摘要:

This paper reviews the history of horns used in loudspeaker systems for audience‐type sound reproduction. It reviews the early pioneering work started around 1910 which used telephone‐type transmitters before vacuum tube amplifiers were available. With the availability of amplifiers more powerful driver type loudspeakers were developed to operate with exponential straight and folded horns. Sound motion pictures created a demand for higher quality, and higher‐powered loudspeaker systems for both voice and music. The many stages of development of two‐way loudspeaker systems are reviewed. The limitations of each stage of development before proceeding to the next improvement is discussed. Photographs of many early horns are included as well as the more recent types.Subject Classification: [43]10.60; [43]85.60.

ISSN:0001-4966    

DOI:10.1121/1.380819

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The orthonormal sets of functions known as Gaussian–Laguerre and Gaussian–Hermite functions (G–L/H) have previously been used in quantum mechanics, electromagnetic waves and laser optics. These functions satisfy a modified wave equation, valid under certain restrictive conditions appropriate to slowly diverging sound fields. The nearfield of an ultrasonic transducer can be expressed as a linear combination of modes, each described by a G–L/H function. Each mode has a slightly different phase velocity in the nearfield but has the same spreading nature and phase velocity in the farfield. Using this formulation, the transition from nearfield to farfield is readily explained.Subject Classification: [43]20.55; [43]35.80; [43]20.15.

ISSN:0001-4966    

DOI:10.1121/1.380832

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The problem of the scattering of a Rayleigh wave by the edge of a thin surface layer of negligible inertia is solved by means of Fourier transforms and the Wiener–Hopf technique. The effect of the layer is approximated by a pair of boundary conditions applied at the surface of the half space. The condition that the traction on theedgeof the layer vanish is explicitly enforced. Expressions are given for amplitudes of the transmitted and reflected surface waves, and the power transmission and reflection coefficients are evaluated.Subject Classification: [43]20.15, [43]20.30, [43]35.54.

ISSN:0001-4966    

DOI:10.1121/1.380844

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Cavitation in liquid helium was induced using an ultrasonic cylindrical transducer resonant at approximately 57 kHz. Interaction of the sound field with superfluid helium during cavitation runs was studied using a low‐power second‐sound signal. This signal was observed to decay when a voltage whose amplitude increased linearly with time was applied to the ultrasonic transducer. This is interpreted as being due to the growth of turbulence in the liquid. It is contended that for the sound field to induce cavitation, the liquid has to be in a state of turbulence. It is possible that there is a threshold for additional attenuation of the second‐sound signal near the onset of cavitation. Some cavitation runs were also made in the presence of a high‐power second‐sound current. Under these circumstances turbulence was generated, either by the second‐sound field or by the associated steady heat current. This turbulence lowered the cavitation threshold.Subject Classification:[43]25.60; [43]35.32, [43]35.47.

ISSN:0001-4966    

DOI:10.1121/1.380846

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

A model for the prediction of wake‐related sound generation by a single airfoil is presented which assumes that the net force fluctuation on an airfoil can be expressed in terms of the net momentum fluctuation in the near wake of the airfoil. The model predicts a forcing function for sound generation which is related to the spectra of the two‐point velocity correlations in the turbulent region near the airfoil trailing edge. Cross spectra were obtained from correlations of the longitudinal and transverse components of turbulence in the wake of a 36‐in. chord NACA series 63‐009 airfoil in the 7×10 ft tunnel at the NASA Ames Research Center usingx‐probe hot‐wire sensors. Additional data were obtained from a 10‐in. chord airfoil in a free‐jet facility at the University of Washington. Based on results from these tests, a scaling model was developed using the turbulent boundary‐layer thickness as the primary length measure. The model was used to predict the sound radiated from a 2‐in. chord airfoil for which acoustical data is available with good agreement both in level and spectral shape. The single airfoil result is extended to a rotor geometry and comparisons studied for various aerodynamic parameters.Subject Classification:[43]28.65; [43]50.55.

ISSN:0001-4966    

DOI:10.1121/1.380821

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Based on the conventional ray‐tracing principle, a fast numerical algorithm is developed that calculates averaged propagation loss and reverberation of underwater sound. The averaging is performed by summing incoherently the contributions of individual ray bundles to each of the rectangular cells into which the range/depth plane is divided. The resulting matrix description of the average intensity field is used to calculate reverberation. The propagation‐loss algorithm is preliminarily tested against mathematical test functions and measured data.Subject Classification: [43]30.20, [43]30.40; [43]85.84.

ISSN:0001-4966    

DOI:10.1121/1.380822

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The work presented in an earlier paper [D. Epstein, J. Geophyos. Res. 72, 3701–3710 (1967)] on the expansion of a high‐pressure gas bubble in a fluid‐saturated porous medium, with the fluid treated as incompressible, is extended by treating the fluid as slightly compressible. Two limiting cases are considered: If the particulate matter is carried along with the fluid, the porous medium acts as a dense liquid, with parameters determined by simple functions of the density and compressibility of the fluid and solid particles, and the usual equations of motion apply. However, if the relative velocity of the fluid with respect to the solid is essentially equal to the fluid velocity, the motion is assumed to be governed by a compressible, generalized Darcy equation. From this equation a pair of integrodifferential equations for the bubble radius as a function of time are derived. The form of the radius‐time curve depends on a nondimensional resistance factorm? = (1/2) β?/c, a combination of quantities characteristic of the fluid, the porous medium, and the gas bubble. The transition regime between ideal and Darcy flow is determined as a function of this parameter. For very small values, the bubble pulsates about the equilibrium raius ?. With increasingm?, the ratio of radiated acoustic energy to energy dissipated decreases, until the Darcy limit is reached, where bubble pulsation no longer occurs, and the expansion radius approaches ? monotonically.Subject Classification: [43]30.60; [43]25.60.

ISSN:0001-4966    

DOI:10.1121/1.380823

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The connection between creeping wave and flat surface wave theory is established by investigating the limit of acoustic scattering from a solid elastic cylinder imbedded in a fluid, as its radius tends to infinity. After applying the Watson–Sommerfeld transformation to the scattering solution, it is shown analytically that the asymptotic expressions for the residue terms (creeping waves) in the Whispering Gallery mode series combine to form separately both the longitudinal and the transverse lateral waves on a flat surface. The cylindrical Rayleigh wave tends individually towards the flat Rayleigh wave, while the Franz and Stoneley waves disappear.Subject Classification: [43]30.35; [43]35.54; [43]20.30.

ISSN:0001-4966    

DOI:10.1121/1.380824

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

By using both continuous and pulsed insonification of a fixed volume at 160 kHz, measurements of scattering strength, scatterer target strengths, and reverberation spectra were made at a single location near San Diego at depths up to 90 m. Scattering strength and density of scatterers decrease slightly with depth while the short‐term fluctuation increases somewhat. Dimensions of the scatterers are estimated to be of the order of millimeters. The scatterers apparently do not migrate vertically over a 24‐hour period and reverberation spectra indicate that they are nonmoving or very slowly moving obstacles carried along with the current. Application of the apparatus in the detection and measurement of turbulence at considerable ranges is considered, and analyses indicate that turbulent speeds of the order of centimeters per second might be observed at ranges of approximately 100 m with a transmitted power of a few hundred watts.Subject Classification: [43]30.40; [43]80.20; [43]28.60.

ISSN:0001-4966    

DOI:10.1121/1.380825

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Acoustic reflection coefficients for reflection from ocean bottoms are sometimes calculated for postulated models of the bottom structure. A published example, using a model based on geophysical data, is examined with attention to possible simplification and to the physical links between model and computer output. It is shown that calculated results at small grazing angles have a physical cause and can be well approximated by a simpler model. It appears, however, that at larger angles the results are not physically reasonable. The cause is that a commonly used ’’pseudolinear’’ gradient of sound speed is unphysical in a half space, because at a certain finite depth the sound speed becomes infinite, and imaginary at greater depths. A physically reasonable profile of sound speed is proposed as a substitute and expressions for the reflection coefficient are derived in the approximation of vanishing absorption. At that stage, the expressions are considerably simpler than those of the published example. An extension incorporating absorption is planned.Subject Classification: [43]30.20, [43]30.30.

ISSN:0001-4966    

DOI:10.1121/1.380826

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP