摘要:

For the scattering of acoustic waves by an elastic shell, the acoustical background coefficients are inherent in the scattering coefficients. The background coefficients for elastic empty shells, named the inherent background, can be obtained from the zero frequency limit of the modal accelerance in the scattering coefficients for analogous liquid shells. In this work, the concept of obtaining the inherent background is applied to multilayered elastic cylindrical structures. The inherent background manifests itself in the sound scattering by the liquid structures. The scatteringS-function and the modal accelerance for the liquid system are determined by considering the incoming and outgoing waves. The accelerance of liquid layers is generalized so that the scattering function can be obtained by the recurrence relation for the accelerances of the adjacent liquid layers. From the zero frequency limit of the generalized accelerance of liquid structures, the constant modal accelerance is extracted and the general expression for the inherent background coefficients is obtained. The background coefficients depend on the densities of the layers and ambient fluid medium, the relative thickness of each layer, and the normal mode number. The acoustical background coefficients for solid cylinders, empty shells, fluid-filled shells, and double-layered shells can be obtained by the appropriate limit of the density ratio and the relative thickness of layers in the generalized inherent background coefficients. The usefulness of the proposed background is demonstrated for several examples of layered structures.

ISSN:0001-4966    

DOI:10.1121/1.421207

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

An analytical model is proposed in this paper for predicting time-averaged energies of boundary structures and enclosed sound field. The sound field is directly driven by an acoustic source and the structures are excited through acoustic-structural coupling. The present model is based on the classical modal coupling method but is developed in such a way to improve the computational efficiency in estimating the bandlimited response of acoustic-structural coupled systems particularly in the medium frequency range where a large number of acoustic and/or structural modes is involved. In this frequency range, one often has to deal with manipulation of large complex matrices in order to obtain full mathematical solutions to the system response using the classical modal coupling method. However, this is avoided if the present bandlimited model is used. This paper describes the mathematical development of the model. Numerical examples for a panel-cavity system are presented and the proposed model is compared with the classical modal coupling method in terms of computational efficiency and accuracy in the prediction of the system energies.

ISSN:0001-4966    

DOI:10.1121/1.421208

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A study on feedback control of the vibration of a clamped elastic plate in the audio frequency range is presented in this paper. The study is specified by a discretization method based on an inverse modal analysis approach, through which a new lumped parameter system can be established to represent the dynamics of the low-order modes of the actual structure. With the discrete model the standard linear quadratic optimal control theory can be formulated in physical state space to design a controller, that will be later implemented in experiments to control the real structure. As an illustration, a lumped parameter model with 9DOFs has been created for the clamped plate. It is demonstrated by experiments under both impulse excitation and stationary random excitation that the corresponding nine vibration modes of the plate are quite well controlled with a collocation of two electromagnetic actuators and five sensors.

ISSN:0001-4966    

DOI:10.1121/1.421209

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Acoustic wave propagation and attenuation in catalytic converters are investigated in the present study. The relationships for wave propagation in a catalytic monolith are derived first and then coupled to the wave propagation in tapered ducts which are commonly placed at either end of the monolith. Analytical and finite element approaches are used to solve the resulting coupled system of equations. Theoretical predictions are then compared with the experimental results for two different (one circular and the other oval) catalyst configurations. The attenuation characteristics of the catalyst with and without the monolith are also investigated.

ISSN:0001-4966    

DOI:10.1121/1.421210

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Previous studies on the local active control of noise in free space show that the size of the quiet zone and the total sound power output of the control system are mainly determined by the spatial arrangement of primary sources, secondary sources, and error sensors. When a nearby reflective surface is introduced, the size of the quiet zone and the total power output of the system will be affected. The research presented in this paper examines the influence of the presence of a nearby reflective plane on the control system with different alignments to the reflective surface. It is found that the reflective surface significantly affects the size of the quiet zone and the sound power output of the system, as compared with the same control system in free space. The effect is related to two important parameters of the control system: the heights of the sources and the error sensor above the surface and the distances between the primary and secondary sources and between the secondary source and the error sensor. While the quiet zones created by most arrangements of the control system in half-space are smaller than those for free space, the quiet zones created by the arrangements of the control system perpendicular to the reflective surface may be larger than those for free space.

ISSN:0001-4966    

DOI:10.1121/1.421211

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The influence of scattering and diffraction on the performance of sound intensity probes has been examined using a boundary element model of an axisymmetric two-microphone probe with the microphones in the usual face-to-face arrangement. On the basis of calculations for a variety of sound field conditions and probe geometries it is concluded that the optimum length of the spacer between the microphones is about one microphone diameter; with this geometry the effect of diffraction and the finite difference error almost counterbalance each other up to about an octave above the frequency limit determined by the finite difference approximation. This seems to be valid under virtually any sound field condition that could be of practical importance in sound power determination. The upper frequency limit corresponds to about 10 kHz for an intensity probe with12-in.microphones, which means that it should be possible to cover most of the audible frequency range, say, from 50 Hz to 10 kHz, with a single probe configuration. The numerical results have been confirmed by a series of experiments.

ISSN:0001-4966    

DOI:10.1121/1.421212

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Random acoustic fields and their energetic quantities (acoustic active and reactive intensities, potential and kinetic energy densities) are described in terms of the mutual coherences between sources. Conditions to correctly construct the coherence matrix of sources in a multivariate random process are given. It is shown that the description of a sound field using the coherence matrix of source is equivalent to the superposition of a number of independent coherent fields, which do not correspond to the original localized sources. A method based on processing the principal components of the coherence matrix of sources is given to reduce the number of necessary fields. The coherence function between acoustic pressure and particle velocity and the curl of active intensity are proposed as two indicators for estimating the degree of coherence and the polarization of acoustic fields. These indicators are analyzed theoretically and experimentally. The description of the structure of partially coherent fields is generalized by the definition of the field matrix whose rank is an indicator of the local complexity of an acoustic field.

ISSN:0001-4966    

DOI:10.1121/1.421213

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

This paper describes a model for generating the hair cell receptor potential based on a second-order Boltzmann function. The model includes only the resistive elements of the hair cell membranes with batteries across them and the series resistance of the external return path of the transducer current through the tissue of the cochlea. The model provides a qualitative description of signal processing by the hair cell transducer and shows that the nonlinearity of the hair cell transducer can give rise to nonlinear phenomena, such as intermodulation distortion products and two-tone suppression with patterns similar to those which have been recorded from the peripheral auditory system. Particular outcomes of the model are the demonstration that two-tone suppression depends not on the saturation of the receptor current, but on the behaviour of the hair cell transducer function close to the operating point. The model also shows that there is non-monotonic growth and phase change for any spectral component, but not for the fundamental of the receptor potential.

ISSN:0001-4966    

DOI:10.1121/1.421214

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

This study varied stimulus frequency and recorded distortion product otoacoustic emissions (DPOAEs) in human newborns and adults. Because of outer and middle ear acoustics, the same auditory input resulted in higher newborn stimulus sound pressure levels across a broad frequency range in the occluded outer ear canal. Noise levels in the canal were 5–15 dB lower for adults at frequencies less than about 3 kHz. The2 f1−f2DPOAE was the most reliably recorded DPOAE except at the lowest frequencies assessed. At the lowest frequencies the2 f2−f1DPOAE was more frequently recorded than any other DPOAE. There were no striking developmental differences in the kinds of DPOAEs that were recorded. The amplitudes of consecutively recorded2 f1−f2DPOAEs were generally within 1.5 dB of each other for all age groups (slightly better reproducibility for adults than newborns). The phases of consecutively recorded2 f1−f2DPOAEs were generally within 15 degrees of each other (often less than 10 and 5 degrees for newborns and adults respectively). At the highest frequencies assessed(f2=4.2–9.9 kHz)all subjects had similar amplitude2 f1−f2DPOAEs. At lower frequencies adult2 f1−f2amplitudes were significantly less than those of newborns. At the lowest frequencies reliably assessed(f2=1.5–2.1 kHz)term newborns had significantly larger2 f1−f2DPOAEs than preterm newborns. Newborn and adult2 f1−f2DPOAE amplitudeX f2/f1functions were quite similar although there were reliable differences. Age related differences in the outer and middle ears may explain some of the differences in DPOAEs that were observed.

ISSN:0001-4966    

DOI:10.1121/1.421215

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

This study varied the levels of the primaries and recorded distortion product otoacoustic emissions (DPOAEs) from human newborns and adults. Preterm as well as term newborns were tested. The2 f1−f2DPOAE was the most reliably recorded DPOAE, especially at low levels of the primaries. Amplitude and phase reproducibility deteriorated with decreasing level of the primaries. Newborn DPOAEs were slightly less reproducible than adult DPOAEs. The underlying DPOAE I/O functions were nonmonotonic for both newborns and adults. Unity gain characterized the initial increasing portion of those functions in most subjects. Although newborn and adult I/O functions were similar, they did differ. In particular, adult functions tended to be more linear with saturation at higher primary levels. Some of the newborn functions saturated at very low stimulus levels. Although differences in cochlear mechanics may explain developmental difference in DPOAE I/O functions, developmental differences in the resonance characteristics of the outer and middle ears may also be involved.

ISSN:0001-4966    

DOI:10.1121/1.421246

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP