[tempestn]

As others have said, the boom is continuous. However, what you could (and presumably would) do is gain altitude at sub-sonic speed, then go supersonic only once cruising altitude is reached. That should help with volume. Also presumably they would avoid flying over major population centers.

[mark-r]

Is the boom attenuated by flying through much less dense air?

[tempestn]

Possibly! Certainly if the plane was traveling in a vacuum it would produce no boom, so one would assume that the boom would be reduced as the air became less dense. However my understanding is that most of the advantage from altitude comes from the attenuation (or really spreading out) of sound with the square of distance.

[mark-r]

Since hearing response is logarithmic, the inverse square law probably isn't as helpful as you'd expect.