our findings indicate that PM can effectively mitigate CMUT harmonic generation, thereby creating new opportunities for wideband transmission and receive operation for BBB opening in clinical and preclinical applications.

Free-field characterization of locally reacting sound absorbers using Bayesian inference with sequential frequency transfer

The in situ absorption characteristics of sound-absorbing samples are generally estimated inversely from sound field measurements above the sample’s surface. However, the entire measurement process is subject to numerous sources of uncertainty, which cannot be accounted for using deterministic inference methods. This paper introduces a Bayesian framework, including sequential frequency transfer to infer point estimates and uncertainty ranges of the frequency-dependent surface admittance, the corresponding sound absorption coefficient and the associated measurement and model uncertainty. The inference is conducted in two phases: in the initial phase, the absorption characteristics and the measurement and model uncertainty at a predefined initial frequency are inferred based on a weakly informative prior. Extensive prior predictive checks are conducted to find the best trade-off between a weak level of informativeness of the prior and physically meaningful predictions of the sound field above the absorber. In the sequential phase, the posterior mean at an analyzed frequency serves as the prior mean at the subsequent frequency. The framework is applied to simulated as well as real free-field measurements of the specific impedance above two different rock wool samples. The measurements are conducted with a combined pressure-particle velocity probe and the samples are assumed to exhibit local reaction behavior. For the investigated samples, a minimum number of four individual measurements of the specific impedance already allows for an accurate estimation of the frequency-dependent absorption characteristics and the associated measurement and model uncertainty, particularly for frequencies above 600 Hz. Due to the proposed sequential frequency transfer, the computational effort of the sampling-based Bayesian inference at a discrete frequency is significantly reduced. This paves the way for efficient inference of frequency-dependent model parameters while overcoming the curse of dimensionality associated with the inference of stochastic processes. The proposed framework lays the foundation for accurate and efficient inference of frequency-dependent parameters in models beyond the field of acoustics.

Investigation of simultaneous effects of noise barriers on near-road noise and air pollutants

Noise barriers are one of the common solutions to control road traffic noise. Many studies have also shown that noise barriers cause reductions in near-road air pollutant concentrations. In this study, the simultaneous effects of a specific noise barrier application on near-road noise and air pollution at a specific location were investigated. In this context, air pollution, noise, and meteorological parameters were measured simultaneously at two points, road and receptor sides of a 50 m long, 4 m high glass fiber reinforced concrete noise barrier on a highway section. Results indicated that the noise barrier has an average 23 % reduction effect on the NOx concentration in addition to the noise level reduction at the receptor side. Besides, bi-weekly average passive sampler measurement results for BTEX pollutants indicate lower values at the receptor side of the barrier compared to the free field measurement results. In addition to real-time and passive sampler measurements, NOx and noise dispersions were modeled using RLINE and SoundPLAN 8.2 software, respectively. Comparisons of the measurement results with the model results indicated strong correlations. Model-calculated NOx and noise values under the free field conditions are highly compatible with a correlation coefficient (r) of 0.78. Although the noise barrier has a reduction effect on both parameters, it has been observed that their dispersion mechanisms are different. This study showed that noise barriers considerably affect the dispersion of road-sourced air pollutants at the receptor side. Further studies are needed to optimize noise barrier designs with different physical and material properties and application scenarios considering noise and air pollutants together.

Towards Auditory Profile-Based Hearing-Aid Fittings: BEAR Rationale and Clinical Implementation.

(1) Background: To improve hearing-aid rehabilitation, the Danish ‘Better hEAring Rehabilitation’ (BEAR) project recently developed methods for individual hearing loss characterization and hearing-aid fitting. Four auditory profiles differing in terms of audiometric hearing loss and supra-threshold hearing abilities were identified. To enable auditory profile-based hearing-aid treatment, a fitting rationale leveraging differences in gain prescription and signal-to-noise (SNR) improvement was developed. This report describes the translation of this rationale to clinical devices supplied by three industrial partners. (2) Methods: Regarding the SNR improvement, advanced feature settings were proposed and verified based on free-field measurements made with an acoustic mannikin fitted with the different hearing aids. Regarding the gain prescription, a clinically feasible fitting tool and procedure based on real-ear gain adjustments were developed. (3) Results: Analyses of the collected real-ear gain and SNR improvement data confirmed the feasibility of the clinical implementation. Differences between the auditory profile-based fitting strategy and a current ‘best practice’ procedure based on the NAL-NL2 fitting rule were verified and are discussed in terms of limitations and future perspectives. (4) Conclusion: Based on a joint effort from academic and industrial partners, the BEAR fitting rationale was transferred to commercially available hearing aids.

The estimation of distance and power of multiple sound sources by combining three-dimensional sound intensity and beamforming

The problem of identifying multiple sound sources occurs in various fields. This study proposes a method for multi-source localization and sound power estimation based on three-dimensional sound intensity (3DSI) measurement combined with high-precision beamforming. First, according to the propagation law of the point source in the free field and 3DSI measurement method, a mathematical model of the sound intensity vector, sound power, and source location in 3D space for multiple sources is derived. The 3DSI components can be measured by 3DSI probes. To reduce the computational cost required to solve the mathematical model, the angles of the sources are estimated via compressive sensing beamforming orthogonal matching pursuit for the method. Moreover, the sound intensity components and angles are used as inputs in the multiple sound source identification model, and the dimensions of the model are significantly reduced. The model was solved to obtain sound source distance and sound power. Two tetrahedral intensity probes and a 16-channel microphone array were used for experiments in a semi-anechoic chamber. The results demonstrate that the method can accurately identify the locations of multiple sound sources and determine the sound power. Considering three sound sources, the maximum angular error is 1.41°, while the maximum distance error is 0.08 m. This method can use a small number of probes to obtain the sound sources spatial position and sound source power, and it improve the resolution and calculation efficiency.

Study of Installation Effects on Automotive Cooling Fan Noise

<div class="section abstract"><div class="htmlview paragraph">Vehicle electrification is one of the biggest trends in the automotive industry. Without the presence of combustion engine, which is the main noise source on conventional vehicles, noise from other components becomes more perceivable; among these components, the cooling fan is one of the major noise sources, especially during battery charging. The design of cooling fan modules is usually carried out in the early stage before building prototype vehicles. Therefore, understanding the installation effects of the cooling fan on the radiated sound is essential to secure good customer satisfaction. In this study, three different measurement setups of cooling fans are carried out: free field, wall mounted, and in-vehicle measurement. Four cooling fan prototypes with different fan blade designs are used in each measurement. Correlations of these measurements are investigated through comparisons of the measurement results. The installation effects are identified through spectrum difference between free field and in-vehicle measurement. A spectral decomposition method is implemented to enable the separation of source strength and propagation effect in the results. One variable is introduced to represent the installation effects of vehicles tested in the present study.</div></div>

Prediction of shooter localization accuracy in an urban environment

The use of acoustic sensor networks for shooter localization can provide a vital contribution to situational awareness in urban environments. During mission planning, the performance prediction for acoustic shooter localization plays a central role as it allows an optimization of the sensor positions in a sensor network. Instead of a sound propagation-based approach, in our work we focus on an information theoretical analysis using the Cramér-Rao bound to predict the achievable shooter localization accuracy. Through this approach, we have shown that accounting for incomplete and heterogeneous acoustic measurement data sets leads to maximization of the fusion gain and consequently to improved achievable localization accuracy. We validated the match between predicted and actual experimental performance in free-field measurements with supersonic gunshots including varying sensor-to-shooter geometries, weapon types, and various measurement types. By measuring signatures of impulsive gas cannon shots in urban terrain, we analyzed the effect of buildings to the sensor performance and achieved a significant improvement in the localization performance prediction by adjusting the sensor model depending on whether line-of-sight or non-line-of-sight conditions to the target exists.

School-age children show poor use of spatial cues in reverberation for speech-in-speech perception

Understanding speech is particularly difficult for children when there is competing speech in the background. When the target and masker talkers are spatially separated, as compared to co-located, the access to corresponding auditory spatial cues can provide release from masking, resulting in an intelligibility gain for speech-in-speech perception. When tested in free-field environments, previous work showed that children demonstrate adult-like spatial release from masking (SRM) by 9–10 years of age. However, in indoor environments where most critical communications occur such as classrooms, reverberation distorts the critical auditory spatial cues that lead to reduced SRM among adults. Little is known about how children process distorted auditory spatial cues for SRM to aid speech-in-speech perception. In this work, we measure SRM in children in virtual reverberant environments that mimic typical learning spaces. We show free-field measurements overestimate SRM maturation in realistic indoor environments. Children show a more protracted development of SRM in reverberation, likely due to immaturities in using distorted auditory spatial cues.

Validation of a coupled atmospheric–aeroelastic model system for wind turbine power and load calculations

Abstract. The optimisation of the power output of wind turbines requires the consideration of various aspects including turbine design, wind farm layout and more. An improved understanding of the interaction of wind turbines with the atmospheric boundary layer is an essential prerequisite for such optimisations. With numerical simulations, a variety of different situations and turbine designs can be compared and evaluated. For such a detailed analysis, the output of an extensive number of turbine and flow parameters is of great importance. In this paper a coupling of the aeroelastic code FAST (fatigue, aerodynamics, structures, and turbulence) and the large-eddy simulation tool PALM (parallelised large-eddy simulation model) is presented. The advantage of the coupling of these models is that it enables the analysis of the turbine behaviour, among others turbine power, blade and tower loads, under different atmospheric conditions. The proposed coupling is tested with the generic National Renewable Energy Laboratory (NREL) 5 MW turbine and the operational eno114 3.5 MW turbine. Simulating the NREL 5 MW turbine allows for a first evaluation of our PALM–FAST coupling approach based on characteristics of the NREL turbine reported in the literature. The basic test of the coupling with the NREL 5 MW turbine shows that the power curve obtained is very close to the one when using FAST alone. Furthermore, a validation with free-field measurement data for the eno114 3.5 MW turbine for a site in northern Germany is performed. The results show a good agreement with the free-field measurement data. Additionally, our coupling offers an enormous reduction of the computing time in comparison to an actuator line model, in one of our cases by 89 %, and at the same time an extensive output of turbine data.

Reliability analysis of damping estimation by random decrement technique for high‐rise buildings

AbstractDamping ratio is one of the most important parameters in the dynamic analysis of high‐rise buildings for structural design. As a practical damping estimation method, the random decrement technique (RDT) has been widely applied to civil structures in the past decades. The inherent limitation of the RDT is that it can only provide a single optimal damping result, while the reliability of its estimated result is unavailable. By combining the RDT with the bootstrap method, this paper presents a reliability analysis scheme for damping estimation. Based on the bootstrap‐based scheme, the reliability of damping results estimated by the RDT using four commonly used triggering conditions is systemically investigated through numerical simulations. The results show that the reliability of damping estimates can be effectively evaluated by providing the confidence interval. Moreover, based on free vibration test results and field measurements of acceleration responses of a 600‐m high skyscraper during a typhoon, the bootstrap‐based scheme is further verified to be applicable and effective for evaluating the reliability of damping results of high‐rise buildings in field measurements. This paper aims to provide useful implications for evaluating the reliability of damping estimates and thereby improve the accuracy of damping evaluation of high‐rise buildings.

Estimating the acoustical properties of locally reactive finite materials using the boundary element method.

The finite size of a sound-absorbing material may lead to inaccurate results when measuring the acoustical properties of the material using the free-field measurement methods. In this study, a method of estimating the acoustical properties of locally reactive finite materials is proposed by combining a sound field model established by the boundary element method with an iteration algorithm. The proposed method takes the finiteness of the material into account, meaning that the size effect is removed and accurate results can be obtained. Numerical simulations and experiments of two kinds of materials, including a rigid floor and a porous material, are carried out to verify the validity of the proposed method. Results demonstrate that the proposed method is effective in estimating the acoustical properties of these two kinds of materials. Besides, a detailed analysis of the influences of the sample size, the source location, and the receiving point position is done in the simulations.

An ultra-wideband 6–14 GHz frequency modulated continuous wave primary radar with 3 cm range resolution

This paper describes the design and verification of an ultra-wideband 6–14 GHz frequency modulated continuous wave (FMCW) primary radar system with very high range resolution. The design and measurement results of the utilized signal generator and receiver are presented. The signal generator features a 86% relative continuous tuning range and average phase noise of −106 dBc/Hz at 1 MHz offset to the carrier. The radar system utilizes a hybrid structure where the voltage controlled oscillator was fabricated using 130 nm SiGe BiCMOS technology and the lower frequency components are off-the-shelf. Free field measurements of the radar showcase an unprecedented combination of 3 cm range resolution, low phase noise and low operating frequency of 6–14 GHz for inherently higher range.

Signal Processing Considerations on the Optical Measurement of Acoustic Particle Velocities in Free-Field Conditions

To overcome the limitations of existing acoustic standards based on microphones, the optical method based on photon correlation has been considered as a potentially new primary standard. In this article, the experimental measurement and signal processing procedures to measure acoustic pressures optically, thus realizing the acoustic pascal in a direct manner, from captured photon sequences are described. The assessment of specific major contributing parameters associated with the signal processing of the photon correlation method is investigated and discussed in detail for free-field measurements. To extract the information relating to the acoustic particle velocities resulting from captured photon sequences, specific signal processing procedures were investigated. First, the captured electrical pulse sequences are converted to a series of binarized photon event sequences. Next, the sequences are divided into time sets related to the time length of individual acoustic cycles at each frequency step; a gating window equal to a short part of the acoustic cycle itself is applied on the aforementioned time sets. The gating process is performed by shifting the time window through the acoustic cycle, with the autocorrelation function calculated for each phase offset step, which yields the acoustic particle velocity and, hence, pressure directly. Single and double gating are applied and their effects are investigated. In addition, the effect of specific signal processing parameters is studied and selection criteria are proposed. Comparison between optical and reciprocity calibrations in terms of sound pressure and device sensitivity in the frequency range of 0.5–16-kHz range revealed discrepancies within 0.01–1.3 dB.

Characterization of HIFU fields inside of a tissue-mimicking HIFU phantom

The use of a tissue-mimicking, scatterer-free polyacrylamide gel phantom for HIFU has been previously described [1]. This transparent material combines with a marker with an absorption of 0.6 dB/cm MHz which turns opaque when the temperature exceeds 65 °C. After a brief review of the material properties and their validation, we discuss typical uses of the phantom as part of QA and protocol development and discuss comparisons between phantom and free-field measurements in water at high intensities. Comparisons are also made to new investigations where in situ pressures have been measured by embedding a fiber-optic probe into a sample of a phantom material. [1] S. Howard, J. Yuen, P. Wegner, and C. Zanelli, “Characterization and FEA simulation for a HIFU phantom material,” in IEEE Ultrasonics Symposium (2003), pp. 1270–1273.

The Effect of Mounting Structure and Piezoelectric Pressure Probe Sensor Incident Angle on the Free-Field Measurement

Free-field shock wave (FSW) in explosion monitored by electrical wired system with piezoelectric pressure probe sensor (PPPS) has been widely investigated and applied. However, it is very difficult to accomplish accurate pressure measurement for shock wave (SW) due to steep rising edge, short pulse width characteristics of peak pressure signal and different incident angle of SW to sensor. This paper aims to find an approach to integrate the mounting structure with PPPS for the FSW testing more accurately. First, we design two mounting structures namely disc structure and wedge structure for sensor installation and evaluate the impacts of different incident angles of SW on PPPS under two installation structures by simulation. Furthermore, the pressure signals are obtained by two measurement system relying on commercial PPPSs from PCB and Kistler Co., Ltd., respectively, and we reconstruct the overpressure waveform by wired measurement system. The results from the experiment demonstrate that the reconstructed waveforms have good uniformities. Depending on the simulation model and experiment data, this paper is conducted to provide new insights and approach into how the integration of mounting structure and PPPS is implemented for acquiring dynamic parameters accurately, and further giving advisable reference to various sensors and structures applied in more FSW monitoring scenarios.

Acoustic performance of noise barrier based on sonic crystals with resonant elements

Recent years have seen a growing interest in the potential for the use of sonic crystals as noise barriers. The frequencies of highest attenuation can be found by assuming that an integer number of half wavelengths fits the distance between the scatterers. However, this leads to a reduction in its usefulness as a viable noise barrier technology, due to the necessary increase in overall crystal size to increase frequency range of noise reduction. Additional attenuation bands may be achieved if the array is composed of resonant elements instead of solid cylinders. The paper presents resonant sonic crystal barrier that could be applied to stationary but movable noise sources eg. power generator. The results of simulation results were confronted with the results of previous work in this field. This work presents also sound insulation measurements conducted over sonic crystal noise barrier according to the European standard EN 1793-6. In most of the reference literature, sound insulation and reflection properties of sonic crystals are measured or a diffuse sound field or in a direct sound field including the top and side edge diffraction effects together with the transmitted (or reflected) components. The aim of this work is to perform free-field measurements over a real-sized sample in order to verify the points of strength and weakness of the application of standardised measurements to sonic crystals.

A new application for reverberation tanks: measurement of underwater impulsive sound characteristics

A new application for reverberation tanks, used to measure the sound energy of impulsive sound sources, is proposed. According to the law of conservation of energy, the impulsive sound energy obtained by sound intensity integration in the time domain is the same as the sound energy density spectrum obtained by integration in the frequency domain. By utilizing both the multipath incoherent processing technique and invariant sound field correction factors, the energy source level of impulsive sound in a reverberation tank can be obtained, and the measurement methods in the frequency and time domains have both been established. The energy source level of the impulsive sound, on the one hand, is calculated based on the sound energy density spectrum in the frequency domain and, on the other hand, is obtained based on self-correlation in the time domain. Both a sinusoidal impulsive sound experiment and a spark source signal experiment were carried out at UATL (Underwater Acoustics Technology Lab, Harbin Engineering University, China) to confirm the validity of the proposed method. Experimental results showed that the energy source level of the impulsive sounds could be far more precisely measured in reverberation tanks than using free field measurements. The largest deviation was less than 1.1 dB for the impulsive test, and the average deviation was less than 1 dB for the spark source signal test with multiple measurements in different tanks. This study provides a useful method for measuring the characteristics of impulsive signals in reverberation tanks, which greatly extends their range of application.