ISSN:0001-4966    

DOI:10.1121/1.1909603

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

Both channel vocoders and formant vocoders have often been used as bandwidth‐compression devices. This paper explores several configurations wherein elements of both are combined into a single system. Frameworks are proposed within which the element forms can be systematically changed. Experimental results from several specific cases are discussed. Most significant of these is the comparison between an 11‐parameter system presented in this paper and a 16‐channel laboratory vocoder. The quality obtained from the two systems was comparable, indicating that data‐rate savings are possible with the new configurations.

ISSN:0001-4966    

DOI:10.1121/1.1909607

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

With a feedback method, two groups were taught to identify, by label, complex sounds. One group continued to hear the sound following response and during identification; the second group experienced a 1‐sec delay between the sound and its label. Better learning and long‐term retention characterized the group for which sound and label overlapped.

ISSN:0001-4966    

DOI:10.1121/1.1909582

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

Light valves based on the ultrasonic‐diffraction effect are capable of producing high‐index intensity modulation of a light beam, but they are subject to a fundamental bandwith limitation associated with the finite time of transit of an ultrasonic wave across the light beam. It is shown theoretically that modulation bandwidth can be maximized by converging the incident light beam to a focus to minimize beam diameter and by confining the ultrasonic beam to the vicinity of the focal plane. Under properly chosen conditions, modulation bandwidth can be as large as 1/7.7 times the ultrasonic carrier frequency. An experimental investigation was carried out using water as the ultrasonic medium to verify the theoretical dependence of modulation bandwidth on ultrasonic‐ and optical‐beam parameters and to evaluate constants of the theory. The experiments also showed that the wavefronts near the focus of a convergent light beam are sufficiently plane to produce Bragg‐type diffraction when conditions are otherwise favorable. Bragg diffraction was used in the modulation bandwidth measurements. The dependence of diffracted‐beam intensity on ultrasonic amplitude was investigated experimentally and, in accordance with theory, it was found that nearly all of the incident (zero‐order) light can be transferred to a selected first‐order diffracted beam. This fact permits either the zero‐order or a first‐order beam to be chosen as the useful output of the modulator.

ISSN:0001-4966    

DOI:10.1121/1.1909601

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

A formal method that is essentially an extension to viscoelasticity of the Karal‐Keller method for elastodynamics is presented for solving three‐dimensional wave‐propagation problems for a wide class of homogeneous, isotropic, and linear viscoelastic media. As applied to high‐frequency time‐harmonic waves, it consists of asymptotically representing the solutions in a series of inverse powers of the frequency. However, the expansion method is not necessarily limited to time‐harmonic waves. The approximation given by the first term in the expansion is called the geometrical theory of viscoelasticity. Subsequent terms provide corrections to this theory. The method can be employed to solve problems for media that contain boundaries. Applications are given to the reflection of incident waves from smooth obstacles and “asymptotic” reflection laws are derived. As an example, the first two terms are determined in the expansion of the reflected waves that result from a time‐harmonic plane dilatational wave striking a rigid plane.

ISSN:0001-4966    

DOI:10.1121/1.1909605

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

Formal solutions for the transient scattering of a monochromatic wave by a scatterer fixed in space are given in the cases where the corresponding steady‐state solutions are known. The characteristic space and time parameters areL, the linear dimension of the scatterer, andL/c, wherecis the speed of sound. The transient waves are found to trail off behind the wavefront, their time of persistence at a given point of observation being of an order of magnitudeL/c. The solutions are applied to yield the transient scattering by a sphere and by a circular cylinder. The quantities governing the convergence of the solutions are found to be the same as in the corresponding steady‐state problems. If the scatterers are perfectly hard, the transient waves extract more energy from an incident wave of very long wavelength than does the accompanying steady‐state wave. The opposite holds for perfectly soft scatterers. For both kinds of boundary conditions, however, the transient waves are of very short wavelength. In the case of the incident waves with short wavelength, the poor convergence of the solutions prevents any explicit evaluation of the scattering. However, an order‐of‐magnitude analysis shows that the amplitude of the transient waves does not exceed that of the steady‐state waves. Moreover, parts of the power radiated as transient waves will be in the long‐wave domain of the spectrum.

ISSN:0001-4966    

DOI:10.1121/1.1909609

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

Determination is made of the longitudinal‐vibration characteristics of a family of tapered cylindrical bars. It is shown that the required solutions may be obtained very conveniently by use of an analog computer.

ISSN:0001-4966    

DOI:10.1121/1.1909611

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

The characteristics of pressure pulses recorded near the surface directly above small explosions at great depths are summarized. Charge weights ranged from 0.04 to 10 lb and burst depths varied from 3000 to 22 000 ft. Significant pressure and time parameters of the initial shock wave, the subsequent negative‐pressure phase, and the first bubble pulse are shown in graphs and Tables. The vertical measurements of shock‐wave peak pressure can be represented by the same power function that Arons previously determined from horizontal measurements of shallow explosions. These data extend the range of measurement for this parameter over an additional logarithmic cycle of the similitude curve to a distanceR(ft) of4×104 W12, whereWis charge weight in pounds. The first period of the bubble oscillation also continues to show the same weight and depth dependence previously reported for depths ⩽500 ft down to depths of 22 000 ft.

ISSN:0001-4966    

DOI:10.1121/1.1909613

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

Energy flux spectra have been digitally computed for near‐surface pressure‐time curves obtained from small (0.067 10 lb) charges detonated at depths ranging from 3000 to 22 000 ft. As charge detonation depth (which is also the range in these experiments) increases, the spectrum reflects changes in relative, as well as absolute, distribution of energy between the initial shock wave and subsequent bubble‐pulse phases of the pressure wave. The peak spectrum level occurs at the bubble‐pulse fundamental frequency , which varies with charge weight and depth. The slope of the high‐frequency end of the spectrum increases from anf−2to anf−3dependency as depth (range) increases. Apparent propagation losses” for deep 1‐lb charges were also estimated from an octave‐band analysis between 250 cps and 16 kc/sec. For midband frequencies near the bubble fundamental, the estimated “losses” as depth (range) increases from 6000 to 22 000 ft are only about half those predicted by spherical spreading.

ISSN:0001-4966    

DOI:10.1121/1.1909615

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP

摘要:

An experiment was carried out to investigate the influence of a surface sound channel on propagation when the receiving site is in relatively deep water and the hydrophone below the duct. To examine this, a two‐ hydrophone vertical array was employed, with one hydrophone in the duct and the other on the sea bed. For frequencies below 200 cps, the propagation was controlled by the nature of the bottom. For frequencies high enough for the first mode to be trapped, propagation to both receivers was controlled by the duct. However, at the higher frequencies higher attenuation and lower energy levels were obtained at the bottomed hydrophone. This behavior can be explained in terms of normal mode theory.

ISSN:0001-4966    

DOI:10.1121/1.1909617

出版商:Acoustical Society of America    

年代:1965

数据来源: AIP