摘要:

The large dynamic range exhibited by the frequency‐wavenumber spectrum of a turbulent boundary layer (TBL) poses a serious problem for estimating the low‐wavenumber region of that spectrum. From the standpoint of sidelobe leakage in beamforming procedures, the convective and sonic regions of the TBL are strong sources of interference which bias the estimate at low wavenumbers. By increasing the number of sensors in the measurement array, the sidelobe‐leakage problem can be minimized. However, this solution is difficult to implement experimentally and to justify theoretically in view of the globally nonhomogeneous nature of the TBL. As an alternative, certain data adaptive procedures can be used which do not have similar problems with sidelobe leakage when the number of sensors is small, but do require that the frequency‐wavenumber spectrum fall within a specified class of spectra. The performance of one such procedure, the maximum likelihood method (MLM), was evaluated under two criteria: spectral fidelity and sensitivity to sampling error. In general, the MLM was superior to the array beamformer in estimating the frequency‐wavenumber spectrum based on simulated observations from a linear array of sensors. [Sponsored by the Office of Naval Research Selected Research Opportunity in Hydroacoustics.]

ISSN:0001-4966    

DOI:10.1121/1.2021809

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Turbulent vortex rings are one of the few examples of nearly stable flow structures in high Reynold's number flows. An investigation of their pressure and flow field characteristics could contribute to our understanding of the processes that couple (dilitational) acoustic fields and rotational flow fields. For this study an electromagnetic method for generating vortex rings of highly repeatable form was developed. The propagating rings were visualized by means of laser sheet illumination and aerosol seeding of the flow. The velocity field associated with a vortex ring was measured by multiple pulse modulation of the illuminating laser beam and direct photographing of the observed particle motions. The pressure field associated with the ring was measured with an array of membranes. Numerical data processing methods were employed to distinguish the acoustic component of the total flow field at various distances from the turbulent core of the vortex ring. Some possible implications of the results for low‐wavenumber components of turbulent boundary layer flow will also be discussed. [Research sponsored by ONR.]

ISSN:0001-4966    

DOI:10.1121/1.2021813

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The accreditation of laboratories is a means to establish minimum acceptance limits by a purchaser of services. As desirable as this may seem there are many laboratories that do not participate in the program. This paper will review one laboratory's status and future plans concerning NVLAP accreditation.

ISSN:0001-4966    

DOI:10.1121/1.2021817

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

This paper will present the views of one consultant, who also heads the Laboratory Accreditation Committee of the National Council of Acoustical Consultants, towards the U.S. National Voluntary Laboratory Accreditation Program. While this program can provide significant benefits to the professional community, it unfortunately has some drawbacks that make it almost prohibitive for the involvement of small consulting businesses.

ISSN:0001-4966    

DOI:10.1121/1.2021818

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A key parameter for network modeling of the human middle ear is the acoustic energy reflection coefficient, or reflectance, at the eardrum. At high frequencies (above 5 kHz) the reflectance is the only unambiguous indicator of acoustic behavior. Using a technique described recently [J. Acoust. Soc. Am. Suppl. 175,S11 (1984)] eardrum reflectance can be determined by measuring the spatial variation of acoustic phase along the ear canal. The main advantage of this technique is the small amount of space required for the measurements, so only shallow penetration of the ear canal by a probe is required. One problem, at times, with the preliminary work was involuntary movement of a subject's head. This has been largely overcome with the use of an improved head brace and microphone manipulator. This new system will be described and experimental results presented for several subjects. As with the earlier study, high eardrum reflectances, between 65% and 85%, are found above 10 kHz.

ISSN:0001-4966    

DOI:10.1121/1.2021823

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Auditory nerve fibers can be categorized by their spontaneous discharge rates (SRs). Although the significance of these categories is not known, it has been suggested that the low and medium SR fibers may contribute to frequency analysis at signal levels at which the more sensitive high SR fibers are saturated. Data obtained from chinchilla auditory nerve fibers in response to two‐tone stimuli are consistent with that possibility. The stimuli were presented in a tone‐on‐tone masking paradigm analogous to that used by Long and Miller [Hear. Res.4,279–285 (1981)] for psychophysical measurements in chinchillas, and average discharge rate data from fibers with medium and high SRs were analyzed separately. Good agreement between the “masked thresholds” of medium SR fibers and the behavioral masked thresholds was observed. Masked thresholds of medium SR fibers were sometimes lower than the masked thresholds of high SR fibers, even though their quiet thresholds tend to be higher. This difference was greatest (10–15 dB) for masker‐probe tone pairs with the smallest frequency separations. High SR fibers were often driven to saturation by the masker, so that no masked threshold could be estimated; however, probe tones nearly always produced rate changes in medium SR fibers. [Supported by NIH.]

ISSN:0001-4966    

DOI:10.1121/1.2021827

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

The development of a special‐purpose microcomputer system (calledPESOGEN= parabolic equation solution generator) that performs high‐speed PE model computations using the split‐step Fourier algorithm was reported previously [L. Nghiem‐Phu, S. C. Daubin, and F. Tappert, J. Acoust. Soc. Am. Suppl. 175,S26 (1984)]. Applications ofPESOGENto three‐variable modeling are reported here. (1) Full‐wave computations of pulse propagation, signal waveforms, and cross‐correlation functions requirep(r,z,t), a function of three variables.PESOGENcomputesp(r,z,ω) in the frequency domain for many values of ω sequentially, and then obtainsp(r,z,t) in the time domain by numerical Fourier transforms [L. Nghiem‐Phu and F. Tappert, J. Acoust. Soc. Am. Suppl. 168,S51 (1980)]. Examples of pulse transmission to long range in deep and shallow range‐dependent oceans are presented, with emphasis on numerical accuracy (tested by time‐domain reciprocity) andPESOGENthroughout times (typically less than 1 h). (2) Full‐wave computations of responses of directional receivers to line‐radiation sources requirep(x,y,z), a function of three variables. Using specified farfield beam patterns,PESOGENcomputesp(x,φ,z] for many values of the azimuthal angle φ sequentially, and then obtains TL′ = TL − ASG, where ASG is the array signal gain [F. Tappert and L. Nghiem‐Phy, J. Acoust. Soc. Am. Suppl. 175,S63 (1984)]. Examples of directional receiver response to long range sources in three dimensions are presented, with emphasis on full‐wave effects in the beam deviation loss andPESOGENthroughput times. Conceptual designs of large special‐purpose computer systems for three‐variable propagation modeling using multiplePESOGENunits in parallel with single‐instruction multiple‐data architectures are also presented.

ISSN:0001-4966    

DOI:10.1121/1.2021833

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

A nonlinear time domain counterpart of the linear frequency domain parabolic wave equation (PE) has been derived for investigation of pulse propagation in a refracting medium, including caustics. Assuming nearly unidirectional propagation, equations of hydrodynamics yield a quadratic correction term for the linear second‐order wave equation. Perturbation analysis about unidirectional wave propagation yields a first‐order nonlinear progressive wave equation (NPE) cast in a wave following frame. This equation explicitly separates terms for the physical processes of refraction, nonlinear steepening, radial spreading, and diffraction. Self‐refraction is manifest through a continuous interaction between steepening and diffraction terms. When the wave is taken to be linear and monochromatic, the NPE reduces to the familiar PE, within appropriate assumptions. A new numerical algorithm of the flux correction type has been constructed for integration of the NPE. Successful applications to date include: (1) development of initially smooth pulses into agingNwave shocks, (2) wideband linear pulse propagation in a slab, and (3) the evolution of a nonlinearNwave incident on a caustic. [Work supported by ONR].

ISSN:0001-4966    

DOI:10.1121/1.2021834

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Asymptotic ray theory can be used to simulate acoustic waves in range‐dependent oceans, but singularities often occur at points of great interest. Transform methods, such as the WKBJ solution used by M.G. Brown [Ph.D. thesis, University of California, San Diego, CA (1982)], describe waves at the singular points, but they are restricted to laterally homogeneous media. Maslov asymptotic theory combines asymptotic ray theory and transform methods to provide a uniform result applicable in range‐dependent environments, and the technique was used by C. H. Chapman and R. Drummond [Bull. Seismol. Soc. Am.72(6), S217–S317 (December 1982)] to compute synthetic body‐wave seismograms. This paper describes a. computer simulation of underwater acoustic signals in range‐dependent oceans that is based upon Maslov asymptotic theory. The theory and computer implementation will be summarized, and examples will be discussed. Preliminary results on range‐dependent signal inversions for underwater acoustic tornography will also be presented.

ISSN:0001-4966    

DOI:10.1121/1.2021841

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP

摘要:

Many early scientists as well as humanists investigated those aspects of music which had traditionally been dealt with by music theory in the medieval quadrivium. Their agenda included a reckoning of tempered scales and to a lesser extent vocal intonation. The main challenge in the 1500's was to overcome certain old precepts, e.g.: that the major third (theditonus) naturally had the monochord ratio (9:8)2= (81:64); that the comma was some kind of “ultimate” interval; that “superparticular” intervals (n:n − 1) could not be divided into equal parts (because music theory must reckon with rational “multitudes” and not “magnitudes,” music being “sonorous number”). In the 1600's, regular temperament and various equal divisions of the octave containing nicely tempered consonances (19, 31, 43,…, as well as 12) were sorted out, often with logarithms; but then keyboard musicians took up irregular temperament, and for some time the sophisticated French‐speaking scientists were so off the track that less urbane German organists outstripped them in developing an adequate account of the contemporary scale.

ISSN:0001-4966    

DOI:10.1121/1.2021847

出版商:Acoustical Society of America    

年代:1984

数据来源: AIP