Showing papers on "Acoustic source localization published in 1986"

Pulse centroid echo method and apparatus for enhanced sound velocity estimation in vivo

Abstract: A method and apparatus of enhancing the accuracy of in vivo sound velocity estimation by identifying segments of different sound velocity along a tracked ultrasound beam. The effects of refraction on the tracked beam at the interface between tissue regions are estimated. Also disclosed is a technique for estimating the refractive effects of naturally occurring and artificially introduced acoustical contrast fluids. The accuracy of the sound velocity measurements is increased by utilizing the centroid-to-centroid distance between the pulses produced by ultrasound sending and receiving transducers in calculating the ultrasound beam velocity.

Method for enhancing the accuracy of in vivo sound velocity estimation

Abstract: A method of enhancing the accuracy of in vivo sound velocity estimation by identifying segments of different sound velocity along a tracked ultrasound beam. The effects of refraction on the tracked beam at the interface between tissue regions are estimated. Also disclosed is a technique for estimating the refractive effects of naturally occurring and artificially introduced acoustical contrast fluids.

Effect of a wind tunnel on the acoustic field from various aeroacoustic sources

Abstract: The effects of the walls of an enclosed test section wind tunnel on measurements of sound fields from various sources has been studied. The acoustic field from a known source in a wind tunnel has been modeled as an infinitely long duct with constant cross section. The model was solved with a numerical panel technique in a control volume near the source, and matched to an outer analytic solution. Several sample problems were studied in a rectangular duct with and without flow. The results indicate that the presence of the duct affects the acoustic field, and that small changes in the product of duct cross dimensions and the source wave number can change the acoustic field significantly. It is also shown that, for low-frequency helicopter rotor harmonic noise, measured in typical wind tunnel rotor tests, the sound levels beyond one rotor diameter from the hub are unreliable indications of the free-field sound levels.

Acoustic calibration device

Abstract: An acoustic calibration device for intensity measuring systems comprising at least two pressure microphones to be calibrated without using an anechoic chamber. The calibration device comprises at least two cavities interconnected by means of one or several acoustic resistance elements. One of the pressure microphones is to be inserted into one of the cavities and the second pressure microphones is to be inserted into the second cavities. A sound source is connected to one of the cavities. In connection with the subsequent cavity the acoustic resistance thus forms an acoustic RC-link providing a phase shift proportional to the frequency corresponding to the conditions in the free field. By a suitable dimensioning of the RC-link a phase shift corresponds to the phase shift over a distance of e.g. 50 mm in the free field. The sound source is able to generate either white noise or pink noise depending on whether measurements are performed over fixed frequency intervals or relative frequency intervals.

Electronic silencer system

Abstract: PURPOSE:To attain silencing with high accuracy for non-stationary noise generating on a propagation path by suppressing an acoustic feedback from an added sound source to a microphone which detects a propagation sound wave from a noise source. CONSTITUTION:In a propagation path 1, sensor microphones M1 and M1', the characteristic of which are made uniform, at the same distance position from an added sound source S are provided, and the phases of outputs are set reversely with each other, then being inputted to an adder circuit 20. The propagation sound waves from the noise source detected with the microphones M1 and M1' are converted to electrical signals, and are inputted through the adder circuit 20 and an AD conversion part 24, and input signals are converted to digital signals based on a transfer function given from a control part 30 by a digital filter 29. A driving signal is impressed through a DA conversion part 26, and the sound wave in order to negate the propagation sound wave emitted from the noise source is radiated from a speaker S. The propagation sound wave from the added sound source S detected by the microphone M1 is erased electrically with the adder circuit 20, thereby an oscillation state being suppressed.

A note on the Helmholtz equations for acoustic waves in inhomogeneous media

Abstract: The paper puts forward a vector Helmholtz equation for the particle velocity of an acoustic perturbation propagating in an inhomoeneous medium. The scalar (acoustic pressure) and vector (acoustic velocity) expressions for the scattered wave are derived and compared. Also, the importance of the source term in the Helmholtz equation is clarified.

Magneto-acoustic effect in n-InSb at low temperatures

Abstract: Using the velocity operator derived from the Heisenberg equation of motion, the authors have investigated the magneto-acoustic effect in n-InSb in the presence of a DC magnetic field B directed along the propagation of acoustic waves. Since they are interested in both high and low frequency regions, an electron relaxation time due to the scatterings in solids is taken into account. It is found that there exists a discontinuous point in the absorption coefficient or change in the sound velocity as a function of the sound frequency. When the sound frequency is below 1.3*1011 rad s-1, the absorption coefficient increases with the sound frequency and the sound velocity decreases with the sound frequency from the sound velocity nu s=4*105 cm s-1. However, when the sound frequency is over 1.3*1011 rad s-1, the absorption coefficient decreases with the sound frequency and the sound velocity increases abruptly with the sound frequency from nu s=4*105 cm s-1.

A new method to locate sound sources by searching the minimum value of error function

Abstract: This paper describes a new method to locate sound sources using many sensors. Spectra of all the sensor outputs are calculated by discrete Fourier transform. In the proposed method, following parameters are assumed; (1) the number of sound sources, (2) a position of each sound source, and (3) the spectrum of the O-th sensor output due to each sound source. The spectra of all the sensor outputs are estimated using the parameters under the assumption of free field. Then, introduced is an error function which is the mean-square value of the difference between the calculated spectra and the estimated ones. By changing the values of the parameters, the minimum value of the error function is searched. When the error function takes the minimum value, the parameters represent the estimates; the number of sound sources, and the positions and spectra of the sound sources. By inverse Fourier transforming the estimated spectra, the wave form of a specified sound source can be obtained.

Method and device for measuring distance using air-borne sound

Abstract: In the method and the device operating using this method for measuring distance by means of air-borne sound, a sound pulse is generated in a sensor and directed onto a reflecting object, and the transit path of the sound, and thus the distance between sensor and object, are determined electrically with the aid of the sound echo pulse. In order to prevent irregularities in the propagation of sound on the transit path, a temporally constant airflow is maintained along the transit path of the sound. For this purpose, there is arranged in a housing of a measuring head a fan which conveys air through an exit opening which is at the same time the exit opening for the sound.

Acoustic controlled rotation and orientation

Abstract: Acoustic energy is applied to a pair of locations spaced about a chamber, to control rotation of an object levitated in the chamber. Two acoustic transducers applying energy of a single acoustic mode, one at each location, can (one or both) serve to levitate the object in three dimensions as well as control its rotation. Slow rotation is achieved by initially establishing a large phase difference and/or pressure ratio of the acoustic waves, which is sufficient to turn the object by more than 45°, which is immediately followed by reducing the phase difference and/or pressure ratio to maintain slow rotation. A small phase difference and/or pressure ratio enables control of the angular orientation of the object without rotating it. The sphericity of an object can be measured by its response to the acoustic energy.

The Helmholtz resonance behavior of single and multiple rooms

Abstract: The results of exploratory measurements of the noise fields inside rooms which are excited to resonance either acoustically or mechanically are presented The data illustrate the nature and extent of the sound pressure level enhancements in single rooms and also how multiple rooms may resonate by means of either acoustic or mechanical coupling Sound pressure level enhancements of about 5 dB were measured during resonance of rooms having flexible walls For such conditions the sound pressure levels in the room were essentially uniform and in phase Variability of up to 20 dB was measured in a room hallway complex having significant acoustic interactions Resonant frequency prediction methods which work well at model scale, give only fair results for rooms

Effects of second sound on acoustic transmission at the solid-liquid 4He interface.

Abstract: We have calculated the reflection and transmission coefficients of acoustic waves propagating across a solid-liquid interface of 4He, using an extension of the treatment of Castaing and Nozieres in which we include pressure changes associated with second sound. These calculations account well for the experimental results obtained over the temperature range 0.83 K.:$ T .:$1.46 K and provide an alternative explanation to the one we offered previously for the lack of agreement between experiment and the theory of Castaing and Nozieres.

Particle velocity detection using a thin membrane

Abstract: Underwater acoustic intensity measurements require the detection of both the acoustic pressure and the fluid particle velocity. The measurement of the acoustic pressure is easily obtained using conventional (pressure‐type) hydrophones. The particle velocity may be obtained by measuring the flexural response of an appropriate membrane (constrained by an inertial structure) to the fluid particle velocity. Several thin (50–300 μm) membranes were constructed from materials which closely match (impedance, density) the acoustic medium. The use of low shear speed materials allowed for the construction of small (< 10 cm) membranes which are resonant (fundamental mode) to low‐frequency fluid oscillations. The response of such membranes to the acoustic particle velocity and pressure is reported in the 0.02 to 0.5 ka range.

Improved acoustic emission locations

Abstract: A technique for locating acoustic emission in brittle rock specimens is described. An array of eight piezoelectric transducers was used to detect acoustic emission and to receive velocity measurement pulses. Axial and transverse velocities were calculated from the transmission data; the use of the velocity data to locate the acoustic emissions is discussed. Waveforms were recorded for about 140 acoustic emissions during the deformation of the Westerly granite specimen and about 100 acoustic emissions were located. The effect of transducer size and incident angle on apparent arrival times is evaluated using a lead zirconium titinate transducer bonded to a small brass rider with a curved surface matched to the brass specimen. The relation between location accuracy in various regions of the specimen and the geometry of the receiver array is studied. It is observed that for incidence angles away from the normal, the velocity appears to be higher than for normal incidences.

Estimation of Plate Vibration and Its Far Field Sound by Means of Near Field Sound Measurement

Abstract: As one approach to noise problem of machinery, advantages of near field sound measurement are discussed for a resonating rectangular steel plate. A proposed technique after Acoustic Intensity Method was applied to measure the distributions of both intensity and particle velocity over a plane placed close to a sound source. Particle velocity in the normal direction of plate proved to be reliable for estimating the vibration of the plate surface. And far field sound pressure is calculated accurately from it. In addition, for the ease of calculating far field sound pressure form a known mode of vibration, a simple approximation by which the sound source can be replaced by the least number of point sources, is also proposed.

Calculating far-field radiated sound pressure levels from NASTRAN output

Abstract: FAFRAP is a computer program which calculates far field radiated sound pressure levels from quantities computed by a NASTRAN direct frequency response analysis of an arbitrarily shaped structure Fluid loading on the structure can be computed directly by NASTRAN or an added-mass approximation to fluid loading on the structure can be used Output from FAFRAP includes tables of radiated sound pressure levels and several types of graphic output FAFRAP results for monopole and dipole sources compare closely with an explicit calculation of the radiated sound pressure level for those sources

Temperature Compensated Doppler Flowmeter for Small Diameter Pipe

Abstract: The paper describes a reduction method of errors caused by changes in the sound velocity and the ultrasonic beam angle of the Doppler flowmeter. Firstly, a correcting method with sound velocity sensor is presented to show that the sound velocity error is effectively reduced. Secondly, the method is compared with the conventional Doppler method, and the various errors resulting from temperature change are discussed. It is concluded that it is possible to compensate sound velocity change in the Doppler method. The sound velocity sensor for the compensation is found to have small errors by its angle misalignment.

Ocean acoustic tomography and acoustic propagation

Abstract: Ocean acoustic tomography is a method for remotely sensing deterministic and stochastic variations in the ocean sound speed field. The fundamental tomographic measurement is sound speed, and therefore tomography directly provides acoustic propagation models with this essential, first‐order, environmental information. In one form or another the various sound propagation codes rely on sound speed data in either 2‐, 3‐ or 4‐dimensional form. Experiments have shown that both mean and range‐varying sound speed data can be obtained using tomography, and that when appropriate assumptions are made about the known horizontal and vertical covariance structure of the ocean, these data are spatially continuous. Thus it is not necessary to choose an interpolation scheme for discrete profiles that may be mathematically sensible, but physically less meaningful. Further, since the tomographic measurement itself uses propagating acoustic waves, it affords an opportunity to obtain simultaneously selected sound propagation ...

Interaction of second sound with acoustic waves in solids

Abstract: An expression has been derived for the collision operator for phonons in a solid, which is valid at very low temperatures. The set of coupled equations for the elastic deformation and the phonon density or second sound has been reduced to a simple tractable form and the dispersion equation for the coupled waves consisting of the acoustic modes and second sound has been derived. It is shown that only the longitudinal mode interacts with the second sound. It is also shown that as a result of the interaction with the second sound, the longitudinal velocity along the principal axis acquires a correction term that is proportional to bothγ 2 andT 4.

Two probabilistic evaluation methods for single and double walls type sound insulation systems with arbitrary random noise excitation

Abstract: In the practical engineering field of noise control, it is as important as the sound source countermeasure to improve the sound propagation characteristics by newly setting a sound insulation system. On the other hand, the noise statistics like an Lx (x=5, 10, 50, 90, ) defined as a (100 -x) percentage point of the sound level distribution are very important as well as the lower order energy moment statistics like an Leq in the actual noise evaluation and regulation problems. In this paper, two kinds of probabilistic evaluation methods for sound insulation systems are theoretically proposed from typically different points of view for the probability distribution of the transmitted sound intensity, when a general stationary random noise having an arbitrary distributionform and frequency characteristics is insulated by a wall. To evaluate its sound transmission coefficient, a somewhat modified statistical energy analysis method has been used.The validity of the theoretical results is experimentally confirmed by applying it to the actually observed data on the output response fluctuation of single and double walls type insulation systems inserted between two reverberation rooms.

Complexity of waveform formulated from Fourier spectrum -- Theory and experiment

Abstract: A new method of spectrum analysis for evaluating the diffuseness of sound fields is presented herein. We apply a concept of entropy, of complexity of waveform, to a Fourier transform of the signal. Thus, a measure of complexity in sound fields is defined as a complex entropy in terms of the frequency components of the complex Fourier transform. We theoretically and experimentally discuss the quantitative difference between two entropies, which is dependent on the distance between two measuring points in free sound and reverberant sound fields, excited by various signals. Computational procedures of signal processing are also described. It is found that this theory predicts the experimental values reasonably exactly in both sound fields. The entropy is considered to be useful for evaluating the diffuseness of sound fields, and to be appropriate for the field of room acoustics.