A spherical implosion converts compression to rarefaction after passing through focus

Evidence

How can we understand the inside-out implosion/explosion? One method requires a little imagination but is fundamentally very simple. We start with a plane pulse consisting of a double layer—a compression followed directly behind by a rarefaction (figure 16.6, left). We construct a spherical imploding wave out of these by adding them up coming from every direction all at once, keeping all the double layers the same distance from a central point. When all the plane waves are added, the inner (orange) compression survives, but the outer (blue) rarefaction gets canceled out by overlapping with orange compression regions. Where orange and blue overlap in equal amounts, the result is white (neutral pressure). Each plane wave translates along at the speed of sound in the direction perpendicular to itself, but suddenly, after they all simultaneously pass through ground zero, the blue rarefaction regions find themselves on the inside of the now expanding sphere, whereas before they were on the outside. The blue regions now survive the superposition of all the waves, and orange has turned to blue! Viewing from inside a cone, the conclusion is that a compression comes back as a rarefaction. — Eric J. Heller, Why You Hear What You Hear- An…, 1464

Categorization

• XREF: Connects to the physics of sound wave inversion at foci, relevant to sonic weaponry and implosion focusing discussed elsewhere in the reader’s knowledge base.
• QUESTION: Does this phase inversion at a focal point have practical implications for how horns or shaped instruments radiate sound? Worth chasing.