摘要:

The intense vibration accompanying the extension of dive brakes and other high‐drag devices on aircraft and missiles has been observed for many years. Recently, considerable vibration has been experienced at the fundamental longitudinal mode and at other internal resonance frequencies of a vehicle which had a blunt base during the maximum dynamic pressure phase of flight. In both cases the vibration is believed to result from the turbulent fluctuations in the wake of the base or drag device.The base pressure fluctuations have been measured at two base positions on a small body of revolution at eleven points in the velocity range of 68 to 352 ft/sec. The results show that: (a) The ratio of(〈p2〉nv/q2)12varies between 0.007 at the center of the base to 0.015 at 65% radius; (b) The spectrum of the pressure fluctuations is a function of the body Strouhal number; (c) The variation of observed vibration response with flight parameters can be predicted from the base pressure fluctuation data.

ISSN:0001-4966    

DOI:10.1121/1.1908404

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

Variations in underwater acoustic ambient noise spectra, from 20 to 1250 cps, were analyzed using a statistical filter technique to examine the spectral density functions in the range from 1 to 80 cycles per week. At frequencies below 100‐cps narrow band components at seven cycles per week were observed in the spectral density functions. In several cases harmonics of the seven‐cycle‐per‐week component were also evident. The period of the cyclic variation agreed more closely with the solar day than with tidal period. The maxima occurred at approximately midnight local zone standard time, and at one location additional maxima were observed at approximately noon. The magnitudes of the periodic variation were small, 1.5 to 5.0 db, showing little evidence of seasonal dependence, except that at one location daily changes of 10 to 20 db were observed during the period of the summer solstice while hardly any periodic change was evident during the winter solstitial season. These periodic changes in the ambient noise levels were not correlated with changes in local wind speed.

ISSN:0001-4966    

DOI:10.1121/1.1908406

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

The development of equipments and techniques for making measurements on underwater transducers is described. The Naval Air Development Center used a circular tank with no special linings, and directed its effort to develop a suitable electronic pulse test equipment. It is concluded that the accuracy and reliability of the calibration facilities, measurement techniques, and the use of the round wooden tank are satisfactory for most underwater transducer evaluations.

ISSN:0001-4966    

DOI:10.1121/1.1908408

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

Hydrographic data (temperature, salinity, depth) and directly measured sound velocities were obtained from a section made at 6°E across the Mediterranean Sea in June and July, 1959. Sound velocities were calculated from the hydrographic data and compared with the measured values. Two methods were used to calculate the sound velocity from the hydrographic data. One method used Del Grosso's zero depth values with the depth correction from Matthew's tables. (This has been the usual method of calculation at the Woods Hole Oceanographic Institution until now.) The other method used the empirical equation of Wilson derived from laboratory measurements made at the Naval Ordnance Laboratory. The significant difference observed in the comparisons appears below 200 m. Here the corrections using Matthew's tables result in too large values for the velocities at depth, the magnitude of the difference being about 2.5 m/sec at 2500 m depth. The calculated values from Wilson's equation agree with the experimental values more closely (about 0.5 m/sec difference at 2200 m depth).

ISSN:0001-4966    

DOI:10.1121/1.1908413

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

A study is reported which examined the efficiency of human observers in the detection of a stimulus waveform which is a train of damped sinusoids. The signal duration and degree of damping (or spectral bandwidth) were varied, with the energy of the signal held constant. Increased bandwidth is shown to decrease detection of relatively long duration signals. A negligible effect of bandwidth was found for short durations. Detection of an amplitude difference, in contrast to detection of the presence of weak signal in noise, is shown to be relatively unaffected either by signal duration or bandwidth over the range of values studied.

ISSN:0001-4966    

DOI:10.1121/1.1908415

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP