I am supposed to have a qualitive knowledge of acoustic peaks for my exam, so none of the maths. After reading around I am still left with a few questions. Is the first acoustic peak from the acoustic oscillations at last scattering? I imagine there were many oscillations before then but we just won't know about them. The first acoustic peak is about 2 degrees in the sky, which is the same size as the particle horizon since last scattering so it would make sense it happened then. (I used 'particle horizon' meaning the distance photons could have travelled since last scattering, I hope I used it right). If this is right: How did the rest of the acoustic peaks get there? The next one is about 0.4 of a degree and that must be waaay past last scattering, so there would be no plasma to oscillate! Thanks for any help. And if what I said above doesn't make any sense, don't blame me, blame my useless course text!