Sonic booms occur, as you know, when air masses are split and then clap together. This occurs during thunderstorms also, after lightning superheats the air so that a semi-vacuum occurs. The air turbulence around a plane about to create a sonic boom is a combination of air pressure in front of the nose or wing, air pressure to the sides of the nose or above or below the wing, and a subsequent lack or pressure behind the wing or tail of the plane. Please note this uneven pressure, as it is the very same mechanism that causes thunder clapping. During thunder, lightning superheats the air it travels through, causing expansion in the path traveled by the lighting. After the electricity stops flowing, the state of the air is that superheated air has pushed away from the lightning path, creating a high pressure area at the sides, and where the lightning passed there is now low pressure. The sides both move toward the low pressure, resulting in two blocks of air bumping into each other and ricocheting away to eventually hit windows or ear drums and result in comment about the thunder clap.
When airplanes “break the sound barrier” they are simply moving fast enough to create turbulence of a sufficient degree that the air pressure closing in on relatively low air pressure places claps together, creating a reverberation that moves toward human ears. Same principle as thunder, different reason for the air turbulence. Such a mass is one or more of these high pressure masses moving outward from the fast moving plane or ricocheting off the earth and returning to meet another high pressure air mass or flowing, as air masses will, to the place of least resistance, inward toward the low pressure areas behind the plane’s wind and tail. Why do you suppose the term is “breaking” the sound barrier, and not “reaching” the sound barrier if the sonic booms continue at all speeds?
Humans have reasoned that the lack of continuous booming is due to the plane accelerating and climbing, so that booms occur at low altitudes and the lack of booms at high altitudes is due to the air turbulence dispersing or perhaps the air being thinner. Rapidly traveling planes slice the air, reducing the disturbance they cause, where the plane approaching the sonic boom point is pushing the air ahead of it, creating turbulence behind the plane and uneven pressure around it. Planes going greater than supersonic speed no long cause a continuing sonic boom, as you also well know. They zoom along, none the wiser on the ground unless they look up. This lack of clapping is due to slicing, rather than pushing, the air masses apart. Cutting with a sharp knife versus cutting with the edge of a fork. With a razor sharp knife, the mass being cut does not move, but with a dull fork, the mass being cut drags back and forth, dragging all attached back and forth with it.
Entertain for a moment the sounds caused by drums, large and small. The booming of the bass drum is caused by the broad area vibrating, creating vibrations that cause relatively large masses of air to move at once, where the tiny drum can barely be heard as it is moving a small air mass and the vibration is relatively rapid. If the vibration gets rapid enough, the ear does not hear it at all, as the ear drum cannot vibrate in sync. Likewise very low frequency sounds are not heard by humans, as the nerves to the ear are not attuned to the gaps in vibration. High or low frequency, thus, is tuned out as noise of one kind or another, and is not considered sound. So what happens when the plane increases speed past what humans erroneously call the sound “barrier”?
Our ships are beyond the speed of your supersonic planes, from the moment they determine to move. Its as simple as that, we skip the sonic boom period.