Acoustic Quantities Research Articles

Reconstructing transient acoustic radiation from an arbitrary object with a uniform surface velocity distribution.

This paper presents the general formulations for reconstructing the transient acoustic field generated by an arbitrary object with a uniformly distributed surface velocity in free space. These formulations are derived from the Kirchhoff-Helmholtz integral theory that correlates the transient acoustic pressure at any field point to those on the source surface. For a class of acoustic radiation problems involving an arbitrarily oscillating object with a uniformly distributed surface velocity, for example, a loudspeaker membrane, the normal surface velocity is frequency dependent but is spatially invariant. Accordingly, the surface acoustic pressure is expressible as the product of the surface velocity and the quantity that can be solved explicitly by using the Kirchhoff-Helmholtz integral equation. This surface acoustic pressure can be correlated to the field acoustic pressure using the Kirchhoff-Helmholtz integral formulation. Consequently, it is possible to use nearfield acoustic holography to reconstruct acoustic quantities in entire three-dimensional space based on a single set of acoustic pressure measurements taken in the near field of the target object. Examples of applying these formulations to reconstructing the transient acoustic pressure fields produced by various arbitrary objects are demonstrated.

An interpolated time-domain equivalent source method for modeling transient acoustic radiation over a mass-like plane based on the transient half-space Green’s function

Interpolated time-domain equivalent source method (ITDESM) is based on the assumption of free space, which makes it not suitable for reconstructing the transient acoustic quantities in the half space. Here, a half-space ITDESM is proposed to model the transient acoustic radiation over a mass-like plane. In this method, the free transient Green’s function existing in the conventional ITDESM is replaced by a closed-form transient half-space Green’s function for a mass-like plane. Such transient Green’s function enables one to take the reflection effect of the mass-like plane into consideration. Modeling acoustic radiation from three transient monopoles above an infinite plane with mass-like behavior is studied by numerical simulations to demonstrate the feasibility of the half-space ITDESM. The proposed method is also examined by comparing the reconstruction accuracy among a free-field model, a rigid plane model and a mass-like plane model. An experiment with an impacted steel plate lying above a table plate is conducted in the semi-anechoic room, and the results further verify the effectiveness of the proposed method.

Comparison of Hanging Panels and Boundary Diffusers in a Reverberation Chamber

Achieving a diffuse sound field in a reverberation chamber is crucial for measurements of acoustic quantities such as sound absorption coefficient, scattering coefficient, and sound power level. Toward this aim, diffusing elements such as hanging panels or rotating diffusers are typically installed in the chamber. However, previous research has suggested that hanging panels violate certain theoretical assumptions regarding diffusivity. Also, rotating diffusers cause the chamber to be a time-variant system, precluding the use of some measurement approaches such as sine-sweep integrated impulse response techniques. Boundary diffusers are offered as an alternative in the current study. The effects of both hanging panels and boundary diffusers on sound field diffusivity in a scale reverberation chamber are systematically and comparatively analyzed. The field diffusivity is characterized based on the guidelines set forth in American and international standards, including ISO 354, ASTM C423, and ASTM E90. Resulting data suggests that these standardized methods do not adequately or rigorously quantify diffusivity. The relative effectiveness of hanging diffusers vs. boundary diffusers will be discussed.

A model for sound propagation between two adsorbing microporous plates.

A model describing the sound propagation between two infinite adsorbing plates is proposed in order to investigate the extension to the audible sound range of the Frequency Response method applied to the measurement of diffusion in micropores. The model relates adsorption parameters (i.e., diffusivity and equilibrium constant) to an acoustic quantity (i.e., propagation constant). The equations describing sound propagation in the presence of adsorbing boundaries are obtained on the basis of the classical Kirchhoff theory [(1868). Ann. Phys. (Leipzig) 134, 177-193]. The solution is derived using the Low Reduced Frequency Approximation method [Tijdeman, (1975). J. Sound Vib. 39, 1-33].

Effect of boundary diffusers in a reverberation chamber: standardized diffuse field quantifiers.

The sound field inside a reverberation chamber must have a high degree of diffusivity to allow for the accurate measurement of various acoustic quantities. Typically, hanging or rotating diffuser panels are installed in the chamber in an effort to achieve this diffusivity. However, both of these diffuser types have certain limitations, and adequate sound field diffusivity is often difficult to realize. A 1:5 scale reverberation chamber has been used to systematically analyze the relative effectiveness of hanging diffusers versus an alternative diffuser type referred to as a boundary diffuser. To characterize sound field diffusivity, three quantifiers from the ASTM E90, ASTM C423, and ISO 354 standards have been used: maximum absorption coefficient, standard deviation of decay rate, and total confidence interval. Analysis of the quantifier data reveals that boundary diffusers and hanging diffusers produce roughly equivalent diffusion in the sound field. The data also show that the standards have certain inconsistencies that can obfuscate the characterization of sound field diffusivity, which may explain reproducibility and repeatability issues previously documented in the literature.

Alternatives to traditional probabilistic approaches for incorporating environmental uncertainty in ocean acoustics

Quantifying environmental uncertainty and its corresponding mapping onto the acoustic field is a subject of considerable importance for underwater acoustics modeling and simulation. The ability to make accurate numerical predictions of basic acoustic quantities, as well as sonar system performance in complex ocean environments depends, in part, on objectively quantifying our imperfect knowledge of the waveguide properties. Over the past few years, a number of approaches have been considered to quantify uncertainty in ocean-acoustics modeling; these methods are grounded in some form of traditional probabilistic reasoning. While probability theory provides a natural framework for representing and quantifying environmental uncertainty, a significant amount of information must be available to specify density functions for the parameters and fields so that the corresponding acoustic field statistics can be computed. This talk will overview alternatives to traditional probability-based reasoning that have been introduced in research areas where conventional specification of probability distributions are not a practical means for describing uncertainty. [Work supported by base funding from ONR.]

Forced Nonlinear Vibrations of a One-Dimensional Bar with Arbitrary Distributions of Hysteretic Damage

An analytical model is presented which predicts the forced, nonlinear response of a bar with arbitrarily distributed damage. Damage, which is either described by quadratic hysteresis, or due to dislocations interacting with point defects distributed along the dislocations’ glide planes, is considered. The wave equation is solved by means of a perturbation approach. Resonance frequency shift caused by damage-induced material softening, nonlinear attenuation, and higher harmonics’ generation are evaluated. For damage which is described by quadratic hysteresis, this model recovers the well-known dependence of the three acoustic quantities mentioned above on the source’s strength. On the other hand, for damage due to dislocations, both frequency shift and nonlinear attenuation present a distinctive nonlinear behavior the origin of which resides in the stress dependence of the fraction of dislocations breaking away from the point defects. Furthermore, different distributions of damage having the same integrated intensity are shown to generate nonlinear effects of increasing magnitude as their spatial extent decreases. Finally, it is suggested that, once the effect of the source’s strength is removed, spectral features may be used to assess the spatial extent of damage.

Comparison of Strategies to Model Spatial Fluctuations of Room Acoustic Single Number Quantities

In a number of independent empiric studies it was shown that room acoustic single number parameters vary severely with small changes of the source and microphone position. Presently there is no evidence that these spatial fluctuations can be modelled using simulated impulse responses. As a result there is limited knowledge about the origin and the contributing influence factors of this variance over space. In this contribution the results of simulations using wave based as well as ray tracing simulations are compared to each other. It will be discussed if these simulations are able to predict the fluctuations that were found in measurement series taken in a number of different auditoria.

Three-dimensional reconstruction of sound fields based on the acousto-optic effect

The acousto-optic effect can be used to measure the pressure fluctuations in air created by acoustic disturbances (the propagation of light is affected by changes in the medium due to the presence of sound waves). This makes it possible to measure an arbitrary sound field using acousto-optic tomography via scanning the field with a laser Doppler vibrometer. Consequently, the spatial characteristics of the sound field are captured in the measurement, implicitly bearing the potential for a full holographic reconstruction in a three-dimensional space. Recent studies have examined the reconstruction of sound pressure fields from acousto-optic measurements in the audible frequency range, based on Fourier transforms and elementary wave expansion methods. The present study examines the complete reconstruction of the sound field from acousto-optic measurements, recovering all acoustic quantities, and compares the results to the ones obtained from conventional microphone array measurements.

The urgent need for standardization of soundscape

In recent years, the soundscape approach gains increasingly in importance. It has shown its potential dealing with environmental noise issues, which conventional noise control approaches frequently fail to cope with. The soundscape concept is applied to comprehend the way humans experience and understand their acoustic environments. It does not simply reduce noise to an averaged acoustic quantity evoking unpleasantness estimated by statistical probabilities, but it considers noise as a valuable resource, which can be purposefully utilized. The standardization of “soundscape” in ISO 12913 is currently in progress. The standard will include definitions, terms and a conceptual framework illustrating that soundscape goes beyond physics and psychoacoustics, since it reflects human perception including cognitive aspects, context and interaction. The paper describes the progress in standardizing soundscape and the implications of the standard with its potential relevance to noise policy. Moreover, it discusses how to meet the challenge to standardize and harmonize the soundscape approach without losing the openness and multidimensionality of the soundscape idea.

Visual Encoding of Acoustic Parameters – Framework and Application to Noise Mapping

Along with an increasing importance and awareness of sonic phenomena related to the human environment, there is a growing need for a comprehensive communication of sound information. The central goal of this contribution is the development of a systematic and comprehensive framework that considers the particular properties of sound phenomena. This framework is structured according to the acoustic quantities that shall be represented and consists of recommendations for their cartographical encoding. A proof of concept is given through the application to official strategic noise maps in the course of the Environmental Noise Directive. Those maps suffer from several weaknesses from a cartographical point of view (e.g. concerning colour schemes or band-like illustrations) which can be eliminated by applying some of the mentioned recommendations. Finally, it will be possible to derive valuable knowledge from our results for multimodal encoding and value transformation between senses.

Cubic spline interpolation-based time-domain equivalent source method for modeling transient acoustic radiation

Transient acoustic radiation from an arbitrarily-shaped source is modeled using a cubic spline interpolation-based time-domain equivalent source method. In the method, the cubic spline interpolation function is introduced to approximate the equivalent source strengths in the time domain for establishing the relationships between the acoustic quantities in the field, the normal acceleration on the source surface and the equivalent sources, and the least-squares solution of the equivalent source strengths at each time step is determined by an iterative solving process with the singular value decomposition technique. The proposed method has the abilities of calculating the acoustic quantities directly in the time domain and applying to arbitrarily-shaped sources. Numerical examples of calculating the transient acoustic fields from a baffled planar piston and an impulsively accelerating sphere demonstrate that the proposed method can model the transient acoustic radiation very well. The experimental results also further verify the validity of the proposed method.

A numerical and experimental validation of the room acoustics diffusion theory inside long rooms

The paper focuses on the validation of the recently proposed room-acoustics diffusion theory by means of numerical simulations and experimental measurements. The analysis aims to verify the equation underlying the theory (Fick’s law of diffusion) which relates the energy density gradient and the sound intensity inside a room through a constant diffusion coefficient. In this work, the acoustic quantities are numerically/experimentally derived under stationary conditions, and their ratio is employed to estimate the effective value of the diffusion coefficient inside long rooms. The numerical study was carried out with particle-tracing simulations. The measurements were performed with a Microflown® three-dimensional sound intensity probe inside a 1:16 scale model of a long room, varying the absorption and the scattering properties at the boundaries. A comparison between numerical and experimental results is carried out with a least-square algorithm, showing a fair agreement between the diffusion coefficients estimated with the two methods. The results lead to the conclusion that the reverberant sound field inside long rooms can be described by a non-homogeneous diffusion process: the local diffusion coefficient is not a constant inside the room but increases with the distance from the source and depends on the acoustical properties of the room boundaries.

Aerodynamical sounding mechanism in flue instruments: Acceleration unbalance between the jet vortex layers

According to particle image velocimetry (PIV) measurement applied to the sound production in organ flue pipes and flutes, the vortex shedding at the pipe edge proposed by Howe (1975) are not observed but the formation of the vortex layer is clearly observed along both sides of the jet flow. This has been confirmed in various sounding conditions with different blowing pressures and resulting pitches. The acceleration unbalance is generated from an incomplete cancelation of the aeroacoustical source term (ω×U) between both sides of the jet, where U is the jet velocity and ω (= rotU) the vorticity. In addition, the vortex layer is essentially unstable because it is formed along the inflection point of the lateral jet-velocity profile. Therefore, the acceleration unbalance and inflection instability of the vortex layer activates the jet wavy motion to reinforce the inward or outward acoustic velocity u at the pipe mouth. Phase relations between acoustic quantities approve conventional acoustical models based on the volume-flow drive and momentum drive. Since ω×U can also activate the jet movement in edge-tone generation, the vortex-layer formation may be regarded as the fluid-dynamical mechanism common to the edge-tone generation and the pipe-tone generation.

Telling it like it is.

With the amazing successes that have been reported over the past 10 years, it is easy for researchers in therapeutic ultrasound to be seduced by the appeal of biological and clinical studies - but understanding the biology alone is insufficient. All good science requires repeatability, and developing a successful therapy is no different. It is imperative from both a scientific and a commercial perspective that we can not only demonstrate the interaction of ultrasound with biological systems but also quantify the relationship between the delivered ultrasound and the biological effect. Moreover, we then need to be able to control the delivered ultrasound - the ultrasound dose, if you like - to achieve consistent and repeatable treatments. A distinguishing feature of our new journal - whose stated goal is to ‘accelerate the adoption of therapeutic ultrasound as a clinical tool’ - should be an insistence that best practice is followed first when planning the ultrasound exposure regime and again when reporting those conditions. We should set a clear example for other journals, and for funding bodies, to follow. This will make it apparent, even to those researchers who are not primarily interested in physics and engineering, that knowledge and control of the delivered ultrasound is not something that can be ignored until writing the final paper. It absolutely must be built into a study even before the proposal is submitted. Too often, even in high-quality and esteemed journals, the exposure description is inadequate, and assumptions (for instance, about the mechanism of interaction) are not pro perly explained. The researchers themselves may often not be fully aware of these weaknesses in their study. It is our belief that a handful of simple guidelines [1] published by the authors of this piece will serve to remind researchers of the importance of good ultrasound measurement and exposure estimation and will lead over time to a greater body of research which can be replicated and tested by others. Our starting principles in preparing these guidelines were simple enough: firstly, that for a quantitative exposure-response study, there must be some measurements of the actual ultrasound field or fields used; and secondly, that the measured values (usually in water, following methods described in international standards) must be reported, and the methods for estimating any other in situ or exposure quantities must be clearly laid out. Of course it is true that the ultrasound field measured in water may be very different from the field in tissue or the sample holder in vitro. Nevertheless, having a proper description of the undisturbed field is the first step to understanding the experimental conditions and to being able to replicate the exposure. Taken together, these two principles will help avoid many of the potential misunderstandings that can arise. The full set of recommendations and example wording are discussed in detail in [1]. As well as reading these, authors and reviewers for our journal should ask themselves some simple questions at all stages of their work. Q1. Does this study attempt to demonstrate correlation between exposure to ultrasound and a physical or biological effect? If it does, it must include acoustic output measurements of the ultrasound field to determine maximum values in the field or, preferably, the acoustic pressure and intensity distributions. Q2. Are the measured values of output power, acoustic pressure, intensity, and other acoustic quantities reported? Have the measurement methods been described in sufficient detail to be replicated by others? Q3. Has the temperature rise in the region of the cells or tissue of interest (or in a suitable substitute) been measured and reported, even if a thermal mechanism is not being tested? Q4. If estimated values of in situ acoustic pressure or intensity are reported (for instance by using a ‘derating’ factor), has the method for calculating the in situ value been fully explained, and a worked example given? Is the estimated transmission loss in the propagation path reasonable? Q5. If a thermal or mechanical index is calculated for use as an exposure quantity, has a reasonable value for the attenuation coefficient of the propagation path been used as the derating factor? Has the calculation been stated mathematically and a worked example given? Asking - and answering! - these questions will help produce a rigorous study which will provide a solid basis for conclusions and for future work. Let us start as we mean to go on with our own journal and demand the highest standards of measurement and reporting of ultrasound fields and exposure levels.

Causality correlation analysis on a cold jet by means of simultaneous particle image velocimetry and microphone measurements

The aeroacoustic sound generation processes of a cold jet were investigated by means of simultaneous Particle-Image-Velocimetry (PIV) in the near-field and microphone measurements in the far field. The measurements were conducted in a synchronized manner so as to enable the calculation of the cross correlation coefficient between the acoustic pressure and flow quantities derived from the measured velocity fluctuations. In this manner the regularities in the near-field fluctuations which are related to the radiated sound field could be identified. The tests were run for different Mach numbers M=0.5, 0.7 and 0.9 in air. Five microphones arranged at angles of 24°<θ<88° to the jet centre line were used. The correlation with the axial velocity was highest. The spatial as well as the temporal distribution of the coefficient was observed to be dominated by coherent structures and showed a strong dependency on M and θ. For the M=0.9 jet the correlation coefficients fell below the confidence interval for θ=67°.

A stochastic response surface formulation for the description of acoustic propagation through an uncertain internal wave field

A modeling and simulation study is performed in a littoral ocean waveguide subject to uncertainty in four quantities: source depth, tidal forcing, initial thermocline structure, and sediment sound speed. In this partially known shelf-break environment, tidal forcing over a density-stratified water column produces internal tides and solitary wave packets. The resulting uncertainty in the space-time oceanographic field is mapped into the sound speed distribution which, in turn, introduces uncertainty into the acoustic wave field. The latter is treated as a stochastic field whose intensity is described by a polynomial chaos expansion. The expansion coefficients are estimated through constrained multivariate linear regression, and an analysis of the chaos coefficients provides insight into the relative contribution of the uncertain acoustic and oceanographic quantities. Histograms of acoustic intensity are estimated and compared to a reference solution obtained through Latin Hypercube sampling. A sensitivity analysis is performed to illustrate the relative importance of the four contributions of incomplete information about the environment. The simulation methodology represents an end-to-end analysis approach including both oceanographic and acoustic field uncertainty where the latter is quantified using stochastic basis expansions in the form of a polynomial chaos representation.

A new method for auralisation of airborne sound insulation

Auralisation techniques have quickly grown in popularity during the last few years, mostly due to their application in virtual reality, the gaming industry and room acoustics. The application of auralisation to sound insulation has also become a valuable tool to assess the sound insulation of buildings, and to suitably describe to the non-acousticians how is the sound experience hidden behind the swarm of acoustic quantities. Auralisation is used now in commercial software to show the relevance of some acoustic features that are often not well-understood by users and constructors until too late.This paper presents a new method to compute the binaural sound transmission between rooms. As the established method, this approach makes use of the EN 12354 sound insulation predictions to build up the auralisation of the sound insulation predictions. In addition, it takes into account the spatial variation of the sound pressure field inside arbitrarily shaped rooms, enabling the user to experience binaural sound from arbitrary sound sources and varying the source and receiver position. Additionally, the method addresses the problem of synthesising the room impulse response from the one-third octave band values of the reverberation time obtained as per ISO 3382-2:2008.The new method increases interaction with the simulated environment creating a deeper, more realistic and immersive scene and leading to more accurate subjective evaluations of sound insulation.

Durational Interaction of Stops and Vowels in English and Korean Child-Directed Speech

The current study observes the durational interaction of tautosyllabic consonants and vowels in the word-initial position of English and Korean child-directed speech (CDS). The effect of phonological laryngeal contrasts in stops on the following vowel duration, and the effect of the intrinsic vowel duration on the release duration of preceding stops in addition to the acoustic realization of the contrastive segments are explored in different prosodic contexts - phrase-initial/medial, focal accented/non-focused - in a marked speech style of CDS. A trade-off relationship between Voice Onset Time (VOT), as consonant release duration, and voicing phonation time, as vowel duration, reported from adult-to-adult speech, and patterns of durational variability are investigated in CDS of two languages with different linguistic rhythms, under systematically controlled prosodic contexts. Speech data were collected from four native English mothers and four native Korean mothers who were talking to their one-word staged infants. In addition to the acoustic measurements, the transformed delta measure is employed as a variability index of individual tokens. Results confirm the durational correlation between prevocalic consonants and following vowels. The interaction is revealed in a compensatory pattern such as longer VOTs followed by shorter vowel durations in both languages. An asymmetry is found in CV interaction in that the effect of consonant on vowel duration is greater than the VOT differences induced by the vowel. Prosodic effects are found such that the acoustic difference is enhanced between the contrastive segments under focal accent, supporting the paradigmatic strengthening effect. Positional variation, however, does not show any systematic effects on the variations of the measured acoustic quantities. Overall vowel duration and syllable duration are longer in English tokens but involve less variability across the prosodic variations. The constancy of syllable duration, therefore, is not found to be more strongly sustained in Korean CDS. The stylistic variation is discussed in relation to the listener under linguistic development in CDS.

Modeling uncertainty in transmission loss due to spatio-temporal variation in environmental parameters

Tactical prediction and decision aid tools require acoustic propagation modeling. The effectiveness of these tools relies on knowledge of the transfer function between model inputs - either measured or predicted - and model outputs. Of particular interest is this transfer when inputs are uncertain and characterized statistically. The Recognized Environmental Picture experiment 2011 (REP11) was designed to provide observations of spatio-temporal variability in oceanographic and acoustic quantities. REP11 was comprised of multiple runs of co-located and contemporaneous oceanographic and acoustic measurements repeated over twenty-four hours. Acoustic measurements were made using a broad-band source towed along radials from a fixed receiver array. The sound speed field in the water column was sampled independently during each run using gliders, towed instruments, and moorings, each having a different spatio-temporal resolution. Due to the nature of the ocean environment, the sound speed field varied both in time and space for each traversal of the track, yielding a suite of realizations representing the sensitivity of acoustic pressure to spatio-temporal variations in the environment. Based on these data, the statistics of transmission loss from propagation simulations using the observed sound speed fields are compared to the statistics of the measured acoustic pressure fields.