This is just an example of leaving out a part of the sentence that is understood from the sentences surrounding it. In this case:
The further off this solid obstruction [is from the source of the noise], the longer time will elapse for the return of the echo.
"this solid obstruction" refers to the wall or mountain mentioned in the previous sentence.
You can make this kind of sentence in English, for another example, in the same context of echoes, we could say; "The further off, the quieter it gets." But this only works in a situation where we can infer what "it" is, and what that thing is further off from. In this case, "it" is the echo, and it is further off from the solid surface, but only if the other text is also there.
It would be more natural in normal speech (in my opinion) to say "The further off this solid obstruction is, or The further off this solid obstruction is from the source of the noise" in order to avoid confusion, but the example you gave is also technically correct.