Acoustic Pressure Waves Research Articles

Pressure waves in a supersaturated bubbly magma

SUMMARY We study the interaction of acoustic pressure waves with an expanding bubbly magma. The expansion of magma is the result of bubble growth during or following magma decompression and leads to two competing processes that affect pressure waves. On the one hand, growth in vesicularity leads to increased damping and decreased wave amplitudes, and on the other hand, a decrease in the effective bulk modulus of the bubbly mixture reduces wave velocity, which in turn, reduces damping and may lead to wave amplification. The additional acoustic energy originates from the chemical energy released during bubble growth. We examine this phenomenon analytically to identify conditions under which amplification of pressure waves is possible. These conditions are further examined numerically to shed light on the frequency and phase dependencies in relation to the interaction of waves and growing bubbles. Amplification is possible at low frequencies and when the growth rate of bubbles reaches an optimum value for which the wave velocity decreases sufficiently to overcome the increased damping of the vesicular material. We examine two amplification phase-dependent effects: (1) a tensile-phase effect in which the inserted wave adds to the process of bubble growth, utilizing the energy associated with the gas overpressure in the bubble and therefore converting a large proportion of this energy into additional acoustic energy, and (2) a compressive-phase effect in which the pressure wave works against the growing bubbles and a large amount of its acoustic energy is dissipated during the first cycle, but later enough energy is gained to amplify the second cycle. These two effects provide additional new possible mechanisms for the amplification phase seen in Long-Period (LP) and Very-Long-Period (VLP) seismic signals originating in magma-filled cracks.

The method of fundamental solutions for acoustic wave scattering by a single and a periodic array of poroelastic scatterers

The method of fundamental solutions (MFS) is now a well-established technique that has proved to be reliable for a specific range of wave problems such as the scattering of acoustic and elastic waves by obstacles and inclusions of regular shapes. The goal of this study is to show that the technique can be extended to solve transmission problems whereby an incident acoustic pressure wave impinges on a poroelastic material of finite dimension. For homogeneous and isotropic materials, the wave equations for the fluid phase and solid phase displacements can be decoupled thanks to the Helmholtz decomposition. This allows for a simple and systematic way to construct fundamental solutions for describing the wave displacement field in the material. The efficiency of the technique relies on choosing an appropriate set of fundamental solutions as well as properly imposing the transmission conditions at the air–porous interface. In this paper, we address this issue showing results involving bidimensional scatterers of various shapes. In particular, it is shown that reliable error indicators can be used to assess the quality of the results. Comparisons with results computed using a mixed pressure–displacement finite element formulation illustrate the great advantages of the MFS both in terms of computational resources and mesh preparation. The extension of the method for dealing with the scattering by an infinite array of periodic scatterers is also presented.

Tonpilz type underwater vector sensor with directional sensitivity to acoustic waves.

Typical Tonpilz-type underwater acoustic transducers making use of piezoceramics detect the magnitude of an acoustic pressure, a scalar quantity, and convert this pressure into a proportional output voltage. The scalar sensor has no directional sensitivity. In this paper, a new Tonpilz transducer structure is proposed to measure both the magnitude and the direction of an incoming acoustic wave with a single transducer unit, which is accordingly referred to as a vector sensor. The piezoceramic stack clamped between the head mass and the tail mass of a Tonpilz transducer is divided into four radial segments, each segment polarized alternately to its neighboring one. An acoustic pressure wave coming from outside causes positive or negative signed electric voltage signals in the piezoceramic segments in accordance with their polarization directions as well as a certain amount of time delay between the electric signals. Proper manipulation of the electric signals, i.e., pairing the segments and either adding or subtracting the signals of the paired segments, can provide the relationship between the signals and the magnitude and direction of the acoustic wave. Feasibility of this structure is confirmed through 3-D simulations of the receiving properties of the transducer with the finite element method.

Theoretical and Experimental Identification of Acoustic Spinning Mode in a Cylindrical Combustor

The analytical solution for the eigenmodes of a cylindrical combustor allows for several realizations of the first tangential mode. The limiting cases of standing and spinning first tangential modes are well known. The general solution for a first tangential mode, however, allows for a large variety of realizations. In this study the analytical solution of the acoustic problem and a numerical time-resolved simulation of the acoustic pressure field are compared with experimental data from hot-fire tests. The experimentally observed pressure data are in excellent agreement with the simulation for all possible theoretical solutions. The approach enables detailed insight into the dynamics of the pressure field and thus constitutes a tool for the characterization of acoustic pressure waves in a cylindrical combustor even when the amplitude of the investigated eigenmode is small as compared with the mean pressure fluctuation.

Utilizing acoustic pressure waves for sensing fluid properties

Ultrasonic sensors can be used to determine physical fluid parameters like viscosity, density, and speed of sound. In this contribution, we present the concept for an integrated sensor utilizing pressure waves to sense the bulk of the fluid. Longitudinal viscosity and speed of sound can be determined using interference effects whereas the acoustic impedance of the liquid can be sensed if interference is suppressed by introducing a wedge reflector in the setup.

Monopole Acoustic Transmitter Comprising A Plurality Of Piezoelectric Discs

A monopole acoustic transmitter with at least one disc assembly with a plurality of piezoelectric discs configured to optimized acoustic pressure output within a desired frequency range while meeting physical restrictions of LWD and MWD logging systems. The transmitter is disposed in a recess or “slot” in the perimeter of a logging tool housing to reduce acoustic pressure waves transmitted directly along the tool housing and to optimize acoustic energy transmission into the borehole environs. In order to increase acoustic pressure output at a desired logging frequency range, the plurality of piezoelectric discs in each of at least one disc assembly are connected electrically in parallel and fired simultaneously. The polarity of the discs and the wiring arrangement are such that each disc expands or contracts in a common direction during simultaneous firing by an applied voltage. The desired output frequency is obtained by selectively polarizing and dimensioning the discs within the one or more disc assemblies. Each disc assembly is preferably disposed within an oil filled pressure housing that is pressure and temperature compensated. The axis of the at least one disc assembly can be parallel or perpendicular to the major axis of the logging tool.

Temporal investigation of acoustic wave pressure spectrum of high repetition rate TEA-CO2 lasers by using a three-dimensional mathematical model

The production of acoustic wave is one of the most important factors on the action of High Repetition Rate TEA-CO2 lasers. In this paper a three-dimensional mathematical modeling has been considered for investigation of temporal and spatial variations of acoustic waves in the laser cavity and its effect on the output of HRR lasers. By calculation of equations obtained from this model and plotting the pressure spectrum in different states, the effect of electrodes dimensions, cavity dimensions, gas flow velocity and repetition rate of laser in different times have been acquired. At last, optimum conditions for performance of laser action and having a good output have been arrived.

A mode matching method for modeling dissipative silencers lined with poroelastic materials and containing mean flow

A mode matching method for predicting the transmission loss of a cylindrical shaped dissipative silencer partially filled with a poroelastic foam is developed. The model takes into account the solid phase elasticity of the sound-absorbing material, the mounting conditions of the foam, and the presence of a uniform mean flow in the central airway. The novelty of the proposed approach lies in the fact that guided modes of the silencer have a composite nature containing both compressional and shear waves as opposed to classical mode matching methods in which only acoustic pressure waves are present. Results presented demonstrate good agreement with finite element calculations provided a sufficient number of modes are retained. In practice, it is found that the time for computing the transmission loss over a large frequency range takes a few minutes on a personal computer. This makes the present method a reliable tool for tackling dissipative silencers lined with poroelastic materials.

Photoacoustic waves generated in blood studied using pulsed digital holography

We studied photoacoustic waves using pulsed digital holography. The acoustic waves were generated in a reindeer blood target by absorption of an IR laser pulse, lambda=1064 nm and pulse length=12 ns. The acoustic pressure waves were then imaged in water using a second collimated laser pulse at lambda=532 nm 2 micros after the first IR pulse. Quantitative information on acoustic wave properties such as three-dimensional shape and pressure distribution was calculated by applying the inverse Radon transform on the recorded projection. The pressure pulse had a flat and sharp front parallel with the blood surface, which indicates that the pressure was generated at the blood surface. The generated pressure was proportional to the laser fluence with the proportionality constant equal to 1.8+/-0.3 cm(-1). According to existing data, the proportionality constant should be 1.4 cm(-1) for oxygenated human blood, which made our calculations probable.

Intensity of oscillation of spark-generated bubbles

In this paper, a new measure of bubble oscillation intensity is introduced, defined as a non-dimensional peak pressure in the first bubble pulse. An iterative method for determining the bubble size and bubble oscillation intensity from a record of the acoustic pressure wave emitted by an oscillating bubble is proposed. Using this procedure the sizes and intensities are determined for a set of pressure records obtained in recent experiments with spark-generated bubbles. It can be seen that in these experiments the bubble sizes, as defined by the first maximum bubble radius, R M 1, ranged from 12.8 to 56.4 mm, and the bubble oscillation intensities, as defined by the non-dimensional peak pressure in the first bubble pulse, p zp 1, ranged from 14.3 to 174. Data obtained in the experiments are compared with data computed in a theoretical model and it is shown that there are differences between the theory and experiment. These differences are attributed to energy losses from the real bubbles not taken into account in the theoretical model.

Numerical study of the direct pressure effect of acoustic waves in planar premixed flames

Recently the unsteady response of 1-D premixed flames to acoustic pressure waves for the range of frequencies below and above the inverse of the flame transit time was investigated experimentally using OH chemiluminescence Wangher (2008) [1]. They compared the frequency dependence of the measured response to the prediction of an analytical model proposed by Clavin et al. (1990) [2], derived from the standard flame model (one-step Arrhenius kinetics) and to a similar model proposed by McIntosh (1991) [3]. Discrepancies between the experimental results and the model led to the conclusion that the standard model does not provide an adequate description of the unsteady response of real flames and that it is necessary to investigate more realistic chemical models. Here we follow exactly this suggestion and perform numerical studies of the response of lean methane flames using different reaction mechanisms. We find that the global flame response obtained with both detailed chemistry (GRI3.0 [4]) and a reduced multi-step model by Peters (1996) [5] lies slightly above the predictions of the analytical model, but is close to experimental results. We additionally used an irreversible one-step Arrhenius reaction model and show the effect of the pressure dependence of the global reaction rate in the flame response. Our results suggest first that the current models have to be extended to capture the amplitude and phase results of the detailed mechanisms, and second that the correlation between the heat release and the measured OH ∗ chemiluminescence should be studied deeper.

Laser-induced optical breakdown applied for laser spark ignition

AbstractIn the present article, the experimental investigation of optical breakdown induced by ns/mJ pulses at two wavelengths, 1064 nm and 532 nm, in air of atmospheric pressure is reported and discussed. The obtained breakdown thresholds were compared with theory and are in good agreement. The generated plasmas have been characterized by their amount of scattered laser light, energy transmission, and change of the transmitted temporal shape. Laser-induced plasma formation in a gas, in air, also generates an acoustic pressure wave. The acoustic energy is compared to the laser pulse energy and is found to be linearly dependent. Moreover, the frequency distribution of the characteristic acoustic pressure wave was analyzed. The experiments described were accomplished in order to optimize a laser ignition system with regard to efficiency and costs. The laser system employed for these investigations is a compact high peak power, passively Q-switched, longitudinally diode-pumped solid-state laser. Such a “laser spark plug” should replace conventional spark plugs in internal combustion engines because conventional ignition has reached its limits in terms of efficiency and durability. Thereby, a reduction of pollutant emission should also be feasible.

Amplification.

In this experiment the students will see a demonstration of how sound waves can be amplified. The setup of the experiment is simple: a tuning fork, a table, and a music box. First the tuning fork is struck by itself and the level of the sound emitted is noted. Then the tuning fork is struck again and held against a table. The amplification of the sound due to propagation of acoustic pressure waves through the larger surface area of the table is noted. The students are encouraged to participate and give their own reasoning for sound amplification. If time permits, students will experiment with the music box as well.

Recognition of vowels in continuous speech by using formants

Speech consists of acoustic pressure waves created by the voluntary movements of anatomical structures in the human speech production system. These waveforms are broadly classified into voiced and unvoiced speech. Voiced sounds (vowels for example), produce quasi-periodic pulses of air which are acoustically filtered as they propagate through the vocal tract. The main distinction between vowels and consonants is that vowels resonate in the throat. Formants are exactly the resonant frequencies of a vocal tract when pronouncing a vowel. In this paper we attempt to carry out Vowel Recognition through Formant Analysis in Serbian language, wherein we detect which of the five Serbian vowels is spoken by the Speaker. Here we describe a standard approach for classification of vowels in continuous speech based on three formants: F1, F2 and F3. We have investigated the correlations between formants in each vowel and developed the algorithm to reduce the overlap of different vowels in F1-F2 and F2-F3 planes.

Experimental Investigation of Low-frequency Wayside Noise from High-speed Train

The authors performed laboratory experiments and field measurements to investigate low-frequency noise generated in wayside environments from high-speed train running. The results indicated the following three types of low-frequency sound sources: pressure fields around the nose and tail parts of the train, low-frequency acoustic pressure waves aerodynamically caused by the train itself, and noise radiation from vibrating concrete railway viaducts. Measurements conducted in a higher-speed section revealed that the major sound source of low-frequency noise in the far field was aerodynamically generated unsteady flow, which is analogous to a line source.

Measuring the local pressure amplitude in microchannel acoustophoresis

A new method is reported on how to measure the local pressure amplitude and the Q factor of ultrasound resonances in microfluidic chips designed for acoustophoresis of particle suspensions. The method relies on tracking individual polystyrene tracer microbeads in straight water-filled silicon/glass microchannels. The system is actuated by a PZT piezo transducer attached beneath the chip and driven by an applied ac voltage near its eigenfrequency of 2 MHz. For a given frequency a number of particle tracks are recorded by a CCD camera and fitted to a theoretical expression for the acoustophoretic motion of the microbeads. From the curve fits we obtain the acoustic energy density, and hence the pressure amplitude as well as the acoustophoretic force. By plotting the obtained energy densities as a function of applied frequency, we obtain Lorentzian line shapes, from which the resonance frequency and the Q factor for each resonance peak are derived. Typical measurements yield acoustic energy densities of the order of 10 J/m(3), pressure amplitudes of 0.2 MPa, and Q factors around 500. The observed half wavelength of the transverse acoustic pressure wave is equal within 2% to the measured width w = 377 microm of the channel.

Wave propagation in a two-dimensional plane straight duct with panels embedded in its sidewalls

This paper presents analytical solutions of the problem of two-dimensional pressure wave propagation in a plane straight duct with rigid spring-supported wall panels. The problems of non-stationary and single-frequency pressure wave propagation in a semi-infinite duct with a moving piston and with panels locally embedded in the sidewalls are studied. Rigid panels are embedded in the sidewalls such that they can carry out small amplitude oscillations perpendicular to the main duct axis. The system of the duct with panels acts as a filter. Based on this filter property, which can be described by the panel parameters, a method of computing the acoustic pressure wave distribution in the duct is presented. A wave localisation effect in the duct caused by trapped modes has been found for particular parameters of the sidewall panels.

Determination of the Instantaneous Fuel Flow Rate Out of a Fuel Nozzle

Reported is a practical method for accurate and fast determination of the instantaneous fuel flow rate out of a fuel injector. Both gaseous and liquid fuels are considered. Unsteady fuel flow rates introduced into a combustor can be caused by both self-excited pressure pulsations and fuel modulations. During combustion instability, the air flow rate into a combustor also varies in response to pressure pulsations. Accurate determination of the instantaneous fuel and air flow rates is important for both modeling and control of combustion instability. The developed method is based on the acoustic wave theory and pressure measurements at two locations upstream of a fuel injector. This method bypasses the complexities and nonlinearities of fuel actuators and fuel nozzles, and works for systems with slow-time-varying characteristics. Acoustic impedance of a gaseous fuel nozzle is found to be a function of multivariables, including the forcing frequency, the acoustic oscillation intensity, and the mean fuel flow rate. Thus, it is not an intrinsic property of the fuel injector alone. In the present study, sharp tubing bending with almost zero radii is found to have minimal effects on the distribution of 1D acoustic wave. This is probably because vortex shedding and recirculation at tubing corners do not alter the globally 1D characteristics of acoustic wave distribution. Different from the traditional two-microphone method, which determines the acoustic velocity at the middle locations of the two microphones, the present method allows the acoustic velocity, the acoustic mass flux, and the specific acoustic impedance to be determined along the fuel tubing or an air pipe.

Negative mass sound shielding structures: Early results

AbstractElastic meta‐materials or meta‐composites can be designed to have a negative effective mass density at certain vibration frequencies, thus blocking wave propagation through the material within that frequency band. The negative mass behaviour is generated by resonant structures within the material that oscillate 180° out of phase with the acoustic pressure waves applied to the surface. As this research is in its initial stages this paper describes work in progress in both the experimental and numerical domains, and some early results from solids containing geometrically simple spring–mass resonant structures. Behaviour is characterized experimentally by dynamic tests of individual resonators as well as impedance tube measurements of panel‐like structures containing multiple resonant elements. The experimental results exhibit clear evidence of the expected resonances, and partial band gap behaviour. Finite element models of both single resonant elements and impedance tube specimens are being developed, and the current status of these models is described. Their results to date show good agreement with the mass law and qualitative agreement with the experimental results.

Roles of Physical Therapists in Wound Management, Part III: Select Biophysical Technologies and Management of Patients With Diabetic Foot Ulceration

Physical therapists (PTs) have several decades of history of using high-frequency (1and 3-MHz) ultrasound (US) to facilitate healing of musculoskeletal tissues (eg muscle, fascia, tendon, ligament, joint capsule) that become wounded or inflamed following many accidents or sports injuries, but not exposed, because the trauma leaves the skin intact. These US frequencies (often referred to as therapeutic US) transmit acoustic energy, which delivers the nonthermal physical properties of stable cavitation and microstreaming to the closed wound tissues via direct contact gel or water coupling to the skin. With megahertz US, the acoustic pressure waves created by stable cavitation and microstreaming are less forceful than the pressure waves created by unstable cavitation that is associated with low-frequency (kilohertz) US. This is because acoustic pressure is inversely proportional to frequency, and since higher frequencies have shorter wavelengths, at the power (wattage) levels used for megahertz US, less acoustic energy is transmitted to tissues per unit of time. The more prominent unstable cavitation produced by low-frequency US (eg 22.5, 25, and 35 kHz) is a major contributing source of energy by which contact US debrides nonviable, adherent fibrin and slough via fragmentation, emulsification, or a combination. The combination of stable cavitation and microstreaming that are characteristic of megahertz US