Showing papers on "Acoustic source localization published in 1987"

Passive source localization employing intersecting spherical surfaces from time-of-arrival differences

Abstract: Problems associated with the use of intersecting hyperboloids for passive source localization from time-of-arrival difference signals are discussed. A closed-form solution for source location is presented given time-of-arrival difference measurements when the distance from the source to any arbitrary reference is unknown.

Determination of Elastic Constants by LFB Acoustic Microscope

Abstract: A new method o f measur ing e l a s t i c cons tan ts o f b u l k s o l i d m a t e r i a l s b y means o f t h e l i ne focus -beam (LFB) a c o u s t i c microscope has been d e v e l o p e d . An LFB a c o u s t i c m i c r o s c o p e h a s b e e n a p p l i e d t o p r e c i s e l y measure t h e v e l o c i t y o f l e a k y s u r f a c e a c o u s t i c waves (LSAWs) propagat ing on waterloaded specimens. The LSAW v e l o c i t y i s d i r e c t l y r e l a t e d t o t h e e l a s t i c p r o p e r t i e s o f specimens, so t h a t i t i s p o s s i b l e t o determine t h e e l a s t i c c o n s t a n t s b y t h e o r e t i c a l ana lys i s . For t h i s study, an a l g o r i t h m has b e e n d e v i s e d t h a t c a n c a l c u l a t e t h e SAW v e l o c i t y f o r a f r e e s u r f a c e c o n d i t i o n e d specimen u s i n g t h e measured LSAW v e l o c i t y . E l a s t i c cons t a n t s have been determined b y c o m p u t e r f i t t i n g o f t h e SAW v e l o c i t y i n n u m e r i c a l c a l c u l a t i o n s . I n t h i s m e t h o d , p r e c i s e measurements o f t h e l o n g i t u d i n a l v e l o c i t y and t h e d e n s i t y a r e a l s o c a r r i e d o u t f o r t h e same specimen, i n o rde r t o reduce t h e number o f unknown p a r a m e t e r s . E x p e r i m e n t s a r e d e m o n s t r a t e d a t a VHF range f o r i s o t r o p i c samples, s u c h as f u s e d q u a r t z (SiO,). a n d some k i n d s o f b o r o s i l i c a t e g l a s s e s . Two independent components o f s t i f f n e s s c o n s t a n t s , C l l and C44 . have b e e n determined w i t h h i g h accuracy and compared w i t h t h e pub l i shed t e c h n i c a l data. 1. I n t r o d u c t i o n I n r e c e n t s t u d i e s on t h e LFB a c o u s t i c m i c r o s c o p e , d e m o n s t r a t i o n a l exper imen ts have been e x t e n s i v e l y done t o show i t s p o s s i b l e a p p l i c a t i o n s t o m a t e r i a l analyses i n t h e research f i e l d s o f m a t e r i a l sc ience and n o n d e s t r u c t i v e eva lua t i on , f o r example, acoust i c a n i s o t r o p y , a c o u s t i c inhomogeneity, s t r u c t u r a l a n a l y s i s o f p o l y c r y s t a l l i n e m a t e r i a l s , f i l m t h i c k n e s s measurement, and v i s c o e l a s t i c a n a l y s i s o f d e n t a l m a t e r i a l s [ l 5 1 . A n o t h e r e x p e c t a b l e app l i c a t i o n p r e s e n t e d h e r e i s f o r d e t e r m i n i n g m a t e r i a l constants . F o r d e t e r m i n a t i o n o f e l a s t i c c o n s t a n t s , i t i s conven t iona l t o make v e l o c i t y measurements o f b o t h l o n g i t u d i n a l and s h e a r waves b y va r ious k i n d s o f u l t r a s o n i c methods i n c l u d i n g t h e o p t i c a l d i f f r a c t i o n method [ 6 ] . The u l t r a s o n i c t ransducers, made o f p i e z o e l e c t r i c p l a t e s a t lower f requenc ies o r ZnO t h i n f i l m s a t h i g h e r f requencies, must be u s u a l l y f a b r i c a t e d on one end o f specimens, o f w h i c h b o t h end su r faces a r e p o l i s h e d w i t h p a r a l l e l i s m . On t h e o t h e r hand, t h e method by t h e LFB a c o u s t i c m i c r o scope has t h e g r e a t a d v a n t a g e t h a t nondes ruc t i ve 0090-5607/87/0000-0817 $1 .OO

Thermoacoustic radiation of sound by a moving laser source

Abstract: The generation of sound by a moving laser source is investigated both theoretically and experimentally. The analysis is restricted to sound waves generated exclusively through the thermal mechanism. The model is based on the impulse response of a thermoacoustic source as described in a previous paper [J. Acoust. Soc. Am. 78, 2074–2082 (1985)]. The results presented here include pressure waveforms, directivity patterns, sound level dependence on source velocity, and spreading curves for subsonic, transonic, and supersonic source velocities. In general, experimental results are in good agreement with theoretical predictions.

Frequency dependence of the speed of sound in air

Abstract: Recent papers demonstrate some confusion over the frequency dependence of the speed of sound in air. The aim here is to correct the misunderstandings that have arisen.

Definition and measurement of sound energy level of a transient sound source

Abstract: Concerning stationary sound sources, sound power level which describes the sound power radiated by a sound source is clearly defined. For its measuring methods, the sound pressure methods using free field, hemi-free field and diffuse field have been established, and they have been standardized in the international and national standards. Further, the method of sound power measurement using the sound intensity technique has become popular. On the other hand, concerning transient sound sources such as impulsive and intermittent sound sources, the way of describing and measuring their acoustic outputs has not been established. In this paper, therefore, “sound energy level” which represents the total sound energy radiated by a single event of a transient sound source is first defined as contrasted with the sound power level. Subsequently, its measuring methods by two kinds of sound pressure method and sound intensity method are investigated theoretically and experimentally on referring to the methods of sound power level measurement.

The active minimisation of harmonic enclosed sound fields, with particular application to propeller induced cabin noise

Abstract: This work considers the use of secondary acoustic sources for the reduction of noise levels in harmonically excited enclosed sound fields. This possibility is studied initially through the use of an analytical model of a single frequency `two-dimensional' rectangular, enclosed sound field of low modal density. Quadratic optimisation theory is used to predict the effectiveness of these active techniques for reducing sound levels. It is shown that if one chooses to minimise the total acoustic potential energy of the sound field then substantial global reductions in sound pressure may be achieved. The conditions required to achieve these reductions are discussed, and in particular the effect of secondary source location is demonstrated. The unsuitability of this as a practical control system cost function is discussed, and as an alternative it is suggested that one can minimise the sum of the squared pressures over a number of discrete sensors. Both theoretical and experimental results from using this cost function are presented. These show good agreement. The importance of both secondary source and error sensor locations is discussed. The reduction of low frequency propeller induced cabin noise is a possible application of active noise control. Simplified analytical models of the structural and internal acoustic response of a 48 seat, twin turboprop passenger aircraft are presented. These consist of the structural response of an isotropic, thin cylindrical shell of finite length and the acoustic response of the enclosed cyclindrical sound field. It is shown that, provided the external acoustic pressure forcing of the shell is modelled representatively of measured propeller pressure fields, then these simple models provide results which demonstrate good agreement with measured data, at least for frequencies encompassing the first two propeller blade passenger harmonics (88 Hz and 176 Hz). These models are used to predict the effectiveness of active noise control when it is applied to reduce the average sound pressure level over a typical seated head height plane in the passenger cabin. Results for the first two blade passage harmonics are presented for systems consisting of up to 24 secondary sources and 48 error sensors. In general the predictions suggest that active noise control shows promise as a potential method of reducing low frequency propeller induced cabin noise.

Method for large-scale multiple source sound reinforcement

Abstract: An improved method for transmitting sound at high power levels over a wide angle zone of dispersion without distortion, comprising the step of emitting sound waves from a plurality of individual sources, each characterized by a relatively narrow, wedge-shaped envelope of sound projection, such that adjacent edges of respective sound projection envelopes are in substantial alignment and do not overlap, whereby the absence of interference between sounds emitted from different sources precludes sound distortion and enables uniform sound dispersion and high sound quality throughout the zone. The sound waves are preferably emitted from electroacoustical loudspeakers having loudspeaker enclosures shaped to conform to the edges of their respective sound envelopes.

Acoustic flow instruments for solid/gas flows

Abstract: Two nonintrusive acoustic flow sensing techniques are reported. One technique, passive in nature, simply measures the bandpassed acoustic noise level produced by particle/particle and particle/wall collisions. The noise levels, given in true root-mean-square voltages or in autocorrelations, show a linear relationship to particle velocity but increase with solid concentration. Therefore, the passive technique requires calibration and a separate measure of solid concentration before it can be used to monitor particle velocity. The second technique is based on the active cross-correlation principle. It measures particle velocity directly by correlating flow-related signatures at two sensing stations. The velocity data obtained by this technique are compared with measurements by a radioactive-particle time-of-flight (TOF) method. A multiplier of 1.53 is required to bring the acoustic data into agreement with the radioactive TOF result. The difference may originate from the difference in flow fields w...

Relations Between the Acoustic Nonlinearity Parameter and Sound Speed and Tissue Composition

Abstract: With the improved thermodynamic method, in which higher frequency was used for lowering the high pressure require& in common thermodynamic method, the acoustic nonlinearity parameter B/A'S in some tieeuea were measured. The results showed that the B/A values of same kind of tiesues were in direct proportion to the reciprocal of the sound speeds of these tissues. In the range of different tiseues, this rule is broken although the B/A value increases with the reciprocal of sound speed. It is also shown that a B/A value is determined both by the composition and by the construction of the tissue. The relation between B/A and tissue composition conformed with Apfel's mixture rule qualitatively.

The Effects of the particle Properties on the Parameters of Impact Sound between Two Particles

Abstract: The parameters of impact sound between two particles are discussed to measure the particle size and velocity of colliding particles by the analysis of that impact sound. The radiation mechanism of the impact sound is considered theoretically, and the pressure waveform of the sound is estimated on the basis of the radiation mechanism. The calculation of pressure waveform takes account of the reflection of the impact sound from the surface of the colliding spherical particle. The estimated waveform agrees well with the measured one.The parameters of impact sound related closely to the particle size and impact velocity between two particles.These results show that it is possible to measure the particle size and velocity of flowing particles by the analysis of flow noise.

Estimation of the directionality pattern of a moving acoustic source

Abstract: In many acoustic measurement situations the sound source is in motion relative to the observer. Source motion can have a considerable effect on the radiated sound field and the proximity of the source to a boundary can also significantly alter the sound field. This paper is concerned with the problem of estimation of the source directionality pattern from sound measurements which are both amplitude and frequency modulated in form due to the range, source directionality and the Doppler effect. The aspects addressed in the paper include (i) the modelling of the sound field; (ii) a description (analytical) of the methods of estimation of the directionality (which includes the Wigner-Ville and evolutionary spectral methods); and (iii) computational results showing the accuracy of estimation of a simple directionality pattern.

In Vivo Sound Velocity in Reflection

Abstract: The measurement accuracy of sound velocity by reflection mode methods is estimated theoretically for the several v ersions of the technique suitable for in vivo application. Focal fields used in versions of the focus adjustment method (FAM) are analyzed b y computer simulation as well as the incoherent crossed-beam method (ICM) and the crossed-transmission-andreception-beam method (CTRM) for the same acoustic conditions. The results show that FAM with a large reception aperture, and both of the crossed-beam methods, produce identical v elocity measurement accuracies. Moreover, a new version of FAM with a modified synthetic aperture produces twice the accuracy of the other methods discussed. In vivo as well as in vitro sound velocity measurements have been made using a high-resolution scanner with pixel-by-pixel real-time dynamic focusing based on the FAM concept with a large reception aperture. Significant sound velocity differences have been detected in normal, cirrhotic, and fatty livers. Since the potential measurement accuracy of FAM is at least twice that of the t ested version, even better instrumentation for measurement of in vivo sound velocity is possible.

Remote Measurement Method of Acoustic Velocity in Liquids by a Pair of Leaky Acoustic Waveguides

Abstract: We propose a new method to measure the elastic characteristics (the acoustic velocity) of liquids by a pair of leaky acoustic waveguides as probes. Transducers are not immersed or inserted into liquids to be measured in this method, s o that it is suitable for remote measurement of the acoustic properties of liquids at high temperatures or in chemically active conditions. In this paper, long steel plates have been investigated as the waveguides. An L1-mode Lamb wave and a Rayleigh wave have been studied experimentally as guided waves. The principle of measurement method, the experimental equipments and the experimental results are described in detail.

Interaction of sound waves. Part II: Plane wave and real beam

Abstract: The nonlinear interaction in a lossless fluid between an infinite plane sound wave and a sound beam generated by a real source is considered theoretically in the quasilinear approximation. The importance of a proper choice of acoustic variables and their correct on‐source values are analyzed. The governing equation for the sum and difference frequency components is solved for prescribed boundary conditions. A solution uniformly valid in space is obtained in the form of a double Fourier integral. In the farfield, the integration can be performed and a simple formula is obtained. This relates the amplitude and directivity at the observation point to the on‐source amplitude and phase of the beam. Relevance to earlier literature on parametric acoustic receiving arrays is also discussed.

Akustische Rastermikroskopie als Methode zur Oberflächenuntersuchung

Abstract: Techniques of scanning acoustic microscopy generally rely on local variations of such solid state parameters influencing generation or propagation of acoustic waves. Depending on the manner of impressing acoustic waves into the sample various methods are distinguished. In conventional scanning acoustic microscopy ultrasound is generated by a lens-transducer arrangement outside the sample and focussed onto or below its surface. Changes in the propagation of this ultrasound wave, like absorption and reflexion or temporal propagation delays, enable analysis of the mechanical or elastic response. At very high frequencies and with additional time-resolving detection techniques applications of this technique to surface analysis become possible. Other scanning acoustic microscopes imply the generation of sound or ultrasound directly within the sample itself due to the impact of temporarily modulated particle or photon beams. These are presently laser, electron, or ion beams. With these methods the acoustic signal as detected by a transducer attached to the sample is on principle affected by propagation properties, too, but it is dominated by local changes of the generation process for the acoustic wave, mainly because the frequency ranges used presently are associated with very long acoustic wavelengths. Depending on the physical nature of the primary probe used many sound generation mechanisms are given resulting in a large amount of different applications. By adjusting the probe parameters in a suitable manner the sound generation process can be confined to the direct vicinity of the specimen surface, which makes this technique feasible for surface characterization. The principles of the various techniques are described, and their usability for surface analysis is discussed.

Acoustic radiation from plates driven by multiple point random forces

Abstract: The problem of acoustic radiation from plates driven by multiple random point forces is analyzed in this study. First, the formulation of the sound power radiated by infinite thin plates (one dimension) under the action of multiple random point forces is derived. The reaction of the acoustic medium on the vibratory response of plates is taken into account. With the input spectrum being assumed as bandlimited white noise, the effects of input force correlation on the radiated sound power level are examined. The analysis is then extended to finite thin plates with four simply supported edges. The fluid loading effects are neglected. The exact expressions for the surface acoustic intensity and the radiated sound power are also derived. Assuming light damping, approximate solutions for acoustic intensity and sound power are obtained. An experimental study is carried out to measure the surface acoustic intensity patterns and compare with the analytical results. The effects of input force correlation on the acoustic intensity patterns and the sound power level are discussed.

Measurement of acoustic impedance of sound absorbing materials in a free field at low frequencies

Abstract: The two‐microphone technique allows the precise measurement of acoustic velocity and consequently the determination of acoustic intensity. Using this principle, a method has been proposed to measure, in a free field, the acoustic impedance of sound absorbing materials. Good results were obtained for frequencies ranging from 700 Hz to 4 kHz [J. F. Allard et al., J. Sound Vib. 114(2) (1987)]. Recently, it has been demonstrated theoretically [Y. Champoux and A.L'Esperance, J. Acoust. Soc. Am. Suppl. 1 80, S56 (1986)] that for low frequencies, even if the source is several meters from the sample, the spherical nature of the waves has to be taken into account. This paper presents experimental evidence that confirms that the modeling correctly describes the phenomena. Further several refinements to the experimental technique, allowing precise measurements for frequencies as low as 250 Hz, will be demonstrated.

Broadband acoustic source localization

Abstract: This paper examines aspects of matched field processing for locating a broadband source in deep water. Concentration is on nearby sources (within 5 km of a vertical receiving array), on frequencies from 100 to 300 Hz, and on deterministic, simulation “data” only. The noise field has been modeled as Gaussian, uncorrelated, white noise while the acoustic field has been modeled by the fast field program, with subsequent processing by conventional and maximum likelihood (ML) methods to generate range‐depth ambiguity surfaces (AMSs). The effects of mismatch in the description of sea surface roughness have been examined and it has been found that it can lead to significant errors in peak location on an AMS, particularly at the higher frequencies. However, incoherent frequency averaging of AMSs can, in the presence of relatively small mismatch, overcome the effect of such mismatch, even though almost all the component frequencies show errors in their predictions. Moreover, consideration of range averaging plus frequency averaging can, under such conditions, improve those cases not resolved by frequency averaging alone. It is hypothesized that frequency and/or range averaging will help only when the mismatch is sufficiently small that the AMSs have at least local maxima near the true source location.

Application of the complex cepstrum to the location of acoustic sources near reflective surfaces

Abstract: The complex cepstrum is used to correct bearing estimations of acoustic sources in the presence of a reflective surface. An automated liftering procedure is used which zeros out a block portion of the cepstrum including the echo information. The problem of the resulting distortion is alleviated by applying a coherence criterion to the recovered direct signals at each microphone. Thus to a large degree the interactive nature of cepstral processing is overcome for this application. For the test signals and geometries considered the cepstrum is shown to accurately correct for bearing errors in acoustic signals contaminated with reflections from nearby surfaces.

Casual constraints on the active control of sound

Abstract: The problem is addressed of evaluating the maximum net reduction in acoustic power output produced by introducing a single point monopole secondary source to control the sound field radiated by a single point monopole primary source in a free field. The secondary source is constrained to act causally with respect to the primary source. The problem reduces to solving a Wiener-Hopf integral equation for the optimal realisable impulse response relating the secondary and primary source outputs. A solution is given which has a well-defined physical significance and demonstrates the need for the secondary source to act as a predictor in order that destructive interference of the primary field is possible at field points closer to the primary source than the secondary source.

Acoustic emission point source locations on thin spherical shell

Abstract: A point source of acoustic emission on a thin spherical shell is defined in spherical and Cartesian coordinates. A required number of acoustic sensors and their locations are postulated for a unique determination of an acoustic emission point source location. Spherical coordinates of an acoustic emission point source are computed using acoustic sensors' measured channel time-differences, radius, and sound propagation velocity in a thin spherical shell.

Parametric Acoustic Array Application Using Liquids with Low Sound Speed

Abstract: Humphrey1 first demonstrated the use of a parametric array to measure the insertion loss of small, half a metre square, panels of material using a 1MHz primary source and positioning the test specimen within the interaction zone of the array at a range of 7OOmm from the primary source. A parametric array is capable of producing a very narrow beam over a wide frequency range with a single, relatively small, projector. Humphrey was thereby able to extend the 50kHz lower frequency limit obtained with conventional methods of measurement to approximately 10kHz. Decreasing source level and increasing diffraction to sound around the specimen with decreasing frequency limits the lowest useable frequency.