摘要:

Laboratory measurements of atmospheric absorption in moist air are reported at frequencies from about 4 to 400 Hz/atm. These measurements provided data for frequencies below the range of any previously published results. They were observed in a resonance tube apparatus at pressures up to 100 atm. Initial measurements of boundary losses in the tube, measured with argon, agreed closely with predicted values based on current theory. This accounted for finite acoustic propagation through the tube wall and “real gas” effects, as well as viscous and thermal losses. The measured values of absorption in air and the corresponding molecular relaxation frequencies provided new validation at very low frequencies for a standard prediction model on air absorption in preparation by the S1‐57 Committee on Sound Propagation. [The research was supported in part by the U. S. Army Research Office, Research Triangle Park, NC.]

ISSN:0001-4966    

DOI:10.1121/1.2003277

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

This paper presents a continuation of the passive range measurement efforts reported at the 90th conference. Previous work included the modeling of passive range determination using a single point receiver based on moving source observables. The outcome as reported last year was a parameterization of a hypothetical system that had the capability to determine range to source passively. This year, we have expanded the work to include building a fully operational system and field testing it against various moving sources. In addition, a pseudo‐Fourier analysis section has been incorporated to accurately and correctly sort out and classify moving sources. This, coupled witha prioriselection information coded into the system, enables it to perform those required functions against preselected sources while ignoring nondesired signals. We have concentrated this year on enlarging our data base of acoustic signatures with an eye to determining the points of commonality between major source classes. This has led to vastly simplified signal processing techniques compared with previously tried methods. Specifically, we have found that within a source class, certain fundamental signature components cannot be masked by purely physical means without materially degrading the operation of the source. Masking efforts employing jamming techniques are not as effective as desired, since jamming means that the total acoustic output has been raised yielding a more visible source. Baffling of noise sources, while effective for high‐frequency noise components, tends to be less than effective when used to suppress the lower components without an attendant loss in source mobility performance.

ISSN:0001-4966    

DOI:10.1121/1.2003281

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Coherent ray‐tracing techniques were applied to describe acoustic propagation through a Gulf Stream Ring 320 km in diameter as measured in 1973. Rays were traced from a fixed source to a fixed receiver 445 km distant incorporating the profile variations of the Ring which had a forward movement of 4 km a day. This anomaly shifted the deep sound channel axis about 400 m and also decreased the minimum sound velocity. These profile variations caused significant changes in the arrival pattern of the ray types and number of cycles. A time history is presented of the relationships between the received signal variations and the passage of the Ring including the effects of the changes in multipath structure.

ISSN:0001-4966    

DOI:10.1121/1.2003286

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

REVGEN provides a computerized ocean acoustic environment for the development of a modern active sonar system. [D, W. Princehouse, “Real‐time reverberation generator,” J. Acoust. Soc. Am.57, S68(A) (1975).] During an exercise, it is important that it run in as near to real time as possible. We have developed a method whereby the numerically difficult and time‐consuming portion of ray tracing can be performed before the exercise starts, and the results used rapidly when needed. We work in the limit of geometrical acoustics and assume that the sound velocity profile depends only on depth, not range or time. While many ray‐tracing techniques can be adapted, we have used those of NISSM II. [Henry Weinberg, Naval Underwater Systems Center, New London, CT 06320. NUSC Technical Report 4527.]A function of two variables is defined, range versus launch depth and vertex velocity. Knowledge of this function allows ray tracing to be performed. This function may be computed prior to its need, a lengthy process. By encoding it in an efficient fashion, however, it may be stored in computer memory for rapid access when needed. Examples of ray tracing, caustic detection, and multipath propagation will be shown.

ISSN:0001-4966    

DOI:10.1121/1.2003294

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Volume attenuation coefficients for one‐third octave frequency bands up to 400 Hz have been determined as part of an underwater sound propagation study conducted in the Labrador Basin in cooperation with the U. S. Naval Research Laboratory. Charge detonation depths, receiver depth and the sound channel axis depth were all roughly 200 m along the 1300‐km track. A late summer maximum in the near‐surface sound velocity and a water depth greater than 3000 m also helped to ensure nearly ideal measurement conditions. The deduced volume attenuation coefficients were greater than the Thorp predictions [W.H. Thorp, J. Acoust. Soc. Am.42, 270 (1967)], with attenuation increasing toward both higher and lower frequencies from a minimum (0.002 dB/km) near 60 Hz. Comparisons with results from other areas will be made. Results on the low frequency dependence of propagation loss will also be discussed.

ISSN:0001-4966    

DOI:10.1121/1.2003302

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The time scale of acoustic fluctuations has been found to depend on source speed for a shallow‐receiver shallow‐sound source configuration in deep water off the coast of St. Croix, U. S. Virgin Islands Measurements were made of a 135‐Hz cw source towed at different speeds on a tangential track through a point 7.5 miles from the receiving hydrophones. Higher source speeds consistently gave rise to shorter correlation decay times. As the data integration period was increased from 5 to 100 sec, the measured correlation decay times increased as well, covering the range of 4–462 sec for various source‐speed—receiver‐depth combinations. The position of the receiver with respect to the thermal layer depth also plays a significant role.

ISSN:0001-4966    

DOI:10.1121/1.2003306

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Recent Legislation in Europe on permissible noise levels in commercial shipping has heightened concern and increased research efforts to solve noise‐control problems. Technisch Physische Dienst TNO‐TH, Delft, Netherlands, has organized an international symposium of invited speakers on this topic in order to encourage dissemnation of new research results. In this summary of the conference by this participant, some of the more significant research results will be discussed. Results will be selected with regard to their expected significance for influencing the design of new commercial ships or modifications of existing ships.

ISSN:0001-4966    

DOI:10.1121/1.2003313

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The U. S. Navy and U. S. Coast Guard have jointly sponsored the development of an airborne noise prediction procedure for military surface ships, which was published in a report entitled “Handbook for Shipboard Airborne Noise Control” dated 13 February 1974. This paper is based on a study aimed at verifying the “Handbook” prediction procedure by comparison with full‐scale, shipboard measurements. The U. S. Navy Spruance Class Destroyer (DD 963) offered an excellent opportunity for making such comparison. During the design phase, predictive calculations of sound pressure levels were made for all manned spaces. During the trials period airborne noise measurements were made in these spaces. The paper compares predicted versus measured sound levels in a limited number of spaces and also examines the source sound power level prediction formulae against measured power levels for various types of machinery. On the basis of this one ship sampling, reasonably good correlation was demonstrated between the predicted and measured sound pressure levels.

ISSN:0001-4966    

DOI:10.1121/1.2003314

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Recently we proposed a theoretical model for cochlear nonlinearity and second filter [J. Acoust. Soc. Am.59, 408–423 (1971)]. From psychophysical two‐tone suppression data we attempted to estimate characteristics of the two filters and of the compressive nonlinearityy = xν. In our first experiments probe frequency (suppressee as well as test probe) was 1 kHz. Masker (suppressor) frequency was a parameter. We used the pulsation‐threshold method, where masker plus suppressee and test probe alternated. The pulsation threshold of the test probe was measured as a function of masker level, suppressee level being a fixed parameter. Data curves typically are characterized by a horizontal part, a descending part, and an ascending part. The descending part, reflecting suppression contains the wanted information. Insofar as our interpretation of the data is valid, we estimate for ν a value from 0.25 to 0.8, depending on musket frequency. The low‐frequency part of the second‐filter characteristic levels off at approximately 10‐dB attenuation. The first‐filter characteristic depends on level. Thus, the data show more nonlinearity than the model accounts for. Possible modifications will be discussed.

ISSN:0001-4966    

DOI:10.1121/1.2003321

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Evidence suggesting that combination tones are not a result of middle‐ear or basilar‐membrane mechanics has been presented in the literature. It has been proposed that the sensory cells in the cochlea are the site of the site of the nonlinearity. However, the precise location of the sensory region responsible for the generation of the combination tones is not known. This important theoretical problem was explored using a tonal masking paradigm with listeners having a localized region of sensorineural hearing loss. Psychoacoustical tuning curves were measured in order to delineate the frequency region of combination tone audibility. By manipulating the frequency of the fixed probe with respect to the listener's hearing loss, the sensory region over which the nonlinear distortion product is generated was determined. Results of this study showed that when the fixed probe was just lower in frequency than the region of the threshold elevation andFm

ISSN:0001-4966    

DOI:10.1121/1.2003331

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP