885095970

peak at near-horizontal anglcs. The parabolic cquation formalism pro-vides a natural basis for this: techniques and results dcve!oped by the author to datę arc discussed and prcsented.

1:15

9UW2. Three-dimensional rough interface scattering of ambient noise. lin-Yuan Liu and Hcnrik Schmidt (Dcpt. of Ocean Eng., MIT, Cambridge, MA 02139)

Based on the previously developed theory of perturbation approach to rough surface scattering [W. A. Kuperman and Henrik Schmidt, J. Acoust. Soc. Am. 86, 1511-1522 (1989) J and a model for the noise field generated by surface random sourccs in an occean waveguide (W. A. Kuperman and F. Ingenito, J. Acoust. Soc. Am. 67, 1988-1996 (1980)], a formulation for rough interfacc scattering of surface-generated ambient noise in a horizontally stratified ocean is established. The result is then applied to study the three-dimensional scattering in a shallow-water waveguide environmcnt boundcd below by a viscoelastic medium. The three-dimensional spatial correlation of the rcvcrberatcd noise in the waveguide and the scattcred noise in the elastic medium are examined in terms of the relation to frequency, water depth, and rough interface statistics. It is demonstrated that even in deep water, the scattering of surface generated noise into interface waves (Scholte waves) is important, showing these waves carriers of ambient noise cnergy con-sistent with experimental observation. (Work supported by NOA RL.J

1:30

9UW3. Trends of wind and noise dependency at Lakę Pend Oreille, Idaho. Jonathan Cummings, II (David Taylor Res. Ctr., Bethesda, MD 20084)

Ambient noise and wind-speed measurements were obtained at Lakę Pend Oreille pcriodically from October 1985 through August 1989. Cyclic trends in noise levels were observed and are summarized. Vari-ations in noise level with time of day and season of the year correlale well with wind speeds and trends in recrcational boating use. Daily averaged data show the contribution of recrcational boating to lakę noise: 10-15 dB average level inerease for midday as compared to night. Average scasonal trends arc gcnerally higher (up to 10 dB) during the summer months as compared to winter months. Noise dependcncies on wind speed were empirically derived and are found to compare well with observations from ocean environments.

1:45

9UW4. Sound radiation from large raindrops: Dependence on salinify and temperaturę. Peter W. Jacobus,'* Herman Medwin, and JefTrey A. Nystuen (Physics Dept.. Naval Postgraduate School, Monterey, CA 93943)

Raindrops are known to produce bubbles by at least two diflferent mechanisms: type I, the pinch-off of the bottom of the crater, and type II, air entrainment from a turbulent water jet formed as a part of the drop splash. Bubble energy is shown to dominate impact energy for the useful rangę of type II drops. Previous work (Snyder et al., J. Acoust. Soc. Am. Suppl. 1 88, S2 (1990)] has shown a relationship between drop size and bubble frequcncy of the type II mechanism. The onset of this mechanism is related to the drop kinetic energy at impact, rather than to velocity. As the drop kinetic energy inereases above a thrcshold of 2 X 10 " ⁴ J, the likelihood of bubble entrainment inereases to approx-imately 65%. Further work reveals the effects of temperaturę, salinity, and surface tension on the sound radiated from large raindrops. The relation between the drop diameter and the spectrum of the underwater acoustic energy is examined for terminal velocity drops. This relation makes possible the remote measurcment of both drop size distribution and rainfall ratę. (Work supported by ONR.] ^ł,LT, USN.

2:00

9UW5. Bubble production by capillary waves. Ali Kolaini, Ronald A. Roy, and Lawrence A. Crum (Natl. Ctr. for Physical Acoust., Univ. of Mississippi, Univcrsity, MS 38677)

Bubblc-producing capillary waves can be casily generated in a lab-oratory tank (8 cmX22 cmX 150 cm) by hlowing air ovcr the water. The lowcst wind speed rcquircd for the occurrcncc of this proccss is measured. The generation of the capillary wavcs depends solcly on the surface tension, which can be changcd by adding surfactants and other Chemicals to the tank water. The cffect of surface tension on the wind-speed threshold for bubble production is discussed. Using a coincidence detector, the bubble production rates per unit area can be measured. Lowering the surface tension, surprisingly, inereases the ratę of bubble production. The dependence of the bubble production ratę on wind speed and wind fetch is also discussed. The underwater acoustic emis-sions from these bubbles are measured. Average power spectrum for several wind speeds arc obtained that exhibic a broad rangę of radiated frequencies (i.e., bubble sizes) and a weak dependence on the wind speed. (Work supported by ONR, ONT, and AEAS.]

2:15

9UW6. The underwater sound produced by the impact of solid objeets. Michael Nicholas, Ronald A. Roy, and Lawrence A. Crum (Natl. Ctr. for Physical Acoust., Univ. of Mississippi, University, MS 38677)

There have been many studics that indicate that there is sigmficant underwater sound produced by precipitation [e.g., J. A. Scrimgcr, Naturę 318, 647 (1985)]. It has also been demonstrated that the major contribution to the underwater sound of rainfall is that due to the entrainment of gas bubbles by the impacting drop [Pumphrcy et al., J. Acoust. Soc. Am. 31, 1080 (1989)]. In an attempt to understand the underwater sound produced by hail, the impact of solid objeets with a piane water surface have been cxamined. With the aid of a high-speed movie camera and underwater transducers, it has been dełermincd that the air entrained by the impacting objcct plays a major role in the sound production. The details of this process as well as the implications con-cerning the underwater sound produced by hail will be presented. (Work supported by ONR.]

2:30

9UW7. The hydrodynamic and acoustic behavior of bubble plumes generated by an impacting water jet. Ali Kolaini, Ronald A. Roy, and Lawrence A. Crum (Natl. Ctr. for Physical Acoust., Univ. of Mississippi, University, MS 38677)

The preliminary results of an cxperimental study of the underwater sound field emitted by a bubble plumc generated by dropping fixcd volume of water, held in a cylindrical Container, onto a still water surface were previously presented (J. Acoust. Soc. Am. Suppl. 1 88, 514 (1990)]. Further studies of the acoustic and hydrodynamic character-istics of the bubble plume are presented. The high-speed video images rcveal the formation of a cylindrical plume that grows in length unlil all of the impacting water vo!ume is injected into the still water. As the leading end of the plume advances, a “substructure” separates from the rest of the plume. The onset of the large-amplitude, low-frequency sound emission occurs at the instant the substructure detaches. The resonance frequencies of the denscly poptilaled suhstructures are in-versely proportional to their radii and are highly dependent on the void fraction. Experimental results are presented which show that dctached plumes undergo damped volume oscillations. The measured damping coefficients are found to be constant and are bclieved to be related to the thermal damping coetficient. The dependence of the dctached plume diameters and undetached plumc lengths on the containers radii, lengths, and heights above the still water level are also discussed. (Work supported by ONR and ONT.]

2013

J. Acoust. Soc. Am., Vol. 89, No. 4, Pt. 2, April 1991

121 sl Meeting: Acoustical Society of America

2013