摘要:

In order to eliminate the risk inherent in the insertion of a measuring probe into the ear canal of a subject, a middle ear impedance transducer should be placed a safe distance away from the eardrum. A technique presented in this paper uses a function which represents the area function of the ear canal and transforms impedance parameters from the measurement plane to a plane at the eardrum. The effects of the geometry of the canal on impedance measurements, and the validity of the approximations over a wide frequency range are studied by means of a computer model. The area functions used to transform the impedance parameters are parabolic horns which are found to provide better results than the commonly used cylindrical approximation. The sensitivity of these approximations to errors in the estimation of the parameters defining the horn is also studied. The parabolic horns give a simple and very reliable correction to the interpretation of middle ear impedance data. [Work supported by the Natural Sciences and Engineering Research Council of Canada, CRD‐8422.]

ISSN:0001-4966    

DOI:10.1121/1.2023749

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

This paper will review the structural and acoustical features of asymptotic solutions recently obtained for fluid‐loaded structure problems. In the cases treated here, the structure is plane (a thin plate or membrane) and driven by a source of concentrated mechanical excitation. The asymptotic results, based on the smallness of the fluid‐loading‐at‐coincidence parameter, hold generally at frequencies below coincidence, but assume particularly simple forms at much lower frequencies. Problems discussed will include semi‐infinite and finite unribbed structures (with details of resonance, mode shapes, and forced response), and infinite and semi‐infinite ribbed structures. An aspect subject to particular study will be the nonlocal (i.e., non‐added‐mass) effects of fluid loading; and generally, the aim is to expose fundamental mechanisms for structural vibration with heavy fluid loading, and to provide benchmarks for computational schemes. [Work supported by ONR.]

ISSN:0001-4966    

DOI:10.1121/1.2023752

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The effects of fluid‐structure interaction on the force transmissibility of mechanical systems elastically mounted in a submerged elastic shell are investigated. An analytical solution for the ease of a harmonic‐force‐excited two‐degrees‐of‐freedom undamped system attached inside a spherical elastic shell is obtained by the separation of variable technique. The transmissibility of the internal system is computed for a broad range of excitation frequencies and compared with that of the same system mounted on a fixed base. Due to the loss of energy through the radiation of the elastic shell, the transmissibility resonance peaks of the undamped system elastically mounted in the submerged shell are reduced to finite values and, for other nonresonance frequencies, the magnitude is in general also reduced. Moreover, the fluid‐structure interaction effect shifts the peaks of the transmissibility towards lower frequencies. This information could be useful for the sound isolation of systems inside a submerged vessel.

ISSN:0001-4966    

DOI:10.1121/1.2023756

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

A set of acoustical arrays has been constructed, the shapes of which are configurated in accordance with the cylindrical geometry of the nearfield of the shells (12–20 in.‐diameter). Some are double layer arrays, and directly give the acoustic intensity of radiated noise through the array. Other give the dynamic pressure in the immediate vicinity of the shell, and reflect both propagating and nonpropagating fields, like those associated with subsonic flexural waves. The studied shells reflect a wide assortment of usual shell technologies in torpedoes or ship building: steel, aluminum alloys, glass, or carbon‐fiber reinforced resins; ribbed shells, more or less disturbed by internal fittings, etc. They are mechanically excited by various arrangements of piezoelectric shakers driven by recurrent pulses. When the measurements are plotted as time/space recordings, they give a descriptive image of wave propagating in the shells: phase and group velocities, damping, and scattering on all discontinuities of the shell, which is generally the main phenomenon to explain the effective propagation of vibrations along realistic shiplike structures.

ISSN:0001-4966    

DOI:10.1121/1.2023757

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Scalar quantization of the LPC parameters requires a high bit rate. Considerably lower rate can be obtained via vector quantization (VQ). However, complexity constraints dictate the use of suboptimal VQ. The use of vector predictive quantization (VPQ) for the LPC parameters is proposed. In VPQ, the suboptimality is compensated for by exploiting the temporal redundancy in the input. VPQ is a two‐stage memory VQ. In the first stage, the input vector is predicted from quantized past vectors, using a set of vector coefficients, held in apredictor codebook. In the second stage, the predicted vector is combined with a residual vector to form the final output. A set of residual vectors is held in aresidual codebook. VPQ was applied to the LPC parameters in the down‐sampled, log‐magnitude spectral domain. The idea was to design an efficient VPQ under the perceptually meaningful log‐likelihood distortion measure, while circumventing the stability problem of the synthesis LPC filter. This VPQ was used in a 4.8 kbit/s CELP coder where only 1.0 kbit/s were allocated to the parameters. The performance was almost indistinguishable from that of a CELP coder with unquantized parameters. [Work supported by NSA.]

ISSN:0001-4966    

DOI:10.1121/1.2023762

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

It has been suggested that codebook coding of LPC residuals offers significant performance advantage over tree coding at very low rates [cf. M. R. Schroeder and B. S. Atal, Proc. ICASSP 85]. An innovation codebook provides a better coveting of the LPC innovation space (in the sense of having a smaller covering radius) than an innovation tree of the corresponding rate. These comparisons usually assume that the branches of the innovation tree are populated with independent identically distributed random variates (i.e., a stochastic tree). In this paper, the methods of selecting the innovation elements on a binary innovation tree such that the branches contain “complementary” sequences are examined. The covering properties of these “complementary” tree codes are compared with the covering properties of stochastic trees and stochastic codebooks. The results on the “complementary” tree codes indicate that (1) the performance of an innovation code did not correlate with its covering radius and (2) tree coding of LPC residuals at very low bit rate (rate 1/4 code) did not result in any performance degradation.

ISSN:0001-4966    

DOI:10.1121/1.2023767

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The three usual techniques employed for benchmarking acoustical propagation models involve measured acoustical data, analytical solutions, and other established acoustical propagation models. Each of these has inherent limitations. The alternative approach described here involves replica correlation to test the accuracy of acoustical propagation models. A received acoustic pressure spectrum, which is generated by the acoustical propagation model to be tested, is multiplied with the source pressure spectrum. The inverse Fourier transform of this product is a replica correlogram, which contains information of the travel time, phase and amplitude of all arrivals. Then a ray‐tracing model is used to determine the travel time of the arrival. If the model to be tested is accurate, the travel times computed by replica correlation will agree with the travel time computed by ray tracing. Results will be presented for an existing acoustical propagation model under realistic ocean conditions. [Work supported by NRL.]

ISSN:0001-4966    

DOI:10.1121/1.2023773

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The importance of early reflected sound from side wall and ceiling reflecting panels in auditoria and other spaces used for musical presentation has been recognized for some 25 years. Quantitative data on the size, shape, and orientation of reflecting panels to achieve wideband spectral reflections have not been widely available. This paper presents some results on the effect of size and aspect ratio on the reflection characteristics of hard panels measured using scale modeling techniques.

ISSN:0001-4966    

DOI:10.1121/1.2023778

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

A variety of measurements using a time‐delay spectrometer in European and American concert halls and multipurpose auditoria will be presented and interpreted. Direct/reverberant energy ratios, reverberation times, lateral energy ratios, orchestra platform effects, and varying sound reflection patterns will be covered.

ISSN:0001-4966    

DOI:10.1121/1.2023785

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Two commercially available single‐channel vibrotactile aids for the heating impaired were compared in tasks using acoustic stimuli of varying levels of complexity. Both devices provide envelope information in an AM signal presented to a single location on the skin; however, one of the devices varies the carrier frequency according to input frequency, providing some additional “spectral” information about the stimulus. Subjects were tested in tasks including sound detection, environmental sound identification, syllable number and stress identification, and phoneme identification of limited‐set vowels and consonants. All stimuli were recorded onto disk and presented through a loudspeaker by computer. Results showed rapid learning of tasks requiring only envelope information (environmental sounds, syllable number and stress), but little or no improvement with training in vowel and consonant identification. No significant differences between the devices were found on any task, suggesting that the additional spectral information provided by one of the devices is not effectively utilized by the tactile system in the absence of visual cues. [Work supported by NSF and NIH.]

ISSN:0001-4966    

DOI:10.1121/1.2023790

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP