Figure 4 : This shows the evolving cosmic sound spectra. The range 0-2000 Hz is shown along the x axis, with time along the y axis from top (Big Bang) to bottom (fog clearing at 380,000 years). Notice (a) at any time the sound is composed of several harmonics; (b) each harmonic is quite broad, containing a range of frequencies; and (c) as time passes the overall pitch of the harmonics drops, settling down after about 100,000 years to a deep roar. (This figure represents the first half of sound1.wav, but does not include the subsequent growth of higher frequencies to make the loud hiss).