The problem with finding the tempo of an acapella is its hard to do without the original. The way I do it is find the original song the acapella is from and then work out the tempo for that by highlighting a few bars with the beat (the more the better) an calculating the bpm from that. Even this way, you will still need to adjust the tempo slightly to keep it on beat the whole way through the song. I normally chop the acapella into a few (normally about 3) sections, that way the acapella is less likely to go off beat if you trigger the sections in the correct place. If you don't have the original with a beat behind it to find it, you will have to trial an error a beat's tempo with the acapella playin over the top an adjust it depending whether it is too fast or too slow. Hope any of that helps...