Energy carried by a travelling wave


        Joe sings into a long cardboard tube of radius r = 0.2 m.
        He hits a perfect middle C (256 Hz), causing the air
        molecules to shake back-and-forth.  At the far end of the
        tube, the sound wave carries a power of 1 Watt = 1 Joule/sec .

        How large is the amplitude of motion of the air molecules
        in the tube?