摘要:

Knowledge of variations in the optical properties of liquids with pressure is required in ultrasonic‐light interaction studies. One parameter used to describe these variations is the piezooptic coefficient, defined as the change in refractive index produced by a unit change in the applied pressure. The relative adiabatic piezooptic coefficients of numerous pure liquids and alcohol‐water mixtures were determined from measurements made on light diffracted by pulsed ultrasonic waves. Absolute values of this parameter for the liquids tested were calculated from a consideration of experimental values previously determined and the results compared with the theoretical and empirical formulas presently used to compute this parameter. [Research sponsored by the Office of Naval Research.]

ISSN:0001-4966    

DOI:10.1121/1.1942735

出版商:Acoustical Society of America    

年代:1966

数据来源: AIP

摘要:

The theory of continuous distribution of oscillating dislocations has been joined with Zener's thermoelastic effects to give a possible estimate of the energy dissipation in metals. This approach to the question of energy dissipation was considered first by Eshelby. In this work, Eshelby made the following simplifying approximations: the inertia of the material was neglected, the interaction effect between the dislocations was not considered, and the inside of the dislocation core was not considered, only the medium outside the dislocation bounded by a “cutoff” lengthlwas considered. Also, difficulties appeared in Eshelby's work because of the singularity of the stress field at the center of a dislocation. With the above restricting assumptions Eshelby was able to obtain that the energy dissipation should increase with frequency. This result disagrees with the limited experimental data available. We attempt to improve upon the basic thesis proposed by Eshelby by considering a continuous distribution of dislocations instead of a discrete dislocation. Also, the inertia of the material is included in the analysis.

ISSN:0001-4966    

DOI:10.1121/1.1942737

出版商:Acoustical Society of America    

年代:1966

数据来源: AIP

摘要:

Sound attenuation and dispersion by small, dense particles suspended in a gas are studied analytically and experimentally. A theory for attenuation and dispersion based on particulate relaxation processes is given. The close relationship between particulate relaxation and relaxation mechanisms due to lagging molecular or atomic internal degrees of freedom is displayed. The particulate relaxation theory predicts attenuation and dispersion, in close agreement with existing, more‐detailed theories in the vicinity of ωτd=1, where the attenuation per unit wavelength is a maximum (ω is the circular frequency, τdis the dynamic relaxation time of the particles). The experimental study consists of measurements of attenuation and dispersion in an acoustic interferometer. The interference tube is filled with an aerosol composed of oleic acid particles in nitrogen. Particle size and concentration are measuredin situby spectrophotometric tests. Dispersion measurements, taken at low values of ωτdwhere no previous measurements have been made, indicate good agreement with the theory. Attenuation measurements taken in the range of ωτdfrom 0.1 to 4.0 show good agreement with the theory. In particular, they confirm the existence of the predicted maximum. [Work supported by the Power Branch of the U. S. Office of Naval Research.]

ISSN:0001-4966    

DOI:10.1121/1.1942744

出版商:Acoustical Society of America    

年代:1966

数据来源: AIP

摘要:

After reviewing the work of American acousticians during the 1930's in respect of the application of normal modes to reverberation, and the second work at UCLA on the diffraction of sound by an array of rectangular reflective panels, this paper gives a preliminary report on an investigation, now in progress, of the effects of suspended panels on the decay of sound in a rectangular reverberation room (19×30×24 ft high), with and without the floor covered with absorptive material. The array consists of 96 panels of plywood1×3 ft × 12 in.thick located in a horizontal plane midway between the floor and ceiling. The panels appear to be completely nonabsorptive at frequencies below about 250 cps. Their absorption increases progressively at higher frequencies. At 4000 cps, they add 23 sabins to the reverberation room, which would correspond to an absorption coefficient of 0.08, if this coefficient is calculated in the usual manner by dividing the added number of sabins by the area of the panels. The absorption coefficient for the array would be 0.04 at 4000 cps, and it diminishes progressively at lower frequencies. As is well known, the covering of the floor with absorptive material results in nonlinear decay rates unless the sound field is diffuse. In the experiments conducted to date, there is no artificial diffusion in the room, except that added by the panels under investigation. The decay curves are characterized by an initial high rate of decay followed by progressively lower rates of decay. The panel array introduces other complications in the decay, and decay rates depend greatly on the positions of the sound source and microphone. We have not yet attempted to explain the findings, but the methods of wave acoustics and normal modes should be able to account for most of the experimental findings. The results to date, however, have a bearing on how suspended panels influence the decay of sound in auditoriums that lack adequate diffusion and have highly reflective walls and ceilings.

ISSN:0001-4966    

DOI:10.1121/1.1942748

出版商:Acoustical Society of America    

年代:1966

数据来源: AIP

摘要:

Absolute and masked binaural thresholds are measured for an ensemble of pulsive periodic signals. Spectral composition, interaural time, and pulse rate are signal parameters. Broad‐band, interaurally in‐phase noise is the masker. From threshold behavior and from masking‐level differences (MLD's), evidence is deduced concerning place of detection on the basilar membrane. The experimental observations are supported by calculations of root‐mean‐square signal‐to‐noise displacement along the membrane, using Flanagan's model for membrane motion. A typical result of the study shows that a periodic train of unipolar impulses, produced at a rate of 250 pps, has (1) an absolute threshold determined by motion near the 1000‐ to 2000‐cps region of the membrane; (2) an homophasic masked threshold determined by motion near the 250‐ to 600‐cps region of the membrane; and (3) an MLD (or homoheterophasic threshold difference) determined by the spectral presence of the fundamental component. In particular, the findings provide further information about the peripheral processing involved in binaural detection of complex signals.

ISSN:0001-4966    

DOI:10.1121/1.1942755

出版商:Acoustical Society of America    

年代:1966

数据来源: AIP

摘要:

Twelve normal hearers were evaluated with a competing letter test. Letters B, C, D, G, etc., or F, L, M, etc., were presented in rhyming pairs, with one letter going to each ear. In the initial experiment, speech was delivered to one ear at a constant (20 dB SL) level while the speech intensity in the opposite ear was increased from 10 to 70 dB. The performance score in the variable ear increased in percent correct up to about 94% at 40 dB. The performance in the constant ear deteriorated as the opposite ear improved. When the intensity of the variable ear reached the 50‐dB level, the curve of the constant ear leveled off at about 30%. At the 60 dB level in the variable ear, both ears improved. At 70 dB, performance was reduced bilaterally. In order to rule out the possibility of a “60‐dB mechanism” in the auditory system, a second group of 12 subjects was tested with the constant ear at 30 dB and the variable ear increasing from 20 to 80 dB. In this experiment, the peak improvement in the constant ear was obvious at 70 dB instead of 60 dB, as observed in the 20‐dB condition. Another experiment demonstrated that the peak improvement in the constant ear occurred at a level 36–38 dB below the intensity of the variable ear.

ISSN:0001-4966    

DOI:10.1121/1.1942761

出版商:Acoustical Society of America    

年代:1966

数据来源: AIP

摘要:

The transmission of analog data, from individual elements of an acoustic array for beam‐forming, places stringent requirements on the transmission system. A 40‐channel telemetry system has been developed, utilizing synchronous sideband frequency‐division multiplexing techniques. The transmitted carrier frequencies are synchronously generated with a PRN generator driven by a master clock. The master‐clock frequency and two unused carrier frequencies are transmitted with the data channels. The receiver carrier frequencies are generated with another PRN generator, driven by a signal at the recovered master‐clock frequency, and are phase‐locked with the two recovered carriers. This technique minimizes the required transmission bandwidth, and affords a dynamic range greater than 40 dB. Measurement and performance data indicate that the transmission system does not affect the generation of beam patterns. [This work was performed under contract with the U. S. Department of the Navy, Bureau of Naval Weapons.]

ISSN:0001-4966    

DOI:10.1121/1.1942766

出版商:Acoustical Society of America    

年代:1966

数据来源: AIP

摘要:

The generation of overtones by finite‐amplitude processes in rigid‐walled cylindrical cavities with large length‐to‐diameter ratios is studied. An extension of the Keck‐Beyer perturbation technique leads to a set of iterative equations valid for standing waves that can be applied to cavities with energy losses occurring at the walls. Necessary features of the theory include assuming one‐dimensional motion, characterizing all absorptive processes by a bulk absorption coefficient, and requiring a boundary‐layer depth much less than the tube diameter. Solutions lead to a steady‐state distribution of harmonics of the fundamental, the relative amplitude of each overtone being inversely proportional to the absorption coefficient characteristic of the relevant normal mode of the cavity. [Work supported in part by the U. S. Office of Naval Research and the U. S. Bureau of Ships.]

ISSN:0001-4966    

DOI:10.1121/1.1942772

出版商:Acoustical Society of America    

年代:1966

数据来源: AIP

摘要:

If Burgers' equation,vt+vvx=δvxx, is used as a descriptor of the propagation of waves of finite amplitude in a viscous fluid, one can give a characterization of the set of boundary functions that are shock‐producing. The boundary conditions assumed arev(0,t)=a(t),vx(0,t)=b(t). Conditions for the appearance of shocks are given in terms of the quadratic forma2(t)−2δb(t)by means of the concept of complete monotonicity. The results obtained are an extension of those obtained by the author in “Mathematical Advances in the Theory of One Dimensional Flow,”in Symposium on Apollo Applications, Huntsville, Alabama, January 1966. [Work supported by the National Aeronautics and Space Administration.]

ISSN:0001-4966    

DOI:10.1121/1.1942775

出版商:Acoustical Society of America    

年代:1966

数据来源: AIP

摘要:

The concept of immittance (impedance or admittance) and transmission matrices for application to the vibration of mechanical systems is reviewed. The experimental measurement of the elements of the matrices is explored from a conceptual standpoint. Included is a discussion of the correction of the data taken under other than desired condition of constraint. Some general applications of the matrices are described.

ISSN:0001-4966    

DOI:10.1121/1.1942782

出版商:Acoustical Society of America    

年代:1966

数据来源: AIP