High speed flights of supersonic aircraft over communities bordering on major jet bases and ordnance explosions at near-shore target bombing ranges brought to light the problem of blast waves. Residents complained about broken windows, crockery, etc., from the blasts and began filing suits against the government to recoup their losses.  

Because of the complaints, studies were conducted to evaluate noise levels under varying atmospheric conditions. From these studies, it was determined that certain atmospheric conditions add to the noise level of blasts, while other conditions lessen the noise level.

The predominant atmospheric conditions that add to the noise level of blasts are temperature increasing with height (inversions), wind speed increasing with height, and wind blowing toward an area of interest. In addition to the atmospheric conditions, the ordnance size, regional topography, and blast altitude also control the intensity of a blast wave.

As an Aerographers Mate, you may be required to work up sound focusing data. Therefore, in this lesson, we will discuss the methods and techniques used in evaluating environmental elements that may increase or decrease the intensity of blast waves.

Learning Objective: Identify the at-mospheric elements that control blast waves in the atmosphere.

Blast waves are generated whenever explosives are detonated or when a jet breaks the sound barrier (a sonic boom). As an air blast moves past a given point, air pressure changes. First, the pressure rises rapidly to a value above the ambient (free air) pressure, then it decreases more slowly to a value below the ambient pressure. Finally, the pressure returns to the ambient value. This pressure change, like that created by a bolt of lightning, is rapid and produces sound. Sound waves accompany blast waves, and the laws of sound propagation discussed earlier apply to both blast and sound waves. The terms blast waves and sound waves are used synonymously in this lesson.

Direction of Blast Waves  

To determine the path of a sound ray through the atmosphere, you need to know two things: (1) the initial direction of the ray when it leaves its source, and (2) the manner in which sound speed varies with altitude.

If sound speed is uniform throughout the air above ground level, sound rays move uniformly in all directions. If sound speed decreases from the surface upward, all sound rays are refracted upward, away from the surface. In this case, sound intensity along the surface decreases rapidly. If sound speed increases with altitude, sound rays are refracted downward, toward the surface. In this case, sound intensity increases at the surface.

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