摘要:

Experiments have been performed in which a piston is vibrated inside a glass tube containing liquid helium. The piston was driven by an electromagnetic shaker in the frequency range 20–80 Hz at an intensity capable of generating cavitation effects. At critical particle velocity amplitudes of a few centimeters per second noise is generated. This threshold velocity appears to be independent of frequency. A much higher velocity, about an order of magnitude greater, is required to produce visible bubbles in superfluid Helium II, but the two thresholds are the same in Helium I. The results are believed to indicate that fluid dynamic effects are important in cavitation nucleation.Subject Classification: [43]25.60; [43]35.32, [43]35.47.

ISSN:0001-4966    

DOI:10.1121/1.380879

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The effect of mechanical angular scanning of a parametric array is examined theoretically and experimentally. The results indicate that difference frequency pressure level and directivity are not affected if the frequency of rotation is small compared to the difference frequency.Subject Classification: [43]25.35; [43]30.75.

ISSN:0001-4966    

DOI:10.1121/1.380880

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The acoustic power transmitted by a variable‐area duct section which carries a steady subsonic flow is investigated under the conditions of both upstream‐ and downstream‐propagating incident plane waves. It is found that the ratio of the power transmitted by incident waves moving against the flow to the power transmitted by incident waves moving with the flow bears the simple relation (1−M2)/(1+M)2.Subject Classification: [43]28.60; [43]20.45.

ISSN:0001-4966    

DOI:10.1121/1.380881

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

There is an extensive body of theory, and some laboratory measurements, of sound propagation over a surface of finite impedance. There are also reliable measurements of outdoor sound propagation in near‐horizontal directions over the ground. In an attempt to relate these more closely, we have made carefully controlled measurements at ranges from 1 to 1000 ft, in most cases over grass‐covered flat surfaces, to demonstrate the several phenomena that are involved. These phenomena depend on, and conversely provide a means of estimating, the values of ground impedance for waves at near‐grazing angles of incidence. Such values obtained for grass‐covered surfaces are in reasonable agreement with each other and with values obtained by conventional means at other angles of incidence. It is suggested that simple but accurate predictions of noise levels can be made by assuming that an excess attenuation due to finite ground impedance would always exist in a certain shadow region near the ground. This shadow region is however penetrated at low frequencies by a ground wave, to an extent that depends principally on distance and ground impedance, and at higher frequencies by interference between direct and ground‐reflected waves to an extent that depends also on source and receiver heights. These phenomena, well established at ranges up to about 1000 ft and in some aspects to over 3000 ft, have been extrapolated theoretically to the order of 10 000 ft so that simple effects of topography and meteorology can be added to show how reflection or refraction acts in conjunction with ground impedance to result in penetration of the shadow region.Subject Classification: [43]28.40; [43]20.55.

ISSN:0001-4966    

DOI:10.1121/1.380882

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The propagation of sound is considered for an atmosphere in which the sound velocity increases linearly with height. For source and receiver separation large compared with their heights above the ground, there are in general four possible ray paths between source and receiver having a given number of ground reflectionsn≳1. Forn=0, i.e., no reflections, there is one ray path; and forn=1 there are three possible ray paths. By suitable grouping of the ray paths it is possible to evaluate the effects of reflection at a ground of known, finite impedance (especially near the source), assuming coherence between neighboring rays; and also to evaluate the net sound intensity of waves traveling via more widely separated paths in the normally turbulent atmosphere assuming incoherence. A temperature inversion is found, using ray theory, to increase the sound level at large horizontal distances inside the shadow region due to finite surface impedance, that is otherwise predicted by wave theory for a neutral atmosphere.Subject Classification: [43]28.40; [43]20.20, [43]20.30.

ISSN:0001-4966    

DOI:10.1121/1.380883

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Forced oscillations of a spherical gas bubble in an incompressible, viscous liquid (water) are calculated numerically. The information gathered is mainly displayed in the form of frequency response curves of the steady‐state solutions showing the harmonics, subharmonics, and ultraharmonics. Bubbles oscillating ultraharmonically at frequencies below the main resonance may emit half the driving frequency. This fact gives rise to a new explanation for the occurrence of the first subharmonic in the spectrum of the cavitation noise in ultrasonic cavitation.Subject Classification: [43]30.70, [43]30.75.

ISSN:0001-4966    

DOI:10.1121/1.380884

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Newman and Thorson have recently proposed and developed a new method which permits the rapid and efficient solution of eigenvalue problems associated with linear second‐order differential equations. Results on applications of the method to problems of interest in underwater acoustics are presented.Subject Classification: [43]30.20; [43]20.40; [43]30.50.

ISSN:0001-4966    

DOI:10.1121/1.380885

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Convincing experimental evidence is presented to show that various ’’decay laws’’ for the decrease of sound intensity with increasing range can give high‐quality representation for shallow water data. However, while a sequence of decay laws may fit data well and are simple, the transition ranges for changeover from one law to another can show a complicated dependence on frequency and environmental parameters. Transition ranges are probably no easier to predict or model than is the transmission loss itself.Subject Classification: [43]30.20, [43]30.25.

ISSN:0001-4966    

DOI:10.1121/1.380865

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Long‐term coherence measurements have been made of the stability of broad‐band acoustic signals transmitted on single paths through the deep ocean over distances of 145, 250, 495, and 635 NM. The stability of individual ray paths was measured by separating the arrivals in time and angle. Twenty‐nine acoustic phase records with durations from 5 to 32 h were analyzed. Typical coherence intervals of 2 to 8 min were found in which the phase of the cross‐correlation of the received signal with a replica varied by no more than ±π/2 rad (±0.6 msec) at 400 Hz. Coherent intervals of 8‐ to 12‐min duration at 400 Hz were common, while occasional long periods on the order of 20 min could be found. The spectra of these phase records possess a dominant low‐frequency peak overlapping the 12‐ and 24‐h tidal frequencies, but also show much weaker peaks at periods of 1 1/2, 3, and 5 h. The latter could be caused by internal waves, nonlinear overtides, or atmospheric pressure variations. No other effects of the moving ocean surface or of higher‐frequency internal ocean instabilities were found.Subject Classification; [43]30.20; [43]60.30; [43]30.82; [43]60.20.

ISSN:0001-4966    

DOI:10.1121/1.380866

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Cavitation inception thresholds in water are measured and correlated with nuclei spectra, measured by the laser‐light‐scattering technique. The effects of temperature, settling time, total air content, various impurity particle additions, and polymer addition are examined and compared with theoretical predictions.Subject Classification: [43]30.60, [43]30.70; [43]35.65; [43]25.60.

ISSN:0001-4966    

DOI:10.1121/1.380867

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP