Sonic-boom noise penetrating under a deep ocean is affected by its time-dependent interaction with the surface waves, which can significantly influence the perceived sound pressure level and tonal content of the disturbances at depth far greater than expected from the flat-ocean (Sawyers) model. The present theory assumes a small surface slope and a high water-to-air density ratio; the ocean surface in the analysis is modelled by a sinusoidal surface-wave train. The analysis shows that a distinct acoustic wave mode in the form of a packet of wavelets emerges in the sound field far below the surface and attenuates with increasing distance in a manner similar to the cylindrical spreading of monochromatic waves. The latter feature renders the surface waviness influence an effect of first-order importance, overwhelming the primary noise field at large depth. Detailed properties of the deep-water wave fields are examined and illustrated for the case of an incident N-wave, for which an explicit, analytic solution is obtained. The result reveals a similarity structure of the wave field with two distinct time scales and the invariance characteristics of the cylindrically spreading waves, in accord with the group-velocity concept of dispersive waves. An example is given of the interaction, illustrating the underwater waveform, sound-pressure and frequency levels.
A sonic boom is a sound associated with shock waves created when an object travels through the air faster than the speed of sound. Sonic booms generate enormous amounts of sound energy, sounding similar to an explosion or a thunderclap to the human ear. A decibel is the primary unit measurement of sound. "A thunderclap is incredibly loud, producing levels between 100 and 120 dBA (decibels A)- the equivalent of standing near a jet during take-off."[2]
Sonic Mechanics – New Wave Deep House 2
Several smaller shock waves can and usually do form at other points on the aircraft, primarily at any convex points, or curves, the leading wing edge, and especially the inlet to engines. These secondary shockwaves are caused by the air being forced to turn around these convex points, which generates a shock wave in supersonic flow.
The later shock waves are somewhat faster than the first one, travel faster and add to the main shockwave at some distance away from the aircraft to create a much more defined N-wave shape. This maximizes both the magnitude and the "rise time" of the shock which makes the boom seem louder. On most aircraft designs the characteristic distance is about 40,000 feet (12,000 m), meaning that below this altitude the sonic boom will be "softer". However, the drag at this altitude or below makes supersonic travel particularly inefficient, which poses a serious problem.
The pressure from sonic booms caused by aircraft is often a few pounds per square foot. A vehicle flying at greater altitude will generate lower pressures on the ground, because the shock wave reduces in intensity as it spreads out away from the vehicle, but the sonic booms are less affected by vehicle speed.
In the late 1950s when supersonic transport (SST) designs were being actively pursued, it was thought that although the boom would be very large, the problems could be avoided by flying higher. This assumption was proven false when the North American XB-70 Valkyrie first flew, and it was found that the boom was a problem even at 70,000 feet (21,000 m). It was during these tests that the N-wave was first characterized.
Seebass and George also worked on the problem from a different angle, trying to spread out the N-wave laterally and temporally (longitudinally), by producing a strong and downwards-focused (SR-71 Blackbird, Boeing X-43) shock at a sharp, but wide angle nose cone, which will travel at slightly supersonic speed (bow shock), and using a swept back flying wing or an oblique flying wing to smooth out this shock along the direction of flight (the tail of the shock travels at sonic speed). To adapt this principle to existing planes, which generate a shock at their nose cone and an even stronger one at their wing leading edge, the fuselage below the wing is shaped according to the area rule. Ideally this would raise the characteristic altitude from 40,000 feet (12,000 m) to 60,000 feet (from 12,000 m to 18,000 m), which is where most SST aircraft were expected to fly.[8]
This remained untested for decades, until DARPA started the Quiet Supersonic Platform project and funded the Shaped Sonic Boom Demonstration (SSBD) aircraft to test it. SSBD used an F-5 Freedom Fighter. The F-5E was modified with a highly refined shape which lengthened the nose to that of the F-5F model. The fairing extended from the nose all the way back to the inlets on the underside of the aircraft. The SSBD was tested over a two-year period culminating in 21 flights and was an extensive study on sonic boom characteristics. After measuring the 1,300 recordings, some taken inside the shock wave by a chase plane, the SSBD demonstrated a reduction in boom by about one-third. Although one-third is not a huge reduction, it could have reduced Concorde's boom to an acceptable level below FM = 1.
As a follow-on to SSBD, in 2006 a NASA-Gulfstream Aerospace team tested the Quiet Spike on NASA-Dryden's F-15B aircraft 836. The Quiet Spike is a telescoping boom fitted to the nose of an aircraft specifically designed to weaken the strength of the shock waves forming on the nose of the aircraft at supersonic speeds. Over 50 test flights were performed. Several flights included probing of the shockwaves by a second F-15B, NASA's Intelligent Flight Control System testbed, aircraft 837.
There has been recent work in this area, notably under DARPA's Quiet Supersonic Platform studies. Research by acoustics experts under this program began looking more closely at the composition of sonic booms, including the frequency content. Several characteristics of the traditional sonic boom "N" wave can influence how loud and irritating it can be perceived by listeners on the ground. Even strong N-waves such as those generated by Concorde or military aircraft can be far less objectionable if the rise time of the over-pressure is sufficiently long. A new metric has emerged, known as perceived loudness, measured in PLdB. This takes into account the frequency content, rise time, etc. A well-known example is the snapping of one's fingers in which the "perceived" sound is nothing more than an annoyance.
The study of such sound waves is sometimes referred to as infrasonics, covering sounds beneath 20 Hz down to 0.1 Hz (and rarely to 0.001 Hz). People use this frequency range for monitoring earthquakes and volcanoes, charting rock and petroleum formations below the earth, and also in ballistocardiography and seismocardiography to study the mechanics of the heart.
When Gavreau and the team attempted to measure an amplitude and pitch, they were shocked when their equipment detected no audible sound. They concluded the sound being generated by the motor was so low in pitch that it was below their biological ability to hear, and that their recording equipment was not capable of detecting these frequencies. Nobody had conceived that sound might exist at such low frequencies, and so no equipment had been developed to detect it. Eventually, it was determined that the sound inducing the nausea was a 7 cycle per second infrasound wave that was inducing a resonant mode in the ductwork and architecture of the building, significantly amplifying the sound.[3] In the wake of this serendipitous discovery, the researchers soon got to work preparing further infrasonic tests in the laboratories. One of his experiments was an infrasonic whistle, an oversized organ pipe.[4][5][6] As a result of this and similar incidents, it has become routine in new architecture construction to inspect for and eliminate any infrasonic resonances in cavities and the introduction of sound-proofing and materials with specialized sonic properties.
Some animals have been thought to perceive the infrasonic waves going through the earth, caused by natural disasters, and to use these as an early warning. An example of this is the 2004 Indian Ocean earthquake and tsunami. Animals were reported to have fled the area hours before the actual tsunami hit the shores of Asia.[34][35] It is not known for sure that this is the cause; some have suggested that it may have been the influence of electromagnetic waves, and not of infrasonic waves, that prompted these animals to flee.[36]
On 31 May 2003, a group of UK researchers held a mass experiment, where they exposed some 700 people to music laced with soft 17 Hz sine waves played at a level described as "near the edge of hearing", produced by an extra-long-stroke subwoofer mounted two-thirds of the way from the end of a seven-meter-long plastic sewer pipe. The experimental concert (entitled Infrasonic) took place in the Purcell Room over the course of two performances, each consisting of four musical pieces. Two of the pieces in each concert had 17 Hz tones played underneath.[60][61]
Many scientists were surprised that a single eruption could produce a Pacific-wide tsunami of about 1 meter (3 feet) that smashed boats in New Zealand and caused an oil spill and two drownings in Peru. Lane said that oceanwide tsunamis are usually triggered by earthquakes that extend across huge areas rather than from a single volcano, essentially a tiny dot in the ocean. She said other factors may have been at play, such as an underwater flank of the volcano collapsing and displacing water. She said one interesting theory is that the shock wave, or sonic boom, from the volcano that traveled twice around the world may have pumped more power into the tsunami waves.
The conference will cover the application of static and dynamic test methods to deep foundations, both driven and cast-in-place, offshore as well as onshore. Subject matters that will be covered include wave mechanics applications to foundations, high strain dynamic testing, low strain integrity testing, rapid load testing, static load testing (compression, tension as well as bi-directional), soil-structure interaction from dynamic, rapid and static testing, lateral testing, vibratory analysis and monitoring of piles and sheet piles, and pile installation effects and drivability in relation to soil investigation methods. 2ff7e9595c
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