Just take a look at the Interactive Frequency Chart
http://www.independentrecording.net/irn/resources/freqchart/main_display.htm
Some but not much happens above 7khz or so musically. That is not a bad tradeoff for a design that I have found to be most robust and durable as well as great sounding and practical for many.
Most conventional 2 way speakers have cross over much lower which if not done really well messes with the sound and coherency of a lot of elements of the music including vocals in particular.
Over the years I find I am seldom a fan of any but the best 3-way speaker designs out there when I hear them. Its those darn multiple crossovers in the business area of the music that is mostly the culprit I think.