Plane Wave Sound Field Research Articles

Interaction between particles and bubbles driven by ultrasound: Acoustic radiation force on an elastic particle immersed in the ideal fluid near a bubble.

A theoretical model is proposed to investigate the acoustic radiation force on the elastic particle for coupled particle-bubble system. Based on the sound scattering theory, an analytical expression of the force function is obtained for the particle in plane wave sound field. Numerical simulations are presented for elastic particle of stainless steel, steel or brass. The results reveal that the presence of bubbles can affect the feature of radiation force curves of particles. The force curve fluctuates, and negative force emerges in the small kR1 region for certain distance between the bubble and particle. There are more sharp peaks and dips in the curves because of the resonance of the elasticity of the system and the resonant peaks of the acoustic radiation force transfer to low frequencies when the size of elastic particle is increased. The approximate positive flat region is shortened because of the presence of bubble, which may help to optimize the size ranges of particle for acoustic screening. This study provides for improvement of the acoustic manipulation theoretical model.

DOA Estimation Performance With Circular Arrays in Sound Fields With Finite Rate of Innovation

A novel Direction-of-Arrival (DOA) estimation method based on a plane wave sound field model was recently proposed for circular microphone arrays [1] . This article presents a detailed theoretical analysis of the method that relies on a Finite Rate of Innovation (FRI) assumption, investigating the impact of different theoretical acoustic source models. The method's estimation accuracy, and robustness against noisy measurement data and deviations from the model are investigated. The estimation performance is validated and assessed on the basis of results obtained from both simulations and experimental data.

Active room compensation for sound reinforcement using sound field separation techniques.

This work investigates how the sound field created by a sound reinforcement system can be controlled at low frequencies. An indoor control method is proposed which actively absorbs the sound incident on a reflecting boundary using an array of secondary sources. The sound field is separated into incident and reflected components by a microphone array close to the secondary sources, enabling the minimization of reflected components by means of optimal signals for the secondary sources. The method is purely feed-forward and assumes constant room conditions. Three different sound field separation techniques for the modeling of the reflections are investigated based on plane wave decomposition, equivalent sources, and the Spatial Fourier transform. Simulations and an experimental validation are presented, showing that the control method performs similarly well at enhancing low frequency responses with the three sound separation techniques. Resonances in the entire room are reduced, although the microphone array and secondary sources are confined to a small region close to the reflecting wall. Unlike previous control methods based on the creation of a plane wave sound field, the investigated method works in arbitrary room geometries and primary source positions.

Room Acoustics Analysis Using Circular Arrays: A Comparison Between Plane-Wave Decomposition and Modal Beamforming Approaches

A complete room acoustics analysis requires not only measurements of the single-room impulse response, but also a rigorous analysis of the sound field's spatial properties. Angle-dependent information for the direct and reflected waves is needed for a more complete sound field description than can be obtained with single microphone measurements. Analysis techniques based on uniform circular arrays (UCAs) have attracted a lot of attention in recent years. There is also active research on beamforming methods based on circular harmonic plane-wave sound field decomposition. However, to date, there has not been a comprehensive experimental study of the acoustics of real rooms that employs both techniques. In this paper, the UCA topology is used in a comparative study for plane-wave decomposition and enhanced modal beamforming techniques. As a result, room echograms can also be obtained by calculating a set of azimuth-steered impulse responses. Experiments in real and simulated rooms are compared and discussed, with an identification of strengths and weaknesses for both measurement methods. The results show the validity of the modal beamforming approach by comparing its performance with that of the plane-wave decomposition method.

In-situ measurement of sound absorbing properties using plane-wave sound field reproduced by virtual loudspeaker array

A new method of measuring the acoustic properties of sound absorbing material in situ under reproduced plane wave sound field is proposed. This method employs a single loudspeaker to synthesize a virtual array of loudspeakers which reproduce the plane wave sound field in front of the material surface, and applies the standard transfer function method to measure acoustic impedance and absorption coefficient. Simulation shows that this method can accurately measure absorption coefficient in the frequency range of 100–1.6 k Hz. This method is also applied to measure the absorption coefficient and acoustic impedance of two test samples in a semi-anechoic chamber, and compared with two other existing in-situ measurement methods. The proposed method demonstrates higher accuracy in measuring both acoustic impedance and absorption coefficient for the strongly-absorbing material sample in the frequency range of 175–1.6 k Hz and for the weakly-absorbing material sample in the frequency range of 250–1.6 k Hz. Experimental results and theoretical analysis also show that insufficient signal-to-noise ratio and unwanted reflected waves contribute to the insufficient accuracy of this method below 175 Hz.

Characterization of directional microphones in an arbitrary sound field

An acoustic characterization method for directional microphones is presented that does not require an anechoic chamber to provide a controlled plane wave sound field. Measurements of a directional microphone under test are performed in a nearly arbitrary sound field for several angles of sound incidence, and the corresponding sound pressure and pressure gradients in the vicinity of the test microphone are measured using an automated probe microphone scanning system. From these measurements, the total acoustic frequency response of the directional microphone can be decomposed into its sensitivities to sound pressure and pressure gradient using a least squares estimation technique. These component responses can then be combined to predict the directional response of the microphone to a plane wave sound field. This technique is demonstrated on a commercially available pressure gradient microphone, and also on a combination sound pressure-pressure gradient microphone. Comparisons with the plane wave responses measured in an anechoic environment show that the method gives accurate results down to 100 Hz.

Acoustic radiation force on coated cylinders in plane progressive waves

The purpose of this paper is to present the theory of the static acoustic radiation force on layered cylinders. The frequency dependence of the acoustic radiation force function Y p (which is the radiation force per unit energy density and unit cross-sectional surface) for coated cylinders suspended in an incident plane wave sound field is analyzed, in relation to the thickness of the outer covering layer and the surrounding fluid. Explicit numerical calculations are presented for layered lossless cylinders immersed in water. The fluid-loading effect on the radiation force function curves is also analyzed by considering a high-density fluid surrounding the cylinders. These results show how absorption and the exterior fluid surrounding the cylinders affect the acoustic radiation force. It is shown here that the theory developed is much broader in scope compared to other existing theories.

706 改良近距離音響ホログラフィ法による平面音波の計測結果

I have proposed a converted Nearfield Acoustic Holography (NAH) method which lightens the workload of measurement at high frequency. In former researches, the theoretical background and the effectiveness of proposing method are explained, and are verified in numerical simulations and experimentations. In former research, the numerical simulations and experimentations are applied to the point sound source(s). However the NAH method is originally based on the plane wave theory. In this research, the experimentations are performed to the plane wave sound field created by vibrated plate. As a result, it is verified that the proposing method can reconstruct almost similar images compared with original NAH method, with lesser workload.

Reproduction of a plane-wave sound field using an array of loudspeakers

Reproduction of a sound field is a fundamental problem in acoustic signal processing. In this paper, we use a spherical harmonics analysis to derive performance bounds on how well an array of loudspeakers can recreate a three-dimensional (3-D) plane-wave sound field within a spherical region of space. Specifically, we develop a relationship between the number of loudspeakers, the size of the reproduction sphere, the frequency range, and the desired accuracy. We also provide analogous results for the special case of reproduction of a two-dimensional (2-D) sound field. Results are verified through computer simulations.

Position-tolerant differential microphones for noisy environments

Directional microphones are best noted for their noise reduction properties in communication systems. Close-talking differential microphones are particularly useful when the noise environment disturbs the ability to communicate without error, such as in public and cellular telephony, aircraft communications, etc. These differential microphones work best when they are placed within 1 cm from the lips of the talker where the sound field has a large gradient. For a plane-wave sound field the sensitivity rises proportional to ωn, where n is the order of the difference. Users of differential microphones do not always correctly position the sensor at the proper distance from the mouth and therefore the sensitivity of the microphone may also rise proportional to ωn especially at high frequencies. A method is described of correcting for this high-frequency gain without significantly degrading the noise canceling properties of first- and second-order differential microphones.

Effects of ultrasound on bilayer lipid membranes

The effects of ultrasound of a frequency of 1 MHz on an oxidized cholesterol bilayer lipid membrane (BLM) were observed for the intensity range from 0 to 1.5 W/cm2. In the experimental arrangement the BLM was formed in a 0.9-mm hole in a teflon support positioned in the farfield of a plane-wave sound field. In a typical experiment the BLM was formed in its support separating two chambers containing KCl or NaCl in aqueous solution. The electrical parameters of the BLM were monitored using platinum electrodes on both sides of the teflon support. At an intensity of 1.5 W/cm2 the sound caused the membrane to rupture, in the intensity range from 0.5 to 1.5 W/cm2 it accelerated the draining of the bulk cholesterol from the teflon during the formation of the BLM and at all intensities the sound had no effect on the conductance, capacitance, or the dependence on each on the voltage applied across the membrane. These results will be discussed in relation to determining the effects of ultrasound on nerve membranes.

Receiving Response of an Arc Segment of a Cylindrical Hydrophone in a Plane-Wave Sound Field

The open-circuit voltage response of an electroded arc segment of a thin-walled complete cylinder varies with electrode dimensions, location relative to the direction of the incident plane wave, and frequency. Since many modes of vibration are excited, the electrical output is the sum of the individual modal components. With symmetrical electrode configurations and/or electrical shading, the relative strength of desired modal outputs can be increased and simpler beam-pattern responses produced. A mathematical expression for arc-segment response was desired to allow further study of beam formation with both symmetrical and asymmetrical electrode configurations. Analysis proved to be extremely difficult, and a measurement program was undertaken. Amplitude and phase measurements are presented for a 15° arc segment of a cylindrical hydrophone. In addition, VILP measurements verifying the symmetry conditions required to accentuate specific modes are presented.

Theory of Passive Linear Electroacoustic Transducers with Fixed Velocity Distribution

A general theory of passive linear electroacoustic transducers for which the surface velocity distribution is independent of the manner of excitation of the transducer is developed and applied to the calculation of important practical parameters of the transducer. A proof of the electroacoustic reciprocity theorem for such transducers is included. Another important theorem, the available acoustic power theorem, which states that the maximum power which can be absorbed by such a transducer from a plane wave sound field is given by the product of the intensity of the wave and the quantity λ2δR/4π where λ is the wave-length of the wave and δR is the receiving directivity factor of the transducer relative to the direction of incidence of the wave is also proved. The paper concludes with a discussion of the available power efficiency of a transducer and its relation to the threshold pressure of the transducer.

The Plane Wave Reciprocity Parameter and Its Application to the Calibration of Electroacoustic Transducers at Close Distances

The reciprocity parameter for a plane wave sound field is shown to be equal to 2A/ρc, where A is the area of the plane piston source and ρc the characteristic acoustic resistance of the medium. It is shown experimentally that the sound field in front of a plane piston source is effectively plane over a distance approximately equal to A/λ, and that in such a region the plane wave parameter may be used to obtain a free field calibration of the transducer by the reciprocity method.

Self-Reciprocity in a Plane Wave Sound Field

An electroacoustic reciprocity parameter is derived for a transducer which transmits and receives plane sound waves. The parameter is shown to have the value J = 2A/ρc, where A is the area of the diaphragm, ρ is the density, and c the velocity of the medium. The plane wave parameter has possible application to the calibrations of transducers at megacycle frequencies by the self-reciprocity method. As illustrated by an example using the pulse-reflections technique, it is simply necessary to measure the impedance of the transducer and the ratio of the amplitudes of the driving and the reflected received pulse as observed on an oscilloscope screen. A correction is necessary for the absorption of the fluid. The validity of the method is dependent upon the extent to which plane waves determine the sound field between transducer and reflector.

Acoustic Pulse Testing

In the testing of electro-acoustic transducers such as microphones one desires to find the generated electrical signal in the presence of an incident plane wave sound field. Likewise, in testing loudspeakers, one wishes to measure the generated sound field. In practical testing systems, reflections which interfere with the measurement are in general present even in the best of “dead” rooms. The pulse testing procedure has been successful in acoustic measurements in eliminating the effects of reflections from the boundaries of the medium. The method consists of generating in the medium a short finite train of sinusoidal waves whose wave-length corresponds to the frequency at which the measurement is desired. The directly transmitted pulse is measured before any reflected signal reaches the sound pickup. The reverberation level is allowed to become negligible before the next pulse is generated and measured. Electronic circuits for generating the applied electrical signal and for continuously recording the received signal level will be described. The system also has been designed so that the direct signal may be discriminated against and any reflection measured, thus providing a means of measuring reflection coefficients of acoustic materials. The limitations of the pulse method will be discussed and compared with those of frequency warble and noise methods of vitiating reflections.