i belive the official name for the oooohhhhmmm sound is a "moose call"? the theory behind it is the same as violin and its bow(pull& release). the vibration you feel on you finger is the "pull" and "release"this is due to friction. each "P"&"R" produces a small sound but it this repeats many times a second and makes the head resonate, which makes the sound much louder.
the pitch change is because of the location were the sound is being produced. on the edge if the drum there is a higher pitch and in the center there is a lower one( same as on all drum.)
it is posible to get this effect on sintetic heads but because the are slik, some times it is more difficult to produce a quality tone.
