Impact of a drop on a water surface is accompanied by a series of sound pulses propagating in air and underwater. Depending on the falling mode (drop size and initial velocity), pulses substantially differ in amplitude, duration, and modulation frequency. We study falling modes in which in addition to conventional sound packets—the shock pulse and single resonance sound packets—several packets are observed. Experiments were conducted with simultaneous recording of sound in air and underwater and were accompanied by synchronous video depiction of currents in the drop impact region. Comparison of videograms and phonograms demonstrate that the sources of sound packets are gas cavities of arbitrary shape detached from the underwater cavern under the action of large accelerations (several km/s2) during a sharp change in its surface area, which gradually achieve equilibrial elliptical and spherical shapes.

Sound Generation as a Drop Falls on a Water Surface

• Desertification is a major environmental problem in arid and semiarid regions. • We evaluated the influence of vegetation cover and rainfall on erosion. • Vegetation cover significantly influenced the total soil carbon and nitrogen stocks. • Rainfall intensity significantly influenced the runoff rate. • The results of this study can be used as a reference for calibrating erosion models. Desertification is a major environmental problem in arid, semiarid, and dry sub-humid regions throughout the world, which causes considerable reductions of ecosystem services, increased food insecurity, and poverty. The influence of vegetation cover and rainfall intensity on the dynamics of water infiltration, surface runoff, and soil loss was measured in situ in a desertification zone in the semi-arid region of Brazil. Twenty-four plots were evaluated in two areas, each with the same type of soil that is predominant in the region: 1) Luvisol with dense vegetation cover (LD) and 2) Luvisol with sparse vegetation cover (LS). A rainfall simulator was used to simulate either 60 or 100 mm h −1 of rainfall. Soil hydraulic conductivity, soil water retention curves, and soil carbon and nitrogen stocks were also determined. Water infiltration was greater in the LD (53 mm h −1 ) than in the LS (15 mm h −1 ), while the runoff rates were 27 mm h −1 for the LD and 61 mm h −1 for the LS. Rainfall intensity significantly influenced the time when runoff started and the runoff rate accelerated soil loss. Vegetation cover significantly influenced the total soil carbon and nitrogen stocks, as well as the saturated hydraulic conductivity of the soil. This is the first study in a semi-arid desertification zone of Brazil that used simulated rainfall to determine effects rain intensity on soil properties. The results of this study can be used as a reference for calibrating erosion models in areas undergoing a desertification process. 

Influence of vegetation cover and rainfall intensity on soil attributes in an area undergoing desertification in Brazil

Sound is produced by many geomorphic and hydrological processes, such as rockfalls and landslides, ocean waves, fluvial flood flows and collisions among moving bedload clasts. In these and other areas of study, acoustic methods have found useful application to detect and quantify the operation of important landscape processes. In some, such as the recording of river discharge, the occurrence of rare events such as exfoliation or the presence and movement of dust devils (willy-willies), the use of acoustic methods is still in a relatively early stage of development and testing. The use of acoustic methods in the recording of rainfall occurrence and intensity is also developing and has the capacity to yield data with higher temporal resolution than can be achieved using conventional rain gauges. Novel acoustic methods include the analysis of the sound recorded by security cameras, which potentially form a vast network of observing stations. The frequencies of sound generated by land-surface processes include audible sound, ultrasound and infrasound at frequencies below the human hearing range. All appear to provide opportunities for further development of useful research tools and methodologies.

Acoustic methods in physical geography: Applications and future development

Abstract Firebrand travel and ignition of spot fires is a major concern in the Wildland-Urban Interface and in wildfire operations overall. Firebrands allow for the efficient breaching across fuel-free barriers such as roads, rivers and constructed fuel breaks. Existing observation-based knowledge on medium-distance firebrand travel is often based on single tree experiments that do not replicate the intensity and convective updraft of a continuous crown fire. Recent advances in acoustic analysis, specifically pattern detection, has enabled the quantification of the rate at which firebrands are observed in the audio recordings of in-fire cameras housed within fire-proof steel boxes that have been deployed on experimental fires. The audio pattern being detected is the sound created by a flying firebrand hitting the steel box of the camera. This technique allows for the number of firebrands per second to be quantified and can be related to the fire's location at that same time interval (using a detailed rate of spread reconstruction) in order to determine the firebrand travel distance. A proof of concept is given for an experimental crown fire that shows the viability of this technique. When related to the fire's location, key areas of medium-distance spotting are observed that correspond to regions of peak fire intensity. Trends on the number of firebrands landing per square metre as the fire approaches are readily quantified using low-cost instrumentation. 

/pdf/quantifying-firebrand-production-and-transport-using-the-23jx8sn2.pdf

Quantifying Firebrand Production and Transport Using the Acoustic Analysis of In-Fire Cameras

Raindrop sizes were indirectly determined through the distinctive vibrations and sounds that occur when raindrops of different sizes impact on a membrane. The relationships between diameter, raindrop kinetic momentum, membrane deflection, and impact force were analyzed. Membrane deflection and vibration models were created, and the optimum membrane diameter and thickness were identified. A prototype membrane disdrometer was developed on the basis of these findings. The relationship between raindrop size and membrane vibration sound pressure was determined experimentally. The fundamental parameter of raindrop size recognition was used. The test results indicate that raindrops 0.4–2.0 mm in diameter can be detected by the proposed membrane disdrometer, and 50 raindrops per second in a zone area of 28.3 cm2 can be detected in natural rain.

http://iopscience.iop.org/article/10.1088/0957-0233/26/11/115103/pdf

A membrane disdrometer based on membrane vibration

Microwave engineers and geomorphologists require rainfall data with a much greater temporal resolution, and a better representation of the numbers of large raindrops than is available from current commercial instruments. This paper describes an acoustic instrument that determines rain parameters from the sound of raindrops falling into a tank of water. There is a direct relationship between the kinetic energy of a raindrop and the acoustic energy that it creates upon impact. Rain kinetic energy flux density (KE) is estimated from measurements of the sound field in the tank and these have been compared to measurements from a co-sited commercial disdrometer. Six months data has been collected in the Eastern UK. Comparisons of rain KE estimated by the two instruments are presented and links between KR and rainfall intensity (RI) are discussed. The sampling errors of the two instruments are analysed to show that the acoustic instrument can produce rain KE measurements with a onesecond integration time with sampling uncertainty of the same size as commercial instruments using a oneminute integration time.

/pdf/1-1-an-acoustic-disdrometer-the-measurement-of-rain-kinetic-57yz6404m8.pdf

Philip N. Winder

Papers

1.1 an acoustic disdrometer: the measurement of rain kinetic energy and rain intensity using an acoustic disdrometer