Infrasonic signals generated by daily supersonic Concorde flights between North America and Europe have been consistently recorded by an array of microbarographs in France. These signals are used to investigate the effects of atmospheric variability on long-range sound propagation. Statistical analysis of wave parameters shows seasonal and daily variations associated with changes in the wind structure of the atmosphere. The measurements are compared to the predictions obtained by tracing rays through realistic atmospheric models. Theoretical ray paths allow a consistent interpretation of the observed wave parameters. Variations in the reflection level, travel time, azimuth deviation and propagation range are explained by the source and propagation models. The angular deviation of a ray's azimuth direction, due to the seasonal and diurnal fluctuations of the transverse wind component, is found to be ϳ5°from the initial launch direction. One application of the seasonal and diurnal variations of the observed phase parameters is the use of ground measurements to estimate fluctuations in the wind velocity at the reflection heights. The simulations point out that care must be taken when ascribing a phase velocity to a turning height. Ray path simulations which allow the correct computation of reflection heights are essential for accurate phase identifications.
[1] Microbaroms are permanent infrasonic waves produced by interacting open ocean swells near low-pressure systems. Continuous infrasound monitoring over 5 years show that microbaroms are globally observed at several middle-and high-latitude infrasound stations that are part of the International Monitoring System (IMS). The arrival azimuths and amplitude of the signals exhibit clear seasonal trends driven primarily by the seasonal reversal of the zonal stratospheric wind. A scaling relation between the signal amplitude and the strength of the upper wind suggests that most of the microbarom energy propagates in the ground to stratosphere waveguide. We show that continuous microbarom measurements can help to evaluate global infrasound detection capabilities, providing new insights on quantitative relationships between infrasonic observables and atmospheric specifications.
The rotary subwoofer is a novel acoustic transducer capable of projecting infrasonic signals at high sound pressure levels. The projector produces higher acoustic particle velocities than conventional transducers which translate into higher radiated sound pressure levels. This paper characterizes measured performance of a rotary subwoofer and presents a model to predict sound pressure levels.
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