Sound in general travels away from a source and decreases in magnitude. The decrease in magnitude is primarily associated with the wave covering a larger area, reducing the energy at any given point further away. For instance, a wave from a point source will have a given amount of energy one foot away from the source. The energy will be distributed through an area surrounding the point source (area of a sphere in an open environment). As the sound moves away, the same energy is distributed through a much larger area reducing the amplitude. Therefore, the sound amplitude at any point 40 ft away is much smaller than at a distance of 1 ft simply due to geometry. In addition, the amplitude of the wave is reduced by the damping in the media, but this effect tends to be much less important in a media such as air.
When the wave is guided through a pipe, the effect of distance away from the source is different than in open air conditions. Instead of spreading out through the environment, the wave is contained in the pipe. Therefore, the amplitude of the wave does not diminish strictly due to distance away from the source. The amplitude is reduced through damping in the media and friction losses in the pipe. Due to these effects, pressure wave fluctuations in a pipe, due to various sources, can travel a long distance without much loss in amplitude. Planar waves can travel in a wide range of frequencies with little restriction.
The other important aspect of a planar wave in a pipe is that the pulse does not directly contact the pipe wall except at bends and reducers or other obstructions. This aspect makes it difficult for a planar wave to couple with a pipe wall or shell mode of a pipe. A planar wave can cause pipe walls to expand and contract due to the change in pressure between the peaks of the pulsation, which will cause the pipe to exhibit a forced response at a defined frequency corresponding to the planar wave.