摘要:

Insertion loss tests were conducted for three different thicknesses of fiberglass pipe insulation and five different jacketings using a proposed test standard. Results indicate that the procedure is a feasible method for testing pipe lagging. The insertion loss increased with increasing thickness of insulation. A significant increase resulted at all test frequencies for the insulation without jacketing and with the aluminum jacketing. The increases at the higher frequencies due to increased insulation thickness diminished with the use of heavier jacketing. When the insulation thickness was held constant at 2 in., each doubling of the weight of the jacketing resulted in increases in insertion loss at all frequencies. For each thicker insulation, these increases due to heavier jacketings were restricted to fewer test frequencies. There was good agreement between the 100‐Hz and 1/3‐oct test data for the 2‐ and 3‐in. pipe insulation with a slight preference for the 100‐Hz data measured at the lower end of each 1/3 oct.

ISSN:0001-4966    

DOI:10.1121/1.391111

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The discrete frequency noise radiated from representative tyes of axial‐flow fans used in electronic equipment is studied in detail. Narow‐band analysis of the discrete frequency noise radiated by these types of fans has been conducted in an anechoic environment. The farfield sound pressure level and radiated directivity of the discrete frequency noise are presented. The influence of operating condition on the radiated sound is determined. The discrete frequency noise dominates the characteristic acoustic spectra at high flow coefficients. The radiated directivity of the discrete frequency noise is generally uniform. Based on the data collected and on the theoretical aspects presented, the principal source of the discrete frequency noise is identified for each of the fans as the interaction of the fan rotor with a circumferentially distorted, nonuniform flow. A collection of prototype design modifications is developed for each representative fan to reduce the radiated discrete frequency noise by improving the uniformity of the local flow field. A modified prototype configuration is developed for each representative fan which is evaluated acoustically and compared to the original baseline data.

ISSN:0001-4966    

DOI:10.1121/1.391112

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A description is given of a technique for measuring small ultrasound velocity differences between two liquids, based on a new high resolution phase comparison method. The technique uses a two‐path acoustic system well suited for velocity dispersion measurements in the frequency range 3.3 to 330 MHz. The phase difference between two acoustic pulses traveling through the liquid is measured with a resolution of a few picoseconds. Velocity differences can be measured with an accuracy of ±1.5 cm/s. The unusually wide frequency capability and accuracy of the method are demonstrated with dispersion measurements on polystyrene samples dissolved in ethylbenzene.

ISSN:0001-4966    

DOI:10.1121/1.391089

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

We quantitatively study the effect that hydrostatic pressures have on the static and dynamic effective material properties of perforated viscoelastic elastomers. A resonance methodology that we had developed earlier is here extended and generalized to account for the distorting effect of added pressures. The distortion essentially consists of a broadening of the spectral response plots and a shifting of the resonance peaks/dips that are present in all the graphs of the effective moduli and/or the effective acoustic/material properties of the pertinent composites. For the lossy, air‐filled, rubberlike materials considered here, the pressure effect seems to be quite noticeable, but smaller than the viscous effect or than that of the nonuniformity in the cavity‐size distribution functions. The model is implemented by a computer code and its numerical predictions are quantitatively displayed for a few of the effective properties of a given perforated elastomer up to pressures of 400 psi. These are the pressures present down to ocean depths of about 900 ft. These perforated elastomers have uses as underwater sound absorbers.

ISSN:0001-4966    

DOI:10.1121/1.391090

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Past efforts to interpret experimental data on the self‐deactivation rate of the ν2(1→0) vibrational mode of water vapor have been thwarted because of the attempt to fit the data to a single relaxation time. In a phenomenological theory proposed here the deactivation takes place by means of three parallel processes: (1) collisional de‐excitation of the excited monomer, (2) a two‐step reaction involving association and spontaneous redissociation of an H2O collision complex, and (3) spontaneous dissociation of the stably bound H2O dimer. In pure H2O and in mixtures without O2, the reaction rate for association of the collision complex is found to be very slow, and the remaining relaxation modes (1) and (3) are effectively decoupled. In mixtures of H2O–air and H2O–O2the association rate of the complex increases strongly, suggesting that O2serves as an effective ‘‘chaperon’’ for this reaction, and the relaxation modes of the monomer (1) and complex (2) become strongly coupled. Fourteen sets of past data, representing a wide variety of test conditions and experimental methods, both acoustical and nonacoustical, are organized into three groups—each corresponding to a relaxation mode predicted by the theory. An analysis of an ultrasonic absorption peak, based on process (3), yields values for the standard entropy and enthalpy of dissociation of the stably bound H2O dimer.

ISSN:0001-4966    

DOI:10.1121/1.391091

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A method for obtaining C‐scan images is reported by using a Lamb wave device with two arched interdigital transducers, in which the phase difference between an input signal as a reference and a delayed signal reflected at a sample surface or inside a test sample is detected as a function of the position. The transducers are used as a sound beam radiator or a detector at a liquid–solid interface. Acoustic imaging results are satisfactory in measuring the stress fields in ceramic samples.

ISSN:0001-4966    

DOI:10.1121/1.391092

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The determination of the continuous spectral contribution for a stratified ocean acoustic model can be a difficult and expensive numerical task. A uniform asymptotic technique has been used to calculate the pressure field for the Pekeris model. It is uniform in the sense that it is valid as a mode passes through cutoff. The basic idea of the technique is to exploit the effect of the various singular points of the integrand representing the continuous spectrum. This idea has been further exploited to describe the continuous spectrum contribution for a general stratified ocean. Numerical examples show that this technique is both accurate and fast.

ISSN:0001-4966    

DOI:10.1121/1.391093

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A compaction model is developed to describe the compressional wave velocity as a function of depth near the seafloor in marine sediments. The model assumes that the porosity decreases with increasing depth of overburden and approaches a limiting value in the first few hundred meters. The limiting value is estimated from sonobuoy data applicable to the deeper sediments where further compaction is negligible compared to age‐dependent effects. Available consolidation data from the western North Atlantic are used in the model to predict the velocity profile, which is in good agreement with hydrophone‐pinger data.

ISSN:0001-4966    

DOI:10.1121/1.391095

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A nonsecular procedure is derived for modifying the intensity distributionI(μ) computed in a channel with a sound speed profile μ, so as to account for small changes Δμ in the profile. This procedure is then employed to relate the statistics of a random ensemble of profiles to the statistics of the corresponding intensity distributions. The difference betweenI(〈μ〉), the intensity distribution for the mean (expected) profile, and 〈I(μ)〉, the expected intensity distribution for the ensemble, is studied: it is taken as a measure of the relevance ofI(〈μ〉) as an approximation. At very short ranges, the two quantities are identical. At very long ranges, 〈I(μ)〉 obliterates entirely the range dependence ofI(〈μ〉). The usefulness ofI(〈μ〉) as an approximation is therefore restricted to ranges up to some transition rangerT. The transition range is inversely proportional to the relative magnitude ε of the deviation from the mean profile 〈μ〉, and also to ratios constructed from the characteristic (focusing) length of the channel, the injection depth of the beam relative to the channel axis, and the correlation length of the ensemble of profiles. Expressions forrTare derived in several limiting situations, and potential applications are discussed.

ISSN:0001-4966    

DOI:10.1121/1.391096

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A technique for acoustically characterizing shallow water waveguides is presented. For a horizontally stratified ocean and bottom, the method consists of measuring the magnitude and phase versus range of the pressure field due to a cw point source and numerically Hankel transforming these data to obtain the depth‐dependent Green’s function versus horizontal wavenumber. It is shown that, in the context of normal mode theory, the Green’s function contains information about the nature of the discrete and continuous modal spectra as well as the plane‐wave reflection coefficients of the waveguide boundaries. Inversions are performed using pressure field data generated synthetically over reasonable experimental apertures (1–5 km) to obtain Green’s functions for the cases of an isovelocity water column overlying both hard and fast isovelocity bottoms (Pekeris waveguide). The Green’s function results show excellent agreement with theory, while the subsequently calculated reflection coefficients of the bottom are of somewhat lower quality. It is shown that features of the Green’s function itself can be used to extract modal properties and characteristics of the bottom. The effects of sediment attenuation and shear in the Pekeris case are discussed, and a comparison of this method with conventional phased array mode resolution techniques is made.

ISSN:0001-4966    

DOI:10.1121/1.391098

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP