No,no,no!
dude its simple,first;
1.get the orignal song with the beat, then
2. use traktor dj or a proffessional bpm counter to clock the bpm,then
3.use that bpm for your acapella
A good free bpm counter is WinBpm version 4 or higher or
use K.A.Z.A.A to find traktor dj or if u hav dsl or...