摘要:

Acoustic pseudo‐waves (leaky waves) along a plane interface are studied in the space‐time domain. Analysis of the Green’s function as derived by the Cagniard–de Hoop method allows a quantitative description of the propagation of a pseudo‐Rayleigh pulse along a fluid/solid interface. This includes a condition which limits the range of existence and an equation for the travel speed. A pseudo‐Rayleigh factor which determines the relative strength of the phenomenon is defined and the decay of pulsed wave motion along the interface is studied.

ISSN:0001-4966    

DOI:10.1121/1.390776

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The rigorous expansion developed by the present authors for the velocity potential of a circular piston source [J. Acoust. Soc. Am.74, 1044–1047 (1983)] is extended to include the space average pressure in the nearfield and the Fresnel diffraction of incident plane waves by an infinitely thin rigid disk. The present theoretical framework has the remarkable advantage that it includes neither approximations nor numerical integrations.

ISSN:0001-4966    

DOI:10.1121/1.390777

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

An explicit time/space finite difference procedure is used to model the propagation of sound in a quasi‐one‐dimensional duct containing high Mach number subsonic flow. Nonlinear acoustic equations are derived by perturbing the time‐dependent Euler equations about a steady, compressible mean flow. The governing difference relations are based on a fourth‐order, two‐step (predictor–corrector) MacCormack scheme. Difference equations for the source and termination boundary conditions are derived from the appropriate characteristic relationships. The solution algorithm functions by switching on a time harmonic source and allowing the difference equations to iterate to a steady state. A significant advantage with this approach is that the nonlinear terms can be retained and evaluated with only modest additional computer cost above that required for a linear model calculation. The principal effect of the nonlinearities was to shift acoustical energy to higher harmonics. With increased source strengths, wave steepening was observed. This phenomenon suggests that the acoustical response may approach a shock behavior at a higher sound pressure level as the throat Mach number approaches unity. Where applicable, comparisons were made with the calculations from a linear finite element algorithm. On a peak level basis, good agreement between the nonlinear finite difference and linear finite element solutions was observed, even though a peak sound pressure level of about 150 dB occurred in the throat region. Nonlinear steady‐state waveform solutions are shown to be in excellent agreement with a nonlinear asymptotic theory.

ISSN:0001-4966    

DOI:10.1121/1.390778

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The time spreading of a spherical‐wave impulse of acoustic power reflecting from a not‐too‐rough fixed boundary (such as an ocean bottom) is computed using a simple geometric‐acoustic model. In that model, a ray reflects specularly from each boundary facet, and arrives at the receiver if and only if the position and slope of the facet provide the requisite specular path. In this way, the probability of reception of a ray from a particular facet is tied to the slope distribution of the reflecting boundary (assuming the depth is constant). A general expression for time spread is derived, and computed time spreads are presented for particular source/receiver geometries. Appreciable time spreads (up to half a second) are shown for benign ocean‐boundary conditions (rms slope ≲tan 15°). Time spreads increase dramatically with rms slope, decrease with increasing range, and increase with increasing depth from source/receiver to the boundary. It is emphasized that, for broadband signals, models must be in three dimensions (as opposed to two) and depart from plane‐wave idealizations.

ISSN:0001-4966    

DOI:10.1121/1.390779

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

One method to estimate the validity of the leading term in the geometrical acoustics expansion is to calculate the second term in the expansion. This second term plays an essential role in the construction of the second term in Ludwig’s uniform smooth caustic ansatz and in the construction of the progressing wave smooth caustic ansatz. Therefore, the determination of this term is important in the assessment of each of these expansions. Expressions for this second term are derived valid for arbitrary index profiles near the source point and near a smooth caustic. Numerical examples are presented for then2‐linear profile. These results show that near the source the second term is not singular, but near the caustic it is more singular than the leading order term.

ISSN:0001-4966    

DOI:10.1121/1.390780

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The Rician probability density function (PDF) accurately describes acoustic scattering by individual live fish. A fish scattering model is proposed which contains concentrated scattering components (representing the swim bladder) and distributed scattering components (representing the fish skeletal structure). We associate the gross directional backscattering cross section with the concentrated component. The ratio of these two components, γ, may serve as a means of identifying different fish species and fish behavior from the scattering PDFs. Monte Carlo simulations of a swimming fish using the acoustic fish model give Rician PDFs.

ISSN:0001-4966    

DOI:10.1121/1.390781

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Ultrasonic pulse echo systems are often limited in range resolution by the bandwidth of the piezoelectric transducer. Significant improvements in the range resolution of such systems can be obtained by minimizing the effects of the transducer’s dynamic response on the overall pulse echo process. An approach to minimize the effects of the transducer is developed from linear system and impulse response techniques. In essence, the pulse echo voltage of interest is deconvolved with a pulse echo reference voltage which is obtained from an air/water interface in the nearfield of the transducer. A computer study of the pulse echo process and the deconvolution process is presented to illustrate the nature of the improvement in range resolution for several cases of interest. Finally, experimental results are presented to illustrate the improvement using commercially available transducers.

ISSN:0001-4966    

DOI:10.1121/1.390782

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The application of multiple input models to analyze structurally generated noise from a forge hammer is discussed. Part I of this article is intended to present the rationale for application and analysis of multiple input models for noise source identification. The development of the empirical models is reviewed and investigated to show how the terms in the model can be interpreted to mathematically simulate the selective wrapping approach to source identification. The interaction of the structural excitation forces and radiated structural noise is examined for a four‐piece forge hammer and provides an indication of the characteristic measurements needed to develop the multiple input model that is representative of the hammer’s sound radiation. The transducer requirements for application to forge hammers are examined through experiments performed on a laboratory test structure and a forge hammer column. The results indicate that a single, well‐placed transducer may be sufficient to characterize the sound radiation from a monolithic element. Part II of this article analyzes the application of the techniques to a Chambersburg ♯8 die forger under production conditions. The experimental results indicate that the ram is the dominant source of sound energy, the columns are secondary sources, and the yoke and anvil are minor sources when detected through a microphone at the operator’s position.

ISSN:0001-4966    

DOI:10.1121/1.390783

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The application of multiple‐input models to analyze structurally generated noise from a forge hammer is discussed. Part I of this article presented the rationale for developing and interpreting multiple‐input models for structural noise source identification. An investigation of the transducer requirements for characterizing the sound radiation from a monolithic element showed that a single, well‐placed accelerometer may be sufficient for each element. Part II of this article analyzes the application of the multiple‐input modeling technique to the structural noise source identification of a Chambersburg ♯8 die forger. A comparison of three‐, five‐, and seven‐input models applied to the forge hammer under production conditions indicates that as few as five transducers would suffice to characterize the sound contributions of the five structural elements. Analysis of these models indicates that the ram is the dominant source of sound energy, the columns are secondary sources, and the yoke and anvil are minor sources when detected through a microphone at the operator’s position. The analysis also shows that the coupling between the hammer structural elements is sufficient to render conventional wrapping identification methods unreliable for analyzing hammer noise.

ISSN:0001-4966    

DOI:10.1121/1.390757

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Two‐dimensional equations of motion of doubly rotated quartz plates for the thickness‐shear, flexure, and extensional vibrations under in‐plane initial stresses are employed to predict changes in the fundamental thickness‐shear frequencies due to initial stresses. Two types of initial stresses are considered: (1) stresses due to a pair of diametral forces, and (2) stresses due to steady accelerations for a three‐point ‘‘T’’‐shaped mount and a four point ‘‘+’’‐shaped mount configurations. Force sensitivity and acceleration sensitivity coefficients are computed and compared with experimental data and existing computed results. For both ‘‘T’’‐ and ‘‘+’’‐shaped mount configurations, mount orientations corresponding to the maximum and minimum of acceleration sensitivity are predicted.

ISSN:0001-4966    

DOI:10.1121/1.390758

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP