摘要:

Effects of tensile loading on the properties of longitudinal-mode elastic-wave propagation in a 1.52-cm-diam, seven-wire strand used for prestressed concrete structures were investigated experimentally. In an unloaded state, the wave propagation properties in strand matched those seen in individual wires comprising the strand, namely, straight center wire and helical outer wires. In the strand, however, extraneous signals were found to be produced from the propagating wave due to physical interactions between the adjacent wires. Under tensile loading, it was observed that a certain portion of the frequency components of the wave became highly attenuative and, thus, absent in the frequency spectrum of the wave. The center frequency of this missing portion, called notch frequency, was found to increase linearly withlog N,whereNis the applied tensile load. In addition, on both sides of the notch frequency, the wave exhibited a large dispersion in a manner similar to the behavior near a cutoff frequency. Possible causes of the observed behavior under tensile loading are discussed.

ISSN:0001-4966    

DOI:10.1121/1.423051

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A new substructuring technique is proposed to perform vibration analysis of line-coupled structures. In dividing the whole structure into a master structure and several auxiliary structures, a variational formulation is used to model the master structure, enabling one to introduce the effects of all auxiliary structures by using their compliance characteristics at several observation points along the junction. Continuous functions of the compliance are obtained via a regression analysis. Given the problem of using the compliance inverse to attain a straightforward formulation, a “Coupling Load Decomposition” technique is proposed since a direct formulation using the compliance inverse is not feasible. By decomposing the interactive load between substructures, relations with displacement decomposition of the master structure can be found. This new formulation permits the direct use of the compliance of the junction, which may be obtained analytically, numerically, or experimentally. Numerical examples using both calculated and experimentally measured compliance data are given. Simulation results are also compared to those obtained experimentally, showing good agreement in low- and medium-frequency ranges.

ISSN:0001-4966    

DOI:10.1121/1.423078

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

This paper presents an analytical framework for understanding and controlling the physical mechanisms which govern power flow to infinitely long cylindrical shells from applied point forces. It has been observed by others that the power flow to an empty shell peaks dramatically when the excitation frequency is near the cutoff frequency of a wave in the shell. In this paper, these observations are explained and generalized by a new analytical expression for power flow based on classic analyses of a shell’s response to an applied point force. This expression quantifies the partition of power among propagating waves excited by an applied force. Furthermore, the expression confirms that power flow increases dramatically as a propagating wave is excited in the vicinity of its cutoff frequency. In the context of this understanding, the attachment of a passive structure to the shell is explored as a means of controlling power flow. The nonlinear problem of designing a structure which satisfies practical constraints and minimizes power flow is formulated in such a way as to be solvable by a variety of optimization techniques. The formulation, which can be extended to other structures, is based on an expression for power flow in which the impedance of the attached structure acts in mechanical parallel with the shell’s impedance at the points of attachment. An example indicates that a significant reduction in power flow can be achieved by the attachment of a passive structure whose parameters have been optimized by a genetic algorithm.

ISSN:0001-4966    

DOI:10.1121/1.423081

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

This paper presents a theoretical approach for constructing a reduced model in the medium-frequency range in the area of structural acoustics for a general three-dimensional dissipative structure made of an anisotropic, inhomogeneous, viscoelastic bounded medium coupled with an external acoustic fluid. All the results presented can be used if the structure is made of beams, plates, and shells. The boundary value problem in the frequency domain and its variational formulation are presented. For a fixed medium-frequency band, an energy operator related to the structural-acoustic system is introduced. This operator is symmetric positive definite and has a countable set of positive eigenvalues. Its dominant eigensubspace allows a reduced model to be constructed using the Ritz–Galerkin method. A finite dimension approximation of the three-dimensional continuous case is presented and an effective construction of the dominant subspace using the subspace iteration method is developed. As an example, the reduced model is used for constructing the time-stationary random response of the structural-acoustic system submitted to a random wall pressure field. Finally, the theory is validated for a finite length circular cylindrical shell coupled with several dashpots and springs and immersed in a gas (air) and in a liquid (water).

ISSN:0001-4966    

DOI:10.1121/1.423052

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A hearing aid fitted to different ears will produce very different sound pressure spectra in the ear canal. In addition, this variation in response is different among hearing aids. A description in terms of an electrical analog model of the ear and hearing aid system is given. The applicability of this model is tested through series of measurements. The measurement and prediction procedure was first verified on a coupler (ear simulator) with good results from 300 to 8000 Hz. Three types of hearing aids were then measured and used on five different human ears. Where the measured and predicted response was compared a fairly good agreement was obtained from 300 Hz to approximately 6000 Hz. A major source of error is probe misalignment. The theoretical description given in the present work is likely to be valid from low frequencies to at least 10 kHz. Although solutions for related problems have been given for low frequencies, a solution of the hearing aid fitting problem at high frequencies has not been published earlier.

ISSN:0001-4966    

DOI:10.1121/1.423082

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

High-resolution hearing threshold and2 f1−f2distortion product otoacoustic emission (DP) were measured with the same in-the-ear sound probe and same calibration at 51 frequencies between 500 and 8000 Hz in 39 sensorineural hearing loss ears associated with tinnitus. Using a primary tone settingL1=0.4L2+39that accounts for the nonlinear interaction of the two primary tones at the DP generation site atf2,DPs were elicited in a wide range fromL2=65to 20 dB SPL. We failed to find a uniform DP behavior in the 39 tinnitus ears tested. Seventeen of them behaved like impaired ears without tinnitus. In these ears a linearized DP growth was observed where the DP level decreased and the slope of the DP I/O functions steepened with increasing hearing loss and as a result both the DP level and the DP slope strongly correlated with hearing threshold. The other population, 22 tinnitus ears, exhibited a poor or even inverse relationship between DP level and hearing threshold, i.e., displayed an increase of DP level with increasing hearing loss. Despite the severe hearing loss but due to the high level, DPs could be recorded well in the frequency range that corresponded to the appearance of the tinnitus. The DP slope, however, increased with increasing hearing loss and, therefore, did still correlate with hearing threshold revealing pathological alteration. The data suggest that the DP level alone is hardly capable of assessing hearing impairment in tinnitus ears and may even be misleading. Thus just the DP slope seems to be the only reliable indicator of cochlear malfunction around the tinnitus frequency. The observed nonuniform DP behavior suggests different cochlear impairments in tinnitus ears. In those ears where the DP level decreases and the slope of the I/O functions increases with hearing loss, cochlear sensitivity and tuning are supposed to be diminished. In those ears where the DP level increases with increasing hearing loss, a reinforced mechanical distortion is hypothetized to be generated by cochlear hyperactivity that can be the source of both the abnormally high DP level and the tinnitus.

ISSN:0001-4966    

DOI:10.1121/1.423053

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The2 f1−f2distortion product otoacoustic emission (DP) was measured in 20 normal hearing subjects and 15 patients with moderate cochlear hearing loss and compared to the pure-tone hearing threshold, measured with the same probe system at thef2frequencies. DPs were elicited over a wide primary tone level range betweenL2=20and 65 dB SPL. With decreasingL2,theL1−L2primary tone level difference was continuously increased according toL1=0.4L2+39 dB,to account for differences of the primary tone responses at thef2place. Above 1.5 kHz, DPs were measurable with that paradigm on average within 10 dB of the average hearing threshold in both subject groups. The growth of the DP was compressive in normal hearing subjects, with strong saturation at moderate primary tone levels. In cases of cochlear impairment, reductions of the DP level were greatest at lowest, but smallest at highest stimulus levels, such that the growth of the DP became linearized. The correlation of the DP level to the hearing threshold was found to depend on the stimulus level. Maximal correlations were found in impaired ears at moderate primary tone levels aroundL2=45dB SPL, but at lowest stimulus levels in normal hearing (L2=25 dBSPL). At these levels,17/20impaired ears and14/15normally hearing ears showed statistically significant correlations. It is concluded that for a clinical application and prediction of the hearing threshold, DPs should be measured not only at high, but also at lower primary tone levels.

ISSN:0001-4966    

DOI:10.1121/1.423054

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

Intracochlear pressure was measuredin vivoin the base of the gerbil cochlea. The measurements were made over a wide range of frequencies simultaneously in scalae vestibuli and tympani. Pressure was measured just adjacent to the stapes in scala vestibuli and at a number of positions spaced by tens of micrometers, including a position within several micrometers of the basilar membrane, in scala tympani. Two findings emerged from the basic results. First, the spatial variation in scala tympani pressure indicated that the pressure is composed of two modes, which can be identified with fast and slow waves. Second, at frequencies between 2 and 46 kHz (the upper frequency limit of the measurements) the scala vestibuli pressure adjacent to the stapes had a gain of approximately 30 dB with respect to the pressure in the ear canal, and a phase which decreased linearly with frequency. Thus, over these frequencies the middle ear and its termination in the cochlea operate as a frequency independent transmission line. A subset of the data was analyzed further to derive the velocity of the basilar membrane, the pressure difference across the organ of Corti complex (defined to include the tectorial and basilar membranes) and the specific acoustic impedance of the organ of Corti complex. The impedance was found to be tuned in frequency.

ISSN:0001-4966    

DOI:10.1121/1.423083

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

The acquisition of adultlike frequency selectivity is generally assumed to be tightly coupled to improvements in threshold sensitivity during cochlear development. In this study, frequency versus threshold (tuning) curves obtained from 1108 auditory-nerve fibers were used to investigate the relationship between tuning and threshold at characteristic frequency (CF) during postnatal development in kittens. At the earliest ages included in this study, sharpness was within the adult range, but thresholds were significantly higher than adult values. Tuning and thresholds improved along different exponential time courses that varied with CF. For units with CFs below 1 kHz, tuning curve slopes below CF matured earliest, followed by CF threshold, and then by slopes above CF. In contrast, for CFs above 1 kHz, the high-frequency slopes matured first, followed by threshold and then by slope below CF. One interpretation of these results is that tuning and thresholdsare nottightly coupled in immature animals. Paradoxically, however, high-frequency slopes were correlated with threshold for individual units at all ages, suggesting that the relationship between tuning and thresholdismaintained during development. This contradiction can be resolved by a developmental model that features a functional separation between cochlear nonlinearities and mechanical/electrical conversion.

ISSN:0001-4966    

DOI:10.1121/1.423055

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP

摘要:

A parametric study of the reduction of threshold shift (toughening phenomena) that takes place during the course of an interrupted noise exposure is described. 266 chinchillas randomly assigned to one of 32 experimental groups were exposed to one of the following: a 400-Hz narrow-band impact noise having a center frequency of 0.5, 1.0, 2.0, 4.0, or 8.0 kHz and peak sound-pressure levels of 109, 115, 121, or 127 dB. The impacts were presented for 5 d, 24 h/d or for 20 d, 6 h/d. Corresponding pairs of exposures had equal energy. Group mean noise effects were estimated from pure-tone thresholds obtained from inferior colliculus evoked potentials and from surface preparation histology. The threshold shift (TS) toughening phenomena is shown to occur in response to all stimuli that produce a TS and at all audiometric test frequencies. The amount of toughening, which is limited to less than 35 dB, varies with noise frequency and intensity. Based on group mean data the auditory system is not protected from the permanent effects of an interrupted noise exposure as a result of the toughening effect but rather differences in permanent effects between the 5- and 20-d exposures are attributed to the spreading of the exposure energy over an extended period of time.

ISSN:0001-4966    

DOI:10.1121/1.423056

出版商:Acoustical Society of America    

年代:1998

数据来源: AIP