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One of the fascinating terms in the Drake equation is L, the length of time that a civilization might radiate detectable signals. I guess I might add another term, the length of time that the one known civilization (us) might be able to detect such signals. Prior to about 1900, we might assume that we were looking for signal fires or some such optical medium. Since that time, we might be listening on radio frequencies. Only since about 1951 have we made the assumption that the best frequency for such intergalactic communication might be the hydrogen window. Only since around 1960 have we been listening at all, with any effort.
Note that both the L term and my new term don't change the number of possible civilaztions, just the likelihood of detecting them.
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